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An Analysis of compound specific carbon isotopes of lipid biomarkers

University of Florida Institutional Repository

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AN ANALYSIS OF COMPOUND SPECI FIC CARBON ISOTOPES OF LIPID BIOMARKERS: A PROXY FOR PALEOEN VIRONMENTAL CHANGE IN THE MAYA LOWLANDS OF PETEN, GUATEMALA By SARAH DAVIDSON NEWELL A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Sarah Davidson Newell

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I find the great thing in this world is not so much where we stand, as in what direction we are movingwe must sail sometimes with th e wind and sometimes against it, but we must sail, and not drift, nor lie at anchor. Oliver Wendell Holmes

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ACKNOWLEDGMENTS First and foremost I would like to thank my advisor, Dr. David A. Hodell, for his constant support, encouragement and inspiration. I appreciate most his faith in my abilities as a research scientist and unspoken prodding to work harder. Over the past two years I have developed a stronger passion for research for which he is unduly responsible. Perhaps the most influential person through my masters research was Dr. Jason H. Curtis, to whom I now owe my first-born child. Over the past two years, Jason has provided constant and unparalleled support both in the lab and in life. Jasons assistance with all aspects of my lab work and method development is the sole reason why I was able to complete my MS in just two years. Jason provided to me a helping hand in the lab (at the drop of a hat, I should note), an ear for listening to complaints and excitement alike, and a shoulder during stressful times. I would especially like to thank him for the hours spent with our heads buried in the GC oven and for putting up with my spastic personality. A number of other individuals were instrumental in the completion of my research. I would like to thank Dr. Mark Brenner for constantly reviewing my work and providing additional advisement. Marks own excitement for this work was an integral part of my completing this project. I would like to thank Dr. Thomas P. Guilderson for allowing me to travel to the CAMS facility at Lawrence Livermore National Laboratory to run my own radiocarbon dates. The experience was incredible and my project has greatly benefited from having the additional radiocarbon dates. I would also like to thank Drs. iv

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Prudence Rice, Katherine Emery and Andrew Zimmerman for the exchange of ideas and thoughtful discussions. I thank Drs. Barbara Leyden, Gerald Haug and Yongsong Huang for sharing their data with me and Drs. Mark Pagani and Yongsong Huang for assistance in the development of my methods. I also thank Jenny Slosek for running all of the bulk isotope data from Lake Sacnab. There are three individuals to whom sincere thanks are owed for their personal support. Dave Buck, Ellie Harrison-Buck and Michael Hillesheim have provided me with friendship that is unequaled. Each one has supported me in their own individual way and I can not imagine having done any of this without their love and understanding. Finally, I would like to thank my parents, Michael and Karen Newell, as well as my siblings, Erik and Jessica Sanderson and Jeremy and Erika Newell. Without their unconditional love and support, I never would have made it! v

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TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES...........................................................................................................viii LIST OF FIGURES...........................................................................................................ix ABSTRACT......................................................................................................................xii CHAPTER 1 INTRODUCTION........................................................................................................1 2 BACKGROUND..........................................................................................................8 Modern Setting.............................................................................................................8 Cultural History of the Petn......................................................................................10 Interactions among the Ancient Maya, Climate and Environment.............................13 Study Sites..................................................................................................................22 Lake Sacnab.........................................................................................................22 Lake Salpetn......................................................................................................23 Compound-Specific Carbon Isotopic Studies.............................................................23 3 METHODS.................................................................................................................27 Core Collection...........................................................................................................27 Lake Sacnab.........................................................................................................27 Lake Salpeten......................................................................................................28 Chronology.................................................................................................................29 Lake Sacnab.........................................................................................................29 Lake Salpeten......................................................................................................29 Bulk Elemental Geochemical Analyses......................................................................29 Bulk Carbon and Nitrogen Isotopic Analyses............................................................30 Compound-specific Carbon Isotopic Analyses..........................................................30 4 RESULTS...................................................................................................................33 Chronology.................................................................................................................33 Lake Sacnab.........................................................................................................33 vi

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Lake Salpeten......................................................................................................33 Elemental Geochemical Analyses..............................................................................38 Lake Sacnab.........................................................................................................38 Bulk Carbon and Nitrogen Isotopes...........................................................................41 Compound-Specific Carbon Isotopes.........................................................................42 Lake Sacnab.........................................................................................................42 Lake Salpeten......................................................................................................44 5 DISCUSSION.............................................................................................................46 Implications for Changes in Land-use........................................................................46 Sources of Organic Matter..........................................................................................50 Evidence for Relative Shifts in Vegetation................................................................58 Comparison with Pollen Records...............................................................................66 Comparison with Population Estimates......................................................................70 Relationship between Climate and Environmental Changes......................................73 Conclusions.................................................................................................................76 APPENDICES A LIPID EXTRACTION PROCEDURE.......................................................................80 B SILICA GEL CHROMATOGRAPHY......................................................................82 C UREA ADDUCTION AND GC ANALYSIS............................................................84 D DATA TABLES.........................................................................................................86 LIST OF REFERENCES...................................................................................................97 BIOGRAPHICAL SKETCH...........................................................................................104 vii

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LIST OF TABLES Table page 1: Perkin Elmer 8500 Gas Chromatograph oven program for sample analyses................31 2: Hewlett Packard 6890 GC oven program for sample analyses.....................................32 3: AMS 14 C dates for samples from sediment core SN-19-VII-97 from Lake Sacnab. For the LLNL-CAMS samples, backgrounds were scaled relative to sample size using Pliocene wood blanks prepared at UF (4g)..............................................34 4: AMS 14 C dates for samples from sediment core SP-12-VI-02-1A from Lake Salpeten....................................................................................................................35 5: Tie points used to correlate the %CaCO 3 record from core SP2-19-VII-99 (Rosenmeier et al., 2002b) and the scanning XRF Ca concentration data from core SP-12-VI-02-1A (correlation coefficient = 0.505)...........................................36 viii

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LIST OF FIGURES Figure page 1: Location map showing the approximate location of the Petn Lake District in northern Guatemala. Map adapted from Map #802723 from the Civil Intelligence Agency (http://www.cia.gov/cia/publications/pubs.html)......................2 2: Impact of long-term Maya settlement on the terrestrial and aquatic environments in the Petn, Guatemala. Modified from Rice (1985).................................................15 3: Population density estimates versus time (cal yr BP) and time periods for Lake Sacnab and Lake Salpetn from the Middle Preclassic (cal yr BP) to the Late Postclassic (cal yr BP). Data from Rice and Rice, 1990. ....6 4: Location map showing: (A) the Yucatan Peninsula and the location of the Petn Lake District within Guatemala, (B) detail of the Petn Lake District, (C) the bathymetry of Lake Sacnab and location of core SN-19-VII-97 and (D) the bathymetry of Lake Salpetn and location of cores SP-12-VI-02 and SP2-99........22 5: Histograms from Ficken et al. (2000) showing the molecular distribution of n-alkanes from the three categories: terrestrial, emergent, and submerged/floating. Only odd carbon number distributions are shown and bars represent 1 standard deviation...................................................................................................................25 6: Depth versus calibrated age (yr BP) for terrestrial wood, seed and charcoal samples in Lake Sacnab core SN-19-VII-97. Squares indicate samples analyzed at LLNL-CAMS and the triangle indicates the sample measured at NOSAMS..........35 7: Correlation of SP2-19-VII-99 and SP-12-VII-02-1A using %CaCO 3 in the SP2-19-VII-99 core and the scanning XRF Ca concentration data from the SP-12-VII-02-1A core................................................................................................................37 8: Calibrated ages (yr BP) versus depth for correlated tie points in core SP-12-VII-02-1A as well as radiocarbon dates of terrestrial samples in Lake Salpeten core SP-12-VII-02-1A. ........................................................................................................38 9: Magnetic susceptibility, percent calcium carbonate (%CaCO3), percent organic matter (% OM), percent other (%Other) and percent nitrogen (%N) versus age in calibrated years before present (cal yr BP) from Lake Sacnab.40 ix

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10: Magnetic susceptibility versus age in calibrated years before present (cal yr BP) for core SP-12-VII-02-1A from Lake Salpeten. The gray highlighted area represents the Maya clay unit...............................................................................41 11: Bulk carbon and nitrogen isotopes for core SN-19-VII-97 from Lake Sacnab. The gray highlighted area represents the Maya clay unit............................................43 12: Compound-specific 13 C results of long chain n-alkanes (C 29 C 31 and C 33 ) from Lake Sacnab, Guatemala versus age in calibrated years before present (cal yr BP)............................................................................................................................44 13: Compound-specific 13 C results of long chain n-alkanes (C 29 C 31 and C 33 ) from Lake Salpeten, Guatemala versus age in calibrated years before present (cal yr BP)............................................................................................................................45 14: Comparison of 13 C of C 33 from Lake Sacnab with magnetic susceptibility..............51 15: Comparison of 13 C of C 33 from Lake Salpetn with magnetic susceptibility............52 16: C:N ratios in weight % and bulk organic matter isotopes from Lake Sacnab.............54 17: Comparison of 13 C values for bulk organic matter, C 31 and C 33 from Lake Sacnab......................................................................................................................55 18: Predominant n-alkane chain length (A) and CPI (B) for Lakes Sacnab and Salpetn....................................................................................................................57 19: Comparison of 13 C records for n-alkane chain C 33 in Lake Sacnab and Lake Salpetn ...................................................................................................................59 20: Diagram comparing the 13 C of C 33 to the 15 N of bulk organic matter in Lake Sacnab......................................................................................................................61 21: Relative shifts in contribution of C 4 vegetation (in %) in Lakes Sacnab and Salpetn over the last ~4500 cal yr BP.....................................................................63 22: Diagram showing the comparison between 13 C values of C 31 from Lake Sacnab, Lake Salpeten and Lake Quexil (Huang et al., 2001)..............................................65 23: Diagram showing % grass pollen versus 13 C of n-alkane C 33 in Lakes Salpetn and Sacnab and the presence of maize pollen in Salpetn (pollen data from Leyden, 1987)...........................................................................................................69 24: Comparison of the compound-specific carbon isotope records (C 33 ) and population density estimates versus time in Lakes Salpetn (top) and Sacnab (bottom)...........71 x

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25: Comparison of the compound-specific carbon isotope records (C 33 ) and percent Ti versus time in Lakes Salpetn and Sacnab. Percent Ti data from Haug et al. (2001).......................................................................................................................75 xi

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science AN ANALYSIS OF COMPOUND SPECIFIC CARBON ISOTOPES OF LIPID BIOMARKERS: A PROXY FOR PALEOENVIRONMENTAL CHANGE IN THE MAYA LOWLANDS OF PETEN, GUATEMALA By Sarah Davidson Newell May 2005 Chair: David A. Hodell Major Department: Geological Sciences The Petn region of northern Guatemala has been occupied by humans for more than 3000 years. During that time, the lowland tropical environment experienced a prolonged period of anthropogenic disturbance. Forest disturbance in the Petn Lake District of northern Guatemala was associated with Maya agricultural practices as well as clearing for urban development, construction, and fuelwood both for cooking and for lime-plaster production. Expansion of the Maya civilization during the Preclassic (~1000 BC to AD 250) and Classic periods (AD 250 to AD 900) was accompanied by increasing deforestation of Petn watersheds and accelerated rates of soil erosion. Palynological data from the Petn Lake District illustrate the near elimination of high forest taxa and prevalence of disturbance taxa (grasses, weeds) during the height of Classic Maya occupation (~AD 500-800). After flourishing during the Classic period between AD 250 xii

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and 800, the Maya civilization in the Petn Lake District experienced a dramatic change between AD 800 and 900 that some have referred to as the collapse. Population densities declined significantly after AD 900, thereby curtailing human pressures on the landscape. This cycle of population expansion and decline in the Petn Lake District provides a natural historical experiment that has been used to study the response of tropical vegetation to long-term changes in land-use by humans. A new line of evidence is used here to complement other archaeological and paleoenvironmental methods: the use of leaf wax biomarkers in palynological inference studies. The molecular and isotopic compositions of leaf waxes have been shown to be reliable indicators of vegetative biomass and are useful for testing palynological inferences. The carbon isotopic composition of long-chain n-alkanes of leaf waxes were used as a geochemical proxy for terrestrial vegetation to test palynological inferences of vegetation change in two lake basins (Lakes Sacnab and Salpetn) in the Petn Lake District of the southern Maya Lowlands over the past ~4500 cal yr BP. Discrepancies between the 13 C of long-chain n-alkanes and vegetation changes inferred from pollen profiles suggest that the two proxies may be recording different characteristics of watershed vegetation as well as different airshed/watershed processes. The data indicate that in the watersheds of Lakes Salpeten and Sacnab, shifts in the proportion of C 3 to C 4 vegetation are most likely controlled by a combination of climate change and human deforestation. The correspondence of 13 C records to independent proxies for climate change from ~4500 cal yr BP until ~3000 cal yr BP suggest that regional drying and increased climate variability caused an increase in the contribution of C 4 vegetation during that time. Following this period and beginning with the first Maya xiii

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occupation in the watersheds, vegetation change was likely a result of human-driven deforestation or perhaps a combination of both climate and human impact. xiv

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CHAPTER 1 INTRODUCTION The Petn region of northern Guatemala (Figure 1) has been occupied by humans for more than 3000 years. During that time, the lowland tropical environment experienced a prolonged period of anthropogenic disturbance. Forest disturbance in the Petn Lake District of northern Guatemala was associated with Maya agricultural practices as well as clearing for urban development, construction, and fuelwood both for cooking and for lime-plaster production. Expansion of the Maya civilization during the Preclassic (~1000 BC to AD 250) and Classic periods (AD 250 to AD 900) was accompanied by increasing deforestation of Petn watersheds and accelerated rates of soil erosion (Brenner, 1983). Palynological data from the Petn Lake District illustrate the near elimination of high forest taxa and prevalence of disturbance taxa (grasses, weeds) during the height of Classic Maya occupation (~AD 500-800) (Islebe et al., 1996; Leyden, 1987, Vaughan et al., 1985, Deevey, 1978). After flourishing during the Classic period between 250 and 800 AD, the Maya civilization in the Petn Lake District experienced dramatic change between 800 and 900 AD that some have referred to as the collapse. Population densities declined significantly after 900 AD, thereby curtailing human pressures on the landscape. This cycle of population expansion and decline in the Petn Lake District provides a natural historical experiment that can be used to study the response of tropical vegetation to long-term changes in land-use by humans (Deevey, 1969). 1

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2 Petn Lake District Figure 1: Location map showing the approximate location of the Petn Lake District in northern Guatemala. Map adapted from Map #802723 from the Civil Intelligence Agency ( http://www.cia.gov/cia/publications/pubs.html ).

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3 The exact timing and extent of deforestation during the period of Maya occupation and the reforestation that followed the collapse is debated among archaeologists and paleoecologists. Evidence currently used in the study of ancient human impact on the Maya lowland environments comes from paleobotanical, environmental archaeological and paleolimnological studies. Much of this evidence is derived from palynological research. However, much of this plant-based evidence is biased in one way or another and none can be used in isolation. For example, vegetation reconstructions inferred from pollen profiles may be biased because the relative abundance of pollen grains in a sediment profile cannot be directly related to species abundance or biomass (Bradley, 1999). Most pollen in lake sediments represents only the small percentage of tropical vegetation that is pollinated by wind and does not reflect those plants that depend on pollination by insects or self-fertilization. Because certain species produce a disproportionately large number of pollen grains, it is difficult to determine the actual composition of vegetation in the landscape. Furthermore, certain plants may produce pollen under conditions of stress rather than optimum growth (Bradley, 1999). Maize pollen has often been used in Mesoamerican studies as a proxy for agriculture and associated deforestation. One potential concern with traditional palynology, however, is that maize pollen is not typically included in total pollen counts, thus causing an over-representation of other taxa. In addition, maize pollen is large and typically only transported short distances. Alternatively, leaf-waxes derived from maize are probably transported readily and should reflect maize cultivation in the watershed accurately. These shortcomings underscore the need to validate palynological interpretations by independent means.

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4 A new line of evidence is used here to complement other archaeological and paleoenvironmental methods: the use of leaf wax biomarkers in palynological inference studies. The molecular and isotopic composition of leaf waxes have been shown to be reliable indicators of vegetative biomass (Hughen et al., 2004; Huang et al., 1999; Huang et al., 2001) and are useful for testing palynological inferences (Hughen et al., 2004). There are several advantages to using leaf wax biomarkers in conjunction with pollen to assess changes in vegetative biomass. Pollen and leaf waxes are derived from different vegetative sources and thus record different aspects of watershed vegetation (Huang et al., 1999). Whereas pollen reflects only reproductive effort for specific groups of plants, leaf waxes provide a more representative measure of vegetative biomass for various plant types within a watershed. Leaf waxes reflect the contribution of all land plants whereas pollen will disproportionately reflect wind-pollinated species. In addition, plants in the watershed that do not reproduce sexually or are dormant will produce a biomarker signal but yield no pollen. The isotopic composition of leaf waxes can be used as a geochemical proxy of terrestrial vegetation (Hughen et al., 2004; Huang et al., 1999, 2001). Long-chain (C 27 -C 33 ) n-alkanes exhibit a strong odd-over-even carbon-numbered dominance and are produced nearly exclusively by vascular plants as components of epicuticular leaf waxes (Meyers, 1997). The carbon isotopic composition of terrestrial plant biomarkers in lake sediments reflects the relative contribution to the sediments of alkanes coming from plants using the C 3 (tropical trees) versus C 4 (grasses) metabolic pathway. Plants that fix carbon by means of the C 3 pathway include all the high forest trees (Huang et al. 2001). C 4 plants include many tropical grasses and maize, which are associated with

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5 cleared land and agriculture in Mesoamerica. The 13 C values of n-alkanes in C 3 plants range from -31 to -38, whereas n-alkanes in leaf waxes of C 4 plants typically range from -19 to -25 (Freeman, 2001). Stratigraphic variations in the 13 C ratio of long-chain n-alkanes in lake sediment cores should therefore reflect changes in the proportion of C 3 to C 4 vegetation in a lakes watershed (Huang et al. 2001; Meyers, 1997). Here, the carbon isotopic composition of leaf waxes are used as a geochemical proxy for terrestrial vegetation type to test palynological inferences of vegetation change in two lake basins (Lakes Sacnab and Salpetn) in the Petn Lake District of the southern Maya Lowlands. Three specific questions are addressed in this study: 1) Is there a significant relationship between the 13 C of long-chain n-alkanes and vegetation changes inferred from pollen profiles? If the signals are correlated, then both long-chain n-alkanes and pollen are most likely recording the same vegetation changes in a watershed. For example, as disturbance taxa (grasses, weeds) replace high forest taxa, one would expect the 13 C of n-alkanes to increase, reflecting a greater proportion of C 4 biomass. Similarly, an increase in the relative proportion of high forest taxa during reforestation should be accompanied by a decrease in the 13 C of n-alkanes. Comparison of pollen taxa and long-chain, n-alkane 13 C records will reveal whether inferred vegetation changes based on the two proxies are in agreement or contradictory. Because pollen and carbon isotopes of leaf waxes may reflect different characteristics of watershed vegetation and different airshed/watershed processes, the two proxies need not necessarily agree. 2) Are 13 C values of long-chain n-alkanes in lake sediments correlated to Maya population density estimates within the same watershed? If agricultural land clearance

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6 was tied to population density, then increased disturbance taxa and maize are expected to increase as populations increased. Because grasses and maize are C 4 plants, one would predict an increase in the 13 C of n-alkanes with population growth. As populations declined and vegetation returned to a C 3 -dominated forest, a decrease in the compound-specific carbon isotopic ratios would be recorded. Population density and the 13 C of long-chain n-alkanes may be decoupled if, for example, agricultural practices changed through time from more extensive to intensive methods, or if alternative crops, other than maize, became important in the diet. The relation between population density and 13 C of long-chain n-alkanes can be addressed because population data are available from archaeological transects in the watersheds of both Lakes Sacnab and Salpetn. 3) What was the relative importance of humanand climate-induced changes in vegetation in the Central Petn It is difficult to assess the effect of climate change during the period of human occupation because it is difficult to tease apart human-induced versus climate-induced changes in vegetation. However, a comparison of the 13 C of n-alkanes with climate proxies that are not confounded by human impact may reveal whether climate played a role in vegetation change at all. If vegetation changes in a watershed were climate-induced, one would predict that long-chain nalkanes would correlate with independent, regional paleoclimate records. For example, a period of increased evaporation/precipitation (E/P) would likely coincide with an increase in the 13 C of n-alkanes, reflecting a greater proportion of C 4 biomass. A period of decreased E/P would likely correspond to a return to a C 3 -dominated forest. This question can be answered by comparing proxy records of climate change from Petn and the Caribbean Sea with

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7 compound-specific analyses, and evaluating those data in light of changes in Maya population densities.

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8 CHAPTER 2 BACKGROUND Modern Setting The Maya area, located in southeastern Mesoamerica, occupies a broad expanse of land and includes parts of the countries of Mexico, Guatemala, Belize, Honduras and El Salvador. The area has typically been both culturally and geographically divided by scholars into the highlands in the south and the lowlands in the north. The highlands, dominated by a volcanic landscape, refer to the area greater than 1000 ft above sea level (a.s.l.). and spread from southeastern Chiapas toward lower Central America. The lowlands consist of the Yucatan Peninsula in Mexico, Guatemala, and Belize. The topography of the lowlands is dominated by a limestone platform that has evolved into porous, karst hills with extensive dissolution features. While there are few permanently flowing rivers and lakes are rare in the northern lowlands, there are numerous lake basins in the department of Petn. Soils are relatively thin and vulnerable to rapid erosion with vegetation removal, making agriculture in this region challenging. Today, the lowland Maya practice shifting, slash-and-burn (swidden) agriculture that permits the forest to regenerate at intervals. The poor soils, however, can only be planted for ~2 years; after which, plots are fallowed for 4 to 7 years in the Petn and 15 to 20 years in other parts of the Yucatan (Coe, 1999). These poor soils, in addition to the stress

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9 added by highly seasonal rainfall, make the lowlands a rather harsh place to practice agriculture (Coe, 1999). The Petn Lake District is located in the Guatemalan Lowlands of the Yucatan Peninsula (Figure 1). The region contains numerous terminal basins that are aligned along a series of east-west aligned en echelon faults (Deevey et al,. 1979). The bedrock is karst limestone of Cretaceous and Tertiary age with elevations ranging from 100 to 300 m. Most of the lakes have deep troughs at the foot of a steep, north-shore fault scarp which gives rise to their distinctive bathymetry. The water table in the area is deep below the ground surface, making groundwater relatively inaccessible. Perched surface waters often result in seasonally inundated topographic depressions, or bajos, that are clay-floored and hold water during the rainy season (Deevey et al., 1979). The major water bodies of the Petn Lake District remain filled throughout the year and extend approximately 100 km from east to west. Soils in the Petn region are dominated by well-drained, mineral-rich mollisols that support a tropical semi-deciduous and evergreen forest (Lundell, 1937). Modern vegetation is variable throughout the region. The central Petn is mostly semi-deciduous subtropical moist forest while the southwest Petn is dominated by extensive savannas with forested hills that support a diverse, fire-resistant herbaceous flora. The savanna vegetation may have been created during the period of Maya occupation when much of the forest was burned for cultivation (Leyden, 1987). Alternatively, it may be a Pleistocene relict or an edaphic assemblage, the consequence of clayey, hydromorphic soils. The low-lying basins in the northeast Petn are dominated by swamp and

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10 marshland. Vegetation here is dominated by palms, grasses and sedges, but also includes a semi-deciduous higher forest (Lundell, 1937). Mean modern annual temperature is approximately 25C and mean annual precipitation is approximately 1600 mm with a pronounced dry season between January and May (Deevey et al., 1980). There is very little surface drainage in the area and most water that falls as rain is either transpired, evaporated, or directly enters the aquifer (Rosenmeier et al., 2002). The Petn typically receives higher rainfall than the rest of the Yucatan; there is a pronounced decrease in rainfall from south to north on the Yucatan Peninsula. There is also considerable interannual precipitation variability. These inter-annual variations force Maya farmers to practice long-term resource planning. Extensive droughts occur periodically in this area and previous studies have shown that this variability is not solely a modern occurrence (Hodell et al., 1995, 2001; Curtis et al., 1998). Cultural History of the Petn The ancient Maya civilization of Mesoamerica arose about 2000 B.C. and spanned a period of 3,000 years before undergoing a period of social and political change in the Late and Terminal Classic, between 750 and 1050 AD. These changes resulted in the development of very different Maya political, economic, and ideological systems and were associated with the cessation of construction of major architecture and elite monuments, the reduction of inland trade systems designed for the movement of elite status markers, and the abandonment of some but not all urban centers as populations dispersed into non-urban village settings, and migrated northward into the northern Yucatan and Belizean region. These political and social changes are inferred from the

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11 archaeological record of ancient Maya settlement. Maya civilization was unique in possessing the only written language in the Americas at the time. It also developed the most sophisticated and accurate calendar of the time, monumental architecture, a hierarchical social class system, sophisticated agricultural systems including intensive agriculture, and trade systems extending from northern Mexico to south of Honduras and El Salvador and perhaps as far as South America. The Maya also had a highly developed religious system based, during the Classic period, on semi-divine kingship and a noble class (Sharer, 1994). The Petn region was continuously occupied by the ancient Maya from the Middle Preclassic (1000 BC) through the Postclassic (AD 900 to 1525), and up to the Spanish conquest in 1697 (Rice and Rice, 1990). The Preclassic period (1000 BC to AD 250) in the Petn region is divided into the Middle Preclassic and Late Preclassic. It is during this period that the Maya are thought to have developed from hunters and gatherers into a complex civilization. This period marked the development of cities, temples and inscribed stone monuments. During the Middle Preclassic, archaeological evidence suggests the presence of social stratification as well as sophisticated religious and economic institutions. The first examples of writing appear in the Late Preclassic (Coe, 1999). The Classic Period was characterized by remarkable growth of the civilization both in terms of population and social complexity. The Classic Period (AD 300AD 900) is also subdivided into three periods: the Early Classic, Late Classic and Terminal Classic. During the Early and Late Classic periods, archaeological evidence suggests the development of states with centralized political and religious authority in addition to

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12 the erection of carved stone monuments (Coe, 1999). An important change in the political structure occurred at the end of this period, as authority became shared by a council of many instead of one individual (Coe, 1999). Populations grew slowly, but exponentially and in many regions displayed peak population densities between AD 700 and 800 AD (Rice and Rice, 1990). The Terminal Classic and Post-Classic Periods (AD 900 to AD 1525) are marked by diverse cultural responses throughout Mesoamerica. Much of the southern lowland region experienced the abandonment of Classic cities, temples and religious centers; subsequent political fragmentation resulted in a massive cultural decline that has been referred to as the collapse (Coe, 1999). The Maya Terminal Classic, however, is perhaps one of the most important periods in the Petn region. The uniqueness of central Petn lies in the ability to study not only the events leading up to the Maya Terminal Classic period, but the continuity of occupation following the collapse of Classic civilization (Rice and Rice, 2004). This period in the central Petn was both a center and a crossroads for Postclassic Maya civilization. Petn-like architecture and iconographic traits in the northern lowlands are evidence of Late Classic, conflict-driven migration from the Petn region. Throughout the Late Classic and Terminal Classic, the Petn region showed demographic loss, whereas the northern lowlands began to be heavily settled. The lakes region in particular suffered population decline, though the area was never completely abandoned. Archaeological evidence from every major Petn lake basin shows continuous occupation from the Late Classic to the Postclassic, with some indication of population migration between the Rio de Pasin region, the Gulf Coast, and the Petn Lake District during the Postclassic (Rice and Rice, 2004).

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13 The nature of settlements changed from the Late Classic to the Postclassic in the Petn Lakes District. Whereas Classic settlements expanded throughout the watershed, most settlement shifted to small, densely populated areas found primarily on islands and peninsulas in the lakes during the Postclassic. This settlement pattern in itself suggests a conflict-driven society. The location of these sites in poor agricultural zones, the presence of defensive structures, as well as the presence of intrusive residential architecture in previously occupied sites, suggests that the Late Classic migration of small groups between the Rio de Pasin and the northern lowlands forced them to settle into an already-established settlement system (Rice and Rice, 2004). Archaeological evidence overwhelmingly suggests that the Maya population in the Petn Lakes District did not collapse, but underwent a significant transformation. While there was a pronounced demographic decline, the collapse in the Petn did not result in complete depopulation. Instead, intersite conflict led to political restructuring and thus a redistribution of population, especially into fragile lands. The region was one of dynamic and contested lands, and remained so until AD ~1200 (Rice and Rice, 2004). Interactions among the Ancient Maya, Climate and Environment The Maya Lowlands have been studied by researchers seeking answers to questions about past human-climate-environment interactions. This research has concentrated on the central Petn, and in particular the Petn Lakes District. In 1972, Edward S. Deevey began a long-term paleoecological project in the Department of Petn, Guatemala. The Central Petn Historical Ecology Project (CPHEP) investigated both the paleolimnology and archaeology of major lake watersheds in the Central Petn (Rice et al., 1985). The objective of the project was to investigate both the social and natural

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14 history of the region through interdisciplinary research, focusing on ecological change as a result of human landscape transformations. One assumption of the CPHEP was that climate change was unimportant during the period of Maya occupation. Recent studies have recognized, however, that climate was not invariant through this period (Hodell et al., 1995; Rosenmeier et al., 2002; Curtis et al., 1998). Research design included paleolimnological studies from other regions that incorporate archaeological perspectives (Dunning et al., 1997, 1998; Demarest, 1997). In particular, lake sediment cores were used to develop long-term, high resolution records of environmental change within a watershed and were used in conjunction with archaeological surveys that estimated the timing and density of human occupation. The impacts of long-term Maya occupation in the Central Petn Lake District on both terrestrial and lacustrine environments were summarized by researchers in the CPHEP (Binford et al., 1987) (Figure 2). The model is generalized and does not necessarily describe the history of human and environmental changes for each and every lake basin. Archaeological surveys consisted of systematic mapping of settlement remains and test-pit excavations, and provide estimates of settlement patterns and population growth in the central Petn (Rice and Rice, 1990). Surveys were completed in each of three twin-lake basins: Sacnab-Yaxha, Macanche-Salpetn, and Quexil-Petnxil. The trends of population growth are notably similar in each basin surveyed, although the true variation was probably not captured by the coarse chronological framework, which was based on ceramic phases, not radiocarbon. Occupation of the region began at approximately 1000 BC and was followed by a steady increase until the end of the Late

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15 Classic (AD 750-800), when populations reached their peak in most basins (Figure 3). Population growth was exponential in the Yaxha-Sacnab basins but not in the other basins, which experienced a small population decline during the Early Classic. Cal yr BP Figure 2: Impact of long-term Maya settlement on the terrestrial and aquatic environments in the Petn, Guatemala. Modified from Rice et al. (1985). Sediment cores from numerous lakes in Petn show similar stratigraphic changes in sediment composition and geochemistry (Figure 2) (Cowgill and Hutchinson, 1966; Deevey et al., 1979; Brenner, 1983; Vaughan et al., 1985; Rice et al., 1985; Binford et al., 1987; Leyden, 1987; Curtis et al., 1998; Islebe et al., 1996). Holocene sediment prior

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16 to Maya occupation is comprised of organic-rich (30-60%) gyttja (Brenner et al., 2002). Overlying the pre-Maya gyttja is a clay-rich horizon known as the Maya clay. The Figure 3: Population density estimates versus time (cal yr BP) and time periods for Lake Sacnab and Lake Salpetn from the Middle Preclassic (cal yr BP) to the Late Postclassic (cal yr BP). Data from Rice and Rice, 1990. base of the Maya clay has been dated to approximately 3,000 yr BP (Brenner, 1994; Rosenmeier et al., 2002) and reflects accelerated erosion associated with early land clearance by humans (Deevey et al., 1979; Brenner, 1983; Vaughan et al., 1985; Rice et al., 1985; Binford et al., 1987; Leyden, 1987; Curtis et al., 1998; Islebe et al., 1996). Studies outside the region per se suggest the increase in colluviation may have been partly related to regional drying (Hodell et al., 1995; Hodell et al., 1996; Curtis et al., 2001). Nutrient-rich soils became unstable as forest cover was removed and replaced by a savanna-like landscape. Increased soil erosion is reflected by high net accumulation rates of lake sediments. As erosion progressed, organic-rich surficial soils were removed

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17 and the underlying bedrock was weathered and transported to the lake basin. This is reflected by the high proportion of inorganic sediments. Overlying the Maya clay unit is another organic-rich layer that is inferred to represent sediments deposited following the Maya collapse (Brenner et al., 2002). Deforestation and environmental disturbance accompanied growth of the Maya population from the Preclassic to the Late Classic (Figure 2). Vaughan et al. (1985) developed pollen stratigraphies for Lakes Quexil and Sacnab that indicate changes in vegetation through the period of Maya occupation. The pre-Maya pollen assemblage was dominated by high forest taxa such as Moraceae, whereas the early to mid-Preclassic is characterized by an open forest and, probably, culturally-induced savanna. Late Preclassic to Late Classic sediments are dominated by clay-rich sediments and 70% non-arboreal pollen. Vaughan et al. (1985) defined three distinct disturbance pollen zones. The Late Preclassic zone is characterized by savanna trees and shrubs. The landscape transitions to nearly grassland during the Early Classic, with only one type of arboreal pollen present (Ramon). The Late Classic to Postclassic contains only grassland taxa. Some increases in high-forest taxa near the top of this interval show evidence of reforestation, as does the pollen spectrum during the Postclassic period. If we assume an anthropogenic cause for the replacement of arboreal with grassland taxa, a sharp rise in arboreal pollen and a decrease in grassland types imply regional depopulation (Vaughan et al., 1985). Although each study is thought to reflect primarily local vegetation changes, multiple subsequent studies from the Petn and surrounding lowlands, with the exception of the work done by Cowgill et al. (1966) and Dunning et al. (1997), have all detected a similar decline in high-forest taxa from the Late Preclassic through the

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18 Terminal Classic that is interpreted to reflect human-driven deforestation (Islebe et al., 1996; Leyden, 1987, Vaughan et al., 1985, Deevey, 1978). The pollen zones were assigned ages by cultural zonation, or the correlation with ages of archaeological periods. Subsequently, they may contain some chronological error. However, these same pollen zones have now been independently dated in other lake cores (Leyden, 1987 and Rosenmeier et al., 2002) and are considered reasonable. Leyden (1987) used a 15 m sediment core from Lake Salpetn, Guatemala to develop a high-resolution pollen record from the basin to reconstruct Holocene vegetation changes. The sediment record spans from the pre-Maya to the present, and shows distinct evidence of Maya land clearance. While there is an apparent lack of disturbance taxa in the early Preclassic, the pollen zones representing the Late Preclassic through the Postclassic show evidence for abundant terrestrial herbs and strongly suggest Maya land clearance. Leyden attributes the lack of disturbance taxa in the early Preclassic to small local populations in the Salpetn basin at that time. Some discrepancies, however, may lie within the issues of chronological control. Leyden suggests that a few high-forest taxa actually increased initially, and declined later as forest removal was intensified. Presence of oak during the Classic period may suggest a savanna landscape, but this is relatively unclear. Leyden suggests that after the initial deforestation, the forest structure was relatively stable. However, greater proportions of maize pollen during the Late Classic through Postclassic indicate intensified agricultural activities. In the gyttja layer above the Maya clay, the concentration of total pollen grains nearly doubles, suggesting forest regrowth after depopulation of the lake basin (or slowed bulk sedimentation rate). Leyden proposes that these forests regenerated rapidly, but

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19 were more open than pre-Maya forests. This may have been related to climate rather than anthropogenic influences; regional drying may be suggested by the Post-Maya continuation of open forests. This period is dominated by secondary growth, as suggested by the increases in successional shrubs and trees. While most vegetation studies have focused on forest clearance, a select few have concentrated on revealing the dynamics of afforestation following the Maya collapse in the central Petn region of Guatemala (Wiseman, 1985; Brenner et al., 1990). Wiseman (1985) used pollen in lake sediments to track vegetation changes and the nature of these changes in response to cultural collapse in the Maya Lowlands. Seven cores from Lakes Petnxil and Quexil were collected and sampled for pollen analyses and used in conjunction with modern pollen and fauna studies that examined 0.1 ha plots in various stages of regrowth. This modern analog guided inferences about forest succession based on the fossil pollen record. All cores that penetrated into Late Classic Period deposits showed a subsequent replacement of agricultural weeds by secondary forest growth. The pollen spectrum during the early Classic to Postclassic was similar to samples taken from soils under swidden agriculture. One notable feature of the sediment cores is that each core shows essentially the same pollen spectra, suggesting that the conditions in the basin were in spatial equilibrium. Wiseman cites the decrease in maize pollen and subsequent recolonization by forest as key evidence for depopulation; however, poor chronological control makes it difficult to determine the precise timing of both environmental and demographic changes. Interpreting vegetation change from palynological data is further complicated by the challenge of obtaining accurate chronological control. Prior to the advent of AMS

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20 dating and the potential to measure 14 C in very small (~20g) samples of organic carbon, many early radiocarbon ages from Petn lakes were based on dating of bulk sediment or carbonate shells. Shells may incorporate carbon derived from the dissolution of ancient limestone and thus appear older than their true age. This problem extends to organic matter that is fixed photosynthetically within the lake, i.e. autochthonous organic matter. Primary producers may incorporate carbon derived from dissolved ancient limestone, and thus display too-old ages. This hard-water lake error has compromised the reliability of early chronologies. To further complicate matters, many subsequent studies assigned ages to the pollen stratigraphy by correlation with ages of archaeological periods. Dating error within one pollen record was thus transferred to records from other lake cores. Chronological imprecision in both sediment cores and archaeological studies creates challenges for linking environmental disturbance and human population sizes (Yaeger and Hodell, in press). Despite the overwhelming evidence for late Holocene vegetation change in the Petn region, it is difficult to distinguish between the impacts of climate and Maya occupation on forest composition. While the CPHEP assumed that climate was relatively constant for the past 10,000 years, recent work has proved otherwise. A sediment core from Lake Chichancanab in northern Yucatan provides evidence for regional drying that occurred beginning approximately 1000 BC with a distinct interval of droughts between AD 800 and AD 1000 that coincided with the Terminal Classic collapse (Hodell et al., 1995, Hodell et al. in press). Another paleoclimate record from Punta Laguna, on the northeastern Yucatan Peninsula, indicates alternating wet-dry shifts in the hydrologic balance through the late Holocene with the driest period lasting from approximately AD

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21 280 to 1080 (Curtis et al., 1998). Four severe drought events were revealed, including one that occurred during the Maya Hiatus (AD 600) and one that occurred concurrently with the Late Classic drought recorded in the Chichancanab record. A climate record from Lake Salpetn indicates a trend similar to the Chichancanab record (Rosenmeier et al., 2002). Lake levels decreased continuously from approximately 1000 BC, with the lowest lake level occurring between AD 800 and 900. This inferred drought coincides with the Terminal Classic collapse of the Classic Maya. The record also showed reduced soil erosion as well as forest recovery after AD 850, likely associated with Maya population decline (Rosenmeier et al., 2002). Rosenmeier argues, however, that these changes may not be entirely climate-related, but rather due to the effects of human-induced vegetation change on the lakes hydrology. The impact of humans on regional vegetation and soil stability, as well as the influence of climate change on the Classic Maya, illustrate the complex interplay among climate, humans, and the environment throughout the Late Holocene in the Maya Lowlands. While it is extremely difficult to separate the signals of human versus climate-controlled changes in the environment, further investigations of vegetation change with precise chronological control may provide additional evidence that the Maya had a profound impact on their lowland tropical environment. The natural experiment provides an excellent opportunity to study the relationship between changes in human population sizes, land-use activities, climate changes, and vegetation responses. Additionally, such studies can help us understand how ecosystems respond once human and climate pressures are curtailed.

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22 Study Sites Paleolimnological cores from two lakes in the Central Petn Lake District, one in the east and one in central Petn, were examined for compound-specific carbon isotopes (Figure 4). Comparison of the records from Lake Sacnab and Lake Salpetn permits a regional assessment of vegetation change. Figure 4: Location map showing: (A) the Yucatan Peninsula and the location of the Petn Lake District within Guatemala, (B) detail of the Petn Lake District, (C) the bathymetry of Lake Sacnab and location of core SN-19-VII-97 and (D) the bathymetry of Lake Salpetn and location of cores SP-12-VI-02 and SP2-99. Lake Sacnab Lake Sacnab (1703N and 8923W) is located in the eastern part of the Petn Lake District near the border between Guatemala and Belize (Figure 4c). The lake has a surface area of 3.9 km 2 and is ~3.5 km long by 1.5 km wide (Deevey et al., 1980). The maximum depth is 13 m. The lake is thermally stratified, but occasional mixing is

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23 indicated by the absence of permanent stagnation. Lake waters are relatively poor in nutrients, with the majority of nutrients being supplied by erosion of upland soils (Deevey et al., 1980). Lake Sacnab has no outflow, thus making the sediments the primary sink for dissolved and particulate matter that enters the lake. Lake Salpetn Lake Salpetn (1658N and 8940W) is located ~35 km to the WSW of Lake Sacnab and has a surface area of approximately 2.6 km 2 (Rosenmeier et al., 2002) (Figure 4d). The lake has a maximum depth of approximately 32 m (Brezonik and Fox, 1974). Lake waters are sulfate-rich and have high total dissolved solids (4.76 g L 1) (Deevey et al. 1980). Surface temperatures range from 27C to 30C throughout the year (Rosenmeier et al., 2002). Lake Salpetn is closed hydrologically and lake-bottom sediments are the primary sink for dissolved and particulate matter that enters the lake. Compound-Specific Carbon Isotopic Studies The major source of organic matter in lake sediments is generally derived from phytoplankton living in the water column or aquatic macrophytes. Land plants may also provide an important source of organic matter to lake sediments. The relative contribution of these three sources is influenced by productivity of lacustrine algae, aquatic macrophytes, and terrestrial plants. Transport processes and preservation may also influence the ultimate contribution of organic compounds from various sources (Meyers, 1997). In order to determine relative changes in algal, macrophyte, or terrestrial plant productivity, it is necessary to discriminate between the sources of organic matter sequestered in lake-bottom sediments. Certain compounds in lake

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24 sediments, commonly called biomarkers, are uniquely derived from specific sources of organic matter. Leaf waxes are produced exclusively by vascular plants and serve as a protective coating on leaves and stems (Eglinton and Hamilton, 1967). The abundance and molecular and isotopic composition of leaf waxes reflect vegetative biomass and have been used to discern relative changes in terrestrial vegetation (Figure 5) (Hughen et al., 2004; Filley et al., 2001; Huang et al., 1999, 2001). The wax particles, introduced into the atmosphere by wind and dust ablation off live vegetation (Simoneit, 1977), have a molecular composition that is generally similar to that of their source vegetation (Conte and Weber, 2002). Leaf waxes can also be remobilized from soils during exposure (Schefub, 2003). Leaf waxes settle onto lake surfaces from the atmosphere and are incorporated into lake-bottom sediments. Leaf waxes thus have atmospheric residence times on the order of days to weeks, making them especially valuable as tracers of abrupt vegetative change. Leaf waxes record biomass of exposed leaf surface area in the watershed and thus have the capability of resolving rapid responses to climate or other environmental changes. The isotopic analysis of long-chain n-alkanes has proven to be a useful new tool for evaluating qualitative changes in terrestrial vegetation (Hughen et al., 2004; Filley et al., 2001; Huang et al., 1999, 2001). Long-chain (C 29 -C 33 ) n-alkanes exhibiting a strong odd-over-even carbon-numbered dominance are produced nearly exclusively by vascular plants as components of epicuticular leaf waxes (Meyers, 1997). In addition, n-alkanes are chemically and biologically resistant and are often found in sediments in quantities sufficient for analysis. Preservation of lipids, and in particular n-alkanes, is generally good. Studies have shown greater preservation of n-alkane terrestrial biomarkers versus

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25 algal biomarkers (Meyers et al., 1984; Meyers and Ishiwatari., 1993; Meyers and Eadie, 1993) suggesting that studies utilizing these biomarkers are more robust. Figure 5: Histograms from Ficken et al. (2000) showing the molecular distribution of n-alkanes from the three categories: terrestrial, emergent, and submerged/floating. Only odd carbon number distributions are shown and bars represent 1 standard deviation. The carbon isotopic composition of long chain n-alkanes (>C 27 ) reflects the relative contribution of C 3 and C 4 plants (Hughen et al., 2004; Filley et al., 2001; Huang et al., 1999; Huang et al., 2001). Plants that fix carbon by means of the C 4 metabolic pathway include the tropical grasses (including maize) and are associated with land

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26 clearance. C 4 plants are more competitive than plants using the C 3 pathway under both high water stress (such as during drought) as well as lower ambient pCO 2 levels. Plants that fix carbon by means of the C 3 pathway include all the high forest trees (Huang et al., 2001). Plants that use the C 3 pathway have bulk 13 C values in the range of -21 to -28, whereas C 4 plants display bulk 13 C values between -11 and -15 (Lajtha and Marshall, 1994). Stratigraphic variations in the 13 C ratio of long-chain n-alkanes in lake sediment cores will thus reflect the changes in the proportion of C 3 to C 4 vegetation in a lakes watershed (Huang et al., 2001). Previous studies have shown that it is possible to recognize C 4 plant expansions by examining the carbon isotopic composition of long-chain n-alkanes in sediment cores. Huang, et al. (2001) examined leaf wax n-alkanes from two sites in Mesoamerica and found contrasting moisture variations over the last 25,000 years. Data indicate that regional climate plays a large role in the relative abundance of C 3 versus C 4 plants. Enriched 13 C values during the Last Glacial Maximum (LGM) suggest an expansion of C 4 plants in the sediments of Lake Quexil during the LGM as a result of low partial pressure of atmospheric carbon dioxide (pCO 2 ) and increased aridity. Results indicate that it is possible to recognize vegetation changes by examining the compound-specific carbon isotopes of preserved organic matter in lake sediments. This study provides information that complements past studies that analyzed only bulk sediment or pollen and thus provide a better record of changes in terrestrial vegetation in the Petn Lake District.

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27 CHAPTER 3 METHODS Core Collection Lake Sacnab On 19 July 1997, two sediment cores were retrieved from Lake Sacnab in 8.0 m of water. A 152-cm-long core (SN-19-VII-97-MWI) was taken using a piston corer designed to recover undisturbed sedime nt-water interface profiles (Fisher et al. 1992). The sediment core was sampled in the field at 1-cm intervals by vertical extrusion into a sampling tray fitted to the top of the core barrel. A total of 3.7 m of sediment was collected in four additional sections (S N-19-VII-97-LEX) using a piston corer with polycarbonate tubing. Cores were extruded and sectioned at 1-cm intervals. Magnetic susceptibility was measured for each section using a GEOTEK mu lti-sensor core logger (MSCL) at Florida State University, Tallahassee. All sample s from core SN-19-VII-97 were freeze-dried and ground in preparation for compound-sp ecific carbon isotopic analyses. Magnetic susceptibility data were used to determine that there was a 25.5-cm offset between the mud-water interface core and the piston core. Al l depths in the deeper core sections were adjusted using the mud-water in terface as the 0-cm datum.

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28 Lake Salpeten On 12 June 2002, two sediment cores were retrieved from 23 meters water depth in Lake Salpeten. A short (93 cm) trigger/gravity core (SP-12-VI-02-1A-MWI) and a longer (550 cm) Kullenberg piston core (SP-12-VI-02-1A) were both collected in polycarbonate liners. The Kullenberg core was cut in the field into approximately 1-m lengths for transport to the University of Florida, where cores were stored in a cooler. Whole core sections were analyzed for magnetic susceptibility, gamma ray attenuation (GRA) bulk density and -wave velocity using a GEOTEK MSCL at the University of Florida. Each section of the Kullenberg core was split lengthwise into archive and sampling halves and described. The sampling half was U-channeled and taken to the Ocean Drilling Program Core Repository in Bremen, Germany for elemental analysis on the X-Ray Fluorescence (XRF) Core Scanner. Additional samples were taken from the remainder of the sampling half at the University of Florida at 10-20 cm intervals for compound-specific carbon isotopic analyses. The mud-water interface core was sampled at 1 cm intervals at the University of Florida by vertical extrusion into a sampling tray fitted to the top of the core barrel. All samples from core SP-12-VI-02 were freeze-dried and ground in preparation for compound-specific carbon isotopic analyses. Magnetic susceptibility and density data were used to determine the 7-cm offset between the mud-water interface core and the Kullenberg core and depths assigned to deeper sections were adjusted accordingly.

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29 Chronology Lake Sacnab Radiocarbon ages for Lake Sacnab sediments were determined by accelerator mass spectrometry (AMS) using terrestrial organic matter (seeds, charcoal and wood) at Lawrence Livermore National Laboratory Center for AMS (LLNL-CAMS) and the National Ocean Science AMS (NOSAMS) facility at Woods Hole Oceanographic Institution. Radiocarbon ages were converted to calendar ages using the program OxCal v 3.9. (Bronk Ramsey, 1995; Bronk Ramsey, 2001) and atmospheric data from Stuiver et al. (1998). The sample that was analyzed at NOSAMS was pretreated on-site whereas the samples analyzed at LLNL-CAMS were pretreated at the University of Florida. For the LLNL-CAMS samples, backgrounds were scaled relative to sample size using UF processed Pliocene wood blanks to determine the modern-C contribution (4g). All radiocarbon ages are adjusted to a 13 C value of -25 per mil. Lake Salpeten Radiocarbon ages for Lake Salpeten sediments were determined by AMS 14 C dating of terrestrial organic matter (seeds, charcoal and wood) at LLNL-CAMS. Radiocarbon ages were converted to calendar ages using the program OxCal v 3.9 (Bronk Ramsey, 1995; Bronk Ramsey, 2001) and atmospheric data from Stuiver et al. (1998). Bulk Elemental Geochemical Analyses Total carbon (TC) and total nitrogen (TN) were measured on all samples from core SN-19-VII-97 using a Carlo Erba NA 1500 CNS elemental analyzer with autosampler. Analytical precision for TC and TN is approximately 0.5%. Total

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30 inorganic carbon (TIC) in the sediments was measured by coulometric titration (Engleman et al., 1985) with a UIC (Coulometrics) Model 5011 CO 2 coulometer coupled with a UIC CM5240-TIC inorganic carbon preparation device. Analytical precision is approximately 0.5% based on analysis of 16 calcium carbonate internal standards. Organic carbon (OC) was calculated by subtracting TIC from TC. Weight percent calcium carbonate (%CaCO 3 ) was calculated by multiplying IC by 8.33. Weight percent organic matter (%OM) was estimated by multiplying OC by 2.5. Bulk Carbon and Nitrogen Isotopic Analyses Bulk organic sediment samples were analyzed on-line for carbon and nitrogen isotopes using a VG PRISM Series II isotope ratio mass spectrometer with a triple trap preparation device linked to a Carlo Erba NA 1500 CNS Elemental. Bulk carbon isotopic results are reported in standard delta () notation relative to the Vienna PeeDee Belemnite (VPDB) standard. Precision for 13 C samples was approximately .15 based on nine analyses of NBS-22. Bulk nitrogen isotopic results are reported in standard delta () notation relative to atmospheric N 2 Precision for samples was approximately .20 based on nine analyses of peptone. Compound-Specific Carbon Isotopic Analyses The extraction and isolation methods used for lipids in this study were patterned after Silliman et al. (2000) and M. Pagani (personal communication, 2003). Prior to extraction, approximately 15 g of C 34 n-alkane was added to each sample as an internal standard for n-alkanes. Lipids were extracted from approximately 3-5 g of dry sediment using 2:1 methylene chloride/methanol in a Dionex Accelerated Solvent Extractor (ASE) (for program see Appendix A). Extraction efficiencies averaged approximately 89% for

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31 all samples. The samples were then evaporated and solvent exchanged to hexane using a hot water bath while adding a stream of dinitrogen gas (N 2 ). Samples were dissolved in 1 mL of hexane and added to a 1 cm x 29 cm glass column filled with 2.5 g of 5% deactivated silica gel. 15 mL of hexane was used to elute the n-alkanes. Full isolation scheme procedures are outlined in Appendix B. Samples were urea adducted to obtain clean n-alkanes for gas chromatography as outlined in Appendix C. After urea adduction, samples were concentrated in 200 L of hexane in preparation for analysis on a Perkin Elmer 8500 Gas Chromatograph (GC) to determine purity and appropriate concentrations for GC-IRMS analyses. Samples of 4 L were injected into the GC with a 30 m DB-1 column (0.25 mm ID). The gas chromatograph was used in split injection mode with a ratio of 20:1 and equipped with a FID detector. The GC oven temperature was programmed to maximize alkane separation (Table 1). The necessary dilution for GC-IRMS analysis was calculated and samples were transferred to a glass auto-sampler vial and sealed with a Teflon crimp cap. Samples were typically dissolved in 20-100 L hexane. Carbon isotopic analyses were performed using a Hewlett Packard 6890 GC connected to a Finnigan MAT Delta + XL Mass Spectrometer via a GC-C III interface. Table 1: Perkin Elmer 8500 Gas Chromatograph oven program for sample analyses. Rate C/min Temperature C Time (min) 50 1 6 300 20 The n-alkanes were separated on a fused silica capillary column (30 m x .32 mm i.d.; .25m film thickness) using helium as the carrier gas. The GC oven temperature was programmed to maximize alkane separation (Table 2). Alkanes were combusted into CO 2 in a ceramic oxidation reactor containing three braided NiO/CuO/Pt wires. Three

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32 pulses of a standard, calibrated CO 2 reference gas were injected via the GC-C II interface to the IRMS for the measurement of 13 C values of individual alkanes. A laboratory working standard (UFIS) consisting of three n-alkane chains (C 19 C 25 and C 30 ) was measured along with unknowns at the beginning and end of each run, as well as after every fourth sample analysis within a run. UFIS was calibrated to a set of standard n-alkanes (Mix A) from Indiana University with known 13 C values. Long-term analytical precision, based on repeated analysis of n-alkanes in UFIS, was .4. Data were acquired and processed using ISODAT NT 2.0 software. All reported carbon isotopic compositions for samples represent averaged values for duplicate analyses. Duplicate analyses had a standard deviation of approximately .5. Long-term analytical precision based on analysis of the internal standard C 34 was .4. Table 2: Hewlett Packard 6890 GC oven program for sample analyses. Rate C/min Temperature C Time (min) 50 2 6 299 0 10 300 4

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CHAPTER 4 RESULTS Chronology Lake Sacnab Twelve AMS 14 C dates were obtained from core SN-19-VII-97 and yielded a maximum age of 4580 14 C yr BP on a sample at 439.5 cm (Table 3). The core chronology was established by converting sediment depths to age using three linear regression equations (Figure 6): 196.5-0 cm: age (cal yr BP) = 11.863 x depth, r 2 = 0.9628; 369.5-196.5 cm: age (cal yr BP) = 4.6821 x depth + 1495, r 2 = 1; 439.5-369.5 cm: age (cal yr BP) = 17.683 x depth 3293.2, r 2 = 0.9317. The age of the base of the core (445 cm) is estimated to be ~4650 cal yr BP based on extrapolation of the linear regression from 369.5 to 439.5. Sedimentation rates for the three intervals are: 196.5-0 cm = 0.081 cm/yr; 369.5-196.5 cm = 0.214 cm/yr; 439.5-369.5 cm = 0.052 cm/yr. The wood date at 363.5 cm had a large error because of its small size and was not used to establish the chronology. Lake Salpeten Two AMS 14 C dates were obtained from core SP-12-VI-02-1A and yielded a maximum age of 3985 14 C yr BP (Table 4). Additional chronological control was 33

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34 obtained by correlating the %CaCO 3 record from core SP2-19-VII-99 (Rosenmeier et al., 2002b) and the scanning XRF Ca concentration data from core SP-12-VI-02-1A (correlation coefficient = 0.505) using AnalySeries v. 1.0 (Paillard et al., 1996). Ten tie points were used to correlate the records (Figure 7, Table 5). Table 3: AMS 14 C dates for samples from sediment core SN-19-VII-97 from Lake Sacnab. For the LLNL-CAMS samples, backgrounds were scaled relative to sample size using Pliocene wood blanks prepared at UF (4g). All radiocarbon ages are adjusted to a 13 C value of -25 Radiocarbon ages were converted to calendar ages using the program OxCal v 3.9 (Bronk Ramsey, 1995; Bronk Ramsey, 2001) and the atmospheric data set of Stuiver et al. (1998). All ages reported in this thesis are in calendar years before present (relative to AD 1950). Sample ID Composite Depth (cm) Accession Number Sample Material Age 1 ( 14 C yr BP) Calibrated Age (cal yr BP) Calibrated Age 95.4% Probability (AD/BC) SN-19-VII-97-MWI_45.5 45 CAMS 58754 280 60 390 AD 1600 115 SN-19-VII-97-LEX1_109.5 84 CAMS 58755 1210 50 1165 AD 800 110 SN19VII97MWI_97-98 97.5 CAMS 106265 seed 990 50 880 AD 1100 115 SN19VII97MWI_121-123 122 CAMS 106266 charcoal, leaf material 1650 60 1560 AD 400 155 SN-19-VII-97-LEX1_152.5 127 CAMS 58756 1620 50 1515 AD 400 120 SN19VII97MWI_145-147 146 CAMS 106267 charcoal, leaf material 1740 100 1650 AD 300 235 SN19VII97LEX2_221-223 196.5 CAMS 106268 seed, charcoal 2370 40 2415 500 BC 90 SN-19-VII-97-LEX4_414.5 363.5 OS 18657 wood 2500 160 2555 600 BC 400 SN19VII97LEX4_394-396 369.5 CAMS 106269 charcoal, leaf material 3030 60 3225 1300 BC 160 SN19VII97LEX4_404-407 379.5 CAMS 106270 charcoal, leaf material 3300 70 3540 1600 BC 155 SN19VII97LEX4_434-436 409.5 CAMS 106271 charcoal, leaf material 3450 40 3740 1800 BC 105 SN19VII97LEX4_464-466 439.5 CAMS 106272 charcoal, leaf material 4050 80 4580 2600 BC 275

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35 y = 11.863xR2 = 0.9628y = 17.683x 3293.2R2 = 0.9317y = 4.6821x + 1495R2 = 1025050075010001250150017502000225025002750300032503500375040004250450047505000050100150200250300350400450500Depth (cm)Calibrated Age (Years BP) Figure 6: Depth versus calibrated age (yr BP) for terrestrial wood, seed and charcoal samples in Lake Sacnab core SN-19-VII-97. Squares indicate samples analyzed at LLNL-CAMS and the triangle indicates the sample measured at NOSAMS. Table 4: AMS 14 C dates for samples from sediment core SP-12-VI-02-1A from Lake Salpeten. Sample ID Sample Material Composite Depth (cm) Accession Number Age 1 (14C yr BP) Calibrated Age (cal yr BP) Calibrated Age 95.4% Probability (AD/BC) SP12VI02 ST1A_110-111 charcoal 110.5 CAMS 106273 210 60 165 1790 AD 170 SP12VI02 ST1A_442-445 charcoal 443.5 CAMS 106274 3985 50 4455 2500 BC 200

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36 Table 5: Tie points used to correlate the %CaCO 3 record from core SP2-19-VII-99 (Rosenmeier et al., 2002b) and the scanning XRF Ca concentration data from core SP-12-VI-02-1A (correlation coefficient = 0.505) using AnalySeries v. 1.0 (Paillard et al., 1996). Included are the respective ages for each set of correlated depths. Ages for depths in core SP2-19-VII-99 were determined using the chronology outlined in Rosenmeier et al. (2002) while ages for depths in SP-12-VI-02-1A were determined using the correlation (Figure 4). Depth in SP-12-VI-02-1A Depth in SP2-19-VII-99 Age (cal yr BP) 152 151 1660 201.5 201 2030 215 213 2090 236.5 236 2210 246 248 2270 265.5 266 2390 293.5 289 2590 310.5 301 2730 330 309.5 2860 376 331 3250 The core chronology was established by converting sediment depths to age with two equations derived by linear regression for two intervals (Figure 8). The top of the core was assumed to be modern. 152-0 cm: age (cal yr BP) = 10.976 x depth, r 2 = 1; 376-152 cm: age (cal yr BP) = 6.8806 x depth + 599.66, r 2 = 0.9947. The age of the base of the core (563.5 cm) was estimated to be ~4500 Cal yr BP based on extrapolation of the linear regression from 152 to 376 cm. The radiocarbon date at 110.5 cm was not used to construct the chronology because of its small sample size and suspicion that the charcoal fragments may have been displaced down core. Based on a comparison between magnetic susceptibility records from SP-12-VI-02-1A and SP2-19-VII-99, the radiocarbon date at 443.5 cm was not used to construct the chronology

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37 because it caused deeper sediments to appear too old based on a comparison with core SP-80-1. Sedimentation rates for the two intervals are as follows: 196.5-0 cm = 0.091 cm/yr; 563.5-196.5 cm = 0.142 cm/yr. 01002003004005001020304050607080Depth (cm)% CaCO3 46810121415-pt Smooth Ca (103)SP-12-VI-02-1ASP2-19-VII-99 6 Figure 7: Correlation of SP2-19-VII-99 and SP-12-VII-02-1A using %CaCO 3 in the SP2-19-VII-99 core and the scanning XRF Ca concentration data from the SP-12-VII-02-1A core.

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38 05001000150020002500300035004000450050000100200300400500Depth (cm)Calibrated Age (yr BP) y=10.976xR2=1y=6.8806x + 599.66R2=0.9947 Figure 8: Calibrated ages (yr BP) versus depth for correlated tie points in core SP-12-VII-02-1A as well as radiocarbon dates of terrestrial samples in Lake Salpeten core SP-12-VII-02-1A. Triangles indicate dates analyzed at LLNL-CAMS while the diamonds represent tie points. Elemental Geochemical Analyses Lake Sacnab From 4500 cal yr BP to ~3300 cal yr BP, sediments are dominated by organic matter as reflected by high %OM and %N concentrations and low magnetic susceptibility and %CaCO 3 (Figure 9). The %Other is low (~20%) at 4500 cal yr BP, and increases continuously to the base of the Maya clay at 3300 cal yr BP. Beginning at the base of the Maya clay unit at ~3300 cal yr BP, magnetic susceptibility and %CaCO 3 begin to increase and %OM and %N decrease abruptly. Sediment composition between ~3300 and 1200 cal yr BP is dominated by inorganic sediment (clay and detrital carbonate) as

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39 reflected by relatively high values of magnetic susceptibility, %CaCO 3 and %Other. While the %OM and %N are relatively constant during this time, the magnetic susceptibility, %CaCO 3 and %Other show some variation. %Other remains relatively unchanged from 3300 to 2600 cal yr BP, reaching maximum values of 90% during this time before decreasing to 50% between 2700 and 2400 cal yr BP. %Other remains relatively constant from 2400 cal yr BP to 1600 cal yr BP, when values begin to increase and reach 60% at 1200 cal yr BP. Following this maximum, %Other decreases continuously from 1200 cal yr BP to the present. At ~3300 cal yr BP, magnetic susceptibility increases, rising to a maximum of ~20 SI at 2700 cal yr BP before decreasing to an average of 10 SI for the remainder of the Maya clay unit. %CaCO 3 increases from 3300 cal yr BP to 2400 cal yr BP, reaching a maximum of ~45%. %CaCO 3 then decreases to ~4% at 1200 cal yr BP and remains low until the present. At the top of the Maya clay unit, %OM and %N begin to increase and do so continuously from 1200 cal yr BP to present. Lake Salpeten Magnetic susceptibility increases from the base of the core to ~4100 cal yr BP (Figure 10). Values remain high from 4100 to ~2000 cal yr BP reflecting the high clay content of the sediment. Beginning at ~2000 cal yr BP, magnetic susceptibility begins to decline with two distinct steps centered at 1700 and 1300 cal yr BP. From 800 cal yr BP to the present, magnetic susceptibility values remain low and relatively unchanged.

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40 Figure 9: Magnetic susceptibility, percent calcium carbonate (%CaCO3), percent organic matter (% OM), percent other (%Other) and percent nitrogen (%N) versus age in calibrated years before present (cal yr BP) from Lake Sacnab. The gray highlighted area represents the Maya clay unit.

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41 Figure 10: Magnetic susceptibility versus age in calibrated years before present (cal yr BP) for core SP-12-VII-02-1A from Lake Salpeten. The gray highlighted area represents the Maya clay unit. Bulk Carbon and Nitrogen Isotopes The carbon and nitrogen isotopic records of bulk organic matter show similar patterns during the last 4650 years (Figure 11). Bulk 13 C values range from -21 to -28 throughout the core, whereas bulk 15 N values vary between 0 and 5. Carbon isotopic values generally increase from -27 at the base of the core to -23 at ~3300 cal yr BP. The 13 C values of bulk organic matter average -23 from 3300 to 2500 cal

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42 yr BP, and then decrease between 2500 and 2000 cal yr BP. Bulk organic 13 C remains unchanged and averages -25 from 2000 to 1300 cal yr BP. A sharp 2.5 decrease in 13 C occurs at 1250 cal yr BP, followed by a slight trend toward increasing values toward present. The 15 N of bulk organic matter averages ~2 from 4500 cal yr BP to 3500 cal yr BP. From 3500 cal yr BP to 3300 cal yr BP, 15 N values increase by 2 and remain high from 3300 cal yr BP to 1200 cal yr BP, averaging 3. The nitrogen isotopic composition then decreases abruptly by 3.5 at 1200 cal yr BP, and averages 1 from 1200 cal yr BP to present. Compound-Specific Carbon Isotopes Lake Sacnab Samples were analyzed for the 13 C of long-chain n-alkanes (C 29 C 31 and C 33 ) in Lake Sacnab sediments at approximately centennial resolution since 4650 cal yr BP (Figure 12). The 13 C signals for C 31 and C 33 show similar trends and will be described in unison, whereas the 13 C of C 29 differs somewhat and will be described separately. The 13 C of C 31 and C 33 averaged -33 at the base of the core and gradually increased to -29 at ~3300 cal yr BP. Values generally remain high from ~3300 cal yr BP to ~2500 cal yr BP and averaged -28. From 2500 to 2100 cal yr BP, 13 C decreases from -28 to -32 and values remain unchanged from 2100 to 1300 cal yr BP. At 1300 cal yr BP, the carbon isotopes of C 31 and C 33 record a rapid decrease with values reaching -39 and -37, respectively. Following this excursion, the 13 C of n

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43 alkane chains C 31 and C 33 increases at 1100 cal yr BP and averages -32 to the top of the core. Figure 11: Bulk carbon and nitrogen isotopes for core SN-19-VII-97 from Lake Sacnab. The gray highlighted area represents the Maya clay unit. The 13 C of C 29 at the base of the core is -32 and generally remains unchanged from ~4500 cal yr BP to ~2500 cal yr BP, with an average ratio of approximately -29. From 2500 cal yr BP to 2100 cal yr BP, 13 C decreases from -29 to -33 and values remain unchanged from 2100 cal yr BP and 1300 cal yr BP. At 1300 cal yr BP, the carbon isotopes of C 29 record a rapid decrease, reaching a value of -35. Following this excursion, the 13 C of C 29 is highly variable, with values ranging from -24 to -33 during the period from 1100 cal yr BP to the present.

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44 Figure 12: Compound-specific 13 C results of long chain n-alkanes (C 29 C 31 and C 33 ) from Lake Sacnab, Guatemala versus age in calibrated years before present (cal yr BP). Lake Salpeten Samples were analyzed for the 13 C of long-chain n-alkanes (C 29 C 31 and C 33 ) in Lake Salpeten sediments at an approximate resolution of 200 years for the past 6000 cal yr BP (Figure 13). The 13 C signals for C 29 C 31 and C 33 show similar trends and will be described in unison; variations in 13 C of C 29 however, are muted relative to C 31 and C 33 The 13 C of long-chain n-alkanes averaged -32 at the base of the core and gradually increased to -28 at ~5200 cal yr BP. Values generally remained unchanged from ~5200 cal yr BP to ~3300 cal yr BP and averaged -30. From 3300 to 2500 cal yr

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45 BP, 13 C increased from -30 to -27. The isotopic ratio decreased from 2500 cal yr BP to the present, reaching minimum values that range from -33 to -36. Figure 13: Compound-specific 13 C results of long chain n-alkanes (C 29 C 31 and C 33 ) from Lake Salpeten, Guatemala versus age in calibrated years before present (cal yr BP).

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CHAPTER 5 DISCUSSION Implications for Changes in Land-use Changes in land-use can be inferred from the lithological composition of lacustrine sediments, which reflect changes in material transfer from the watershed to the lake. With increased land clearance, the detrital load to a lake basin will increase. Sediment cores taken from Lakes Sacnab and Salpetn show similar changes in sediment composition. This common pattern has been documented throughout the central Petn lake district and is interpreted to reflect sediment compositional and rate changes related to the history of Maya settlement and population in the region (Cowgill and Hutchinson, 1966; Deevey et al., 1979; Brenner, 1983; Vaughan et al., 1985; Rice et al., 1985; Binford et al., 1987; Leyden, 1987; Curtis et al., 1998; Islebe et al., 1996). The sediment composition and magnetic susceptibility records in Lake Sacnab (Figure 6) document changes in material transfer from the catchment to the lake. Magnetic susceptibility is a proxy for the concentration of magnetic minerals contained within the sediment, which are derived from the erosion of soil and bedrock in the watershed and subsequently transported to the lake. CaCO 3 in sediments can be derived from either authigenic precipitation within the lake or by the weathering and transport of detrital carbonate from soils or limestone bedrock. There is little to no observed lacustrine carbonate in the sediments of Lake Sacnab, which may suggest that most of the CaCO 3 is detrital. Inorganic carbon and oxygen isotopes measured on bulk material from the lake have values that range from -8 to 0 and -4 to 0, respectively. These 46

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47 values are generally consistent with measured mean values of Petn soils surrounding Lake Quexil ( 13 C -10 to -7, 18 O -3 to 0,) (Leyden et al., 1993) and indicate that the source for sediment CaCO 3 is dominantly allochthonous CaCO 3 as opposed to diffuse authigenic CaCO 3 Thus, relative shifts in %CaCO 3 may serve as an additional indicator of erosion in the catchment. Organic matter in lacustrine sediments is derived from multiple sources including terrestrial vegetation, lacustrine algae and bacteria, and aquatic macrophytes. The %Other may also serve as a proxy indicator of landscape erosion. Because there is relatively little contribution to the sediments other than CaCO 3 and organic matter, the %Other is a likely indicator of non-carbonate clastic material (clay) eroded from the watershed. It is important to note the effects of the closed-sum problem inherent in using weight % data. Dilution may play a major role in down-core variations of any of the three components. As the % of any component increases, the other two components will respond by decreasing in % to sum to 100%. Therefore, variation in the delivery of any of the sediment components, especially during the deposition of the Maya clay, would significantly alter the apparent input of the other sediment components. To avoid the closed-sum problem, accumulation rates (g/cm 2 / yr) can be calculated. Accumulation rates of the three individual sediment components provide a better indication of erosion and sediment deposition. Unfortunately, the density data necessary to calculate accumulation rates are not available for Core SN-19-VII-97 from Lake Sacnab. Anselmetti et al. (in prep), however, calculated accumulation rates for specific time intervals in Lake Salpetn for the period between 8500 cal yr BP and the present. Data indicate that erosion rates were lowest during the early to mid-Holocene and increased beginning in the Early Preclassic. Erosion rates were highest during the

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48 Late Preclassic period (2200 to 1700 cal yr BP) and subsequently decreased over time even during the height of Maya occupation in the watershed (1700 and 1100 cal yr BP). The sedimentological history and associated watershed erosional characteristics determined by Anselmetti et al. (in prep) are consistent with those determined in this study. In Lake Sacnab, Holocene sediment prior to Maya occupation is classified as gyttja. Overlying the pre-Maya gyttja is a clay-rich horizon known as the Maya clay, associated with increased sedimentation rates. The onset of Maya clay deposition appears to have begun at 3300 cal yr BP and lasted until ~1200 cal yr BP. This unit is common in all the central Petn lakes and has been interpreted previously to reflect accelerated erosion associated with Maya land clearance (Deevey et al., 1979; Brenner, 1983; Vaughan et al., 1985; Rice et al., 1985; Binford et al., 1987; Leyden, 1987; Curtis et al., 1998; Islebe et al., 1996) and/or possibly regional drying (Hodell et al., 1995; Curtis et al., 1998). In the Lake Sacnab watershed, soils likely became unstable as forest cover was removed at the onset of Maya occupation. The common soil type in the Petn are mollisols, consisting thin (typically <1 m) mineral soils that develop over CaCO 3 -rich material called sascab (Brenner et al., 2002). Down-profile erosion of Petn soils following deforestation is evidenced in the sedimentological history. Changes in the ratio of %Other to %CaCO 3 suggest that the organicand clay-rich surface horizon was eroded between 3300 and 2500 cal yr BP. Once the watershed had been denuded of organic-rich top soils, the weathering and erosion of more carbonate-rich, deep soils (sascab) ensued from 2500 to 1300 cal yr BP, as evidenced by the increase in the influx of CaCO 3 and decrease in %Other (Figure 6).

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49 This period (between 3300 and 1200 cal yr BP) was characterized by enhanced delivery of detrital material to the lake and resulted in sediments with a low organic content, which is likely a dilution effect. Overlying the Maya clay unit is another organicand clay-rich layer that is inferred to represent decreased erosion following the cessation of agriculture and increased contribution of lacustrine organic matter to the sediment. Stratigraphic changes in sediment composition in Lake Sacnab are similar to those in other lakes in the Petn determined in previous studies (Deevey et al., 1979; Brenner, 1983; Vaughan et al., 1985; Rice et al., 1985; Binford et al., 1987; Leyden, 1987; Curtis et al., 1998; Islebe et al., 1996) and suggests that the history of human and environmental changes is similar throughout the central Petn region. The magnetic susceptibility record from Lake Salpetn shows a similar trend to Lake Sacnab in the transfer of watershed material to the lake but with a distinct difference in timing (Figure 10). The onset of Maya clay deposition, indicated by the enhanced delivery of watershed-derived detrital material resulting from landscape denudation, appears to have begun at 4100 cal yr BP and lasted until ~1100 cal yr BP. Previous work (Rosenmeier et al., 2002) in Lake Salpetn, however, has indicated that the Maya Clay was deposited between ~3300 and 1100 cal yr BP. The apparent discrepancy between the two cores is most likely related to the chronology of core SP-12-VI-02-1A (this study), which was correlated to core SP2-19-VII-99 (Rosenmeier et al., 2002) using ten tie points (see Chapter 4). In using this method, there is the possibility for miscorrelations that would cause error in the chronology. Core SP-12-VI-02-1A was dated indirectly and ages for the Maya clay from Rosenmeier et al. (2002) are probably

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50 more accurate. Archaeological settlements in the Lake Salpetn watershed are documented beginning at ~3000 cal yr BP (Figure 3) (Rice and Rice, 1990) and roughly coincided with the sedimentological evidence for human occupation of Rosenmeier et al. (2002). For the most part, compound-specific carbon isotopes track magnetic susceptibility closely throughout the records for both Lake Sacnab and Lake Salpetn (Figures 14, 15). The magnetic susceptibility record is generally considered a proxy for changes in amount of detrital material transported from the landscape into the lake that, in turn, is affected by the amount of vegetation in a watershed. In both lakes, periods of inferred increased erosion occur simultaneously with periods of higher relative contributions of C 4 vegetation. As vegetation shifts from high-forest taxa to a more savanna-like landscape during forest removal, rapid erosion of watershed soils resulting from soil destabilization ensues. Because both magnetic susceptibility and the compound-specific records are influenced by vegetation cover and type and are indicative of landscape conditions, the correlation of these proxies was not unexpected. Sources of Organic Matter Organic matter in lacustrine sediments is derived from multiple sources including terrestrial vegetation, lacustrine algae, and aquatic macrophytes. The C/N ratio can provide information about the proportions of algal versus terrestrial plant contribution to organic matter (Prahl et al., 1980; Meyers, 1994; Kaushal and Bindford, 1999). Organic matter from algae and bacteria has C/N weight ratios ranging between 5 and 12, whereas organic matter from vascular land plants usually has weight ratios of 24 and greater (Meyers, 1994). While C/N ratios between 36 and 48 are generally characteristic of

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51 cellulose-rich vascular plants, weight ratios between ~14 and 20 suggest a mixture of both algal and vascular plant material (Ertel and Hedges, 1985). Figure 14: Comparison of 13 C of C 33 from Lake Sacnab with magnetic susceptibility.

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52 Figure 15: Comparison of 13 C of C 33 from Lake Salpetn with magnetic susceptibility. In Lake Sacnab, the C:N ratio suggests that the source of organic matter to the lake may have changed over the last 4500 years (Figure 16). Values range from 10 to 20 in the earliest part of the record, from 4500 to 2700 cal yr BP, and suggest a mixture of both algal and terrestrial plant material. From 2700 cal yr BP to 1300 cal yr BP, increased ratios of C:N suggest the predominant contribution of terrestrial vegetation to

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53 sediment organic matter. Values are low from 1300 cal yr BP to the present, ranging only between 9 and 13.5, suggesting a return to dominantly aquatic organic matter. The distinguishing C:N ratios among the different types of organic matter generally survive both sinking and sedimentation in lacustrine environments. However, certain diagenetic processes such as dissolution, oxic/anoxic cycles and bacterial processes may modify the original ratios (Mller and Mathesius, 1999). For this reason, C:N values are often used together with the carbon isotope ratio of bulk organic matter to determine both the source(s) and composition of organic matter. The period between 4500 and 2700 cal yr BP has bulk isotope values that suggest an increasing contribution of a mix of C 3 and C 4 land plants while the C:N ratio suggests additional contribution from algae. From 2700 to 1300 cal yr BP, both the C:N and the bulk isotopes suggest a dominant contribution from C 4 terrestrial plants and is followed by a period between 1300 cal yr BP and the present in which there is a higher contribution from algae, aquatic macrophytes and C 3 terrestrial vegetation.

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54 Figure 16: C:N ratios in weight % and bulk organic matter isotopes from Lake Sacnab.

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55 Figure 17: Comparison of 13 C values for bulk organic matter, C 31 and C 33 from Lake Sacnab. The 13 C of bulk organic matter in Lake Sacnab sediments shows similar trends as the compound-specific record which might indicate that the bulk organic matter within the lake sediment is dominated by terrestrial organic matter with only a small contribution from aquatic macrophytes and algae (Figure 17). However, the C:N ratios

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56 suggest that the source(s) of organic matter changed over time and is likely a better proxy for the source(s) of organic matter. The bulk and compound-specific isotopes show the most distinct variation in trend in the latest part of the record, from ~1200 cal yr BP to the present and may be explained by the increased contribution from aquatic macrophytes and lacustrine algae as demonstrated by the C:N ratio. Carbon isotopic values of bulk organic matter are enriched by ~5 to 7, however, relative to individual leaf wax n-alkanes. This is expected because lipids, including n-alkanes, are commonly depleted in 13 C relative by ~6 to 8 relative to other biosynthetic products (Hayes, 2001). Thus, much of the terrestrial organic matter in the lake deposits owes its origin to non-lipid sources. An important concern in analyzing the isotopes of individual n-alkanes is preservation potential. The carbon preference index (CPI) is often used as a proxy for the preservation potential of the organic matter when there is a clear predominance of epicuticular leaf waxes of terrestrial plants (Hedges and Prahl, 1993). CPI values are generally highest in living plants and surface sediments. Lowered values indicate increasing maturity and degradation and tend to decrease to a final value of 1 (Hedges and Prahl, 1993). However, this index cannot be used as a preservation potential proxy in all cases. For example, a CPI of 1 may indicate immature organic matter with a low contribution from higher plants rather than mature organic matter. In order to determine whether the CPI can be used as a proxy for preservation potential, the examination of the predominant n-alkane chain length can provide some insight into the provenance of organic matter. The predominant n-alkane chain length is shown in Figure 18a, which is interpreted as an indicator of the origin of the organic debris input (algae, aquatic

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57 macrophytes or land plants) (Cranwell et al., 1987; Ficken et al., 2000). The predominance of n-alkanes of high-molecular-weight indicates that terrestrial plants are the predominant organic matter source and suggests that the CPI can be used as a proxy for preservation potential. With respect to the CPI, most of the values are greater than 1.5 with few exceptions (Figure 18b). The CPI shows a predominance of immature, plant-derived material and, therefore, very little removal of components during transport and postdeposition (Ortiz et al., 2004). Figure 18: Predominant n-alkane chain length (A) and CPI (B) for Lakes Sacnab and Salpetn.

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58 Evidence for Relative Shifts in Vegetation Sediments from Lakes Sacnab and Salpetn show a range of values (-38 to -27,) during the last 4500 years that indicate changing relative proportions of C 3 to C 4 vegetation in the watershed (Figure 11). A comparison of the 13 C records of C 33 from Lake Sacnab and Lake Salpetn reveals similar absolute values and long-term trends over the past 4500 cal yr BP (Figure 19). At the base of the core, more depleted values suggest a higher proportion of C 3 versus C 4 vegetation. A shift towards more enriched values from 4500 cal yr BP to 3300 cal yr BP reflects an increase in the relative abundance of C 4 (grasses, etc.) to C 3 (trees) vegetation. Decreasing 13 C values from 2500 cal yr BP to 2000 cal yr BP indicates some forest regrowth and a higher proportion of C 3 taxa than in the previous interval. The proportion of C 3 to C 4 vegetation is relatively unchanged from 2000 cal yr BP to 1300 cal yr BP, which is surprising given the population changes that were occurring in this period. At 1300 cal yr BP in the Sacnab watershed only, a sharp decrease in the 13 C indicates a rapid transition (~100 years) to a highly C 3 -dominated landscape. During the last ~1200 cal yr BP, the carbon isotopic record from both lakes suggests a gradual shift towards greater contribution of C 3 vegetation toward present. There are several distinct differences in the 13C records of the two cores. A peak in the 13 C in Lake Salpetn at approximately 3900 cal yr BP and subsequent decrease until 3300 cal yr BP is not observed in Lake Sacnab. Rather, values are continually increasing during this time in Lake Sacnab. In addition, the 13 C peaks at approximately 3100 cal yr BP in Lake Sacnab, whereas maximum 13C values in Lake

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59 Salpetn do not occur until 2800 cal yr BP. The rapid transition in 13 C at 1300 cal yr BP in Lake Sacnab is not present in the Salpetn record. The differences in the 13C records may be due partly to sampling resolution, which is approximately two times higher in Lake Sacnab than in Lake Salpetn. In addition, some variation may be expected in the history of vegetation changes in the two watersheds. While the general trends of land-use and human occupation are thought to be similar between watersheds, these systems are dynamic and the history of land use was probably variable throughout the region. Figure 19: Comparison of 13 C records for n-alkane chain C 33 in Lake Sacnab and Lake Salpetn

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60 The rapid decline in 13 C in the Lake Sacnab core at 1300 cal yr BP may reflect increased dominance of a single species with a relatively depleted carbon isotopic value after agricultural abandonment. Various chronosequence studies of abandoned agricultural fields have determined that soil nitrogen is often the limiting factor during the entire succession of vegetative regrowth (Tilman, 1984; Tilman 1987; Knops and Tilman, 2000). In many cases, legumes are the first plants to colonize abandoned fields because they are able to fix atmospheric nitrogen. If the pioneering species were legumes, then it would have a low 15 N value because of nitrogen fixation ( 15 N = 0), which involves little isotopic fractionation between plant tissues and air. In fact, the rapid transition in the 13 C record in Lake Sacnab occurs at the same time (~1300 cal yr BP) as a rapid decrease in the 15 N of bulk organic matter (Figure 20). These data support an increased contribution of sediment organic nitrogen from N 2 fixers because sedimentary 15 N values at this time are close to zero. While the average 13 C of legumes is not necessarily more negative than other plants, they are exclusively C 3 plants and thus have low 13 C values relative to C 4 vegetation. An alternative explanation for the rapid transition at 1300 cal yr BP is the result of dominance of a non-leguminous pioneer species. For example, it has been documented that in certain systems, bracken fern often dominates the vegetation in a field immediately following abandonment (Pakemen et al., 1994). Eventually, other high forest taxa out-compete this weed and would thus change the isotopic composition of sediments gradually over time. While the 13 C value of bracken fern is not reported in the literature and is not necessarily the source for the depleted values in this study, previous work shows that one species may dominate following agricultural abandonment.

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61 It may be possible to determine whether a pioneer species accounts for the dramatic isotope shift by analyzing pollen samples at close intervals above, within, and below the transition. 13C C33 (, PDB) -39-37-35-33-31-29-27-250500100015002000250030003500400045005000Age (cal yr BP)-101234515N Bulk (, air) Carbon Nitrogen Figure 20: Diagram comparing the 13 C of C 33 to the 15 N of bulk organic matter in Lake Sacnab.

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62 Reported 13 C values of n-alkanes in C 3 plants range from -31 to -38, whereas n-alkanes in leaf waxes of C 4 plants typically range from -19 to -25 (Freeman, 2001). The 13 C values for n-alkanes in this study, however, range from -27 to -38 which might suggest that C 3 vegetation dominated throughout much of the study period. Previous studies of aerosols and sediments have translated 13 C values into percentages of C 3 versus C 4 plants using a two-component mixing equation (Huang et al., 2000; Schefub et al., 2003). It has been found, however, that the isotopic values of C 3 plants usually become more depleted with increasing carbon number while C 4 plants have isotopic values that are consistent over all chain lengths (Collister et al., 1994). The variation is suggested to result from the production of different leaf wax lipids in different proportions during a leafs growth cycle and averages approximately 2.4. This variation translates into a potential 16% error in the calculation of C 3 to C 4 abundance. This characteristic makes it difficult to quantify the contribution of C 3 versus C 4 plants. In order to account for this effect, the values for the lowest-number terrestrial n-alkane chain, C 29 are most often used to translate 13 C values into % contributions. Because aquatic macrophytes can contribute significant amounts of C 29 it is preferable to calculate %C 4 using an n-alkane of greater chain length. For this study, values were calculated using a two-component mixing equation assuming end-member 13 C values of n-alkane C 33 of -36.4 for C 3 plants and -19 for C 4 plants, respectively. These values were adapted from those of Collister et al. (1994), who reported -34 for the C 3 -endmember and -19 for the C 29 C 4 -endmember. For Lake Salpetn, the contribution of C 4 vegetation to the C 33 n-alkane pool ranges between 14% and 57.5% (Figure 21). The % contribution of C 4 taxa increases

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63 between 4500 and ~2700 cal yr BP when the greatest proportion of C4 occurred. The contribution of C 4 biomass declines from nearly 58% at 2700 cal yr BP to just 14% at present. 050010001500200025003000350040004500010203040506070C4 Vegetation (%)Age (cal yr BP) Lake Sacnab Lake Salpeten Figure 21: Relative shifts in contribution of C 4 vegetation (in %) in Lakes Sacnab and Salpetn over the last ~4500 cal yr BP. Values were calculated using a two-component mixing equation with C 3 and C 4 inputs represented by 13 C values of n-alkane C 33 of -36.4 and -19, respectively (Collister et al., 1994).

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64 Lake Sacnab shows similar shifts to Lake Salpetn with respect to % contribution of C 4 biomass. From 4500 cal yr BP until ~3300 cal yr BP, the % contribution increases, reaching peak values of ~55 between 3300 and 2700 cal yr BP. From 2700 cal yr BP to 1300 cal yr BP, the contribution of C 4 vegetation generally decreases. At 1300 cal yr BP, the contribution of C 4 vegetation is 0%, but rapidly increases to a greater contribution (30%) by 1200 cal yr BP. The contribution of C 4 vegetation gradually decreases from 1200 cal yr BP to the present. Changes in water-use efficiency (WUE), i.e. the ratio of carbon gained to water lost during gas exchange, may also affect the 13C of vegetation. A negative correlation exists between precipitation and 13 C of vegetation among tropical sites (Leffler and Enquist, 2002). Consequently, the 13 C of C 3 vegetation would increase during periods when the precipitation decreased significantly. Consequently the calculated % C 4 contribution may be greater than the true value. It is thus necessary to consider the potential influence of climate on the 13 C record. The 13 C values of n-alkanes in Lakes Sacnab and Salpetn are similar to values obtained by Huang et al. (2001) in Lake Quexil (Figure 22). The 13 C values during the Maya clay interval in Quexil averaged -31 and are similar to values measured in the Maya clay from Lake Sacnab (average = -30). Values from Lake Quexil in the pre-Maya gyttja average -34 and increased by ~5 between 3300 and 1100 cal yr BP, with a peak in 13 C during the time of the Maya Terminal Classic period. They attribute this increase to both anthropogenic forest clearance and regional drying. The 13 C from Lake Sacnab, however, reaches peak values much earlier at ~3000 cal yr BP than in Lake

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65 Quexil. The Quexil carbon isotope record is much lower resolution than either Lakes Sacnab or Salpetn, and may not capture the structure observed in the higher-resolution 13 C records. Figure 22: Diagram showing the comparison between 13 C values of C 31 from Lake Sacnab, Lake Salpeten and Lake Quexil (Huang et al., 2001).

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66 In the records from Lake Salpetn and Sacnab, the 13 C values of n-alkanes from 3300 cal yr BP to 2700 cal yr BP are comparable to values during the LGM in Lake Quexil, further supporting the interpretation of enhanced C 4 input. The data imply that vegetation during the period between 3300 cand 2700 cal yr BP may have been quite similar to the vegetation during the Last Glacial Maximum, when climate was significantly cooler. It is remarkable that vegetation may have changed as drastically during the period of human occupation as it did during a glacial-interglacial cycle. Comparison with Pollen Records Compound-specific carbon isotope records can be compared with pollen records from the same or nearby lakes to test if vegetation changes inferred from the 13 C of long-chain n-alkanes are the same as those inferred from pollen profiles. A comparison of the % disturbance taxa from Lake Salpetn (Leyden, 1987) and the long-chain n-alkane 13 C record from Lake Sacnab reveals significant differences in the two proxies (Figure 23). Disturbance taxa include grasses, sedges, and herbs from the following families: Amaranthaceae (C 3 and C 4 ), Ambrosia (C 3 ), Compositae (C 3 and C 4 ), Cyperaceae (C 3 and C 4 ) and Gramineae (C 3 and C 4 ). While these pollen data may provide an accurate representation of changes in the relative abundance of the selected taxa, they are not ideal for distinguishing changes in the relative contributions of C 3 versus C 4 plants because both plant types are represented by the disturbance taxa. Perhaps a better way to compare the compound-specific and disturbance taxa is to compare 13 C to grasses only, which would be most representative of C 4 contribution.

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67 A comparison of compound-specific carbon isotopes to % grasses reveals that while both records increase from the base of the record to 3300 cal yr BP, only the 13 C of C 33 peaks between 3300 and 2500 cal yr BP (Figure 24). The % grasses in the pollen record, however, is relatively low during this period (9%) and does not peak until ~2300 cal yr BP. Both the % grass and isotope records generally decline from 2300 cal yr BP to present. The pollen (% grasses) record is lower resolution than the isotope record; which may explain some of the discrepancy between the two vegetation proxies. Perhaps the largest potential reason for the discrepancy may lie in the fact that maize pollen is not included in the total pollen count because of its large size, thus causing an over-representation of other taxa. Maize pollen has often been used in Mesoamerican studies as a proxy for agriculture and associated deforestation and it is well-documented that maize pollen is abundant during the period of Maya occupation (Leyden, 1987; Islebe et al., 1996). The 13 C of n-alkanes, however, should be very sensitive to large stands of maize in the watershed. There are additional potential shortcomings in using the 13 C of n-alkanes as a proxy for vegetation change. For example, maize was probably an important (C 4 ) plant in the vegetation of the Petn Lake District, especially during times of high population density. Shoreline cultivation of maize would strongly influence the compound-specific 13 C while not altering the pollen profile. The higher contribution of C 4 vegetation during the Preclassic (~3300 cal yr BP to 2500 cal yr BP) may represent early shoreline maize cultivation in the watershed. As populations increased into the Classic period, the more desirable shorelines may have become residential areas as opposed to agricultural areas, which would have then been moved further from the lake. Abandonment of near-shore

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68 fields would alter the compound-specific 13 C but would not be apparent in the pollen profile. Alternatively, maize cultivation on shorelines may have ceased in the Late Preclassic due to soil depletion. As the soils immediately surrounding the lake were exhausted, the ancient Maya may have moved agricultural fields further into the surrounding watershed. While there are potential explanations for the discrepancy between pollen and compound-specific 13 C values, it is necessary to fully understand that both records are recording different aspects of watershed vegetation. Vegetation inferences from pollen percentages represent the relative abundance of pollen grains in a sediment profile and do not necessarily reflect species abundance or biomass on the landscape (Bradley, 1999) whereas compound-specific carbon isotopes do not reveal any information about forest composition. Pollen and leaf waxes are derived from different vegetative sources and thus record different aspects of watershed vegetation (Huang et al., 1999). Whereas pollen is a measure of only reproduction, leaf waxes provide a more representative measure of vegetative biomass within a watershed. The exclusion of maize pollen provides yet another complication for interpreting pollen profiles. Compound-specific carbon isotopes, on the other hand, do not reveal any detailed information about forest composition. They simply allow estimation of the relative contribution of C 3 versus C 4 n-alkanes to the sedimented organic matter. When examined in conjunction with pollen accumulation rates, however, leaf waxes and pollen may provide a better estimate of vegetative biomass. The carbon isotopic composition of leaf waxes is a good

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69 geochemical proxy for testing palynological inferences for vegetation changes within a watershed. 05101520250500100015002000250030003500400045005000Age (cal yr BP) % Grasses-40-35-30-2 5 13C C33 (, PDB) MAIZE % Grasses Salpeten Isotopes Sacnab Isotopes Figure 23: Diagram showing % grass pollen versus 13 C of n-alkane C 33 in Lakes Salpetn and Sacnab and the presence of maize pollen in Salpetn (pollen data from Leyden, 1987).

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70 Comparison with Population Estimates The correlation between Maya population densities and 13 C values of long-chain n-alkanes in lake sediments suggests that while vegetation change over the past 4500 cal yr BP may have been tied to changes in population density, additional factors may have affected vegetation in the watershed in the early part of the record. Pollen records (Leyden, 1987; Islebe et al., 1996) reveal that disturbance taxa and maize increased as populations increased. Pollen changes were most likely tied to agricultural land clearance. Because grasses and maize are C 4 plants, one would predict an increase in the 13 C of n-alkanes as populations grew. The largest shifts in vegetation, however, in both Lakes Sacnab and Salpetn, occurred well before the peak in late Classic Maya populations (Figure 24). The highest 13 C values, indicating the largest contribution from C 4 vegetation, occurs in Lake Sacnab between 3300 cal yr BP and 2700 cal yr BP and in Lake Salpetn at 2700 cal yr BP, during the Preclassic Period. In both lakes there appears to be a decrease in C 4 biomass between 2700 cal yr BP and 1300 cal yr BP, the time period during which population density was greatest. This decoupling of population density and the 13 C of long-chain n-alkanes between 4500 cal yr BP and 1300 cal yr BP may be a result of several factors. For example, if agricultural practices changed through time from more extensive to intensive methods, a return to a C 3 -dominated landscape may coincide with both population growth and agricultural developments. The question regarding whether the Maya maintained economic trees and house gardens within cities remains unanswered, but researchers have made speculations in some instances. For example, Leyden (1987) notes that the presence of ramon (Bromsimum) pollen suggests that the Maya were arboriculturists,

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71 possibly growing tree gardens in typically residential and common areas. Selective clearing and/or tree planting, however, is not indicated by the rest of the pollen record. Instead, greater proportions of corn pollen during the Late Classic through Postclassic indicate intensified agricultural activities. The interpretations made from the pollen record (that ramon does not necessarily indicate arboriculture) are supported by the work of Lambert and Arnason (1982), who showed that ramon is attracted to constructions of limestone and is positively correlated with Maya sites; this would indicate not arboriculture, but instead settlement expansion. Figure 24: Comparison of the compound-specific carbon isotope records (C 33 ) and population density estimates versus time in Lakes Salpetn (top) and Sacnab (bottom).

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72 It is also possible that, as discussed in the previous section, the higher contribution of C 4 vegetation during the Preclassic (~3300 cal yr BP to 2500 cal yr BP) may represent early shoreline maize cultivation in the watershed. As populations increased into the Classic period, the more desirable shorelines may have shifted to residential areas as opposed to agricultural areas, which would have then been relocated further from the lake. Abandonment of near-shore fields would be represented by a decoupling of the compound-specific 13 C and population because the agricultural fields may have expanded away from the lake shore as populations expanded in the Classic Period. Another possibility for the lack of coherence between population estimates and the compound-specific vegetation record may lie within the population estimates themselves. The population estimates in the Petn are based on a limited number of survey transects and house-mound excavations. There is a possibility that existing sites may not have been revealed during survey transects. In addition, older Preclassic population may have been underestimated if house mounds of the period were poorly preserved in the archaeological record. In addition, the chronology for population estimates was based on ceramic phases and inherently has a significant amount of error associated with it. While estimates of population densities in the watersheds may not be entirely accurate and may potentially be off by orders of magnitude the relative changes in population density are correct, and populations were certainly greater in the Classic than Preclassic Periods. Despite the divergent trends in population density and proxy vegetation in the early part of the record, the records display similar trends between 1200 cal yr BP and the present. A decrease in the compound-specific carbon isotopic ratios begins at 1200 cal

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73 yr BP and correlates with a significant decline in population associated with the decline of the Maya civilization. As human pressures on the landscape were curtailed and fields were abandoned, watershed vegetation shifted from a C 4 to a C 3 -dominated landscape. This indicates that the decline in population density was likely the main cause of late Holocene vegetation change. Relationship between Climate and Environmental Changes Finally, the relative importance of humans versus climate for vegetation change in the Central Petn can be determined by comparing the 13 C of n-alkanes with climate proxies that are not confounded by human impact. These comparisons may reveal whether climate played any role in environmental change. If vegetation changes in a watershed were climate-induced, long-chain n-alkanes should correlate with independent evidence for regional climate change. Proxy records of climate change are available for both the Yucatan and the Caribbean Sea and can be compared with compound-specific analyses and evaluated in regards to changes in Maya population densities. A comparison with the %Ti record from the Cariaco basin (Haug et al., 2001) (Figure 25) reveals a relationship between %Ti and the compound-specific records from each lake during the early part of the record. In the Cariaco Basin, %Ti in the sediments is used as a proxy for terrigenous sediment input, which is influenced by regional dry/wet cycles. Higher %Ti reflects higher terrigenous input and greater precipitation. The %Ti record from 10,000 cal yr BP to 4000 cal yr BP is high and relatively unchanging, indicating relatively mesic conditions. At approximately 4000 cal yr BP, however, %Ti values decrease and show greater variability indicating generally reduced

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74 precipitation, perhaps related to the southward migration of the Intertropical Convergence Zone (ITCZ). Increased variability in %Ti is especially pronounced from 4000 cal yr BP to 2000 cal yr BP. The apparent relationship of 13 C of n-alkanes to climate suggests that vegetation may have been partly controlled by changes in regional climate rather than local cultural changes over some time period. Increased climate variability during the period between 4000 cal yr BP and 2000 cal yr BP may have altered precipitation patterns and the vegetation in Peten. An extended dry period with sporadic periods of greater precipitation is an effective way to stress vegetation and to erode the landscape. Prior to 2000 cal yr BP, changes in regional climate may have influenced vegetation in addition to human disturbance. Further evidence for climate-influenced vegetation change can be found in sediment records from numerous lakes in northern Yucatan. A sediment core from Lake Chichancanab provides evidence for regional drying that occurred beginning at approximately 3000 cal yr BP with a distinct interval of droughts between 1300 cal yr BP and 1100 cal yr BP, which coincided with the Terminal Classic collapse (Hodell et al., 1995; Hodell et al., in press). Compound-specific carbon isotopes (Figure 15 ) during the period between 3300 and 2700 cal yr BP in both Lakes Sacnab and Salpetn represent the highest proportion of C 4 versus C 3 vegetation, as would be expected during a period of increased climate variability. One would expect to see continued increased contribution of C 4 vegetation throughout the period of increased climate variability but this is not the case. This discrepancy may suggest that while climate influenced vegetation change beginning at 3000 cal yr BP, human land-use change exacerbated the deforestation process from 3000 cal yr BP until Maya populations decreased at 1100 cal yr BP.

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75 00.050.10.150.20.250.30.350.40.45010002000300040005000600070008000900010000Age (cal yr BP)Ti (%, 3-pt smooth)-38-36-34-32-30-28-26-24-22-2013C C33 (, PDB) % Ti Salpeten Isotopes Sacnab Isotopes Figure 25: Comparison of the compound-specific carbon isotope records (C 33 ) and percent Ti versus time in Lakes Salpetn and Sacnab. Percent Ti data from Haug et al. (2001).

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76 The 18 O of ostracod carbonate in Lake Miragoane, Haiti (Hodell et al., 1991) records a similar climate history to that for the Cariaco basin (Haug et al., 2001). The isotope record, indicating changes in evaporation/precipitation and lake level, indicates a switch from dry forest vegetation to more mesic conditions at approximately 8200 cal yr BP. Beginning approximately 3800 cal yr BP, however, the isotopic value of shell carbonate begins to increase coincident with a loss of mesic forest trees and a regional drying event is suggested. These data correspond to local vegetation changes in the Sacnab and Salpetn basins; as climate became drier, vegetation shifted from a C 3 to a C 4 -dominated landscape. This further supports the interpretation that (~4000 cal yr BP to 3000 cal yr BP), climate may have played some role in vegetation change. Conclusions The questions addressed in this study explore the dynamics of environmental change with respect to ancient Maya population shifts and late Holocene climate changes. While the analysis of compound-specific 13 C values to study vegetation shifts is still in its infancy, this is the first late Holocene record for the Petn and the first to compare the 13 C of n-alkanes with a local pollen record. The data indicate that in the watersheds of Lakes Salpetn and Sacnab, shifts in the proportion of C 3 to C 4 are most likely controlled by a combination of climate change and human deforestation. The correspondence of 13 C records to independent proxies for climate change from ~4500 cal yr BP until ~3000 cal yr BP suggest that regional drying and increased climate variability caused an increase in the contribution of C 4 vegetation

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77 during that time. Following this period and beginning with the first Maya occupation in the watersheds, vegetation change was likely a result of human-driven deforestation or perhaps a combination of both climate and human impact. The lack of concordance between the compound-specific carbon isotopic record and the pollen profiles between 3000 cal yr BP and 1100 cal yr BP may be due to the underlying ambiguities in comparing percent pollen data with compound-specific carbon isotopic measurements. The only true assessment of the C 4 pollen signal is to examine shifts in the absolute values of grass and maize pollen influx. The higher contribution of C 4 vegetation during the Preclassic inferred from compound-specific 13 C (~3300 cal yr BP to 2500 cal yr BP) may represent early shoreline maize cultivation in the watershed. As populations increased into the Classic period and shorelines shifted to residential as opposed to agricultural areas, cultivated fields would have then been moved further into the surrounding watershed. This abandonment of fields and subsequent cessation of maize cultivation on the shoreline may strongly alter the compound-specific 13 C but would not be apparent in the pollen profile. Discrepancies between vegetation inferred from pollen profiles versus 13 C of n-alkanes may also lie within the interpretation of the compound-specific 13 C record itself. While 13 C of n-alkanes is an excellent proxy for estimating the relative changes in biomass contribution of C 3 and C 4 vegetation, it cannot be used to interpret forest composition or the dynamics of tropical reforestation. In addition, there is no clear understanding of how issues such as differential leaf production and the amount of leaf wax a plant produces may affect the 13 C of n-alkanes. Another potential problem in interpreting 13 C values is that variation in canopy density that alters both light regimes

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78 and the 13 C of source CO 2 air for plants, which in turn affects the 13 C of plant material. While an important factor, it is difficult to quantitatively assess the canopy effect on 13 C values of n-alkanes. Lastly, the relationship between 13 C and water-use efficiency (WUE) in C 3 plants may cause 13 C values to appear more depleted and thus suggest a higher apparent contribution of C 3 vegetation. With these limitations in mind, it is necessary to interpret compound-specific carbon isotope records with caution. While the decoupling of population density and the 13 C of long-chain n-alkanes may suggest that agricultural practices changed through time from more extensive to intensive methods, it is impossible to determine this from the 13 C from long-chain n-alkanes alone. The 13 C from long-chain n-alkanes need not necessarily track population change if a shift in the proximity of agricultural lands (and proximity of maize pollen, which is large and does not travel far) to the lake was influencing the 13 C. The movement of fields and subsequent cessation of maize cultivation on the shoreline would strongly alter the compound-specific 13 C but would not necessarily be reflected by changes in population densities. The interpretation of compound-specific carbon isotopes of long-chain n-alkanes is dependent upon the area surrounding the lake over which the compounds integrate. There is unfortunately very little information regarding transport and deposition of n-alkanes in lacustrine studies. If compound-specific carbon isotopes of long-chain n-alkanes are more strongly influenced by vegetation on water-edge lands, than they may not provide an accurate picture of overall watershed vegetation and land-use. Studies that attempt to calibrate the compound-specific carbon isotopic value of modern sediments

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79 with the surrounding forest structure may greatly aid in interpreting sediment records of paleoenvironmental change. Compound-specific carbon isotopes and pollen profiles record represents different aspects of watershed vegetation and are best used in tandem to infer past changes in watershed vegetation. While this study complements past studies that analyzed only bulk sediment or pollen and further supports the record of changes in terrestrial vegetation in the Petn region of the Maya Lowlands, further research is needed to understand the dynamic changes in forest structure associated with deforestation and reforestation. In addition, calculating grass and maize pollen accumulation rates from the same core as the 13 C may provide a more robust assessment of the C 4 pollen signal. And lastly, it is important to utilize new archaeological information from surrounding watersheds to better understand how changes in land-use correlate to environmental change.

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APPENDIX A LIPID EXTRACTION PROCEDURE Supplies: 33 mL stainless steel cell Glass filters Quartz sand 2:1 methylene chloride (DCM)/methanol Scintillation vials Notes: -All quartz sand should be ashed at 450C for at least two hours in a muffle furnace. ASE cells should be cleaned after each use by washing with DI water and solvent rinsing with methanol 3 times. Store ASE cells in oven, or with top and bottom screwed in place. 1) Screw base onto 33 ml Accelerated Solvent Extractor cell. Place two filters at the base and add ~5-10 mL of sand. 2) Weigh out and add approximately 10-20 g of an internal standard (C 34 ) to each sample so as to allow calculation of yields and quantification of lipid compounds later in the process. 3) Weight out approximately 3-5 g of sample that has been frozen, freeze dried and crushed. 4) Record the weight and then add the sample to the cell. 5) Fill remaining space in the cell with sand and screw on the top. 6) Place cells in the Accelerated Solvent Extractor and use the following method to extract the phospholipids: Solvent: 2:1 DCM/Methanol Pressure: 1500 PSI Temperature: 100 C Heat: 5 mins Static: 5 mins Flush: 60% Purge: 200 sec Cycles: 3 80

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81 7) Solvent exchange to hexane using a hot water bath under a stream of nitrogen gas. This is performed by evaporating samples to dryness and then adding ~10 mL of hexane. To ensure full solvent exchange, repeat this process three times. 8) Transfer residue to a labeled 20 mL glass scintillation vial using ~15ml of hexane. For maximum compound retention, do this in multiple wash/transfer steps. 9) Evaporate sample in a hot water bath under a stream of nitrogen gas. 10) Samples (in scintillation vials) are now ready for silica gel chromatography.

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APPENDIX B SILICA GEL CHROMOTAGRAPHY Supplies: Glass column Teflon stopcock Glass rod Glass funnels Glass wool GC Resolv or Optima grade Hexane GC Resolv or Optima grade methylene chloride (DCM) Teflon squirt bottle Small stainless steel spatula 5% deactivated silica gel Pasteur pipette with long pipette tips 20 mL beaker 2 graduates cylinders Notes: -All glassware, glass wool, and silica gel should be solvent rinsed and ashed at 450C for at least two hours in a muffle furnace. 1) To make 5% deactivated silica gel, use the following relation: a. (%deactivation/ (100-%deactivation)) = (mL water/weight of silica (g)) b. (%deactivation/(100-%deactivation)) (weight of silica (g)) = mL of water Add 1 mL of DI water to 20g of 100% activated silica gel c. Shake bottle for 10 minutes. d. Store in dessicator (good for only three days). 2) Assemble stopcock to the base of the column to regulate flow. 3) Set up column by placing glass wool at the base of a glass column and pushing it down with a glass rod until the column is clogged. 4) Set up column vertically in the hood with a clamp. 5) Add approximately one column full of hexane and allow the hexane to drain to clean the glass plug. 6) Weight out 2.5 grams of 5% deactivated silica gel in a 20-ml beaker. Cover with hexane quickly to keep if from absorbing any moisture from the air. 82

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83 7) Place a glass funnel in the top of the column. Mix silica gel/hexane thoroughly with the spatula creating a suspension. Add the mixture to the column using the funnel and allow the silica to settle while tapping gently to avoid gas bubbles. Be sure to scrape all silica gel from the sides of the beaker into the funnel. Squirt the gel with hexane to force it through the funnel. 8) Remove the funnel and fill with hexane. 9) Drain excess hexane while tapping the column to guarantee full settling. Close the stopcock when the hexane level reaches just above the silica gel. Free any remaining silica gel adhering to the sides by tapping the column gently. 10) Rinse the column with ~10ml of hexane to wash remaining silica gel, again stopping when the solvent level reaches just above the silica-gel. Rinse the stopcock tip with hexane. 11) Place a collection vial labeled for n-alkanes under the column. 12) Fill a graduated cylinder with 16-17 mL of hexane. Use a pipette to bring the level to 15 mL, thus rinsing the pipette. 13) Add 1 mL of hexane from the graduated cylinder to the scintillation vial containing the sample. Rotate the sample vial between fingers while holding the vial at a 45 angle to dissolve the residue. 14) Transfer the sample in hexane to the column, gently dispensing sample ~1cm above silica gel. 15) Drain and collect the sample and solvent. 16) Repeat steps 13-15 twice more before adding the remaining hexane (~12 mL) in the graduated cylinder to the column. 17) Drain the sample and solvent to just above the level of the silica gel. 18) Remove the collection vial and a place second vial labeled for the non-n-alkane organic fraction under the column. 19) Using a graduated cylinder, measure 15 mL of DCM and add it to the column. 20) Drain the DCM into the collection vial, thus eluting all other lipid compounds. 21) Archive for potential later use. 22) Concentrate n-alkane fractions in collection vials by evaporating to dryness under a stream of N 2 23) Re-dissolve n-alkanes in 200 L of hexane. 24) Run samples on a Perkin Elmer 8500 Gas Chromatograph (PE 8500 GC) injecting 4 L of sample in order to determine purity and approximate concentrations using the following oven program: T ramp Rate C/min Hold Temp. C Hold Time (min) 50 1 6 300 20

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APPENDIX C UREA ADDUCTION AND GC ANALYSIS Supplies: GC Resolv or Optima grade hexane GC Resolv or Optima grade acetone GC Resolv or Optima grade pentane GC Resolv or Optima grade urea-saturated methanol DCM-extracted DI water GC Resolv or Optima grade methanol Pasteur pipette with long pipette tips 20 mL beaker 2 graduates cylinders gas-tight Teflon coated syringes Notes: -All glassware should be solvent rinsed and ashed at 450C for at least two hours in a muffle furnace. All of the following solvent transfers are performed using glass pipettes or Teflon-coated gas-tight syringes. 1) After running each alkane sample on the PE 8500 GC, transfer samples to a 4 mL glass screw top vial and evaporate to dryness under a stream of nitrogen. 2) Add 1 mL each of acetone, pentane, and urea-saturated methanol to each sample. Replace caps. 3) Place in a -4C freezer for ~30 minutes. 4) After 30 minutes, remove samples from freezer. Evaporate excess solvent under a stream of nitrogen. The urea crystals that remain contain the straight-chain n-alkanes only, while the non-adducts, branched and cyclic compounds, remain outside of the crystals. 5) Wash urea crystals with hexane three times, with the wash hexane being pipetted into a separate 4 mL vial labeled as non-adducts to be archived for potential later use. 6) Dissolve the remaining urea crystals in 1 mL of methanol and 1 mL of DCM-extracted DI water. This fraction contains the straight-chain n-alkanes. 7) Add approximately 2 mL of hexane to this fraction. Shake vigorously using a Vortex vibrator to extract hydrocarbons. 8) Centrifuge vials at 500 rpm for 5 minutes or until hexane and urea/methanol/water separate. 84

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85 9) Remove the hexane from the top with a gas-tight syringe and place into a separate 4 mL vial labeled as adducts. Repeat the hydrocarbon extraction (Steps 7-8) twice more to guarantee full recovery of n-alkanes. 10) Evaporate excess solvent from the adduct fraction under a stream of nitrogen. 11) Repeat steps 2-10 twice more on the adduct fraction. 12) After triple adduction, evaporate the adduct fraction to dryness under a stream of nitrogen and re-dissolve in 200 L of hexane. 13) Run samples on a Perkin Elmer 8500 GC using the following oven program and injecting 4 L of sample: Rate C/min Temperature C Time (min) 50 1 6 300 20 14) Evaporate the sample to dryness again under a stream of nitrogen and transfer to a 1 mL glass crimp-top autosampler vial using multiple washes to ensure full transfer of n-alkanes. 15) Evaporate the sample to dryness under a stream of nitrogen. Dissolve in a known amount of hexane according to obtain the approximate concentration necessary for GC-IRMS analysis (ex.: samples that had a voltage of ~20-30 mV on the PE 8500 GC should be dissolved in 100 L of hexane whereas samples with a voltage of ~10 should be dissolved in 25 L of hexane). 16) The adduct fraction containing straight-chain n-alkanes is now ready for GC-Isotope Ratio Mass Spectrometry (IRMS) analysis.

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APPENDIX D DATA TABLES

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Table 6: Values for %N, %C, C:N, %TIC, %CaCO3, %TOC, %Organic matter and %Other from Sacnab.Lake 87

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88 Table 6: Continued.

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Table 6: Continued. 89

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Table 7: Values for bulk organic matter isotopes from Lake Sacnab. 90

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91 .Table 8: Values for compound-specific carbon isotopes from Lake Sacnab.

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92 .Table 8: Continued.

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93 Table 9: Values for compound-specific carbon isotopes from Lake Salpeten.

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94 Table 10: Concentrations of n-alkanes in g/g from Lake Sacnab.

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95 .Table 10: Continued

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96 Table 11: Concentrations of n-alkanes in g/g from Lake Salpetn.

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LIST OF REFERENCES Anselmetti, F., Ariztegui, D., Hodell, D.A., Brenner, M., Curtis, J.H., Guilderson, T.P., and Rosenmeier, M., in preparation. Enhanced soil erosion rates in the southern Maya Lowlands: The Lago Salpeten record, Guatemala. Binford, M.W., Brenner, M., Whitmore, T.J., Higuera-Gundy, A., Deevey, E.S., and Leyden, B.W., 1987. Ecosystems, paleoecology and human disturbance in subtropical and tropical America. Quaternary Science Reviews 6: 115-128. Bradley, R.S. 1999. Paleoclimatology. Academic Press, San Diego, CA. Brenner, M., 1983. Paleolimnology of the Petn Lake District, Guatemala, 2: Mayan population density and sediment and nutrient loading of Lake Quexil. Hydrobiologia 103: 205-210. Bronk Ramsey, C., 1995. Radiocarbon calibration and analysis of stratigraphy: The OxCal Program. Radiocarbon 37(2): 425-430. Bronk Ramsey C., 2001. Development of the Radiocarbon Program OxCal. Radiocarbon 43 (2A): 355-363. Brezonik, P.L. and Fox, J.L., 1974. The limnology of selected Guatemalan lakes. Hydrobologia 45: 467-487. Butzer, K.W., 1992. The Americas before and after 1492: An introduction to current geographical research. Annals of the Association of American Geographers 82(3): 345-368. Coe, M.D., 1999. The Maya: Sixth Edition. Thames and Hudson, New York, New York. Collister, J.W., Rieley, G., Stern, B., Eglinton, G., and Fry, B., 1994. Compound-specific 13 C analyses of leaf lipids from plants with differing carbon dioxide metabolism. Organic Geochemistry 21: 619-627. Conte, M.H and Weber, J.C, 2002. Plant biomarkers in aerosols record isotopic discrimination of terrestrial photosynthesis. Nature 417: 639-641. Cowgill, U.M. and Hutchinson, G.E., 1966. A general account of the basin and the chemistry and mineralogy of the sediment cores. In: U.M Cowgill, C.E. Goulden, G.E. Hutchinson, R. Patrick. A.A. Racek and M. Tsukada (eds): The history of 97

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98 Laguna de Petenxil. Memoirs of the Connecticut Academy of Arts and Sciences, New Haven, 17: 2-62. Cranwell, P.A., 1981. Diagenesis of free and bound lipids in terrestrial detritus deposited in a lacustrine sediment. Organic Geochemistry 3:79-89. Cranwell, P.A., Eglinton, G. and Robinson, N., 1987. Lipids of aquatic organisms as potential contributors to lacustrine sediments-II, Organic Geochemistry 11: 513527 Curtis, J.H., Brenner, M., Hodell, D.A., Balser, R.A., Islebe, G.A., Hooghiemstra, H., 1998. A multi-proxy study of Holocene environmental change in the Maya Lowlands of Petn, Guatemala. Journal of Paleolimnology 19: 139-159. Deevey, E.S., 1969. Coaxing history to conduct experiments. Bioscience 19 (1):40-43. Deevey, E.S., 1978. Holocene forests and Maya disturbance near Quexil Lake, Petn, Guatemala. Polish Archives of Hydrobiology 25 (1/2): 117-129. Deevey, E.S., Brenner, M., and Binford, M.W., 1983. Paleolimnology of the Petn Lake District, Guatemala. III. Late Pleistocene and Gamblian environments of the Maya area. Hydrobiologia 103: 211-216. Deevey, E.S., Brenner, M., Flannery, M.S., and Yezdani, G.H., 1980. Lakes Yaxha and Sacnab, Petn, Guatemala: Limnology and hydrology. Arch. Hydrobiol. 57: 418-460. Deevey, E.S., Rice, D.S., Rice, R.M., Vaughan, H.H., Brenner, M., and Flannery, M.S., 1979. Mayan urbanism: Impact on a tropical karst environment. Science 206: 298-306. Demarest, A.A., 1997. The Vanderbuilt Petexbatun Regional Archaeological Project 1989-1994: Overview, history, and major results of a multidisciplinary study of the Classic Maya collapse. Ancient Mesoamerica 8: 209-227. Dunning, N., Beach, T., and Rue, D., 1997. The paleoecology and ancient settlement of the Petexbatun region, Guatemala. Ancient Mesoamerica 8: 255-266. Dunning, N., Rue, D.J., Beach, T., Covich, A., and Traverse, A., 1998. Human-environment interactions in a tropical watershed: The paleoecology of Laguna Tamarindito, El Petn, Guatemala. Journal of Field Archaeology 25:139-151. Emery, K.F. (in press). Dietary, environmental, and societal implications of ancient Maya animal use in the Petexbatun: A zooarchaeological perspective on the collapse. Vanderbilt University Press, Nashville.

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99 Eglinton, G. and Hamilton, R.J., 1967. Leaf epicuticular waxes. Science 156: 1322-1335. Ertel. J.R. and Hedges, J.I., 1985. Sources of sedimentary humic substances: vascular plant debris. Geochimica et Cosmochimica Acta 49: 2097. Ficken, K.J., Li, B., Swain, D.L., and Eglinton, G., 2000. An n-alkane proxy for the sedimentary input of submerged/floating freshwater aquatic macrophytes. Organic Geochemistry 31: 745-749. Filley T. R., Freeman K. H., Bianchi T., Colarusso L. A. and P. Hatcher (2001) An isotopic biogeochemical assessment of shifts in organic matter input to Holocene sediments from Mud Lake, Florida. Organic Geochemistry 32: 1153-1167. Hayes, J.M., 1993. Factors controlling 13 C contents of sedimentary organic compounds: principles and evidence. Marine Geology 113: 111-125. Hayes, J.M., 2001. Fractionation of the isotopes carbon and hydrogen in biosynthetic processes; prepared for short course of Mineralogical Society of America, GSA 2001. Haug, G.H., Hughen, K.A., Sigman, D.M., Peterson, L.C., and Rohl, U., 2001. Southward migration of the Intertropical Convergence Zone through the Holocene. Science 293: 1304-1308. Hedges, J.I. and Prahl, F.G., 1993. Early diagenesis: consequences for applications of molecular biomarkers In: M.H. Engel and S.A. Macko, Editors, Organic Geochemistry. Principles and Applications, Plenum Press, New York, pp. 237253. Hodell, D.A, Brenner, M., Curtis, J.H., and Guilderson, T., 2001. Solar forcing of drought in the Maya Lowlands. Science 292: 1367-1370. Hodell, D.A, Curtis, J.H., and Brenner, M., 1995. Possible role of climate in the collapse of the Classic Maya civilization. Nature 375: 391-394. Huang, Y., Street-Perrott, F.A., Metcalfe, S.E., Brenner, M., Moreland, M., and Freeman, K.H., 2001. Climate change as the dominant control on Glacial-Interglacial variations in C 3 and C 4 plant abundance. Science: 293 (5535). Huang, Y., Street-Perrott, F.A., Perrott, A., Metzger, P., and Eglinton, G., 1999. Glacial-interglacial environmental changes inferred from molecular and compound-specific d 13 C analysis of sediments from Sacred Lake, Mt. Kenya. Geochimica et Cosmochimica Acta 63: 1383-1404.

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100 Hughen, K. A., Eglinton, T.I., Xu, L., and Makou, M., 2004. Abrupt tropical vegetation response to rapid climate change. Science 304: 1955-1959. Islebe, G.A., Hooghiemstra, H., Brenner, M., Curtis, J.H., and Hodell, D.A, 1996. A Holocene vegetation history from lowland Guatemala The Holocene 6: 265-271. Kaushal, S. and Bindford, M.W., 1999. Relationship between C:N ratios of lake sediments, organic matter sources, and historical deforestation of Lake Pleasant, Massachusetts, USA. Journal of Paleolimnology 22: 439. Knops, J.M.H. and Tilman, D., 2000. Dynamics of soil nitrogen and carbon accumulation for 61 years after agricultural abandonment. Ecology 81: 88-98. Lajtha, K. and Marshall, J.D., 1994. Sources of variation in the stable isotopic composition of plants. In K. Lajtha and R.H. Michener, eds., Stable Isotopes in Ecology and Environmental Science. Oxford, Great Britain: Blackwell Scientific Publications, Oxford, pp 1-21. Lambert, J.D.H.; Arnason, J. T. 1982. Ramon and Maya ruins: An ecological, not an economic, relation. Science 216: 298. Leffler, A.J. and Enquist, B.J., 2002. Carbon isotope composition of tree leaves from Guanacaste, Costa Rica: Comparison across tropical forests and tree life history. Journal of Tropical Ecology 18: 151-159. Leyden, B.W., 1987. Man and climate in the Maya Lowlands. Quaternary Research 28:407-414. Leyden, B.W., 2002. Pollen evidence for climatic variability and cultural disturbance in the Maya Lowlands. Ancient Mesoamerica 13: 85-101. Leyden, B.W., Brenner, M., and Dahlin, B.H., 1998. Cultural and climatic history of Coban a lowland Maya city in Quintana Too, Mexico. Quaternary Research 49: 111-122. Leyden, B. W., Brenner, M., Hodell, D.A., and Curtis, J.H., 1993. Late Pleistocene climate in the Central American lowlands. In Climate change in continental isotopic records. Geophysical Monograph 78, Washington, DC: pp.165-178. Lundell, C.L., 1937. The vegetation of Petn. Carnegie Institute, Washington, DC. Meyers, P.A., 1994. Preservation of elemental and isotopic source identification of sedimentary organic matter. Chemical Geology 144: 289.

PAGE 115

101 Meyers, P. A., 1997. Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic processes. Organic Geochemistry 27: 213-250. Meyers, P.A. and Eadie, B.J., 1993. Sources, degradation and recycling of organic matter associated with sinking particles in Lake Michigan. Organic Geochemistry 20: 47. Meyers, P.A., and R. Ishiwatari. 1993. Lacustrine organic geochemistry An overview of indicators of organic matter sources and diagenesis in lake sediments. Organic Geochemistry 20: 867-900. Meyers, P.A., Kawka, O.E., and Whitehead, D.R., 1984. Geolipid, pollen and diatom stratigraphy in postglacial lacustrine sediments. Organic Geochemistry 6: 727732. Mller, A. and Mathesius, U., 1999. The palaeoenvironments of coastal lagoons in the southern Baltic Sea. The application of sedimentary C org /N ratios as source indicators of organic matter. Palaeogeography, Palaeoclimatology, Palaeoecology 145: 1. Paillard D., L. Labeyrie, and P. Yiou, 1996. Macintosh program performs time-series analysis. Eos, Transactions, American Geophysical Union 77: 379. Pakeman, R.J., Marrs, R.H., and Jacob, P.J., 1994. A model of bracken (Pteridium aquilinum) growth and the effects of control strategies and changing climate. Journal of Applied Ecology 31: 145-154. Prahl, F.G., Bennett, J.T. and Carpenter, R., 1980. The early diagenesis of aliphatic hydrocarbons and organic matter in sedimentary particulates from Dabob Bay, Washington. Geochimica et Cosmochimica Acta 44: 1967. Rice, D.S., 1993. Eighth-Century Physical Geography, Environment, and Natural Resources in the Maya Lowlands. In Sabloff, J.A. and Henderson, J.S. (ed.): Lowland Maya Civilization in the Eighth Century A.D.: 11-64. Dumbarton Oaks Research Library and Collection, Washington D.C. Rice, D.S., and P.M. Rice, 1990. Population Size and Population Change in the Central Petn Lakes Region, Guatemala. In: Precolumbian Population History in the Maya Lowlands, edited by T. Patrick Culbert and Don S. Rice, pp. 123-148. University of New Mexico Press, Albuquerque, New Mexico. Rice, P.M. and Rice, D.S., 2004. Late Classic to Postclassic Transformations in the Petn Lakes Region, Guatemala. In: A.A. Demarest, P.M. Rice, and D.S. Rice (eds.): The terminal Classis in the Maya lowlands: Collapse, transition, and transformation. University Press of Colorado, Colorado.

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102 Rice, D.S., Rice, P.M., and Deevey, E.S., 1985. Paradise Lost: Classic Maya Impact on a Lacustrine environment. In M. Pohl, ed., Prehistoric Lowland Maya Environment and Subsistence Economy, pp. 91-105, Peabody Museum Papers 77. Harvard University Press, Cambridge, MA. Robinson, N., Cranwell, P.A., Finlay, B.J., and Englinton, G., 1984. Lipids of aquatic organisms as potential contributors to lacustrine sediments. Organic Geochemistry 6: 143-152. Rosenmeier, M.F., Hodell, D.A., Brenner, M., Curtis, J.H., Martin, J.B., Anselmetti, F.A., Ariztegui, D., and Guilderson, T.P., 2002. Influence of vegetation change on watershed hydrology: Implications for paleoclimatic interpretation of lacustrine 18 O records. Journal of Paleolimnology 27: 117-131. Schefub, E., Ratmeyer, V., Stuut, J.-B. W., Jansen, J.H.F., and Sinninghe Damst, J.S., 2003. Carbon isotope analyses of n-alkanes in dust from the lower atmosphere over the central eastern Atlantic. Geochimica et Cosmochimica Acta 67: 1757-1767. Schmitt, J., Glaser, B., Zech, W., 2003. Amount-dependent isotopic fractionation during compound-specific isotope analysis. Rapid Communications in Mass Spectrometry, 17: 970-977. Sharer, R.J., 1994. The ancient Maya: Fifth Edition. Stanford University Press, Stanford, California. Simoneit, B.R.T., 1977. Organic matter in eolian dusts over the Atlantic Ocean. Marine Chemistry 5: 443-464. Tilman, D., 1984. Plant dominance along an experimental nutrient gradient. Ecology 65: 1445-1453. Tilman, D., 1987. Secondary succession and the pattern of plant dominance along experimental nitrogen gradients. Ecological Monographs 57: 189-214. Turner, B.L., Klepeis, P., and Schneider, L.C., 2003. Three Millennia in the Southern Yucatan Peninsula: Implications for Occupancy, Use, and Carrying Capacity. In A. Gomez-Pompa, M.F. Allen, S.L. Fedick, and J.J. Jiminez-Osornio: The lowlands Maya area: Three millennia at the human-wildland interface: Food Products Press, New York, pp. 361-387. Vaughan, H.H., Deevey, E.S. and Garrett-Jones, S.E., 1985. Pollen stratigraphy of two cores from the Petn Lake District. In M. Pohl, ed., Prehistoric lowland Maya

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103 environment and subsistence economy. Peabody Museum Papers 77. Harvard University Press, Cambridge, MA, pp. 73-89. Wiseman, F.M., 1985. Agriculture and Vegetation Dynamics of the Maya Collapse in Central Petn, Guatemala. In M. Pohl, ed., Prehistoric lowland Maya environment and subsistence economy. Peabody Museum Papers 77. Harvard University Press, Cambridge, MA, pp. 63-71.

PAGE 118

BIOGRAPHICAL SKETCH Sarah Davidson Newell was born on 31 March 1980 in Schenectady, NY. She lived her first 22 years in upstate New York and spent most of her childhood years covered in mud. Eventually, she grew out of trudging through the woods and spent her teenage years at Galway High School as a typical teen involved in academics, athletics, drama and student government. She enrolled at Union College in September of 1998 and, after briefly contemplating a classics major, rediscovered her passion for being covered in mud and chose a major in geology. After working with Dr. Don Rodbell for three years in the Core Lab at Union, Sarah graduated cum laude from Union College in 2002 with a BS in geology. Sarah continued her education at the University of Florida where she pursued a MS degree in geological sciences under the supervision of Dr. David A. Hodell. At the University of Florida, Sarah spent much of her time developing methods for compound-specific isotopic studies and worked on sediments from northern Guatemala. Sarah will graduate with her MS degree in May 2005 and has already taken a position at Stanford University, beginning work towards her PhD in ocean sciences. Sarah is working with Dr. Robert Dunbar, developing a multi-centennial record of climate variability in the South Pacific using stable isotopes and trace metals from Easter Island corals. 104


Permanent Link: http://ufdc.ufl.edu/UFE0009584/00001

Material Information

Title: An Analysis of compound specific carbon isotopes of lipid biomarkers : a proxy for paleoenvironmental change in the maya lowlands of Peten, Guatemala
Physical Description: Mixed Material
Language: English
Creator: Newell, Sarah D. ( Dissertant )
Adviser: Hodell, David A. ( Thesis advisor )
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2005
Copyright Date: 2005

Subjects

Subjects / Keywords: Geological Sciences thesis, M.S
Dissertations, Academic -- UF -- Geological Sciences
Spatial Coverage: Guatemala--Peten

Notes

Abstract: The Petén region of northern Guatemala has been occupied by humans for more than 3000 years. During that time, the lowland tropical environment experienced a prolonged period of anthropogenic disturbance. Forest disturbance in the Pete acuten Lake District of northern Guatemala was associated with Maya agricultural practices as well as clearing for urban development, construction, and fuelwood both for cooking and for lime-plaster production. Expansion of the Maya civilization during the Preclassic (~1000 BC to AD 250) and Classic periods (AD 250 to AD 900) was accompanied by increasing deforestation of Pete acuten watersheds and accelerated rates of soil erosion. Palynological data from the Petén Lake District illustrate the near elimination of high forest taxa and prevalence of disturbance taxa (grasses, weeds) during the height of Classic Maya occupation (~AD 500-800). After flourishing during the Classic period between AD 250 and 800, the Maya civilization in the Petén Lake District experienced a dramatic change between AD 800 and 900 that some have referred to as the "collapse." Population densities declined significantly after AD 900, thereby curtailing human pressures on the landscape. This cycle of population expansion and decline in the Petén Lake District provides a natural historical "experiment" that has been used to study the response of tropical vegetation to long-term changes in land-use by humans. A new line of evidence is used here to complement other archaeological and paleoenvironmental methods: the use of leaf wax biomarkers in palynological inference studies. The molecular and isotopic compositions of leaf waxes have been shown to be reliable indicators of vegetative biomass and are useful for testing palynological inferences. The carbon isotopic composition of long-chain n-alkanes of leaf waxes were used as a geochemical proxy for terrestrial vegetation to test palynological inferences of vegetation change in two lake basins (Lakes Sacnab and Salpete acuten) in the Petén Lake District of the southern Maya Lowlands over the past ~4500 cal yr BP. Discrepancies between the delta¹³ carbon isotopic composition of long-chain n-alkanes and vegetation changes inferred from pollen profiles suggest that the two proxies may be recording different characteristics of watershed vegetation as well as different airshed/watershed processes. The data indicate that in the watersheds of Lakes Salpeten and Sacnab, shifts in the proportion of C₃ to C₄ vegetation are most likely controlled by a combination of climate change and human deforestation. The correspondence of carbon isotopic records to independent proxies for climate change from ~4500 cal yr BP until ~3000 cal yr BP suggest that regional drying and increased climate variability caused an increase in the contribution of C₄ vegetation during that time. Following this period and beginning with the first Maya occupation in the watersheds, vegetation change was likely a result of human-driven deforestation or perhaps a combination of both climate and human impact.
Subject: biomarkers, carbon, climate, Guatemala, isotopes, lipids, Maya, paleoenvironment, pollen, vegetation
General Note: Title from title page of source document.
General Note: Document formatted into pages; contains 118 pages.
General Note: Includes vita.
Thesis: Thesis (M.S.)--University of Florida, 2005.
Bibliography: Includes bibliographical references.
General Note: Text (Electronic thesis) in PDF format.

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0009584:00001

Permanent Link: http://ufdc.ufl.edu/UFE0009584/00001

Material Information

Title: An Analysis of compound specific carbon isotopes of lipid biomarkers : a proxy for paleoenvironmental change in the maya lowlands of Peten, Guatemala
Physical Description: Mixed Material
Language: English
Creator: Newell, Sarah D. ( Dissertant )
Adviser: Hodell, David A. ( Thesis advisor )
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2005
Copyright Date: 2005

Subjects

Subjects / Keywords: Geological Sciences thesis, M.S
Dissertations, Academic -- UF -- Geological Sciences
Spatial Coverage: Guatemala--Peten

Notes

Abstract: The Petén region of northern Guatemala has been occupied by humans for more than 3000 years. During that time, the lowland tropical environment experienced a prolonged period of anthropogenic disturbance. Forest disturbance in the Pete acuten Lake District of northern Guatemala was associated with Maya agricultural practices as well as clearing for urban development, construction, and fuelwood both for cooking and for lime-plaster production. Expansion of the Maya civilization during the Preclassic (~1000 BC to AD 250) and Classic periods (AD 250 to AD 900) was accompanied by increasing deforestation of Pete acuten watersheds and accelerated rates of soil erosion. Palynological data from the Petén Lake District illustrate the near elimination of high forest taxa and prevalence of disturbance taxa (grasses, weeds) during the height of Classic Maya occupation (~AD 500-800). After flourishing during the Classic period between AD 250 and 800, the Maya civilization in the Petén Lake District experienced a dramatic change between AD 800 and 900 that some have referred to as the "collapse." Population densities declined significantly after AD 900, thereby curtailing human pressures on the landscape. This cycle of population expansion and decline in the Petén Lake District provides a natural historical "experiment" that has been used to study the response of tropical vegetation to long-term changes in land-use by humans. A new line of evidence is used here to complement other archaeological and paleoenvironmental methods: the use of leaf wax biomarkers in palynological inference studies. The molecular and isotopic compositions of leaf waxes have been shown to be reliable indicators of vegetative biomass and are useful for testing palynological inferences. The carbon isotopic composition of long-chain n-alkanes of leaf waxes were used as a geochemical proxy for terrestrial vegetation to test palynological inferences of vegetation change in two lake basins (Lakes Sacnab and Salpete acuten) in the Petén Lake District of the southern Maya Lowlands over the past ~4500 cal yr BP. Discrepancies between the delta¹³ carbon isotopic composition of long-chain n-alkanes and vegetation changes inferred from pollen profiles suggest that the two proxies may be recording different characteristics of watershed vegetation as well as different airshed/watershed processes. The data indicate that in the watersheds of Lakes Salpeten and Sacnab, shifts in the proportion of C₃ to C₄ vegetation are most likely controlled by a combination of climate change and human deforestation. The correspondence of carbon isotopic records to independent proxies for climate change from ~4500 cal yr BP until ~3000 cal yr BP suggest that regional drying and increased climate variability caused an increase in the contribution of C₄ vegetation during that time. Following this period and beginning with the first Maya occupation in the watersheds, vegetation change was likely a result of human-driven deforestation or perhaps a combination of both climate and human impact.
Subject: biomarkers, carbon, climate, Guatemala, isotopes, lipids, Maya, paleoenvironment, pollen, vegetation
General Note: Title from title page of source document.
General Note: Document formatted into pages; contains 118 pages.
General Note: Includes vita.
Thesis: Thesis (M.S.)--University of Florida, 2005.
Bibliography: Includes bibliographical references.
General Note: Text (Electronic thesis) in PDF format.

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0009584:00001


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AN ANALYSIS OF COMPOUND SPECIFIC CARBON ISOTOPES OF LIPID
BIOMARKERS: A PROXY FOR PALEOENVIRONMENTAL CHANGE IN THE
MAYA LOWLANDS OF PETEN, GUATEMALA














By

SARAH DAVIDSON NEWELL


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2005

































Copyright 2005

by

Sarah Davidson Newell

































I find the great thing in this world is not so much where we stand, as in what direction we
are moving...we must sail sometimes with the wind and sometimes against it, but we
must sail, and not drift, nor lie at anchor.





Oliver Wendell Holmes















ACKNOWLEDGMENTS

First and foremost I would like to thank my advisor, Dr. David A. Hodell, for his

constant support, encouragement and inspiration. I appreciate most his faith in my

abilities as a research scientist and unspoken prodding to work harder. Over the past two

years I have developed a stronger passion for research for which he is unduly responsible.

Perhaps the most influential person through my master's research was Dr. Jason H.

Curtis, to whom I now owe my first-born child. Over the past two years, Jason has

provided constant and unparalleled support both in the lab and in life. Jason's assistance

with all aspects of my lab work and method development is the sole reason why I was

able to complete my MS in just two years. Jason provided to me a helping hand in the

lab (at the drop of a hat, I should note), an ear for listening to complaints and excitement

alike, and a shoulder during stressful times. I would especially like to thank him for the

hours spent with our heads buried in the GC oven and for putting up with my spastic

personality.

A number of other individuals were instrumental in the completion of my research.

I would like to thank Dr. Mark Brenner for constantly reviewing my work and providing

additional advisement. Mark's own excitement for this work was an integral part of my

completing this project. I would like to thank Dr. Thomas P. Guilderson for allowing me

to travel to the CAMS facility at Lawrence Livermore National Laboratory to run my

own radiocarbon dates. The experience was incredible and my project has greatly

benefited from having the additional radiocarbon dates. I would also like to thank Drs.









Prudence Rice, Katherine Emery and Andrew Zimmerman for the exchange of ideas and

thoughtful discussions. I thank Drs. Barbara Leyden, Gerald Haug and Yongsong Huang

for sharing their data with me and Drs. Mark Pagani and Yongsong Huang for assistance

in the development of my methods. I also thank Jenny Slosek for running all of the bulk

isotope data from Lake Sacnab.

There are three individuals to whom sincere thanks are owed for their personal

support. Dave Buck, Ellie Harrison-Buck and Michael Hillesheim have provided me

with friendship that is unequaled. Each one has supported me in their own individual

way and I can not imagine having done any of this without their love and understanding.

Finally, I would like to thank my parents, Michael and Karen Newell, as well as my

siblings, Erik and Jessica Sanderson and Jeremy and Erika Newell. Without their

unconditional love and support, I never would have made it!
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES .......................................... viii

LIST OF FIGURES ........................................ .............. ix

A B S T R A C T ...................................................................................................................... x ii

CHAPTER

1 IN T R O D U C T IO N ................................................. .............................................. .

2 BACKGROUND .............................. .. .......... .............................8

M o d em S ettin g ..................................................................................................... 8
Cultural History of the Peten ........................ .................. .................... 10
Interactions among the Ancient Maya, Climate and Environment.......................... 13
S tu d y S ite s .............................................................................................................. . 2 2
L ak e S acn ab ......................................................................................................... 2 2
Lake Salpeten .............. ...... ............ .............. ............... 23
Compound-Specific Carbon Isotopic Studies ................ ...................................23

3 M E T H O D S ................................................................................................................ 2 7

C o re C o llectio n ........................................................................................................... 2 7
L ak e S acn ab ......................................................................................................... 2 7
L ak e S alp eten ...................................................................................................... 2 8
C h ro n o lo g y ................................................................................................................ 2 9
L ak e S acn ab ......................................................................................................... 2 9
L ake Salpeten ............................................................................................. 29
Bulk Elemental Geochemical Analyses.................................................................29
Bulk Carbon and Nitrogen Isotopic Analyses ....................................................... 30
Compound-specific Carbon Isotopic Analyses .............. ....................................30

4 R E S U L T S ................................................................................................................. .. 3 3

C h ro n o lo g y ................................................................................................................ 3 3
L ak e S acn ab ......................................................................................................... 3 3









L a k e S a lp e te n ......................................................................................................3 3
E lem ental G eochem ical A nalyses ......................................................... ................ 38
L ake Sacnab ......................................................................................................... 38
B ulk C arbon and N itrogen Isotopes ...................................................... ................ 41
Com pound-Specific Carbon Isotopes.................................................... ................ 42
L ake Sacnab ................................................................................................... 42
L ake Salpeten .......... ................................. ...................... ... .... ... 44

5 D ISC U SSIO N ............................................................................... ...................... 46

Implications for Changes in Land-use.................................................... 46
Sources of O rganic M atter..................................................................... ............... 50
Evidence for R elative Shifts in V egetation ........................................... ................ 58
C om prison w ith Pollen R records .......................................................... ................ 66
Com prison w ith Population Estim ates................................................. ................ 70
Relationship between Climate and Environmental Changes................................73
C o n c lu sio n s............................................................................................................... .. 7 6

APPENDICES

A LIPID EXTRACTION PROCEDURE..................................................................80

B SILICA GEL CHROM ATOGRAPHY ......................................................................82

C UREA ADDUCTION AND GC ANALYSIS ............................. ..................... 84

D D A T A T A B L E S ............................................................ ............................................. 8 6

L IST O F R E FE R E N C E S .................................................. ........................................... 97

BIOGRAPH ICAL SKETCH .................. .............................................................. 104















LIST OF TABLES


Table page

1: Perkin Elmer 8500 Gas Chromatograph oven program for sample analyses .............31

2: Hewlett Packard 6890 GC oven program for sample analyses. ...............................32

3: AMS 14C dates for samples from sediment core SN-19-VII-97 from Lake Sacnab.
For the LLNL-CAMS samples, backgrounds were scaled relative to sample size
using Pliocene wood blanks prepared at UF (4+1tg)......................... ................ 34

4: AMS 14C dates for samples from sediment core SP-12-VI-02-1A from Lake
S a lp e te n ............................................................................................................... .. 3 5

5: Tie points used to correlate the %CaCO3 record from core SP2-19-VII-99
(Rosenmeier et al., 2002b) and the scanning XRF Ca concentration data from
core SP-12-VI-02-1A (correlation coefficient = 0.505)......................................36















LIST OF FIGURES


Figure page

1: Location map showing the approximate location of the Peten Lake District in
northern Guatemala. Map adapted from Map #802723 from the Civil
Intelligence Agency (http://www.cia.gov/cia/publications/pubs.html)...................2...

2: Impact of long-term Maya settlement on the terrestrial and aquatic environments in
the Peten, Guatemala. M odified from Rice (1985). ........................... ................ 15

3: Population density estimates versus time (cal yr BP) and time periods for Lake
Sacnab and Lake Salpeten from the Middle Preclassic (cal yr BP) to the Late
Postclassic (cal yr BP). Data from Rice and Rice, 1990. ........................... 16

4: Location map showing: (A) the Yucatan Peninsula and the location of the Peten
Lake District within Guatemala, (B) detail of the Peten Lake District, (C) the
bathymetry of Lake Sacnab and location of core SN-19-VII-97 and (D) the
bathymetry of Lake Salpeten and location of cores SP-12-VI-02 and SP2-99 .......22

5: Histograms from Ficken et al. (2000) showing the molecular distribution of n-
alkanes from the three categories: terrestrial, emergent, and submerged/floating.
Only odd carbon number distributions are shown and bars represent 1 standard
d e v ia tio n ............................................................................................................... ... 2 5

6: Depth versus calibrated age (yr BP) for terrestrial wood, seed and charcoal samples
in Lake Sacnab core SN-19-VII-97. Squares indicate samples analyzed at
LLNL-CAMS and the triangle indicates the sample measured at NOSAMS ..........35

7: Correlation of SP2-19-VII-99 and SP-12-VII-02-1A using %CaCO3 in the SP2-19-
VII-99 core and the scanning XRF Ca concentration data from the SP-12-VII-
02-1A core .............................................................................. ......................... 37

8: Calibrated ages (yr BP) versus depth for correlated tie points in core SP-12-VII-02-
lA as well as radiocarbon dates of terrestrial samples in Lake Salpeten core SP-
12 -V II-0 2 -1A ........................................................................................................ 3 8

9: Magnetic susceptibility, percent calcium carbonate (%CaCO3), percent organic
matter (% OM), percent other (%Other) and percent nitrogen (%N) versus age in
calibrated years before present (cal yr BP) from Lake Sacnab........................ 40









10: Magnetic susceptibility versus age in calibrated years before present (cal yr BP)
for core SP-12-VII-02-1A from Lake Salpeten. The gray highlighted area
represents the "M aya clay" unit. ..............................................................................4 1

11: Bulk carbon and nitrogen isotopes for core SN-19-VII-97 from Lake Sacnab. The
gray highlighted area represents the "Maya clay" unit. .....................................43

12: Compound-specific 613C results of long chain n-alkanes (C29, C31, and C33) from
Lake Sacnab, Guatemala versus age in calibrated years before present (cal yr
B P ) ....................................................................................................... . ....... .. 4 4

13: Compound-specific 613C results of long chain n-alkanes (C29, C31, and C33) from
Lake Salpeten, Guatemala versus age in calibrated years before present (cal yr
B P ) ....................................................................................................... . ....... .. 4 5

14: Comparison of 613C of C33 from Lake Sacnab with magnetic susceptibility. ............51

15: Comparison of 613C of C33 from Lake Salpeten with magnetic susceptibility............52

16: C:N ratios in weight % and bulk organic matter isotopes from Lake Sacnab.............54

17: Comparison of 613C values for bulk organic matter, C31 and C33 from Lake
S a c n a b ........................................................... ...... ................................................ ... 5 5

18: Predominant n-alkane chain length (A) and CPI (B) for Lakes Sacnab and
S a lp e te n ............................................................................................................... .. 5 7

19: Comparison of 613C records for n-alkane chain C33 in Lake Sacnab and Lake
S a lp e te n .............................................................................................................. .. 5 9

20: Diagram comparing the 613C of C33 to the 615N of bulk organic matter in Lake
S a c n a b ........................................................... ...... ................................................ ... 6 1

21: Relative shifts in contribution of C4 vegetation (in %) in Lakes Sacnab and
Salpeten over the last -4500 cal yr BP................................................ ................ 63

22: Diagram showing the comparison between 613C values of C31 from Lake Sacnab,
Lake Salpeten and Lake Quexil (Huang et al., 2001) .........................................65

23: Diagram showing % grass pollen versus 613C of n-alkane C33 in Lakes Salpeten
and Sacnab and the presence of maize pollen in Salpeten (pollen data from
L eyden, 1987) ........................................................................... ........ ............... 69

24: Comparison of the compound-specific carbon isotope records (C33) and population
density estimates versus time in Lakes Salpeten (top) and Sacnab (bottom)...........71









25: Comparison of the compound-specific carbon isotope records (C33) and percent Ti
versus time in Lakes Salpeten and Sacnab. Percent Ti data from Haug et al.
(2 0 0 1 ) ................................................................... ................................................ ... 7 5















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

AN ANALYSIS OF COMPOUND SPECIFIC CARBON ISOTOPES OF LIPID
BIOMARKERS: A PROXY FOR PALEOENVIRONMENTAL CHANGE IN THE
MAYA LOWLANDS OF PETEN, GUATEMALA

By

Sarah Davidson Newell

May 2005



Chair: David A. Hodell
Major Department: Geological Sciences



The Peten region of northern Guatemala has been occupied by humans for more

than 3000 years. During that time, the lowland tropical environment experienced a

prolonged period of anthropogenic disturbance. Forest disturbance in the Peten Lake

District of northern Guatemala was associated with Maya agricultural practices as well as

clearing for urban development, construction, and fuelwood both for cooking and for

lime-plaster production. Expansion of the Maya civilization during the Preclassic (-1000

BC to AD 250) and Classic periods (AD 250 to AD 900) was accompanied by increasing

deforestation of Peten watersheds and accelerated rates of soil erosion. Palynological

data from the Peten Lake District illustrate the near elimination of high forest taxa and

prevalence of disturbance taxa (grasses, weeds) during the height of Classic Maya

occupation (-AD 500-800). After flourishing during the Classic period between AD 250









and 800, the Maya civilization in the Peten Lake District experienced a dramatic change

between AD 800 and 900 that some have referred to as the "collapse." Population

densities declined significantly after AD 900, thereby curtailing human pressures on the

landscape. This cycle of population expansion and decline in the Peten Lake District

provides a natural historical "experiment" that has been used to study the response of

tropical vegetation to long-term changes in land-use by humans. A new line of evidence

is used here to complement other archaeological and paleoenvironmental methods: the

use of leaf wax biomarkers in palynological inference studies. The molecular and isotopic

compositions of leaf waxes have been shown to be reliable indicators of vegetative

biomass and are useful for testing palynological inferences. The carbon isotopic

composition of long-chain n-alkanes of leaf waxes were used as a geochemical proxy for

terrestrial vegetation to test palynological inferences of vegetation change in two lake

basins (Lakes Sacnab and Salpeten) in the Peten Lake District of the southern Maya

Lowlands over the past -4500 cal yr BP.

Discrepancies between the 613C of long-chain n-alkanes and vegetation changes

inferred from pollen profiles suggest that the two proxies may be recording different

characteristics of watershed vegetation as well as different airshed/watershed processes.

The data indicate that in the watersheds of Lakes Salpeten and Sacnab, shifts in the

proportion of C3 to C4 vegetation are most likely controlled by a combination of climate

change and human deforestation. The correspondence of 613C records to independent

proxies for climate change from -4500 cal yr BP until -3000 cal yr BP suggest that

regional drying and increased climate variability caused an increase in the contribution of

C4 vegetation during that time. Following this period and beginning with the first Maya









occupation in the watersheds, vegetation change was likely a result of human-driven

deforestation or perhaps a combination of both climate and human impact.














CHAPTER 1
INTRODUCTION

The Peten region of northern Guatemala (Figure 1) has been occupied by humans

for more than 3000 years. During that time, the lowland tropical environment

experienced a prolonged period of anthropogenic disturbance. Forest disturbance in the

Peten Lake District of northern Guatemala was associated with Maya agricultural

practices as well as clearing for urban development, construction, and fuelwood both for

cooking and for lime-plaster production. Expansion of the Maya civilization during the

Preclassic (-1000 BC to AD 250) and Classic periods (AD 250 to AD 900) was

accompanied by increasing deforestation of Peten watersheds and accelerated rates of soil

erosion (Brenner, 1983). Palynological data from the Peten Lake District illustrate the

near elimination of high forest taxa and prevalence of disturbance taxa (grasses, weeds)

during the height of Classic Maya occupation (-AD 500-800) (Islebe et al., 1996;

Leyden, 1987, Vaughan etal., 1985, Deevey, 1978).

After flourishing during the Classic period between 250 and 800 AD, the Maya

civilization in the Peten Lake District experienced dramatic change between 800 and 900

AD that some have referred to as the "collapse". Population densities declined

significantly after 900 AD, thereby curtailing human pressures on the landscape. This

cycle of population expansion and decline in the Peten Lake District provides a natural

historical "experiment" that can be used to study the response of tropical vegetation to

long-term changes in land-use by humans (Deevey, 1969).





















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The exact timing and extent of deforestation during the period of Maya

occupation and the reforestation that followed the collapse is debated among

archaeologists and paleoecologists. Evidence currently used in the study of ancient

human impact on the Maya lowland environments comes from paleobotanical,

environmental archaeological and paleolimnological studies. Much of this evidence is

derived from palynological research. However, much of this plant-based evidence is

biased in one way or another and none can be used in isolation. For example, vegetation

"reconstructions" inferred from pollen profiles may be biased because the relative

abundance of pollen grains in a sediment profile cannot be directly related to species

abundance or biomass (Bradley, 1999). Most pollen in lake sediments represents only

the small percentage of tropical vegetation that is pollinated by wind and does not reflect

those plants that depend on pollination by insects or self-fertilization. Because certain

species produce a disproportionately large number of pollen grains, it is difficult to

determine the actual composition of vegetation in the landscape. Furthermore, certain

plants may produce pollen under conditions of stress rather than optimum growth

(Bradley, 1999). Maize pollen has often been used in Mesoamerican studies as a proxy

for agriculture and associated deforestation. One potential concern with traditional

palynology, however, is that maize pollen is not typically included in total pollen counts,

thus causing an over-representation of other taxa. In addition, maize pollen is large and

typically only transported short distances. Alternatively, leaf-waxes derived from maize

are probably transported readily and should reflect maize cultivation in the watershed

accurately. These shortcomings underscore the need to validate palynological

interpretations by independent means.









A new line of evidence is used here to complement other archaeological and

paleoenvironmental methods: the use of leaf wax biomarkers in palynological inference

studies. The molecular and isotopic composition of leaf waxes have been shown to be

reliable indicators of vegetative biomass (Hughen et al., 2004; Huang et al., 1999; Huang

et al., 2001) and are useful for testing palynological inferences (Hughen et al., 2004).

There are several advantages to using leaf wax biomarkers in conjunction with pollen to

assess changes in vegetative biomass. Pollen and leaf waxes are derived from different

vegetative sources and thus record different aspects of watershed vegetation (Huang et

al., 1999). Whereas pollen reflects only reproductive effort for specific groups of plants,

leaf waxes provide a more representative measure of vegetative biomass for various plant

types within a watershed. Leaf waxes reflect the contribution of all land plants whereas

pollen will disproportionately reflect wind-pollinated species. In addition, plants in the

watershed that do not reproduce sexually or are dormant will produce a biomarker signal

but yield no pollen.

The isotopic composition of leaf waxes can be used as a geochemical proxy of

terrestrial vegetation (Hughen et al., 2004; Huang et al., 1999, 2001). Long-chain (C27-

C33) n-alkanes exhibit a strong odd-over-even carbon-numbered dominance and are

produced nearly exclusively by vascular plants as components of epicuticular leaf waxes

(Meyers, 1997). The carbon isotopic composition of terrestrial plant biomarkers in lake

sediments reflects the relative contribution to the sediments of alkanes coming from

plants using the C3- (tropical trees) versus C4- (grasses) metabolic pathway. Plants that

fix carbon by means of the C3 pathway include all the high forest trees (Huang et al.

2001). C4 plants include many tropical grasses and maize, which are associated with









cleared land and agriculture in Mesoamerica. The 613C values of n-alkanes in C3 plants

range from -31%o to -38%o, whereas n-alkanes in leaf waxes of C4 plants typically range

from -19%o to -25%o (Freeman, 2001). Stratigraphic variations in the 613C ratio of long-

chain n-alkanes in lake sediment cores should therefore reflect changes in the proportion

of C3 to C4 vegetation in a lake's watershed (Huang et al. 2001; Meyers, 1997).

Here, the carbon isotopic composition of leaf waxes are used as a geochemical

proxy for terrestrial vegetation type to test palynological inferences of vegetation change

in two lake basins (Lakes Sacnab and Salpeten) in the Peten Lake District of the southern

Maya Lowlands. Three specific questions are addressed in this study:

1) Is there a significant relationship between the 613C of long-chain n-alkanes and

vegetation changes inferred from pollen profiles? If the signals are correlated, then both

long-chain n-alkanes and pollen are most likely recording the same vegetation changes in

a watershed. For example, as disturbance taxa (grasses, weeds) replace high forest taxa,

one would expect the 613C of n-alkanes to increase, reflecting a greater proportion of C4

biomass. Similarly, an increase in the relative proportion of high forest taxa during

reforestation should be accompanied by a decrease in the 613C of n-alkanes. Comparison

of pollen taxa and long-chain, n-alkane 613C records will reveal whether inferred

vegetation changes based on the two proxies are in agreement or contradictory. Because

pollen and carbon isotopes of leaf waxes may reflect different characteristics of

watershed vegetation and different airshed/watershed processes, the two proxies need not

necessarily agree.

2) Are 613C values of long-chain n-alkanes in lake sediments correlated to Maya

population density estimates within the same watershed? If agricultural land clearance









was tied to population density, then increased disturbance taxa and maize are expected to

increase as populations increased. Because grasses and maize are C4 plants, one would

predict an increase in the 613C of n-alkanes with population growth. As populations

declined and vegetation returned to a C3-dominated forest, a decrease in the compound-

specific carbon isotopic ratios would be recorded. Population density and the 613C of

long-chain n-alkanes may be decoupled if, for example, agricultural practices changed

through time from more extensive to intensive methods, or if alternative crops, other than

maize, became important in the diet. The relation between population density and 613C

of long-chain n-alkanes can be addressed because population data are available from

archaeological transects in the watersheds of both Lakes Sacnab and Salpeten. 3) What

was the relative importance of human- and climate-induced changes in vegetation in the

Central Peten? It is difficult to assess the effect of climate change during the period of

human occupation because it is difficult to tease apart human-induced versus climate-

induced changes in vegetation. However, a comparison of the 613C of n-alkanes with

climate proxies that are not confounded by human impact may reveal whether climate

played a role in vegetation change at all. If vegetation changes in a watershed were

climate-induced, one would predict that long-chain n-alkanes would correlate with

independent, regional paleoclimate records. For example, a period of increased

evaporation/precipitation (E/P) would likely coincide with an increase in the 613C of n-

alkanes, reflecting a greater proportion of C4 biomass. A period of decreased E/P would

likely correspond to a return to a C3-dominated forest. This question can be answered by

comparing proxy records of climate change from Peten and the Caribbean Sea with






7


compound-specific analyses, and evaluating those data in light of changes in Maya

population densities.




















CHAPTER 2
BACKGROUND

Modern Setting

The Maya area, located in southeastern Mesoamerica, occupies a broad expanse of

land and includes parts of the countries of Mexico, Guatemala, Belize, Honduras and El

Salvador. The area has typically been both culturally and geographically divided by

scholars into the highlands in the south and the lowlands in the north. The highlands,

dominated by a volcanic landscape, refer to the area greater than 1000 ft above sea level

(a.s.1.). and spread from southeastern Chiapas toward lower Central America. The

lowlands consist of the Yucatan Peninsula in Mexico, Guatemala, and Belize.

The topography of the lowlands is dominated by a limestone platform that has

evolved into porous, karst hills with extensive dissolution features. While there are few

permanently flowing rivers and lakes are rare in the northern lowlands, there are

numerous lake basins in the department of Peten. Soils are relatively thin and vulnerable

to rapid erosion with vegetation removal, making agriculture in this region challenging.

Today, the lowland Maya practice shifting, slash-and-burn (swidden) agriculture that

permits the forest to regenerate at intervals. The poor soils, however, can only be planted

for ~2 years; after which, plots are fallowed for 4 to 7 years in the Peten and 15 to 20

years in other parts of the Yucatan (Coe, 1999). These poor soils, in addition to the stress









added by highly seasonal rainfall, make the lowlands a rather harsh place to practice

agriculture (Coe, 1999).

The Peten Lake District is located in the Guatemalan Lowlands of the Yucatan

Peninsula (Figure 1). The region contains numerous terminal basins that are aligned

along a series of east-west aligned en echelon faults (Deevey et al,. 1979). The bedrock

is karst limestone of Cretaceous and Tertiary age with elevations ranging from 100 to 300

m. Most of the lakes have deep troughs at the foot of a steep, north-shore fault scarp

which gives rise to their distinctive bathymetry. The water table in the area is deep below

the ground surface, making groundwater relatively inaccessible. Perched surface waters

often result in seasonally inundated topographic depressions, or bajos, that are clay-

floored and hold water during the rainy season (Deevey et al., 1979). The major water

bodies of the Peten Lake District remain filled throughout the year and extend

approximately 100 km from east to west.

Soils in the Peten region are dominated by well-drained, mineral-rich mollisols that

support a tropical semi-deciduous and evergreen forest (Lundell, 1937). Modern

vegetation is variable throughout the region. The central Peten is mostly semi-deciduous

subtropical moist forest while the southwest Peten is dominated by extensive savannas

with forested hills that support a diverse, fire-resistant herbaceous flora. The savanna

vegetation may have been created during the period of Maya occupation when much of

the forest was burned for cultivation (Leyden, 1987). Alternatively, it may be a

Pleistocene relict or an edaphic assemblage, the consequence of clayey, hydromorphic

soils. The low-lying basins in the northeast Peten are dominated by swamp and









marshland. Vegetation here is dominated by palms, grasses and sedges, but also includes

a semi-deciduous higher forest (Lundell, 1937).

Mean modern annual temperature is approximately 25C and mean annual

precipitation is approximately 1600 mm with a pronounced dry season between January

and May (Deevey et al., 1980). There is very little surface drainage in the area and most

water that falls as rain is either transpired, evaporated, or directly enters the aquifer

(Rosenmeier et al., 2002). The Peten typically receives higher rainfall than the rest of the

Yucatan; there is a pronounced decrease in rainfall from south to north on the Yucatan

Peninsula. There is also considerable interannual precipitation variability. These inter-

annual variations force Maya farmers to practice long-term resource planning. Extensive

droughts occur periodically in this area and previous studies have shown that this

variability is not solely a modern occurrence (Hodell et al., 1995, 2001; Curtis et al.,

1998).



Cultural History of the Peten

The ancient Maya civilization of Mesoamerica arose about 2000 B.C. and spanned

a period of 3,000 years before undergoing a period of social and political change in the

Late and Terminal Classic, between 750 and 1050 AD. These changes resulted in the

development of very different Maya political, economic, and ideological systems and

were associated with the cessation of construction of major architecture and elite

monuments, the reduction of inland trade systems designed for the movement of elite

status markers, and the abandonment of some but not all urban centers as populations

dispersed into non-urban village settings, and migrated northward into the northern

Yucatan and Belizean region. These political and social changes are inferred from the









archaeological record of ancient Maya settlement. Maya civilization was unique in

possessing the only written language in the Americas at the time. It also developed the

most sophisticated and accurate calendar of the time, monumental architecture, a

hierarchical social class system, sophisticated agricultural systems including intensive

agriculture, and trade systems extending from northern Mexico to south of Honduras and

El Salvador and perhaps as far as South America. The Maya also had a highly developed

religious system based, during the Classic period, on semi-divine kingship and a noble

class (Sharer, 1994).

The Peten region was continuously occupied by the ancient Maya from the Middle

Preclassic (1000 BC) through the Postclassic (AD 900 to 1525), and up to the Spanish

conquest in 1697 (Rice and Rice, 1990). The Preclassic period (1000 BC to AD 250) in

the Peten region is divided into the Middle Preclassic and Late Preclassic. It is during

this period that the Maya are thought to have developed from hunters and gatherers into a

complex civilization. This period marked the development of cities, temples and

inscribed stone monuments. During the Middle Preclassic, archaeological evidence

suggests the presence of social stratification as well as sophisticated religious and

economic institutions. The first examples of writing appear in the Late Preclassic (Coe,

1999).

The Classic Period was characterized by remarkable growth of the civilization both

in terms of population and social complexity. The Classic Period (AD 300- AD 900) is

also subdivided into three periods: the Early Classic, Late Classic and Terminal Classic.

During the Early and Late Classic periods, archaeological evidence suggests the

development of "states" with centralized political and religious authority in addition to









the erection of carved stone monuments (Coe, 1999). An important change in the

political structure occurred at the end of this period, as authority became shared by a

council of many instead of one individual (Coe, 1999). Populations grew slowly, but

exponentially and in many regions displayed peak population densities between AD 700

and 800 AD (Rice and Rice, 1990).

The Terminal Classic and Post-Classic Periods (AD 900 to AD 1525) are marked

by diverse cultural responses throughout Mesoamerica. Much of the southern lowland

region experienced the abandonment of Classic cities, temples and religious centers;

subsequent political fragmentation resulted in a massive cultural decline that has been

referred to as the "collapse" (Coe, 1999). The Maya Terminal Classic, however, is

perhaps one of the most important periods in the Peten region. The uniqueness of central

Peten lies in the ability to study not only the events leading up to the Maya Terminal

Classic period, but the continuity of occupation following the collapse of Classic

civilization (Rice and Rice, 2004). This period in the central Peten was both a center and

a crossroads for Postclassic Maya civilization. Peten-like architecture and iconographic

traits in the northern lowlands are evidence of Late Classic, conflict-driven migration

from the Peten region. Throughout the Late Classic and Terminal Classic, the Peten

region showed demographic loss, whereas the northern lowlands began to be heavily

settled. The lakes region in particular suffered population decline, though the area was

never completely abandoned. Archaeological evidence from every major Peten lake

basin shows continuous occupation from the Late Classic to the Postclassic, with some

indication of population migration between the Rio de Pasi6n region, the Gulf Coast, and

the Peten Lake District during the Postclassic (Rice and Rice, 2004).









The nature of settlements changed from the Late Classic to the Postclassic in the

Peten Lakes District. Whereas Classic settlements expanded throughout the watershed,

most settlement shifted to small, densely populated areas found primarily on islands and

peninsulas in the lakes during the Postclassic. This settlement pattern in itself suggests a

"conflict-driven" society. The location of these sites in poor agricultural zones, the

presence of defensive structures, as well as the presence of intrusive residential

architecture in previously occupied sites, suggests that the Late Classic migration of

small groups between the Rio de Pasi6n and the northern lowlands forced them to settle

into an already-established settlement system (Rice and Rice, 2004).

Archaeological evidence overwhelmingly suggests that the Maya population in the

Peten Lakes District did not "collapse", but underwent a significant transformation.

While there was a pronounced demographic decline, the "collapse" in the Peten did not

result in complete depopulation. Instead, intersite conflict led to political restructuring

and thus a redistribution of population, especially into fragile lands. The region was one

of dynamic and contested lands, and remained so until AD -1200 (Rice and Rice, 2004).



Interactions among the Ancient Maya, Climate and Environment

The Maya Lowlands have been studied by researchers seeking answers to

questions about past human-climate-environment interactions. This research has

concentrated on the central Peten, and in particular the Peten Lakes District. In 1972,

Edward S. Deevey began a long-term paleoecological project in the Department of Peten,

Guatemala. The Central Peten Historical Ecology Project (CPHEP) investigated both the

paleolimnology and archaeology of major lake watersheds in the Central Peten (Rice et

al., 1985). The objective of the project was to investigate both the social and natural









history of the region through interdisciplinary research, focusing on ecological change as

a result of human landscape transformations. One assumption of the CPHEP was that

climate change was unimportant during the period of Maya occupation. Recent studies

have recognized, however, that climate was not invariant through this period (Hodell et

al., 1995; Rosenmeier et al., 2002; Curtis et al., 1998). Research design included

paleolimnological studies from other regions that incorporate archaeological perspectives

(Dunning et al., 1997, 1998; Demarest, 1997). In particular, lake sediment cores were

used to develop long-term, high resolution records of environmental change within a

watershed and were used in conjunction with archaeological surveys that estimated the

timing and density of human occupation.

The impacts of long-term Maya occupation in the Central Peten Lake District on

both terrestrial and lacustrine environments were summarized by researchers in the

CPHEP (Binford et al., 1987) (Figure 2). The model is generalized and does not

necessarily describe the history of human and environmental changes for each and every

lake basin.

Archaeological surveys consisted of systematic mapping of settlement remains

and test-pit excavations, and provide estimates of settlement patterns and population

growth in the central Peten (Rice and Rice, 1990). Surveys were completed in each of

three twin-lake basins: Sacnab-Yaxha, Macanche-Salpeten, and Quexil-Petenxil. The

trends of population growth are notably similar in each basin surveyed, although the true

variation was probably not captured by the coarse chronological framework, which was

based on ceramic phases, not radiocarbon. Occupation of the region began at

approximately 1000 BC and was followed by a steady increase until the end of the Late









Classic (AD 750-800), when populations reached their peak in most basins (Figure 3).

Population growth was exponential in the Yaxha-Sacnab basins but not in the other

basins, which experienced a small population decline during the Early Classic.


High
MAYA POPULATION
DENSITY
I
Low

Forest
t
VEGETATION
I
Savanna

High
SOIL EROSION
I
Low

High
T
SEDIMENTATION
RATE
L
Low


N


10,000 4000 3000 2000 1000 0
Cal yr BP


Figure 2: Impact of long-term Maya settlement on the terrestrial and aquatic
environments in the Peten, Guatemala. Modified from Rice et al. (1985).

Sediment cores from numerous lakes in Peten show similar stratigraphic changes

in sediment composition and geochemistry (Figure 2) (Cowgill and Hutchinson, 1966;

Deevey et al., 1979; Brenner, 1983; Vaughan et al., 1985; Rice et al., 1985; Binford et

al., 1987; Leyden, 1987; Curtis et al., 1998; Islebe et al., 1996). Holocene sediment prior









to Maya occupation is comprised of organic-rich (30-60%) gyttja (Brenner et al., 2002).

Overlying the pre-Maya gyttja is a clay-rich horizon known as the "Maya clay." The







-,













pal yrelateda freo ring an d ecet al.1al 16rti
ap 150







2500 2000 1SO 1000 Soo 0
cal is. BP
Figure 3: Population density estimates versus time (cal yr BP) and time periods for Lake
Sacnab and Lake Salpeten from the Middle Preclassic (cal yr BP) to the Late Postclassic
(cal yr BP). Data from Rice and Rice, 1990.


base of the Maya clay has been dated to approximately 3,000 yr BP (Brenner, 1994;

Rosenmeier et al., 2002) and reflects accelerated erosion associated with early land

clearance by humans (Deevey et al., 1979; Brenner, 1983; Vaughan et al., 1985; Rice et

al., 1985; Binford etal., 1987; Leyden, 1987; Curtis etal., 1998; Islebe etal., 1996).

Studies outside the region per se suggest the increase in colluviation may have been

partly related to regional drying (Hodell et al., 1995; Hodell et al., 1996; Curtis et al.,

2001). Nutrient-rich soils became unstable as forest cover was removed and replaced by

a savanna-like landscape. Increased soil erosion is reflected by high net accumulation

rates of lake sediments. As erosion progressed, organic-rich surficial soils were removed









and the underlying bedrock was weathered and transported to the lake basin. This is

reflected by the high proportion of inorganic sediments. Overlying the Maya clay unit is

another organic-rich layer that is inferred to represent sediments deposited following the

Maya collapse (Brenner et al., 2002).

Deforestation and environmental disturbance accompanied growth of the Maya

population from the Preclassic to the Late Classic (Figure 2). Vaughan et al. (1985)

developed pollen stratigraphies for Lakes Quexil and Sacnab that indicate changes in

vegetation through the period of Maya occupation. The pre-Maya pollen assemblage was

dominated by high forest taxa such as Moraceae, whereas the early to mid-Preclassic is

characterized by an open forest and, probably, culturally-induced savanna. Late

Preclassic to Late Classic sediments are dominated by clay-rich sediments and 70% non-

arboreal pollen. Vaughan et al. (1985) defined three distinct disturbance pollen zones.

The Late Preclassic zone is characterized by savanna trees and shrubs. The landscape

transitions to nearly grassland during the Early Classic, with only one type of arboreal

pollen present (Ramon). The Late Classic to Postclassic contains only grassland taxa.

Some increases in high-forest taxa near the top of this interval show evidence of

reforestation, as does the pollen spectrum during the Postclassic period. If we assume an

anthropogenic cause for the replacement of arboreal with grassland taxa, a sharp rise in

arboreal pollen and a decrease in grassland types imply regional depopulation (Vaughan

et al., 1985). Although each study is thought to reflect primarily local vegetation

changes, multiple subsequent studies from the Peten and surrounding lowlands, with the

exception of the work done by Cowgill et al. (1966) and Dunning et al. (1997), have all

detected a similar decline in high-forest taxa from the Late Preclassic through the









Terminal Classic that is interpreted to reflect human-driven deforestation (Islebe et al.,

1996; Leyden, 1987, Vaughan et al., 1985, Deevey, 1978). The pollen zones were

assigned ages by "cultural zonation", or the correlation with ages of archaeological

periods. Subsequently, they may contain some chronological error. However, these

same pollen zones have now been independently dated in other lake cores (Leyden, 1987

and Rosenmeier et al., 2002) and are considered reasonable.

Leyden (1987) used a 15 m sediment core from Lake Salpeten, Guatemala to

develop a high-resolution pollen record from the basin to reconstruct Holocene

vegetation changes. The sediment record spans from the pre-Maya to the present, and

shows distinct evidence of Maya land clearance. While there is an apparent lack of

disturbance taxa in the early Preclassic, the pollen zones representing the Late Preclassic

through the Postclassic show evidence for abundant terrestrial herbs and strongly suggest

Maya land clearance. Leyden attributes the lack of disturbance taxa in the early

Preclassic to small local populations in the Salpeten basin at that time. Some

discrepancies, however, may lie within the issues of chronological control. Leyden

suggests that a few high-forest taxa actually increased initially, and declined later as

forest removal was intensified. Presence of oak during the Classic period may suggest a

savanna landscape, but this is relatively unclear. Leyden suggests that after the initial

deforestation, the forest structure was relatively stable. However, greater proportions of

maize pollen during the Late Classic through Postclassic indicate intensified agricultural

activities. In the gyttja layer above the Maya clay, the concentration of total pollen grains

nearly doubles, suggesting forest regrowth after depopulation of the lake basin (or slowed

bulk sedimentation rate). Leyden proposes that these forests regenerated rapidly, but









were more open than pre-Maya forests. This may have been related to climate rather than

anthropogenic influences; regional drying may be suggested by the Post-Maya

continuation of open forests. This period is dominated by secondary growth, as

suggested by the increases in successional shrubs and trees.

While most vegetation studies have focused on forest clearance, a select few have

concentrated on revealing the dynamics of afforestation following the Maya collapse in

the central Peten region of Guatemala (Wiseman, 1985; Brenner et al., 1990). Wiseman

(1985) used pollen in lake sediments to track vegetation changes and the nature of these

changes in response to cultural collapse in the Maya Lowlands. Seven cores from Lakes

Petenxil and Quexil were collected and sampled for pollen analyses and used in

conjunction with modern pollen and fauna studies that examined 0.1 ha plots in various

stages of regrowth. This modern analog guided inferences about forest succession based

on the fossil pollen record. All cores that penetrated into Late Classic Period deposits

showed a subsequent replacement of agricultural weeds by secondary forest growth. The

pollen spectrum during the early Classic to Postclassic was similar to samples taken from

soils under swidden agriculture. One notable feature of the sediment cores is that each

core shows essentially the same pollen spectra, suggesting that the conditions in the basin

were in spatial equilibrium. Wiseman cites the decrease in maize pollen and subsequent

recolonization by forest as key evidence for depopulation; however, poor chronological

control makes it difficult to determine the precise timing of both environmental and

demographic changes.

Interpreting vegetation change from palynological data is further complicated by

the challenge of obtaining accurate chronological control. Prior to the advent of AMS









dating and the potential to measure 14C in very small (-20.g) samples of organic carbon,

many early radiocarbon ages from Peten lakes were based on dating of bulk sediment or

carbonate shells. Shells may incorporate carbon derived from the dissolution of ancient

limestone and thus appear older than their true age. This problem extends to organic

matter that is fixed photosynthetically within the lake, i.e. autochthonous organic matter.

Primary producers may incorporate carbon derived from dissolved ancient limestone, and

thus display "too-old" ages. This hard-water lake error has compromised the reliability

of early chronologies. To further complicate matters, many subsequent studies assigned

ages to the pollen stratigraphy by correlation with ages of archaeological periods. Dating

error within one pollen record was thus transferred to records from other lake cores.

Chronological imprecision in both sediment cores and archaeological studies creates

challenges for linking environmental disturbance and human population sizes (Yaeger

and Hodell, in press).

Despite the overwhelming evidence for late Holocene vegetation change in the

Peten region, it is difficult to distinguish between the impacts of climate and Maya

occupation on forest composition. While the CPHEP assumed that climate was relatively

constant for the past 10,000 years, recent work has proved otherwise. A sediment core

from Lake Chichancanab in northern Yucatan provides evidence for regional drying that

occurred beginning approximately 1000 BC with a distinct interval of droughts between

AD 800 and AD 1000 that coincided with the Terminal Classic collapse (Hodell et al.,

1995, Hodell et al. in press). Another paleoclimate record from Punta Laguna, on the

northeastern Yucatan Peninsula, indicates alternating wet-dry shifts in the hydrologic

balance through the late Holocene with the driest period lasting from approximately AD









280 to 1080 (Curtis et al., 1998). Four severe drought events were revealed, including

one that occurred during the Maya Hiatus (AD 600) and one that occurred concurrently

with the Late Classic drought recorded in the Chichancanab record.

A climate record from Lake Salpeten indicates a trend similar to the

Chichancanab record (Rosenmeier et al., 2002). Lake levels decreased continuously

from approximately 1000 BC, with the lowest lake level occurring between AD 800 and

900. This inferred drought coincides with the Terminal Classic collapse of the Classic

Maya. The record also showed reduced soil erosion as well as forest recovery after AD

850, likely associated with Maya population decline (Rosenmeier et al., 2002).

Rosenmeier argues, however, that these changes may not be entirely climate-related, but

rather due to the effects of human-induced vegetation change on the lake's hydrology.

The impact of humans on regional vegetation and soil stability, as well as the

influence of climate change on the Classic Maya, illustrate the complex interplay among

climate, humans, and the environment throughout the Late Holocene in the Maya

Lowlands. While it is extremely difficult to separate the signals of human versus

climate-controlled changes in the environment, further investigations of vegetation

change with precise chronological control may provide additional evidence that the Maya

had a profound impact on their lowland tropical environment. The "natural experiment"

provides an excellent opportunity to study the relationship between changes in human

population sizes, land-use activities, climate changes, and vegetation responses.

Additionally, such studies can help us understand how ecosystems respond once human

and climate pressures are curtailed.










Study Sites

Paleolimnological cores from two lakes in the Central Peten Lake District, one in

the east and one in central Peten, were examined for compound-specific carbon isotopes

(Figure 4). Comparison of the records from Lake Sacnab and Lake Salpeten permits a

regional assessment of vegetation change.


B I Lake Yaxha-

Lake Sacnab
Lake Perdida deran,
Lake Peten Itza Lake SapeIn
Lake Salpeten

~u-e'ra .yr Lake Macanche t
Lake Sacpuy Lake Queil

I I ake Pethnxil N

Figure 4: Location map showing: (A) the Yucatan Peninsula and the location of the Peten
Lake District within Guatemala, (B) detail of the Peten Lake District, (C) the
bathymetry of Lake Sacnab and location of core SN-19-VII-97 and (D) the
bathymetry of Lake Salpeten and location of cores SP-12-VI-02 and SP2-99.


Lake Sacnab

Lake Sacnab (1703'N and 8923'W) is located in the eastern part of the Peten

Lake District near the border between Guatemala and Belize (Figure 4c). The lake has a

surface area of 3.9 km2 and is -3.5 km long by 1.5 km wide (Deevey et al., 1980). The

maximum depth is 13 m. The lake is thermally stratified, but occasional mixing is


Lake Sacnab
8(tifr'


SN 1~VII-~1

~x I~w~ -


S12VI-0r2
5P2.9


CCVIxi tf 4n I t









indicated by the absence of permanent stagnation. Lake waters are relatively poor in

nutrients, with the majority of nutrients being supplied by erosion of upland soils

(Deevey et al., 1980). Lake Sacnab has no outflow, thus making the sediments the

primary sink for dissolved and particulate matter that enters the lake.



Lake Salpeten

Lake Salpeten (1658'N and 8940'W) is located -35 km to the WSW of Lake

Sacnab and has a surface area of approximately 2.6 km2 (Rosenmeier et al., 2002) (Figure

4d). The lake has a maximum depth of approximately 32 m (Brezonik and Fox, 1974).

Lake waters are sulfate-rich and have high total dissolved solids (4.76 g L-1) (Deevey et al.

1980). Surface temperatures range from 27C to 30C throughout the year (Rosenmeier et

al., 2002). Lake Salpeten is closed hydrologically and lake-bottom sediments are the

primary sink for dissolved and particulate matter that enters the lake.



Compound-Specific Carbon Isotopic Studies

The major source of organic matter in lake sediments is generally derived from

phytoplankton living in the water column or aquatic macrophytes. Land plants may also

provide an important source of organic matter to lake sediments. The relative

contribution of these three sources is influenced by productivity of lacustrine algae,

aquatic macrophytes, and terrestrial plants. Transport processes and preservation may

also influence the ultimate contribution of organic compounds from various sources

(Meyers, 1997). In order to determine relative changes in algal, macrophyte, or

terrestrial plant productivity, it is necessary to discriminate between the sources of

organic matter sequestered in lake-bottom sediments. Certain compounds in lake









sediments, commonly called biomarkers, are uniquely derived from specific sources of

organic matter. Leaf waxes are produced exclusively by vascular plants and serve as a

protective coating on leaves and stems (Eglinton and Hamilton, 1967). The abundance

and molecular and isotopic composition of leaf waxes reflect vegetative biomass and

have been used to discern relative changes in terrestrial vegetation (Figure 5) (Hughen et

al., 2004; Filley et al., 2001; Huang et al., 1999, 2001). The wax particles, introduced

into the atmosphere by wind and dust ablation off live vegetation (Simoneit, 1977), have

a molecular composition that is generally similar to that of their source vegetation (Conte

and Weber, 2002). Leaf waxes can also be remobilized from soils during exposure

(Schefub, 2003). Leaf waxes settle onto lake surfaces from the atmosphere and are

incorporated into lake-bottom sediments. Leaf waxes thus have atmospheric residence

times on the order of days to weeks, making them especially valuable as tracers of abrupt

vegetative change. Leaf waxes record biomass of exposed leaf surface area in the

watershed and thus have the capability of resolving rapid responses to climate or other

environmental changes.

The isotopic analysis of long-chain n-alkanes has proven to be a useful new tool

for evaluating qualitative changes in terrestrial vegetation (Hughen et al., 2004; Filley et

al., 2001; Huang et al., 1999, 2001). Long-chain (C29-C33) n-alkanes exhibiting a strong

odd-over-even carbon-numbered dominance are produced nearly exclusively by vascular

plants as components of epicuticular leaf waxes (Meyers, 1997). In addition, n-alkanes

are chemically and biologically resistant and are often found in sediments in quantities

sufficient for analysis. Preservation of lipids, and in particular n-alkanes, is generally

good. Studies have shown greater preservation of n-alkane terrestrial biomarkers versus









algal biomarkers (Meyers et al., 1984; Meyers and Ishiwatari., 1993; Meyers and Eadie,

1993) suggesting that studies utilizing these biomarkers are more robust.




n-ALKANES

40



60
TERRESTRIAL 3B w.





50 -











SUBMERGED 3








submerged/floating. Only odd carbon number distributions are shown and
bars represent 1 standard deviation.



The carbon isotopic composition of long chain n-alkanes (>C27) reflects the

relative contribution of C3 and C4 plants (Hughen et al., 2004; Filley et al., 2001; Huang

et al., 1999; Huang et al., 2001). Plants that fix carbon by means of the C4 metabolic

pathway include the tropical grasses (including maize) and are associated with land









clearance. C4 plants are more competitive than plants using the C3 pathway under both

high water stress (such as during drought) as well as lower ambient pCO2 levels. Plants

that fix carbon by means of the C3 pathway include all the high forest trees (Huang et al.,

2001). Plants that use the C3 pathway have bulk 613C values in the range of -21%o to -

28%o, whereas C4 plants display bulk 613C values between -11%o and -15%o (Lajtha and

Marshall, 1994). Stratigraphic variations in the 6 13C ratio of long-chain n-alkanes in lake

sediment cores will thus reflect the changes in the proportion of C3 to C4 vegetation in a

lake's watershed (Huang et al., 2001).

Previous studies have shown that it is possible to recognize C4 plant expansions by

examining the carbon isotopic composition of long-chain n-alkanes in sediment cores.

Huang, et al. (2001) examined leaf wax n-alkanes from two sites in Mesoamerica and

found contrasting moisture variations over the last 25,000 years. Data indicate that

regional climate plays a large role in the relative abundance of C3 versus C4 plants.

Enriched 613C values during the Last Glacial Maximum (LGM) suggest an expansion of

C4 plants in the sediments of Lake Quexil during the LGM as a result of low partial

pressure of atmospheric carbon dioxide (pCO2) and increased aridity. Results indicate

that it is possible to recognize vegetation changes by examining the compound-specific

carbon isotopes of preserved organic matter in lake sediments. This study provides

information that complements past studies that analyzed only bulk sediment or pollen and

thus provide a better record of changes in terrestrial vegetation in the Peten Lake District.















CHAPTER 3
METHODS

Core Collection

Lake Sacnab

On 19 July 1997, two sediment cores were retrieved from Lake Sacnab in 8.0 m

of water. A 152-cm-long core (SN-19-VII-97-MWI) was taken using a piston corer

designed to recover undisturbed sediment-water interface profiles (Fisher et al., 1992).

The sediment core was sampled in the field at 1-cm intervals by vertical extrusion into a

sampling tray fitted to the top of the core barrel. A total of 3.7 m of sediment was

collected in four additional sections (SN-19-VII-97-LEX) using a piston corer with

polycarbonate tubing.

Cores were extruded and sectioned at 1-cm intervals. Magnetic susceptibility was

measured for each section using a GEOTEK multi-sensor core logger (MSCL) at Florida

State University, Tallahassee. All samples from core SN-19-VII-97 were freeze-dried

and ground in preparation for compound-specific carbon isotopic analyses. Magnetic

susceptibility data were used to determine that there was a 25.5-cm offset between the

mud-water interface core and the piston core. All depths in the deeper core sections were

adjusted using the mud-water interface as the 0-cm datum.









Lake Salpeten

On 12 June 2002, two sediment cores were retrieved from 23 meters water depth

in Lake Salpeten. A short (93 cm) trigger/gravity core (SP-12-VI-02-1A-MWI) and a

longer (550 cm) Kullenberg piston core (SP-12-VI-02-1A) were both collected in

polycarbonate liners. The Kullenberg core was cut in the field into approximately 1-m

lengths for transport to the University of Florida, where cores were stored in a cooler.

Whole core sections were analyzed for magnetic susceptibility, gamma ray

attenuation (GRA) bulk density and p-wave velocity using a GEOTEK MSCL at the

University of Florida. Each section of the Kullenberg core was split lengthwise into

archive and sampling halves and described. The sampling half was U-channeled and

taken to the Ocean Drilling Program Core Repository in Bremen, Germany for elemental

analysis on the X-Ray Fluorescence (XRF) Core Scanner. Additional samples were

taken from the remainder of the sampling half at the University of Florida at 10-20 cm

intervals for compound-specific carbon isotopic analyses. The mud-water interface core

was sampled at 1 cm intervals at the University of Florida by vertical extrusion into a

sampling tray fitted to the top of the core barrel. All samples from core SP-12-VI-02

were freeze-dried and ground in preparation for compound-specific carbon isotopic

analyses. Magnetic susceptibility and density data were used to determine the 7-cm offset

between the mud-water interface core and the Kullenberg core and depths assigned to

deeper sections were adjusted accordingly.









Chronology

Lake Sacnab

Radiocarbon ages for Lake Sacnab sediments were determined by accelerator

mass spectrometry (AMS) using terrestrial organic matter (seeds, charcoal and wood) at

Lawrence Livermore National Laboratory Center for AMS (LLNL-CAMS) and the

National Ocean Science AMS (NOSAMS) facility at Woods Hole Oceanographic

Institution. Radiocarbon ages were converted to calendar ages using the program OxCal

v 3.9. (Bronk Ramsey, 1995; Bronk Ramsey, 2001) and atmospheric data from Stuiver et

al. (1998).

The sample that was analyzed at NOSAMS was pretreated on-site whereas the

samples analyzed at LLNL-CAMS were pretreated at the University of Florida. For the

LLNL-CAMS samples, backgrounds were scaled relative to sample size using UF

processed Pliocene wood blanks to determine the modern-C contribution (41 pg). All

radiocarbon ages are adjusted to a 613C value of-25 per mil.

Lake Salpeten

Radiocarbon ages for Lake Salpeten sediments were determined by AMS 14C

dating of terrestrial organic matter (seeds, charcoal and wood) at LLNL-CAMS.

Radiocarbon ages were converted to calendar ages using the program OxCal v 3.9 (Bronk

Ramsey, 1995; Bronk Ramsey, 2001) and atmospheric data from Stuiver et al. (1998).

Bulk Elemental Geochemical Analyses

Total carbon (TC) and total nitrogen (TN) were measured on all samples from

core SN-19-VII-97 using a Carlo Erba NA 1500 CNS elemental analyzer with

autosampler. Analytical precision for TC and TN is approximately 0.5%. Total









inorganic carbon (TIC) in the sediments was measured by coulometric titration

(Engleman et al., 1985) with a UIC (Coulometrics) Model 5011 CO2 coulometer coupled

with a UIC CM5240-TIC inorganic carbon preparation device. Analytical precision is

approximately 0.5% based on analysis of 16 calcium carbonate internal standards.

Organic carbon (OC) was calculated by subtracting TIC from TC. Weight percent

calcium carbonate (%CaCO3) was calculated by multiplying IC by 8.33. Weight percent

organic matter (%OM) was estimated by multiplying OC by 2.5.

Bulk Carbon and Nitrogen Isotopic Analyses

Bulk organic sediment samples were analyzed on-line for carbon and nitrogen

isotopes using a VG PRISM Series II isotope ratio mass spectrometer with a triple

trap preparation device linked to a Carlo Erba NA 1500 CNS Elemental. Bulk carbon

isotopic results are reported in standard delta (6) notation relative to the Vienna PeeDee

Belemnite (VPDB) standard. Precision for 613C samples was approximately 0.15%o

based on nine analyses of NBS-22. Bulk nitrogen isotopic results are reported in

standard delta (6) notation relative to atmospheric N2. Precision for 615N samples was

approximately 0.20%o based on nine analyses of peptone.

Compound-Specific Carbon Isotopic Analyses

The extraction and isolation methods used for lipids in this study were patterned

after Silliman et al. (2000) and M. Pagani (personal communication, 2003). Prior to

extraction, approximately 15 .g of C34 n-alkane was added to each sample as an internal

standard for n-alkanes. Lipids were extracted from approximately 3-5 g of dry sediment

using 2:1 methylene chloride/methanol in a Dionex Accelerated Solvent Extractor (ASE)

(for program see Appendix A). Extraction efficiencies averaged approximately 89% for









all samples. The samples were then evaporated and solvent exchanged to hexane using a

hot water bath while adding a stream of dinitrogen gas (N2). Samples were dissolved in 1

mL of hexane and added to a 1 cm x 29 cm glass column filled with 2.5 g of 5%

deactivated silica gel. 15 mL of hexane was used to elute the n-alkanes. Full isolation

scheme procedures are outlined in Appendix B. Samples were urea adducted to obtain

clean n-alkanes for gas chromatography as outlined in Appendix C. After urea

adduction, samples were concentrated in 200 [.L of hexane in preparation for analysis on

a Perkin Elmer 8500 Gas Chromatograph (GC) to determine purity and appropriate

concentrations for GC-IRMS analyses. Samples of 4 U.L were injected into the GC with a

30 m DB-1 column (0.25 mm ID). The gas chromatograph was used in split injection

mode with a ratio of 20:1 and equipped with a FID detector. The GC oven temperature

was programmed to maximize alkane separation (Table 1). The necessary dilution for

GC-IRMS analysis was calculated and samples were transferred to a glass auto-sampler

vial and sealed with a Teflon crimp cap. Samples were typically dissolved in 20-100 UL

hexane. Carbon isotopic analyses were performed using a Hewlett Packard 6890 GC

connected to a Finnigan MAT Delta+ XL Mass Spectrometer via a GC-C III interface.

Table 1: Perkin Elmer 8500 Gas Chromatograph oven program for sample analyses.
Rate Temperature Time
C/min 0C (min)
50 1
6 300 20



The n-alkanes were separated on a fused silica capillary column (30 m x .32 mm

i.d.; .25uim film thickness) using helium as the carrier gas. The GC oven temperature

was programmed to maximize alkane separation (Table 2). Alkanes were combusted into

CO2 in a ceramic oxidation reactor containing three braided NiO/CuO/Pt wires. Three









pulses of a standard, calibrated CO2 reference gas were injected via the GC-C II interface

to the IRMS for the measurement of 613C values of individual alkanes. A laboratory

working standard (UFIS) consisting of three n-alkane chains (C19, C25 and C30) was

measured along with unknowns at the beginning and end of each run, as well as after

every fourth sample analysis within a run. UFIS was calibrated to a set of standard n-

alkanes (Mix A) from Indiana University with known 613C values. Long-term analytical

precision, based on repeated analysis of n-alkanes in UFIS, was 0.4%o. Data were

acquired and processed using ISODAT NT 2.0 software. All reported carbon isotopic

compositions for samples represent averaged values for duplicate analyses. Duplicate

analyses had a standard deviation of approximately 0.5%o. Long-term analytical

precision based on analysis of the internal standard C34 was 0.4%o.

Table 2: Hewlett Packard 6890 GC oven program for sample analyses.


Rate Temperature Time
C/min 0C (min)
50 2
6 299 0
10 300 4














CHAPTER 4
RESULTS

Chronology

Lake Sacnab

Twelve AMS 14C dates were obtained from core SN-19-VII-97 and yielded a

maximum age of 4580 14C yr BP on a sample at 439.5 cm (Table 3). The core

chronology was established by converting sediment depths to age using three linear

regression equations (Figure 6):

196.5-0 cm: age (cal yr BP) = 11.863 x depth, r2 = 0.9628;

369.5-196.5 cm: age (cal yr BP) = 4.6821 x depth + 1495, r2 = 1;

439.5-369.5 cm: age (cal yr BP) = 17.683 x depth 3293.2, r2 = 0.9317.

The age of the base of the core (445 cm) is estimated to be -4650 cal yr BP based on

extrapolation of the linear regression from 369.5 to 439.5. Sedimentation rates for the

three intervals are:

196.5-0 cm = 0.081 cm/yr;

369.5-196.5 cm = 0.214 cm/yr;

439.5-369.5 cm = 0.052 cm/yr.

The wood date at 363.5 cm had a large error because of its small size and was not used to

establish the chronology.

Lake Salpeten

Two AMS 14C dates were obtained from core SP-12-VI-02-1A and yielded a

maximum age of 3985 14C yr BP (Table 4). Additional chronological control was











obtained by correlating the %CaCO3 record from core SP2-19-VII-99 (Rosenmeier et al.,


2002b) and the scanning XRF Ca concentration data from core SP-12-VI-02-1A


(correlation coefficient = 0.505) using AnalySeries v. 1.0 (Paillard et al., 1996). Ten tie


points were used to correlate the records (Figure 7, Table 5).


Table 3: AMS 14C dates for samples from sediment core SN-19-VII-97 from Lake
Sacnab. For the LLNL-CAMS samples, backgrounds were scaled relative to
sample size using Pliocene wood blanks prepared at UF (4+1 pg). All
radiocarbon ages are adjusted to a 613C value of -25 %o. Radiocarbon ages
were converted to calendar ages using the program OxCal v 3.9 (Bronk
Ramsey, 1995; Bronk Ramsey, 2001) and the atmospheric data set of Stuiver
et al. (1998). All ages reported in this thesis are in calendar years before
present (relative to AD 1950).

Calibrated
Composi Calibrated Age 95.4%
te Depth Accession Sample Age 1 a Age (cal Probability
Sample ID (cm) Number Material (14C yr BP) yr BP) (AD/BC)

AD 1600
SN-19-VII-97-MWI_45.5 45 CAMS 58754 280 60 390 115

SN-19-VII-97-LEX1 109.5 84 CAMS 58755 1210 50 1165 AD 800 110
AD 1100
SN19VII97MWI 97-98 97.5 CAMS 106265 seed 990 50 880 115
charcoal,
leaf
SN19VII97MWI 121-123 122 CAMS 106266 material 1650 60 1560 AD 400 155

SN-19-VII-97-LEX1 152.5 127 CAMS 58756 1620 50 1515 AD 400 120
charcoal,
leaf
SN19VII97MWI 145-147 146 CAMS 106267 material 1740 100 1650 AD 300 235
seed,
SN19VII97LEX2 221-223 196.5 CAMS 106268 charcoal 2370 40 2415 500 BC 90

SN-19-VII-97-LEX4_414.5 363.5 OS 18657 wood 2500 160 2555 600 BC 400
charcoal,
leaf 1300 BC
SN19VII97LEX4 394-396 369.5 CAMS 106269 material 3030 60 3225 160
charcoal,
leaf 1600 BC
SN19VII97LEX4 404-407 379.5 CAMS 106270 material 3300 70 3540 155
charcoal,
leaf 1800 BC
SN19VII97LEX4 434-436 409.5 CAMS 106271 material 3450 40 3740 105
charcoal,
leaf 2600 BC
SN19VII97LEX4 464-466 439.5 CAMS 106272 material 4050 80 4580 275











5000
4750
4500 -
4250 y = 17.683x 3293.2
4000- R2 = 0.9317
3750 m
3500 -
r 3250 -y = 4.6821x + 1495
S 3000 R2
? 2750 -
) 2500 -
2250 -
2000 -
1750 y =11.863x
o 1500- R2 0.9628
1250 -
1000 -
750 -
500 -
250
0
0 50 100 150 200 250 300 350 400 450 500
Depth (cm)

Figure 6: Depth versus calibrated age (yr BP) for terrestrial wood, seed and charcoal
samples in Lake Sacnab core SN-19-VII-97. Squares indicate samples
analyzed at LLNL-CAMS and the triangle indicates the sample measured at
NOSAMS.



Table 4: AMS 14C dates for samples from sediment core SP-12-VI-02-1A from Lake
Salpeten.

Calibrated
Composite Age 1 a Calibrated Age 95.4%
Sample Depth Accession (14C yr Age (cal Probability
Sample ID Material (cm) Number BP) yr BP) (AD/BC)

SP12VI02 CAMS 1790 AD
STA_110-111 charcoal 110.5 106273 210 60 165 170
ST1A 110-111 106273 170

SP12VI02 CAMS 2500 BC
STA_442-445 charcoal 443.5 106274 3985 50 4455 200
ST1A 442-445 106274 200










Table 5: Tie points used to correlate the %CaCO3 record from core SP2-19-VII-99
(Rosenmeier et al., 2002b) and the scanning XRF Ca concentration data from
core SP-12-VI-02-1A (correlation coefficient = 0.505) using AnalySeries v.
1.0 (Paillard et al., 1996). Included are the respective ages for each set of
correlated depths. Ages for depths in core SP2-19-VII-99 were determined
using the chronology outlined in Rosenmeier et al. (2002) while ages for
depths in SP-12-VI-02-1A were determined using the correlation (Figure 4).

Depth in
Depth in SP- SP2-19-VII-
12-VI-02-1A 99 Age (cal yr BP)
152 151 1660
201.5 201 2030
215 213 2090
236.5 236 2210
246 248 2270
265.5 266 2390
293.5 289 2590
310.5 301 2730
330 309.5 2860
376 331 3250


The core chronology was established by converting sediment depths to age with

two equations derived by linear regression for two intervals (Figure 8). The top of the

core was assumed to be modem.

152-0 cm: age (cal yr BP) = 10.976 x depth, r2 = 1;

376-152 cm: age (cal yr BP) = 6.8806 x depth + 599.66, r2 = 0.9947.

The age of the base of the core (563.5 cm) was estimated to be -4500 Cal yr BP based on

extrapolation of the linear regression from 152 to 376 cm. The radiocarbon date at 110.5

cm was not used to construct the chronology because of its small sample size and

suspicion that the charcoal fragments may have been displaced down core. Based on a

comparison between magnetic susceptibility records from SP-12-VI-02-1A and SP2-19-

VII-99, the radiocarbon date at 443.5 cm was not used to construct the chronology






37

because it caused deeper sediments to appear too old based on a comparison with core

SP-80-1. Sedimentation rates for the two intervals are as follows:

196.5-0 cm = 0.091 cm/yr;

563.5-196.5 cm = 0.142 cm/yr.


SP2-19-VII-99 SP-12-VI-02-1A
0



100



200



300



400



500


10 20 30 40 50 60 70 80 4 6 8 10 12 14 16
% CaCO3 5-pt Smooth Ca (103)


Figure 7: Correlation of SP2-19-VII-99 and SP-12-VII-02-1A using %CaCO3 in the SP2-
19-VII-99 core and the scanning XRF Ca concentration data from the SP-12-
VII-02-1A core.



















3000-

< 2500-

r 2000-

1500- y=10.976x

1000-

500-

0
0 100 200 300 400 500
Depth (cm)


Figure 8: Calibrated ages (yr BP) versus depth for correlated tie points in core SP-12-
VII-02-1A as well as radiocarbon dates of terrestrial samples in Lake Salpeten
core SP-12-VII-02-1A. Triangles indicate dates analyzed at LLNL-CAMS
while the diamonds represent tie points.



Elemental Geochemical Analyses

Lake Sacnab

From 4500 cal yr BP to -3300 cal yr BP, sediments are dominated by organic

matter as reflected by high %OM and %oN concentrations and low magnetic susceptibility

and %CaCO3 (Figure 9). The %Other is low (-20%) at 4500 cal yr BP, and increases

continuously to the base of the Maya clay at 3300 cal yr BP. Beginning at the base of the

Maya clay unit at -3300 cal yr BP, magnetic susceptibility and %CaCO3 begin to

increase and %OM and /oN decrease abruptly. Sediment composition between -3300

and 1200 cal yr BP is dominated by inorganic sediment (clay and detrital carbonate) as









reflected by relatively high values of magnetic susceptibility, %CaCO3 and %Other.

While the %OM and %N are relatively constant during this time, the magnetic

susceptibility, %CaCO3 and %Other show some variation. %Other remains relatively

unchanged from 3300 to 2600 cal yr BP, reaching maximum values of 90% during this

time before decreasing to 50% between 2700 and 2400 cal yr BP. %Other remains

relatively constant from 2400 cal yr BP to 1600 cal yr BP, when values begin to increase

and reach 60% at 1200 cal yr BP. Following this maximum, %Other decreases

continuously from 1200 cal yr BP to the present. At -3300 cal yr BP, magnetic

susceptibility increases, rising to a maximum of -20 SI at 2700 cal yr BP before

decreasing to an average of 10 SI for the remainder of the Maya clay unit. %CaCO3

increases from 3300 cal yr BP to 2400 cal yr BP, reaching a maximum of -45%.

%CaCO3 then decreases to -4% at 1200 cal yr BP and remains low until the present. At

the top of the Maya clay unit, %OM and %N begin to increase and do so continuously

from 1200 cal yr BP to present.

Lake Salpeten

Magnetic susceptibility increases from the base of the core to -4100 cal yr BP

(Figure 10). Values remain high from 4100 to -2000 cal yr BP reflecting the high clay

content of the sediment. Beginning at -2000 cal yr BP, magnetic susceptibility begins to

decline with two distinct steps centered at 1700 and 1300 cal yr BP. From 800 cal yr BP

to the present, magnetic susceptibility values remain low and relatively unchanged.











Magnetic Susceptibilily 1SlI
-10 -5 0 5 10 15 20 25
0





1000





2000


C-


% OM
0 20 40 60 80 100


%N
0 0.5 1 1.5 2 2.5 3
0





1000





2000





3000


4000


0 20 40 60 80 100 0 20 40 60 80 100
%CaCOi % Other
Figure 9: Magnetic susceptibility, percent calcium carbonate (%CaCO3), percent organic
matter (% OM), percent other (%Other) and percent nitrogen (%N) versus age in
calibrated years before present (cal yr BP) from Lake Sacnab. The gray highlighted area
represents the "Maya clay" unit.









0





1000 -





2000 -











4000 -





5(000 I I I I I
-1 -0.5 0 0.5 1 1.5 2 2.5 3
Magnetic Susceptibility (cgs)
Figure 10: Magnetic susceptibility versus age in calibrated years before present (cal yr
BP) for core SP-12-VII-02-1A from Lake Salpeten. The gray highlighted area
represents the "Maya clay" unit.

Bulk Carbon and Nitrogen Isotopes

The carbon and nitrogen isotopic records of bulk organic matter show similar

patterns during the last 4650 years (Figure 11). Bulk 613C values range from -21%o to -

28%o throughout the core, whereas bulk 615N values vary between 0%o and 5%o. Carbon

isotopic values generally increase from -27%o at the base of the core to -23%o at -3300

cal yr BP. The 613C values of bulk organic matter average -23%o from 3300 to 2500 cal









yr BP, and then decrease between 2500 and 2000 cal yr BP. Bulk organic 613C remains

unchanged and averages -25%o from 2000 to 1300 cal yr BP. A sharp 2.5%o decrease in

613C occurs at 1250 cal yr BP, followed by a slight trend toward increasing values toward

present.

The 615N of bulk organic matter averages -2%o from 4500 cal yr BP to 3500 cal

yr BP. From 3500 cal yr BP to 3300 cal yr BP, 615N values increase by 2%o and remain

high from 3300 cal yr BP to 1200 cal yr BP, averaging 3%o. The nitrogen isotopic

composition then decreases abruptly by 3.5%o at 1200 cal yr BP, and averages 1%o from

1200 cal yr BP to present.

Compound-Specific Carbon Isotopes

Lake Sacnab

Samples were analyzed for the 613C of long-chain n-alkanes (C29, C31, and

C33) in Lake Sacnab sediments at approximately centennial resolution since 4650 cal yr

BP (Figure 12). The 613C signals for C31 and C33 show similar trends and will be

described in unison, whereas the 613C of C29 differs somewhat and will be described

separately.

The 613C of C31 and C33 averaged -33%o at the base of the core and gradually

increased to -29%o at -3300 cal yr BP. Values generally remain high from -3300 cal yr

BP to -2500 cal yr BP and averaged -28%o. From 2500 to 2100 cal yr BP, 613C

decreases from -28%o to -32%o and values remain unchanged from 2100 to 1300 cal yr

BP. At 1300 cal yr BP, the carbon isotopes of C31 and C33 record a rapid decrease with

values reaching -39%o and -37%o, respectively. Following this excursion, the 613C of n-










alkane chains C31 and C33 increases at 1100 cal yr BP and averages -32%o to the top of the

core.

0




1000




2000




< 3000




4000




5000
-1 0 1 2 3 4 5 -28 -26 -24 -22 -20
6 -N (%96, air) 8' C (%., PDB)

Figure 11: Bulk carbon and nitrogen isotopes for core SN-19-VII-97 from Lake Sacnab.
The gray highlighted area represents the "Maya clay" unit.


The 613C of C29 at the base of the core is -32%o and generally remains unchanged

from -4500 cal yr BP to -2500 cal yr BP, with an average ratio of approximately -29%o.

From 2500 cal yr BP to 2100 cal yr BP, 613C decreases from -29%o to -33%o and values

remain unchanged from 2100 cal yr BP and 1300 cal yr BP. At 1300 cal yr BP, the

carbon isotopes of C29 record a rapid decrease, reaching a value of -35%o. Following this

excursion, the 613C of C29 is highly variable, with values ranging from -24%o to -33%o

during the period from 1100 cal yr BP to the present.










8~"C C, (%., PDB)
-39 -36 -33 -30 -27 -24




1000




2000









4000




5000
-39 -36 -33 -30 -27 -24 -21 -39 -36 -33 -30 -27 -24
S13C C29 (%., PDB) &g1C C13 (%', PDB)

Figure 12: Compound-specific 613C results of long chain n-alkanes (C29, C31, and C33)
from Lake Sacnab, Guatemala versus age in calibrated years before present
(cal yr BP).




Lake Salpeten

Samples were analyzed for the 613C of long-chain n-alkanes (C29, C31, and C33) in

Lake Salpeten sediments at an approximate resolution of 200 years for the past 6000 cal

yr BP (Figure 13). The 613C signals for C29, C31, and C33 show similar trends and will be

described in unison; variations in 613C of C29, however, are muted relative to C31 and C33.

The 613C of long-chain n-alkanes averaged -32%o at the base of the core and

gradually increased to -28%o at -5200 cal yr BP. Values generally remained unchanged

from -5200 cal yr BP to -3300 cal yr BP and averaged -30%o. From 3300 to 2500 cal yr











BP, 613C increased from -30%o to -27%o. The isotopic ratio decreased from 2500 cal yr


BP to the present, reaching minimum values that range from -33%o to -36%o.


a8 C31 (%o, PDB)
-35 -34 -33 -32 -31 -30 -29 -28
0



1000



2000



S3000 ..



4000



5000


-34 -32 -30 -28
8 C C2, (%o, PDB)


-26


-36 -34 -32 -30 -28
61C C33 (%o, PDB)


Figure 13: Compound-specific 613C results of long chain n-alkanes (C29, C31, and C33)
from Lake Salpeten, Guatemala versus age in calibrated years before present
(cal yr BP).


6000
-36


-26














CHAPTER 5
DISCUSSION

Implications for Changes in Land-use

Changes in land-use can be inferred from the lithological composition of

lacustrine sediments, which reflect changes in material transfer from the watershed to the

lake. With increased land clearance, the detrital load to a lake basin will increase.

Sediment cores taken from Lakes Sacnab and Salpeten show similar changes in sediment

composition. This common pattern has been documented throughout the central Peten

lake district and is interpreted to reflect sediment compositional and rate changes related

to the history of Maya settlement and population in the region (Cowgill and Hutchinson,

1966; Deevey et al., 1979; Brenner, 1983; Vaughan et al., 1985; Rice et al., 1985;

Binford etal., 1987; Leyden, 1987; Curtis etal., 1998; Islebe etal., 1996).

The sediment composition and magnetic susceptibility records in Lake Sacnab

(Figure 6) document changes in material transfer from the catchment to the lake.

Magnetic susceptibility is a proxy for the concentration of magnetic minerals contained

within the sediment, which are derived from the erosion of soil and bedrock in the

watershed and subsequently transported to the lake. CaCO3 in sediments can be derived

from either authigenic precipitation within the lake or by the weathering and transport of

detrital carbonate from soils or limestone bedrock. There is little to no observed

lacustrine carbonate in the sediments of Lake Sacnab, which may suggest that most of the

CaCO3 is detrital. Inorganic carbon and oxygen isotopes measured on bulk material from

the lake have values that range from -8%o to 0%o and -4%o to 0%o, respectively. These









values are generally consistent with measured mean values of Peten soils surrounding

Lake Quexil (613C -10%o to -7%o, 6180 -3%o to 0%o,) (Leyden et al., 1993) and

indicate that the source for sediment CaCO3 is dominantly allochthonous CaCO3 as

opposed to diffuse authigenic CaCO3. Thus, relative shifts in %CaCO3 may serve as an

additional indicator of erosion in the catchment. Organic matter in lacustrine sediments is

derived from multiple sources including terrestrial vegetation, lacustrine algae and

bacteria, and aquatic macrophytes. The %Other may also serve as a proxy indicator of

landscape erosion. Because there is relatively little contribution to the sediments other

than CaCO3 and organic matter, the %Other is a likely indicator of non-carbonate clastic

material (clay) eroded from the watershed. It is important to note the effects of the

closed-sum problem inherent in using weight % data. Dilution may play a major role in

down-core variations of any of the three components. As the % of any component

increases, the other two components will respond by decreasing in % to sum to 100%.

Therefore, variation in the delivery of any of the sediment components, especially during

the deposition of the Maya clay, would significantly alter the apparent input of the other

sediment components. To avoid the closed-sum" problem, accumulation rates (g/cm2/yr)

can be calculated. Accumulation rates of the three individual sediment components

provide a better indication of erosion and sediment deposition. Unfortunately, the density

data necessary to calculate accumulation rates are not available for Core SN-19-VII-97

from Lake Sacnab. Anselmetti et al. (in prep), however, calculated accumulation rates

for specific time intervals in Lake Salpeten for the period between 8500 cal yr BP and the

present. Data indicate that erosion rates were lowest during the early to mid-Holocene

and increased beginning in the Early Preclassic. Erosion rates were highest during the









Late Preclassic period (2200 to 1700 cal yr BP) and subsequently decreased over time

even during the height of Maya occupation in the watershed (1700 and 1100 cal yr BP).

The sedimentological history and associated watershed erosional characteristics

determined by Anselmetti et al. (in prep) are consistent with those determined in this

study.

In Lake Sacnab, Holocene sediment prior to Maya occupation is classified as

gyttja. Overlying the pre-Maya gyttja is a clay-rich horizon known as the "Maya clay",

associated with increased sedimentation rates. The onset of Maya clay deposition

appears to have begun at 3300 cal yr BP and lasted until -1200 cal yr BP. This unit is

common in all the central Peten lakes and has been interpreted previously to reflect

accelerated erosion associated with Maya land clearance (Deevey et al., 1979; Brenner,

1983; Vaughan et al., 1985; Rice et al., 1985; Binford et al., 1987; Leyden, 1987; Curtis

et al., 1998; Islebe et al., 1996) and/or possibly regional drying (Hodell et al., 1995;

Curtis et al., 1998). In the Lake Sacnab watershed, soils likely became unstable as forest

cover was removed at the onset of Maya occupation. The common soil type in the Peten

are mollisols, consisting thin (typically <1 m) mineral soils that develop over CaCO3-rich

material called sascab (Brenner et al., 2002).

Down-profile erosion of Peten soils following deforestation is evidenced in the

sedimentological history. Changes in the ratio of %Other to %CaCO3 suggest that the

organic- and clay-rich surface horizon was eroded between 3300 and 2500 cal yr BP.

Once the watershed had been denuded of organic-rich top soils, the weathering and

erosion of more carbonate-rich, deep soils (sascab) ensued from 2500 to 1300 cal yr BP,

as evidenced by the increase in the influx of CaCO3 and decrease in %Other (Figure 6).









This period (between 3300 and 1200 cal yr BP) was characterized by enhanced delivery

of detrital material to the lake and resulted in sediments with a low organic content,

which is likely a dilution effect.

Overlying the Maya clay unit is another organic- and clay-rich layer that is

inferred to represent decreased erosion following the cessation of agriculture and

increased contribution of lacustrine organic matter to the sediment. Stratigraphic changes

in sediment composition in Lake Sacnab are similar to those in other lakes in the Peten

determined in previous studies (Deevey et al., 1979; Brenner, 1983; Vaughan et al.,

1985; Rice et al., 1985; Binford et al., 1987; Leyden, 1987; Curtis et al., 1998; Islebe et

al., 1996) and suggests that the history of human and environmental changes is similar

throughout the central Peten region.

The magnetic susceptibility record from Lake Salpeten shows a similar trend to

Lake Sacnab in the transfer of watershed material to the lake but with a distinct

difference in timing (Figure 10). The onset of Maya clay deposition, indicated by the

enhanced delivery of watershed-derived detrital material resulting from landscape

denudation, appears to have begun at 4100 cal yr BP and lasted until -1100 cal yr BP.

Previous work (Rosenmeier et al., 2002) in Lake Salpeten, however, has indicated that

the Maya Clay was deposited between -3300 and 1100 cal yr BP. The apparent

discrepancy between the two cores is most likely related to the chronology of core SP-12-

VI-02-1A (this study), which was correlated to core SP2-19-VII-99 (Rosenmeier et al.,

2002) using ten tie points (see Chapter 4). In using this method, there is the possibility

for miscorrelations that would cause error in the chronology. Core SP-12-VI-02-1A was

dated indirectly and ages for the Maya clay from Rosenmeier et al. (2002) are probably









more accurate. Archaeological settlements in the Lake Salpeten watershed are

documented beginning at -3000 cal yr BP (Figure 3) (Rice and Rice, 1990) and roughly

coincided with the sedimentological evidence for human occupation of Rosenmeier et al.

(2002).

For the most part, compound-specific carbon isotopes track magnetic

susceptibility closely throughout the records for both Lake Sacnab and Lake Salpeten

(Figures 14, 15). The magnetic susceptibility record is generally considered a proxy for

changes in amount of detrital material transported from the landscape into the lake that,

in turn, is affected by the amount of vegetation in a watershed. In both lakes, periods of

inferred increased erosion occur simultaneously with periods of higher relative

contributions of C4 vegetation. As vegetation shifts from high-forest taxa to a more

savanna-like landscape during forest removal, rapid erosion of watershed soils resulting

from soil destabilization ensues. Because both magnetic susceptibility and the

compound-specific records are influenced by vegetation cover and type and are indicative

of landscape conditions, the correlation of these proxies was not unexpected.

Sources of Organic Matter

Organic matter in lacustrine sediments is derived from multiple sources including

terrestrial vegetation, lacustrine algae, and aquatic macrophytes. The C/N ratio can

provide information about the proportions of algal versus terrestrial plant contribution to

organic matter (Prahl et al., 1980; Meyers, 1994; Kaushal and Bindford, 1999). Organic

matter from algae and bacteria has C/N weight ratios ranging between 5 and 12, whereas

organic matter from vascular land plants usually has weight ratios of 24 and greater

(Meyers, 1994). While C/N ratios between 36 and 48 are generally characteristic of









cellulose-rich vascular plants, weight ratios between -14 and 20 suggest a mixture of

both algal and vascular plant material (Ertel and Hedges, 1985).


I5"C C-, J%. PDI
-30 -,"


-34 -36 -IS


SI I I I I I
W 25 20 15 10 5 0 -5 -10
Magnetic Susceptibility (Sl)

Figure 14: Comparison of 613C of C33 from Lake Sacnab with magnetic susceptibility.


-26 -"









6~C vy~., P 1B1

-32


3 2.5 2 1.5 I 0.5 0 -0.5 -I
Magnetic Susceptibility (cgs)

Figure 15: Comparison of 613C of C33 from Lake Salpeten with magnetic susceptibility.



In Lake Sacnab, the C:N ratio suggests that the source of organic matter to the

lake may have changed over the last 4500 years (Figure 16). Values range from 10 to 20

in the earliest part of the record, from 4500 to 2700 cal yr BP, and suggest a mixture of

both algal and terrestrial plant material. From 2700 cal yr BP to 1300 cal yr BP,

increased ratios of C:N suggest the predominant contribution of terrestrial vegetation to









sediment organic matter. Values are low from 1300 cal yr BP to the present, ranging

only between 9 and 13.5, suggesting a return to dominantly aquatic organic matter.

The distinguishing C:N ratios among the different types of organic matter

generally survive both sinking and sedimentation in lacustrine environments. However,

certain diagenetic processes such as dissolution, oxic/anoxic cycles and bacterial

processes may modify the original ratios (Muller and Mathesius, 1999). For this reason,

C:N values are often used together with the carbon isotope ratio of bulk organic matter to

determine both the sources) and composition of organic matter.

The period between 4500 and 2700 cal yr BP has bulk isotope values that suggest

an increasing contribution of a mix of C3 and C4 land plants while the C:N ratio suggests

additional contribution from algae. From 2700 to 1300 cal yr BP, both the C:N and the

bulk isotopes suggest a dominant contribution from C4 terrestrial plants and is followed

by a period between 1300 cal yr BP and the present in which there is a higher

contribution from algae, aquatic macrophytes and C3 terrestrial vegetation.









54




C:N

0 5 10 15 20 25 30 35 40 45 50


C



C=





0 I
C,
CT



















LO


CD
CD




C -2

-C3




SCO



Co
m I-
d 28


Figure 16: C:N ratios in weight % and bulk organic matter isotopes from Lake Sacnab.


-27 -26 -25 -24 -23 -22 -21 -20

a'C (%o, PDB)
















1000 -





2000 -





< 3000 -





4000 -


o Bulk Organic Matter
-- C33
------ C31
5000 C 1 1
-40 -35 -30 -25 -20

813C (%o, PDB)

Figure 17: Comparison of 613C values for bulk organic matter, C31 and C33 from Lake
Sacnab.


The 613C of bulk organic matter in Lake Sacnab sediments shows similar trends

as the compound-specific record which might indicate that the bulk organic matter within

the lake sediment is dominated by terrestrial organic matter with only a small

contribution from aquatic macrophytes and algae (Figure 17). However, the C:N ratios









suggest that the sources) of organic matter changed over time and is likely a better proxy

for the sources) of organic matter. The bulk and compound-specific isotopes show the

most distinct variation in trend in the latest part of the record, from -1200 cal yr BP to

the present and may be explained by the increased contribution from aquatic macrophytes

and lacustrine algae as demonstrated by the C:N ratio. Carbon isotopic values of bulk

organic matter are enriched by -5%o to 7%o, however, relative to individual leaf wax n-

alkanes. This is expected because lipids, including n-alkanes, are commonly depleted in

13C relative by ~6%o -to 8%o relative to other biosynthetic products (Hayes, 2001). Thus,

much of the terrestrial organic matter in the lake deposits owes its origin to non-lipid

sources.

An important concern in analyzing the isotopes of individual n-alkanes is

preservation potential. The carbon preference index (CPI) is often used as a proxy for the

preservation potential of the organic matter when there is a clear predominance of

epicuticular leaf waxes of terrestrial plants (Hedges and Prahl, 1993). CPI values are

generally highest in living plants and surface sediments. Lowered values indicate

increasing maturity and degradation and tend to decrease to a final value of 1 (Hedges

and Prahl, 1993). However, this index cannot be used as a preservation potential proxy in

all cases. For example, a CPI of 1 may indicate immature organic matter with a low

contribution from higher plants rather than mature organic matter. In order to determine

whether the CPI can be used as a proxy for preservation potential, the examination of the

predominant n-alkane chain length can provide some insight into the provenance of

organic matter. The predominant n-alkane chain length is shown in Figure 18a, which is

interpreted as an indicator of the origin of the organic debris input (algae, aquatic








57



macrophytes or land plants) (Cranwell et al., 1987; Ficken et al., 2000). The


predominance of n-alkanes of high-molecular-weight indicates that terrestrial plants are


the predominant organic matter source and suggests that the CPI can be used as a proxy


for preservation potential.


With respect to the CPI, most of the values are greater than 1.5 with few


exceptions (Figure 18b). The CPI shows a predominance of immature, plant-derived


material and, therefore, very little removal of components during transport and post-


deposition (Ortiz et al., 2004).


Dominant n-alkane
20.00 2200 24.00 26_00 2B.00 30 00 3200



500


1000
Sacrnb
USalpeten
1500


2000


2500


3000


3500


4000


4500

A
5000


3400 0
0


500


1000


1500


2000


2500


3000


3500


4000


4500


5000


5 10 15 20 25


Figure 18: Predominant n-alkane chain length (A) and CPI (B) for Lakes Sacnab and
Salpeten.











Evidence for Relative Shifts in Vegetation

Sediments from Lakes Sacnab and Salpeten show a range of values (-38%o to -

27%o,) during the last 4500 years that indicate changing relative proportions of C3 to C4

vegetation in the watershed (Figure 11). A comparison of the 613C records of C33 from

Lake Sacnab and Lake Salpeten reveals similar absolute values and long-term trends over

the past 4500 cal yr BP (Figure 19). At the base of the core, more depleted values

suggest a higher proportion of C3 versus C4 vegetation. A shift towards more enriched

values from 4500 cal yr BP to 3300 cal yr BP reflects an increase in the relative

abundance of C4 (grasses, etc.) to C3 (trees) vegetation. Decreasing 613C values from

2500 cal yr BP to 2000 cal yr BP indicates some forest regrowth and a higher proportion

of C3 taxa than in the previous interval. The proportion of C3 to C4 vegetation is

relatively unchanged from 2000 cal yr BP to 1300 cal yr BP, which is surprising given

the population changes that were occurring in this period. At 1300 cal yr BP in the

Sacnab watershed only, a sharp decrease in the 613C indicates a rapid transition (-100

years) to a highly C3-dominated landscape. During the last -1200 cal yr BP, the carbon

isotopic record from both lakes suggests a gradual shift towards greater contribution of

C3 vegetation toward present.

There are several distinct differences in the 613C records of the two cores. A

peak in the 613C in Lake Salpeten at approximately 3900 cal yr BP and subsequent

decrease until 3300 cal yr BP is not observed in Lake Sacnab. Rather, values are

continually increasing during this time in Lake Sacnab. In addition, the 613C peaks at

approximately 3100 cal yr BP in Lake Sacnab, whereas maximum 613C values in Lake










Salpeten do not occur until 2800 cal yr BP. The rapid transition in 613C at 1300 cal yr BP

in Lake Sacnab is not present in the Salpeten record. The differences in the 613C records

may be due partly to sampling resolution, which is approximately two times higher in

Lake Sacnab than in Lake Salpeten. In addition, some variation may be expected in the

history of vegetation changes in the two watersheds. While the general trends of land-use

and human occupation are thought to be similar between watersheds, these systems are

dynamic and the history of land use was probably variable throughout the region.


0




1000




1 2000




S3000




4000


5000 L
-38


-36 -34 -32 -30 -28 -26
S3C C33(?o, PDB)


Figure 19: Comparison of 613C records for n-alkane chain C33 in Lake Sacnab and Lake
Salpeten .









The rapid decline in 613C in the Lake Sacnab core at 1300 cal yr BP may reflect

increased dominance of a single species with a relatively depleted carbon isotopic value

after agricultural abandonment. Various chronosequence studies of abandoned

agricultural fields have determined that soil nitrogen is often the limiting factor during

the entire succession of vegetative regrowth (Tilman, 1984; Tilman 1987; Knops and

Tilman, 2000). In many cases, legumes are the first plants to colonize abandoned fields

because they are able to fix atmospheric nitrogen. If the pioneering species were

legumes, then it would have a low 615N value because of nitrogen fixation (615N = 0),

which involves little isotopic fractionation between plant tissues and air. In fact, the

rapid transition in the 613C record in Lake Sacnab occurs at the same time (-1300 cal yr

BP) as a rapid decrease in the 615N of bulk organic matter (Figure 20). These data

support an increased contribution of sediment organic nitrogen from N2 fixers because

sedimentary 615N values at this time are close to zero. While the average 613C of

legumes is not necessarily more negative than other plants, they are exclusively C3 plants

and thus have low 613C values relative to C4 vegetation.

An alternative explanation for the rapid transition at 1300 cal yr BP is the result of

dominance of a non-leguminous pioneer species. For example, it has been documented

that in certain systems, bracken fern often dominates the vegetation in a field

immediately following abandonment (Pakemen et al., 1994). Eventually, other high

forest taxa out-compete this weed and would thus change the isotopic composition of

sediments gradually over time. While the 613C value of bracken fern is not reported in

the literature and is not necessarily the source for the depleted values in this study,

previous work shows that one species may dominate following agricultural abandonment.









61




It may be possible to determine whether a pioneer species accounts for the dramatic


isotope shift by analyzing pollen samples at close intervals above, within, and below the


transition.


613C C33 (%o, PDB)

-39 -37 -35 -33 -31
C 1--I I-I


-29 -27 -25


C
C



;51
C
C



C
C




C



C
C
C
0







C-

C



C
CO







C-1
0y
0D
0D

C1D


Figure 20: Diagram comparing the 613C of C33 to the 685N of bulk organic matter in Lake

Sacnab.


0 1 2 3 4

815N Bulk (%o, air)









Reported 813C values of n-alkanes in C3 plants range from -31%o to -38%o,

whereas n-alkanes in leaf waxes of C4 plants typically range from -19%o to -25%o

(Freeman, 2001). The 613C values for n-alkanes in this study, however, range from -27%o

to -38%o which might suggest that C3 vegetation dominated throughout much of the study

period. Previous studies of aerosols and sediments have translated 613C values into

percentages of C3 versus C4 plants using a two-component mixing equation (Huang et al.,

2000; Schefub et al., 2003). It has been found, however, that the isotopic values of C3

plants usually become more depleted with increasing carbon number while C4 plants have

isotopic values that are consistent over all chain lengths (Collister et al., 1994). The

variation is suggested to result from the production of different leaf wax lipids in

different proportions during a leafs growth cycle and averages approximately 2.4%o.

This variation translates into a potential 16% error in the calculation of C3 to C4

abundance. This characteristic makes it difficult to quantify the contribution of C3 versus

C4 plants. In order to account for this effect, the values for the lowest-number terrestrial

n-alkane chain, C29, are most often used to translate 613C values into % contributions.

Because aquatic macrophytes can contribute significant amounts of C29, it is preferable to

calculate %C4 using an n-alkane of greater chain length. For this study, values were

calculated using a two-component mixing equation assuming end-member 613C values of

n-alkane C33 of -36.4%o for C3 plants and -19%o for C4 plants, respectively. These values

were adapted from those of Collister et al. (1994), who reported -34%o for the C3-

endmember and -19%o for the C29 C4-endmember.

For Lake Salpeten, the contribution of C4 vegetation to the C33 n-alkane pool

ranges between 14% and 57.5% (Figure 21). The % contribution of C4 taxa increases







63


between 4500 and -2700 cal yr BP when the greatest proportion of C4 occurred. The

contribution of C4 biomass declines from nearly 58% at 2700 cal yr BP to just 14% at

present.


C4 Vegetation (%)


0 10 20 30 40 50 60 70


500



1000



1500



5 2000



< 2500



3000



3500



4000



4500


Figure 21: Relative shifts in contribution of C4 vegetation (in %) in Lakes Sacnab and
Salpeten over the last -4500 cal yr BP. Values were calculated using a two-
component mixing equation with C3 and C4 inputs represented by 613C values
of n-alkane C33 of-36.4%o and -19%o, respectively (Collister et al., 1994).











Lake Sacnab shows similar shifts to Lake Salpeten with respect to % contribution

of C4 biomass. From 4500 cal yr BP until -3300 cal yr BP, the % contribution increases,

reaching peak values of ~55%o between 3300 and 2700 cal yr BP. From 2700 cal yr BP

to 1300 cal yr BP, the contribution of C4 vegetation generally decreases. At 1300 cal yr

BP, the contribution of C4 vegetation is 0%, but rapidly increases to a greater contribution

(30%) by 1200 cal yr BP. The contribution of C4 vegetation gradually decreases from

1200 cal yr BP to the present.

Changes in water-use efficiency (WUE), i.e. the ratio of carbon gained to water

lost during gas exchange, may also affect the 613C of vegetation. A negative correlation

exists between precipitation and 613C of vegetation among tropical sites (Leffler and

Enquist, 2002). Consequently, the 613C of C3 vegetation would increase during periods

when the precipitation decreased significantly. Consequently the calculated % C4

contribution may be greater than the true value. It is thus necessary to consider the

potential influence of climate on the 613C record.

The 613C values of n-alkanes in Lakes Sacnab and Salpeten are similar to values

obtained by Huang et al. (2001) in Lake Quexil (Figure 22). The 613C values during the

Maya clay interval in Quexil averaged -31%o and are similar to values measured in the

Maya clay from Lake Sacnab (average = -30%o). Values from Lake Quexil in the pre-

Maya gyttja average -34%o and increased by -5%o between 3300 and 1100 cal yr BP,

with a peak in 613C during the time of the Maya Terminal Classic period. They attribute

this increase to both anthropogenic forest clearance and regional drying. The 613C from

Lake Sacnab, however, reaches peak values much earlier at -3000 cal yr BP than in Lake












Quexil. The Quexil carbon isotope record is much lower resolution than either Lakes


Sacnab or Salpeten, and may not capture the structure observed in the higher-resolution


613C records.


1"C C, (%o, PDB)
-40.0 -35.0 -30.0
0



5000



10000



15000



20000



25000



30000



35000



40000
-Lake Sacnab
--Lake Salpulern
45000


-25.0 -40.0
- 0 ---


500


1000


1500


2000


2500


2000


2500


4000


4500


5000


13 C 31 (%o, PDB)
-35.0 -30.0


Figure 22: Diagram showing the comparison between 613C values of C31 from Lake
Sacnab, Lake Salpeten and Lake Quexil (Huang et al., 2001).


-25.0









In the records from Lake Salpeten and Sacnab, the 613C values of n-alkanes from

3300 cal yr BP to 2700 cal yr BP are comparable to values during the LGM in Lake

Quexil, further supporting the interpretation of enhanced C4 input. The data imply that

vegetation during the period between 3300 cand 2700 cal yr BP may have been quite

similar to the vegetation during the Last Glacial Maximum, when climate was

significantly cooler. It is remarkable that vegetation may have changed as drastically

during the period of human occupation as it did during a glacial-interglacial cycle.



Comparison with Pollen Records

Compound-specific carbon isotope records can be compared with pollen records

from the same or nearby lakes to test if vegetation changes inferred from the 613C of

long-chain n-alkanes are the same as those inferred from pollen profiles. A comparison

of the % disturbance taxa from Lake Salpeten (Leyden, 1987) and the long-chain n-

alkane 613C record from Lake Sacnab reveals significant differences in the two proxies

(Figure 23). Disturbance taxa include grasses, sedges, and herbs from the following

families: Amaranthaceae (C3 and C4), Ambrosia (C3), Compositae (C3 and C4),

Cyperaceae (C3 and C4) and Gramineae (C3 and C4). While these pollen data may

provide an accurate representation of changes in the relative abundance of the selected

taxa, they are not ideal for distinguishing changes in the relative contributions of C3

versus C4 plants because both plant types are represented by the disturbance taxa.

Perhaps a better way to compare the compound-specific and disturbance taxa is to

compare 613C to grasses only, which would be most representative of C4 contribution.









A comparison of compound-specific carbon isotopes to % grasses reveals that

while both records increase from the base of the record to 3300 cal yr BP, only the 613C

of C33 peaks between 3300 and 2500 cal yr BP (Figure 24). The % grasses in the pollen

record, however, is relatively low during this period (9%) and does not peak until -2300

cal yr BP. Both the % grass and isotope records generally decline from 2300 cal yr BP to

present. The pollen (% grasses) record is lower resolution than the isotope record; which

may explain some of the discrepancy between the two vegetation proxies.

Perhaps the largest potential reason for the discrepancy may lie in the fact that

maize pollen is not included in the total pollen count because of its large size, thus

causing an over-representation of other taxa. Maize pollen has often been used in

Mesoamerican studies as a proxy for agriculture and associated deforestation and it is

well-documented that maize pollen is abundant during the period of Maya occupation

(Leyden, 1987; Islebe et al., 1996). The 613C of n-alkanes, however, should be very

sensitive to large stands of maize in the watershed.

There are additional potential shortcomings in using the 613C of n-alkanes as a

proxy for vegetation change. For example, maize was probably an important (C4) plant

in the vegetation of the Peten Lake District, especially during times of high population

density. Shoreline cultivation of maize would strongly influence the compound-specific

613C while not altering the pollen profile. The higher contribution of C4 vegetation during

the Preclassic (-3300 cal yr BP to 2500 cal yr BP) may represent early shoreline maize

cultivation in the watershed. As populations increased into the Classic period, the more

desirable shorelines may have become residential areas as opposed to agricultural areas,

which would have then been moved further from the lake. Abandonment of near-shore









fields would alter the compound-specific 613C but would not be apparent in the pollen

profile. Alternatively, maize cultivation on shorelines may have ceased in the Late

Preclassic due to soil depletion. As the soils immediately surrounding the lake were

exhausted, the ancient Maya may have moved agricultural fields further into the

surrounding watershed.

While there are potential explanations for the discrepancy between pollen and

compound-specific 613C values, it is necessary to fully understand that both records are

recording different aspects of watershed vegetation. Vegetation inferences from pollen

percentages represent the relative abundance of pollen grains in a sediment profile and do

not necessarily reflect species abundance or biomass on the landscape (Bradley, 1999)

whereas compound-specific carbon isotopes do not reveal any information about forest

composition.

Pollen and leaf waxes are derived from different vegetative sources and thus

record different aspects of watershed vegetation (Huang et al., 1999). Whereas pollen is

a measure of only reproduction, leaf waxes provide a more representative measure of

vegetative biomass within a watershed. The exclusion of maize pollen provides yet

another complication for interpreting pollen profiles. Compound-specific carbon

isotopes, on the other hand, do not reveal any detailed information about forest

composition. They simply allow estimation of the relative contribution of C3 versus C4

n-alkanes to the sedimented organic matter. When examined in conjunction with pollen

accumulation rates, however, leaf waxes and pollen may provide a better estimate of

vegetative biomass. The carbon isotopic composition of leaf waxes is a good







69


geochemical proxy for testing palynological inferences for vegetation changes within a

watershed.


% Grasses
) 15


0



500



1000



1500



2000

CD
L. 2500



3000



3500



4000



4500


5000


-35

S130 033 (%o, PDB)


Figure 23: Diagram showing % grass pollen versus 613C of n-alkane C33 in Lakes
Salpeten and Sacnab and the presence of maize pollen in Salpeten (pollen data
from Leyden, 1987).









Comparison with Population Estimates

The correlation between Maya population densities and 613C values of long-chain

n-alkanes in lake sediments suggests that while vegetation change over the past 4500 cal

yr BP may have been tied to changes in population density, additional factors may have

affected vegetation in the watershed in the early part of the record. Pollen records

(Leyden, 1987; Islebe et al., 1996) reveal that disturbance taxa and maize increased as

populations increased. Pollen changes were most likely tied to agricultural land

clearance. Because grasses and maize are C4 plants, one would predict an increase in

the 613C of n-alkanes as populations grew. The largest shifts in vegetation, however, in

both Lakes Sacnab and Salpeten, occurred well before the peak in late Classic Maya

populations (Figure 24). The highest 613C values, indicating the largest contribution

from C4 vegetation, occurs in Lake Sacnab between 3300 cal yr BP and 2700 cal yr BP

and in Lake Salpeten at 2700 cal yr BP, during the Preclassic Period. In both lakes there

appears to be a decrease in C4 biomass between 2700 cal yr BP and 1300 cal yr BP, the

time period during which population density was greatest.

This decoupling of population density and the 613C of long-chain n-alkanes

between 4500 cal yr BP and 1300 cal yr BP may be a result of several factors. For

example, if agricultural practices changed through time from more extensive to intensive

methods, a return to a C3-dominated landscape may coincide with both population growth

and agricultural developments. The question regarding whether the Maya maintained

economic trees and house gardens within cities remains unanswered, but researchers have

made speculations in some instances. For example, Leyden (1987) notes that the

presence of ramon (Bromsimum) pollen suggests that the Maya were "arboriculturists",









possibly growing tree gardens in typically residential and common areas. Selective

clearing and/or tree planting, however, is not indicated by the rest of the pollen record.

Instead, greater proportions of corn pollen during the Late Classic through Postclassic

indicate intensified agricultural activities. The interpretations made from the pollen

record (that ramon does not necessarily indicate arboriculture) are supported by the work

of Lambert and Arnason (1982), who showed that ramon is attracted to constructions of

limestone and is positively correlated with Maya sites; this would indicate not

arboriculture, but instead settlement expansion.

-26 250

-28 200
150 "
-30 0
100
-32
50
Lake Salpeten 0
-34 0




Q -26
120 0

-30 80


-34 40 I

Lake Sacnab
-38
0 1000 2000 3000 4000 5000
Age (cal yr BP)

Figure 24: Comparison of the compound-specific carbon isotope records (C33) and
population density estimates versus time in Lakes Salpeten (top) and Sacnab
(bottom).









It is also possible that, as discussed in the previous section, the higher

contribution of C4 vegetation during the Preclassic (-3300 cal yr BP to 2500 cal yr BP)

may represent early shoreline maize cultivation in the watershed. As populations

increased into the Classic period, the more desirable shorelines may have shifted to

residential areas as opposed to agricultural areas, which would have then been relocated

further from the lake. Abandonment of near-shore fields would be represented by a

decoupling of the compound-specific 613C and population because the agricultural fields

may have expanded away from the lake shore as populations expanded in the Classic

Period.

Another possibility for the lack of coherence between population estimates and the

compound-specific vegetation record may lie within the population estimates themselves.

The population estimates in the Peten are based on a limited number of survey transects

and house-mound excavations. There is a possibility that existing sites may not have

been revealed during survey transects. In addition, older Preclassic population may have

been underestimated if house mounds of the period were poorly preserved in the

archaeological record. In addition, the chronology for population estimates was based on

ceramic phases and inherently has a significant amount of error associated with it. While

estimates of population densities in the watersheds may not be entirely accurate and may

potentially be off by orders of magnitude the relative changes in population density are

correct, and populations were certainly greater in the Classic than Preclassic Periods.

Despite the divergent trends in population density and proxy vegetation in the early

part of the record, the records display similar trends between 1200 cal yr BP and the

present. A decrease in the compound-specific carbon isotopic ratios begins at 1200 cal









yr BP and correlates with a significant decline in population associated with the decline

of the Maya civilization. As human pressures on the landscape were curtailed and fields

were abandoned, watershed vegetation shifted from a C4 to a C3-dominated landscape.

This indicates that the decline in population density was likely the main cause of late

Holocene vegetation change.




Relationship between Climate and Environmental Changes

Finally, the relative importance of humans versus climate for vegetation change in

the Central Peten can be determined by comparing the 613C of n-alkanes with climate

proxies that are not confounded by human impact. These comparisons may reveal

whether climate played any role in environmental change. If vegetation changes in a

watershed were climate-induced, long-chain n-alkanes should correlate with independent

evidence for regional climate change. Proxy records of climate change are available for

both the Yucatan and the Caribbean Sea and can be compared with compound-specific

analyses and evaluated in regards to changes in Maya population densities.

A comparison with the %Ti record from the Cariaco basin (Haug et al., 2001)

(Figure 25) reveals a relationship between %Ti and the compound-specific records from

each lake during the early part of the record. In the Cariaco Basin, %Ti in the sediments

is used as a proxy for terrigenous sediment input, which is influenced by regional dry/wet

cycles. Higher %Ti reflects higher terrigenous input and greater precipitation.

The %Ti record from 10,000 cal yr BP to 4000 cal yr BP is high and relatively

unchanging, indicating relatively mesic conditions. At approximately 4000 cal yr BP,

however, %Ti values decrease and show greater variability indicating generally reduced









precipitation, perhaps related to the southward migration of the Intertropical Convergence

Zone (ITCZ). Increased variability in %Ti is especially pronounced from 4000 cal yr BP

to 2000 cal yr BP. The apparent relationship of 613C of n-alkanes to climate suggests that

vegetation may have been partly controlled by changes in regional climate rather than

local cultural changes over some time period.

Increased climate variability during the period between 4000 cal yr BP and 2000

cal yr BP may have altered precipitation patterns and the vegetation in Peten. An

extended dry period with sporadic periods of greater precipitation is an effective way to

stress vegetation and to erode the landscape. Prior to 2000 cal yr BP, changes in regional

climate may have influenced vegetation in addition to human disturbance.

Further evidence for climate-influenced vegetation change can be found in

sediment records from numerous lakes in northern Yucatan. A sediment core from Lake

Chichancanab provides evidence for regional drying that occurred beginning at

approximately 3000 cal yr BP with a distinct interval of droughts between 1300 cal yr BP

and 1100 cal yr BP, which coincided with the Terminal Classic collapse (Hodell et al.,

1995; Hodell et al., in press). Compound-specific carbon isotopes (Figure 15 ) during the

period between 3300 and 2700 cal yr BP in both Lakes Sacnab and Salpeten represent the

highest proportion of C4 versus C3 vegetation, as would be expected during a period of

increased climate variability. One would expect to see continued increased contribution

of C4 vegetation throughout the period of increased climate variability but this is not the

case. This discrepancy may suggest that while climate influenced vegetation change

beginning at 3000 cal yr BP, human land-use change exacerbated the deforestation

process from 3000 cal yr BP until Maya populations decreased at 1100 cal yr BP.












Ti (%, 3-pt smooth)
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45


0




1000




2000




3000




4000


5000


6000




7000




8000




9000


10000 I-
-20


-22 -24 -26 -28 -30 -32 -34 -36 -38


S130 033 (%o, PDB)


Figure 25: Comparison of the compound-specific carbon isotope records (C33) and
percent Ti versus time in Lakes Salpeten and Sacnab. Percent Ti data from
Haug et al. (2001).











The 6180 of ostracod carbonate in Lake Miragoane, Haiti (Hodell et al., 1991)

records a similar climate history to that for the Cariaco basin (Haug et al., 2001). The

isotope record, indicating changes in evaporation/precipitation and lake level, indicates a

switch from dry forest vegetation to more mesic conditions at approximately 8200 cal yr

BP. Beginning approximately 3800 cal yr BP, however, the isotopic value of shell

carbonate begins to increase coincident with a loss of mesic forest trees and a regional

drying event is suggested. These data correspond to local vegetation changes in the

Sacnab and Salpeten basins; as climate became drier, vegetation shifted from a C3 to a

C4-dominated landscape. This further supports the interpretation that (-4000 cal yr BP to

3000 cal yr BP), climate may have played some role in vegetation change.



Conclusions

The questions addressed in this study explore the dynamics of environmental

change with respect to ancient Maya population shifts and late Holocene climate changes.

While the analysis of compound-specific 613C values to study vegetation shifts is still in

its infancy, this is the first late Holocene record for the Peten and the first to compare the

613C of n-alkanes with a local pollen record.

The data indicate that in the watersheds of Lakes Salpeten and Sacnab, shifts in the

proportion of C3 to C4 are most likely controlled by a combination of climate change and

human deforestation. The correspondence of 613C records to independent proxies for

climate change from -4500 cal yr BP until -3000 cal yr BP suggest that regional drying

and increased climate variability caused an increase in the contribution of C4 vegetation









during that time. Following this period and beginning with the first Maya occupation in

the watersheds, vegetation change was likely a result of human-driven deforestation or

perhaps a combination of both climate and human impact.

The lack of concordance between the compound-specific carbon isotopic record

and the pollen profiles between 3000 cal yr BP and 1100 cal yr BP may be due to the

underlying ambiguities in comparing percent pollen data with compound-specific carbon

isotopic measurements. The only true assessment of the C4 pollen signal is to examine

shifts in the absolute values of grass and maize pollen influx. The higher contribution of

C4 vegetation during the Preclassic inferred from compound-specific 613C (-3300 cal yr

BP to 2500 cal yr BP) may represent early shoreline maize cultivation in the watershed.

As populations increased into the Classic period and shorelines shifted to residential as

opposed to agricultural areas, cultivated fields would have then been moved further into

the surrounding watershed. This abandonment of fields and subsequent cessation of

maize cultivation on the shoreline may strongly alter the compound-specific 613C but

would not be apparent in the pollen profile.

Discrepancies between vegetation inferred from pollen profiles versus 613C of n-

alkanes may also lie within the interpretation of the compound-specific 613C record itself

While 613C of n-alkanes is an excellent proxy for estimating the relative changes in

biomass contribution of C3 and C4 vegetation, it cannot be used to interpret forest

composition or the dynamics of tropical reforestation. In addition, there is no clear

understanding of how issues such as differential leaf production and the amount of leaf

wax a plant produces may affect the 613C of n-alkanes. Another potential problem in

interpreting 613C values is that variation in canopy density that alters both light regimes









and the 613C of source CO2 air for plants, which in turn affects the 613C of plant material.

While an important factor, it is difficult to quantitatively assess the canopy effect on 613C

values of n-alkanes. Lastly, the relationship between 613C and water-use efficiency

(WUE) in C3 plants may cause 613C values to appear more depleted and thus suggest a

higher apparent contribution of C3 vegetation. With these limitations in mind, it is

necessary to interpret compound-specific carbon isotope records with caution.

While the decoupling of population density and the 613C of long-chain n-alkanes

may suggest that agricultural practices changed through time from more extensive to

intensive methods, it is impossible to determine this from the 613C from long-chain n-

alkanes alone. The 613C from long-chain n-alkanes need not necessarily track population

change if a shift in the proximity of agricultural lands (and proximity of maize pollen,

which is large and does not travel far) to the lake was influencing the 613C. The

movement of fields and subsequent cessation of maize cultivation on the shoreline would

strongly alter the compound-specific 613C but would not necessarily be reflected by

changes in population densities.

The interpretation of compound-specific carbon isotopes of long-chain n-alkanes is

dependent upon the area surrounding the lake over which the compounds integrate.

There is unfortunately very little information regarding transport and deposition of n-

alkanes in lacustrine studies. If compound-specific carbon isotopes of long-chain n-

alkanes are more strongly influenced by vegetation on water-edge lands, than they may

not provide an accurate picture of overall watershed vegetation and land-use. Studies that

attempt to calibrate the compound-specific carbon isotopic value of modem sediments









with the surrounding forest structure may greatly aid in interpreting sediment records of

paleoenvironmental change.

Compound-specific carbon isotopes and pollen profiles record represents different

aspects of watershed vegetation and are best used in tandem to infer past changes in

watershed vegetation. While this study complements past studies that analyzed only bulk

sediment or pollen and further supports the record of changes in terrestrial vegetation in

the Peten region of the Maya Lowlands, further research is needed to understand the

dynamic changes in forest structure associated with deforestation and reforestation. In

addition, calculating grass and maize pollen accumulation rates from the same core as the

613C may provide a more robust assessment of the C4 pollen signal. And lastly, it is

important to utilize new archaeological information from surrounding watersheds to

better understand how changes in land-use correlate to environmental change.














APPENDIX A
LIPID EXTRACTION PROCEDURE

Supplies:

33 mL stainless steel cell

Glass filters
Quartz sand
2:1 methylene chloride (DCM)/methanol
Scintillation vials

Notes:

-All quartz sand should be ashed at 450C for at least two hours in a muffle
furnace. ASE cells should be cleaned after each use by washing with DI water
and solvent rinsing with methanol 3 times. Store ASE cells in oven, or with top
and bottom screwed in place.


1) Screw base onto 33 ml Accelerated Solvent Extractor cell. Place two filters at the
base and add -5-10 mL of sand.
2) Weigh out and add approximately 10-20 tg of an internal standard (C34) to each
sample so as to allow calculation of yields and quantification of lipid compounds
later in the process.
3) Weight out approximately 3-5 g of sample that has been frozen, freeze dried and
crushed.
4) Record the weight and then add the sample to the cell.
5) Fill remaining space in the cell with sand and screw on the top.
6) Place cells in the Accelerated Solvent Extractor and use the following method to
extract the phospholipids:

Solvent: 2:1 DCM/Methanol
Pressure: 1500 PSI
Temperature: 100 C
Heat: 5 mins
Static: 5 mins
Flush: 60%
Purge: 200 sec
Cycles: 3






81


7) Solvent exchange to hexane using a hot water bath under a stream of nitrogen gas.
This is performed by evaporating samples to dryness and then adding -10 mL of
hexane. To ensure full solvent exchange, repeat this process three times.
8) Transfer residue to a labeled 20 mL glass scintillation vial using ~15ml of hexane.
For maximum compound retention, do this in multiple wash/transfer steps.
9) Evaporate sample in a hot water bath under a stream of nitrogen gas.
10) Samples (in scintillation vials) are now ready for silica gel chromatography.














APPENDIX B
SILICA GEL CHROMOTAGRAPHY

Supplies:

Glass column
Teflon stopcock
Glass rod
Glass funnels
Glass wool
GC Resolv or Optima grade Hexane
GC Resolv or Optima grade methylene chloride (DCM)
Teflon squirt bottle
Small stainless steel spatula
5% deactivated silica gel
Pasteur pipette with long pipette tips
20 mL beaker
2 graduates cylinders


Notes:

-All glassware, glass wool, and silica gel should be solvent rinsed and ashed at
450C for at least two hours in a muffle furnace.



1) To make 5% deactivated silica gel, use the following relation:
a. (%deactivation/ (100-%deactivation)) = (mL water/weight of silica (g))
b. (%deactivation/(100-%deactivation)) (weight of silica (g)) = mL of water
Add 1 mL of DI water to 20g of 100% activated silica gel
c. Shake bottle for 10 minutes.
d. Store in dessicator (good for only three days).
2) Assemble stopcock to the base of the column to regulate flow.
3) Set up column by placing glass wool at the base of a glass column and pushing it
down with a glass rod until the column is clogged.
4) Set up column vertically in the hood with a clamp.
5) Add approximately one column full of hexane and allow the hexane to drain to
clean the glass plug.
6) Weight out 2.5 grams of 5% deactivated silica gel in a 20-ml beaker. Cover with
hexane quickly to keep if from absorbing any moisture from the air.









7) Place a glass funnel in the top of the column. Mix silica gel/hexane thoroughly
with the spatula creating a suspension. Add the mixture to the column using the
funnel and allow the silica to settle while tapping gently to avoid gas bubbles. Be
sure to scrape all silica gel from the sides of the beaker into the funnel. Squirt the
gel with hexane to force it through the funnel.
8) Remove the funnel and fill with hexane.
9) Drain excess hexane while tapping the column to guarantee full settling. Close
the stopcock when the hexane level reaches just above the silica gel. Free any
remaining silica gel adhering to the sides by tapping the column gently.
10) Rinse the column with -10ml of hexane to wash remaining silica gel, again
stopping when the solvent level reaches just above the silica-gel. Rinse the
stopcock tip with hexane.
11) Place a collection vial labeled for n-alkanes under the column.
12) Fill a graduated cylinder with 16-17 mL of hexane. Use a pipette to bring the
level to 15 mL, thus rinsing the pipette.
13) Add 1 mL of hexane from the graduated cylinder to the scintillation vial
containing the sample. Rotate the sample vial between fingers while holding the
vial at a 45 angle to dissolve the residue.
14) Transfer the sample in hexane to the column, gently dispensing sample -lcm
above silica gel.
15) Drain and collect the sample and solvent.
16)Repeat steps 13-15 twice more before adding the remaining hexane (-12 mL) in
the graduated cylinder to the column.
17) Drain the sample and solvent to just above the level of the silica gel.
18) Remove the collection vial and a place second vial labeled for the "non-n-alkane
organic fraction" under the column.
19) Using a graduated cylinder, measure 15 mL of DCM and add it to the column.
20) Drain the DCM into the collection vial, thus eluting all other lipid compounds.
21) Archive for potential later use.
22) Concentrate n-alkane fractions in collection vials by evaporating to dryness under
a stream of N2.
23) Re-dissolve n-alkanes in 200 [tL of hexane.
24) Run samples on a Perkin Elmer 8500 Gas Chromatograph (PE 8500 GC) injecting
4 [tL of sample in order to determine purity and approximate concentrations using
the following oven program:


T ramp Hold
Rate Hold Temp. Time
C/min 0C (min)
50 1
6 300 20














APPENDIX C
UREA ADDUCTION AND GC ANALYSIS

Supplies:

GC Resolv or Optima grade hexane
GC Resolv or Optima grade acetone
GC Resolv or Optima grade pentane
GC Resolv or Optima grade urea-saturated methanol
DCM-extracted DI water
GC Resolv or Optima grade methanol
Pasteur pipette with long pipette tips
20 mL beaker
2 graduates cylinders
gas-tight Teflon coated syringes


Notes:

-All glassware should be solvent rinsed and ashed at 450C for at least two hours
in a muffle furnace. All of the following solvent transfers are performed using
glass pipettes or Teflon-coated gas-tight syringes.



1) After running each alkane sample on the PE 8500 GC, transfer samples to a 4 mL
glass screw top vial and evaporate to dryness under a stream of nitrogen.
2) Add 1 mL each of acetone, pentane, and urea-saturated methanol to each sample.
Replace caps.
3) Place in a -4C freezer for -30 minutes.
4) After 30 minutes, remove samples from freezer. Evaporate excess solvent under a
stream of nitrogen. The urea crystals that remain contain the straight-chain n-
alkanes only, while the non-adducts, branched and cyclic compounds, remain
outside of the crystals.
5) Wash urea crystals with hexane three times, with the wash hexane being pipetted
into a separate 4 mL vial labeled as "non-adducts" to be archived for potential
later use.
6) Dissolve the remaining urea crystals in 1 mL of methanol and 1 mL of DCM-
extracted DI water. This fraction contains the straight-chain n-alkanes.
7) Add approximately 2 mL of hexane to this fraction. Shake vigorously using a
Vortex vibrator to extract hydrocarbons.
8) Centrifuge vials at 500 rpm for 5 minutes or until hexane and urea/methanol/water
separate.









9) Remove the hexane from the top with a gas-tight syringe and place into a separate
4 mL vial labeled as "adducts". Repeat the hydrocarbon extraction (Steps 7-8)
twice more to guarantee full recovery of n-alkanes.
10) Evaporate excess solvent from the adduct fraction under a stream of nitrogen.
11) Repeat steps 2-10 twice more on the adduct fraction.
12) After triple adduction, evaporate the adduct fraction to dryness under a stream of
nitrogen and re-dissolve in 200 UL of hexane.
13) Run samples on a Perkin Elmer 8500 GC using the following oven program and
injecting 4 UL of sample:


Rate Temperature Time
C/min 0C (min)
50 1
6 300 20


14) Evaporate the sample to dryness again under a stream of nitrogen and transfer to a
1 mL glass crimp-top autosampler vial using multiple washes to ensure full
transfer of n-alkanes.
15) Evaporate the sample to dryness under a stream of nitrogen. Dissolve in a known
amount of hexane according to obtain the approximate concentration necessary
for GC-IRMS analysis (ex.: samples that had a voltage of -20-30 mV on the PE
8500 GC should be dissolved in 100 UL of hexane whereas samples with a
voltage of -10 should be dissolved in 25 UL of hexane).
16) The adduct fraction containing straight-chain n-alkanes is now ready for GC-
Isotope Ratio Mass Spectrometry (IRMS) analysis.















APPENDIX D
DATA TABLES