Bulk and Compound-Specific Isotope Analysis of Long-Chain n-Alkanes from a 85,000 Year Sediment Core from Lake Peten Itz...

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Bulk and Compound-Specific Isotope Analysis of Long-Chain n-Alkanes from a 85,000 Year Sediment Core from Lake Peten Itza, Guatemala
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english
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Mays, Jennifer L
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University of Florida
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Degree:
Master's ( M.S.)
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University of Florida
Degree Disciplines:
Geology, Geological Sciences
Committee Chair:
Brenner, Mark
Committee Members:
Lambeck, Andrea Dutton
Zimmerman, Andrew R

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Subjects / Keywords:
carbon -- compound-specific -- core -- geochemistry -- guatemala -- isotopes -- lake -- n-alkanes -- neotropics -- paleoclimate -- paleolimnology
Geological Sciences -- Dissertations, Academic -- UF
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Geology thesis, M.S.
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theses   ( marcgt )
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Abstract:
Drill cores obtained from Lake Peten Itza, Guatemala, contain a ~85,000 year (kyr) record of terrestrial climate from lowland Central America. Variations in sediment lithology suggest rapid changes in precipitation during the last glacial and deglacial periods. Previous work in nearby Lake Quexil demonstrated the utility of using the carbon isotopic composition of leaf wax n-alkanes to infer changes in terrestrial vegetation (Huang et al., 2001). Forty-nine samples were taken from one composite core (PI-6) to record carbon isotopes of bulk organic and long-chain n-alkanes. Changes in delta13C values indicate shifts in the relative proportion of C3 to C4 biomass. Our record shows largest variations in delta13C occurring in association with Heinrich Events. Carbon isotopic values in the Last Glaical Maximum (LGM) sediment indicate environmental conditions with relatively moderate precipitation with little variation. The most arid periods occur in the deglacial associated with large-scale climate events (Younger Dryas and Bolling-Allerod) and are interpreted by samples with the largest proportion of C4 plants, as well as independent measurements obtained from ostracods (delta13C) and pollen data from our same samples sites within PI-6.
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by Jennifer L Mays.
Thesis:
Thesis (M.S.)--University of Florida, 2012.
Local:
Adviser: Brenner, Mark.
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RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2013-02-28

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1 BULK AND COMPOUND SPECIFIC ISOTOPE ANALYSIS OF LONG CHAIN N ALKANES FROM A 85,000 YEAR SEDIMENT CORE FROM LAKE PETN ITZ, GUATEMALA By JENNIFER LYN MAYS A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORI DA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012

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2 2012 Jennifer Lyn Mays

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3 To my Mom ; t he beat goes on

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4 ACKNOWLEDGMENTS I wish to thank my committee: Mark Brenner, David Hodell, An drew Zimmerman, and Andrea Dutton for their un wavering support and patience. This project would not have been possible without the guidance of Jason Curtis and Kathy Curtis and their extraordinary help with lab methods and testing. I sincerely thank Jaime Escobar and Alex Correa Metrio f or sharing their data with me. I also wish to thank the numerous agencies and individuals in Guatemala who assisted with the Lake Petn Itz Scientific Drilling Project (PISDP) as well as the Drilling, Observation and Sampli ng of Continental Crust program ( DOSECC ). Many thanks go to Anders Noren, Kristina Brady and Amy Myrbo at the National Lacustrine Core Facility (LacCore ), University of Minnesota Twin Cities for their assistance with core curation and sample pr eparation. This project was funded by grants from the United S tates National Science Foundation ( under ATM 0502030), the International Continental Scientific Drilling Program (ICDP) the Swiss Federal Institute of Technology, and the Swiss National Scienc e Fo undation. Radiocarbon analyses were perfor med under the auspices of the United States Department of Energy by Lawrence Livermore National Laboratory under contract DE AC52 07NA27344. I thank my mom and da d, my dear grandma, Velma, and my aunts an d un cles for their u nconditional love and support. I also thank my fellow students, Jen Gifford Gilly Rosen and Helen Evans. Finally, I am ever grateful to Amy Miller, and Kevin D. for their continual encouragement and friendship.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 6 LIST OF FIGURES ................................ ................................ ................................ .......... 7 ABSTRACT ................................ ................................ ................................ ..................... 8 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 10 Objective ................................ ................................ ................................ ................. 10 Background ................................ ................................ ................................ ............. 11 Study Site ................................ ................................ ................................ ............... 13 2 MATERIALS AND METHODS ................................ ................................ ................ 14 Sa mple Preparation ................................ ................................ ................................ 14 Geochemical and Compound Specific Stable Carbon Isotope Analyses ................ 14 3 RESULTS ................................ ................................ ................................ ............... 18 Chronology ................................ ................................ ................................ ............. 18 Lithology ................................ ................................ ................................ ................. 18 Geochemistry ................................ ................................ ................................ .......... 19 4 DISCUSSION ................................ ................................ ................................ ......... 25 Carbon Isotopes Upcore by Unit ................................ ................................ ............. 25 Unit VIII, VII, and VI ................................ ................................ .......................... 25 Unit V, and IV ................................ ................................ ................................ ... 26 Pre LGM Chronozone ................................ ................................ ...................... 26 Unit III: LGM Chronozone ................................ ................................ ................. 27 Unit II: Deglacial ................................ ................................ ............................... 27 Deglacial/Holocene Transition ................................ ................................ .......... 28 Unit I: Holocene ................................ ................................ ................................ 28 Comparison of Compound Specific and Bulk Carbon Isotope Values .................... 29 5 CONCLUSION ................................ ................................ ................................ ........ 36 LIS T OF REFERENCES ................................ ................................ ............................... 37 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 40

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6 LIST OF TABLES Table page 3 1 Section ID, de pths, ages, and units for PI 6 ................................ ....................... 20 3 2 Bulk geochemical results for PI 6 ................................ ................................ ....... 21 3 3 Bulk and compound specific carbon results ................................ ....................... 23

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7 LIST OF FIGURES Figure page 1 1 Location of Lake Petn Itz Guatemala and site PI 6 (Hodell et al. 2008) ........ 13 2 1 Lithology, magnetic susceptibility and sample locations by depth (mcd) for PI 6 (Mueller et al. 2010) ................................ ................................ ......................... 17 4 1 Composite core PI 6 magnetic susceptibility (red line) an d 13 C TOC (green line with triangles) by meter composite depth (mcd) show tightly coupled variation indicating an agreement between lithology and 13 C TOC values. .......... 32 4 2 Values of 13 C TOC (red line with triangles) and 13 C ostracod (blue line) through the deglacial are likely representing the same carbon source. ........................... 32 4 3 PI 6 13 C TOC (green line with triangles) and 13 C 33 (orange line with circles) during the deglacial show general agreement indicating bulk and compound specific measurements show similar trends. ................................ ...................... 33 4 4 Percent Poaceae (green line with circles at left), perc ent total organic carbon (red line with triangles in center, note scale direction), and 13 C TOC (blue line with circles at right) by meter composite depth (mcd) show agreement throughout length of core indicating positive relationship between amount of grass, amount of carbon and relative percentage of C 4 plants in environment. .. 33 4 5 Bulk and compound specific carbon isotopes: 13 C TOC (green line with circles at left) 13 C 29 blue line with squares second from left) 13 C 31 red line with diamonds second from right) and 13 C 33 (pu rple line with triangles at right) by meter composite depth (mcd) values show good fit indicating possible use of 13 C TOC to demonstrate 13 C n alk trend. ................................ ................................ 34 4 6 Bulk and compound specific carb on isotopes show largest shifts at climatic boundaries. Values shown from 0 43 kyr BP; climatic events in grey bars: A=Holocene, B=Younger Dryas & Preboreal C=Bolling Allerod, D=Heinrich Stadial 1, E=LGM, F=Heinrich Stadial 2, G= MIS3. ................................ ............ 34 4 7 This scatter plot delineates the source of organic matter using 13 C TOC and C:N (carbon to nitrogen ratio). The resulting plot shows a dominance of C 3 plants in the source material. ................................ ................................ .............. 35 4 8 The dominant n alkane values and carbon preference index (CPI) by m eter composite depth (mcd) for PI 6 show largest variations during the deglacial just prior to Holocene transition. ................................ ................................ ......... 35

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8 Abstract of Thesis Presented to the Graduate School of the University of Florida in Par tial Fulfillment of the Requirements for the Degree of Master of Science BULK AND COMPOUND SPECIFIC ISOTOPE ANALYSIS OF LONG CHAIN N ALKANES FROM A 85,000 YEAR SEDIMENT CORE FROM LAKE PETN ITZ, GUATEMALA By Jennifer Lyn Mays August 2012 Chair: Mark Br enner Major: Geology Drill cores from Lake Petn Itz, Guatemala contain a ~85,000 year ( 85 k a ) record of terrestrial climate from lowland Central America Variations in sediment lithology suggest rapid pronounced changes in precipitation during the las t glacial and deg lacial periods. Previous work in nearby Lake Quexil demonstrated the utility of using the carbon isotopic composition of leaf wax n alkanes to infer changes in terrestrial vegetation (Huang et al., 2001). Forty nine samples w ere taken from one composite Petn Itz core (PI 6) to re cord carbon isotopes of bulk organic and long chain n alkanes Changes in 13 C values indicate shifts in t he relative proportion of C 3 to C 4 biomass The record show s largest 13 C variations in association with He inrich Events Carbon isotopic values in the Last Glacial Maximum (LGM) sediment indicate environmental conditions with relatively moderate precipitation and little variability The deglacial was a period of pronounced climate variability (e.g. Bolling All erod and Younger Dryas ), and arid times of the deglacial were inferred from samples with the greatest 13 C values i n organic matter, i.e. the largest proportion of C 4 plants Such

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9 inferences were supported by isotope measurements on ostracods and analysis of pollen from the same sample depths in core PI 6.

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10 CHAPTER 1 INTRODUCTION Objective Lake Petn Itz, in the lowlands of northern Guatemala, wa s targeted in this study to explore pre great depth (>160 m) suggested a likely source for continuous, terrestrial sediment records spanning the l ast glacial period. In 2006, a suite of cores was recovered from seven sites in the lake (Hodell et al. 2008). Cores from one site, PI 6, were found to span the last 85 ka based on dates obtained from radiocarbon and tephra analyses. I took samples through out composite core PI 6 and measured 13 C of bulk organic carbon ( 13 C TOC ) and compound specific, long chain n alkanes ( 13 C n alk ). My objective was to discern climate driven shifts in local vegetation using the changing isotopic signature of or ganic matte r in the sediments. The 13 C values indicate the relative abundance of C 3 /C 4 plants in the watershed because the two plant types possess different photosynthetic pathways and discriminate differentially with respect to their carbo n source, i.e. carbon diox ide. Because C 3 and C 4 plants thrive in different environments, stratigraphic changes in 13 C values reveal paleoclimate variation, especially with respect to precipitation. I compared my results to previous findings inferred from pollen (Leyden 1984; Bush et al. 2009; Correa Metria et al. 2012), density and magnetic susceptibility (Hodell et al. 2008), lithology (Mueller et al. 2010), and stable isotope measures ( 13 C and 18 O) in ostracods (Escobar et al. 2012). I used my carbon isotope data in sediment o rganic matter as an alternate line of evidence to evaluate the timing and magnitude of vegetation response to chan ges in regional precipitation.

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11 Background Vascular plant leaves contain waxes on their epicuticular surface that are constantly ablated and de posited in the immediate and regional environm ent (Eglinton and Hamilton 1967; Schess 2003 ). One component of leaf wax is n alkanes, hydrocarbons that record the 13 C signature of environmental car bon at the time of primary growth. The highly refractory nature of n alkanes enables them to be well preserved in lake sediment. Two main means of carbon fixation have been identified in plants, the C 3 (Calvin Benson) and the C 4 (Hatch S Because of differences in plant physiology and carbon fixation, the 13 C signatures of the two plant groups fall into distinct ranges: C 3 plants typically have 13 C values between 4 pla nts have less negative values, between and et al.1982 ). Tropical grasses and sedges, including maize, are C 4 plants, which outcompete C 3 plants in dry conditions. Most trees, shrubs, and cool season grasses and sedges are C 3 plants, which dominate in wetter envir onments (Cerling et al. 1993). Stratigraphic changes in 13 C values of organic matter in lake sediment cores have been used to infer past shifts in the relative abundance of C 3 versus C 4 plants in the surrounding watershed. Because the two types of plants display different preferred growing conditions, the carb on isotope signature of organic carbon in sediments can be used to infer past climate, especially regional aridity (Collister et al. 1994; Hughen et al. 2004). Whereas atmospheric carbon dioxide concentration ( p CO 2 ), temperature, and aridity all influence the ratio of C 3 to C 4 plants, previous research showed that aridity may be the main factor influencing vegetation type (Huang et al. 2001; Castaeda et al. 2007).

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12 The isotope signature of bulk organic c arbon in lake sediments can be used to evaluate past c lake trophic status (Brenner et al. 1999; Gu et al. 1996). Organic carbon in lake sediment, however, comes from multiple sources, both terrestrial and aquatic. Hence, the 13 C value of bulk organic carbon provides a single signature for the mixed carbon sources, often making it difficult to distinguish between past cha nges on land and in the water. Selective diagenesis of organic compounds in the water column and sediment is also a conce rn for using bulk 13 C values to reconstruct paleo environmental conditions (Meyers 1994). Measurement of compound specific stable carbon isotopes, using GC IRMS, enables investigators to isolate and analyze molecules of known provenience, thereby making it possible to inde pendently track te rrestrial and aquatic changes. Long chain n alkanes, specifically C 29 C 31 and C 33 are found almost exclusively in terrestrial plants (Lichtfouse et al.1994 b ), whereas short chain alkanes (C 15 C 19 ) are produced largely by algae (Chika raishi and Naraoka 2003). Aquatic macrophytes typically show intermediate values, between C 23 C 25 (Ficken et al. 2000). Although compound specific 13 C analysis enables one to distinguish between terrestrial and aquatic carbon sources in lake sediments, the approach is costly and time consuming. To justify use of the compound specific approach, it must be demonstrated that the same information cannot be obtained from the bulk carbon 13 C record. I addressed two main questions in my carbon isotope study of sediments f rom Lake Petn Itz core PI 6. First, I compared 13 C results from bulk organic carbon and specific compounds to determine whether extr action and measurement of n alkanes is warr anted in Petn Itz sediments. Second, I compared my results to other lines of

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13 paleoclimate evidence and inferred shifts in vegetation and climate in the area over the last ~85,000 years. Study Site Lake Petn It 2 ), effectively closed basin lake in central, lowland Guatemala Variation in lake level is closely associated with the seasonal migration of the Intertropical Convergence Zone (ITCZ) (Hodell et al. 2008). Wetter con ditions are associated with more northward migration of the ITCZ, whereas drier conditions reflect a southward position of the ITCZ. Members of the Lake Petn Itz Scientific Drilling Project (PISDP) recovered 1,327 meters of lake sediment core from seven sites in Lake Petn Itz during February and March 2006, using the GLAD 800 drilling system. Site PI 6, the focus of my study, was located at a water depth of 71 m (Fig. 1 1). Three holes were drilled at PI 6, primarily using a hydraulic piston corer, t o a maximum depth of 75.9 mblf (meters below lake floor). Cores were taken in 3 m sections and the average recovery rate was 94.9% (Hodell et al. 2008). Figure 1 1. Location of Lake Petn Itz Guatemala and site PI 6 ( Hodell et al. 2008)

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14 CHAPTER 2 MATE RIALS AND METHODS Sample Preparation Forty nine samples of 20 cc each were sampled along the length of core PI 6 at the LacCore facility at the University of Minnesota. Sample depths for my carbon isotope study were selected using the lithology, magnetic s usceptibility and density re cords of composite core PI 6. Samples were collected at 3 m intervals to obtain a general, albeit relatively low resolution 13 C profile for the core, and then additional samples were taken at transitions between lithologic uni ts (Fig. 2 1 ). I ran preliminary analyses to determine the minimum amount of sediment needed to obt ain sufficient organic carbon. Solvent rinsed metal tools were used for sampling to avoid contami nation with organic materials. To avoid contamination, s edim ent in direct contact with the polycarbonate core liner was not sampled. Solvent rinsed, ashed (450 C for 5 hours) glassware was u sed to store sediment samples. Samples were shipped overnight to the University o f Florida in 20 ml glass scintillation vials Upon arrival, samples were freeze dried, a nd repacked into glass vials in preparation for further analysis. Geochemical a nd Compound Specific Stable Carbon Isotope Analyses Approximately 1 g of freeze dried, ground sediment from each sample was reserved for bulk geochemical measuremen ts. Total carbon (TC) and total nitrogen (TN) were measured on a Carlo Erba N A 1500 CNS elemental analyzer. Total inorganic carbon (TIC) was measured using a UIC (Coulometrics) 5011 CO 2 coulometer coupled with an AutoMate aut omated carbonate preparation device (AutoMateFX.com). Approximately 15 mg of sample was weighed into septum top tubes and placed into the

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15 AutoMate carousel. A double needle assembly was used to purge the sample vial of atmospheric gas, using CO 2 free nitro gen carrier gas. Acid was injected into the sample vial and evolved CO 2 was carried through a silver nitrate scrubber to the coul ometer where TIC was measured. Percent CaCO 3 was calculated by using a standard curve constructed using CO 2 evolution measureme nts from pure Ca carbonate standards. Percent organic carbon (OC) was calculated by subtracting IC from TC. The organic carbon to total nitrogen ratio (C:N) was determined from the calculated OC and measured TN values. For bulk stable organic carbon and ni trogen isotope measurements, a pproximately 200 mg was placed in a container with ~250 ml HCl to remove sediment carbonate. After 48 h the material was processed through a cellulose fiber filter with the aid of a vacuum pump. Remaining sediment was oven dri ed, separated from the filter, and stored in 20 ml glass scintil lation vials. These de carbonated samples were analyzed for bulk 15 N and 13 C on a Thermo Finnigan Delta Plus XL isotope ratio mass spectrometer with a ConFlo III interface linked to a Costec h ECS 4010 Elemental Combustion System (elemental analyzer). Carbon and nitrogen stable isotope results are reported as per mil ( ) in standard delta notation relative to Vienna PeeDee Belemnite (VPDB) and atmospheric N 2 respectively. Precision for 13 C b ulk samples an internal lab standard Methods for compound specific isotopic analyses used in my study follow the protocols described in Newell (2005). Because of the high potential for contamination from organic materials, such as plastics, strict handling and storage guidelines were obse rved. Only solvent rinsed, ashed glassware an d metal instruments were used. All

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16 solvents we re stored in glass containers. Lipids were extracted into a solution of 2:1 dichloromet hane (DCM)/methanol using an Accelerated Solvent Extractor (ASE) Approximately 15 g of a standard (C 34 alkane) was added to each sample prior to extraction to verify n alkane recovery. Lipids were solvent exchanged in to hexane by evaporation under a N 2 s tream in a warm water bath. Silica gel chromatography was then used to separate long chain n alkanes fro m branched and cyclic alkanes. Urea adduction was completed three times for each sample to isolate the n alkanes before injecting into a gas chromatog raph (GC) to check for purity and to determine the di lution amount needed for isotopic analysis. Samples were then transferred to 1 ml glass crimp vials and diluted with hexane to a pre determined volume. Compound specific carbon isotope ratios were measur ed on a Thermo Finnigan MAT Delta Plus XL isotope ratio mass spectrometer with a GC/C III interface linked to an Agilent/HP GC Samples in hexane dilutions of <10 ml were run manually, while those >10 ml were p rocessed using an autosampler. A l aboratory st andard, University of Florida Internal Standard (UFIS ) was calibrated to a set of standard n alkanes, Mix B, with known 13 C values. Duplicate analyses had a standard devi Results are reported per otation relative to Vienna PeeDee Belemnite (VPDB).

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17 Figure 2 1 Lithology, magnetic susceptibility and carbon isotope sample locations by depth (mcd) for PI 6 ( Mueller et al. 2010)

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18 CHAPTER 3 RESULTS C hronol ogy Magnetic susceptibility and gamma ray a ttenuation bulk density were recorded in Guatemala on whole cores using a GEOTEK multi sensor core logger. Cores were shipped to LacCore, University of Minnesota, Minneapolis, and were split into working and archive halves during sampling efforts in summer s 2006 and 2007. Exposed surfaces were scraped clean and photographed with a calibrated GEOTEK high resolution digital color linescan camera. T he chronology of the top 45 m composite depth (mcd) (~38.1 ka) of PI 6 was determined by 44 AMS 14 C dates on terr estrial material that yielded 36 age depth points Hodell developed the chronology in part, from dates on material in cores from sites PI 2 and PI 3, which were projected to stratigraphically equivalent depths in the PI 6 section by correlation of magneti c susceptibility records and utilization of the software program Splicer (Escobar et al. 2012). Lithology Lithology of the PI 6 core was described by M ueller et al. (2010) (Fig. 2). Sediment from the top 10.8 m composite depth (mcd), Unit I, consists of or ganic rich clay silt, with gra y clay and carbonate clay mud. Unit II begins at 10.8 mcd with a sharp transition to gypsum sand that is interbedded with clay m ud, and continues to 21.2 mcd. This unit is assoc iated with the last deglacial. From 21.2 to 25.4 mcd, a unit of homogenous gray clay mud, Unit III, is dated to t he Last Glacial Maximum (LGM). Unit IV/V consists of gypsum sand interbedded with clay mud, and green clay mud ranging from 25.4 to 50.3 mcd. Unit VI is gray clay mud a nd ranges from 50.3 to 5 5 mcd. From 55 to

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19 67.25 mcd, the sediment is composed of organic rich, light and dark carbonate mud, Unit VII. Unit VIII is course carbonate sand ranging from 67.25 to 71.8 mcd and directly overlies bedrock (Table 3 1). Geochemistry Total carbon (TC) val ue s range from 1.3 to 11.5 %. Values <3% occur primarily in the gypsum rich layers of the deglacial, Un it II. Total nitrogen (TN) values were gener ally very low, ranging from 0.0 to 0.45% throughout the core. Inorganic carbon (TIC) val ues ranged from 0.4 to 9 .4 %. The majority of low TIC values, i. e. <1%, also occur in Unit II. Organic carbon (TOC) ra nges from 0.1 to 8.4 %. Values <1 % are found mainly in Unit II. The TOC to TN ratios, reported as C:N in atomic weight, ranged from 5.8 to 187.4 (Table 3 2). The 15 N values range from 1.2 to 10.0 Values in Un it I range from 0.9 to 2.5 Unit II va lues range from 3.0 to 10.0 Unit III resu lts range from 1.2 to 7.6 Unit IV/V values range from 1.2 to 8.0 Bulk 13 C TOC va ries fro m 29.4 to 18.5 Unit I values range from 28.9 to 24.1 Unit II 13 C TOC values are generally higher, with a range of 27.2 to 18.5 Unit III results range from 26.1 to 19.9 Un it IV/V values range from 26.8 to 21.8 3 3). Compound specif ic 13 C values for long chain n alkanes C 29 and C 31 are very similar, whereas C 33 values follow the same tr end, but are generally higher. Results for C 29 range from 35.4 (Un it VIII) to 25.5 C 31 values range from 35.5 (Unit VII) to 27.3 it II), whereas C 33 results range from 34.8 (Unit VII) to 25.8 (Table 3 3)

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20 Table 3 1 Section ID, depths, ages, and units for PI 6 UF ID section ID depth in section (cm ) mcd Peten Itza composite age model v. 1.0 (cal yr BP) Unit 1263 GLAD9 PI06 6B 2H1 90.2 2.925 1292 I 1264 GLAD9 PI06 6B 3H1 81 5.791 3052 I 1265 GLAD9 PI06 6C 3H2 53.8 8.689 4868 I 1266 GLAD9 PI06 6C 4H1 47.1 10.317 8554 I 1267 GLAD9 PI06 6C 4H1 53.6 10.382 8578 I 1268 GLAD9 PI06 6C 4H1 134.1 11.187 10648 II 1269 GLAD9 PI06 6C 4H1 139.1 11.237 10713 II 1270 GLAD9 PI06 6C 4H1 145.6 11.302 10792 II 1277 GLAD9 PI06 6C 4H2 25.8 11.596 11187 II 1278 GLAD9 PI06 6B 5H1 118.7 12.821 12792 II 1279 GLAD9 PI06 6B 5H2 67.2 13.696 13260 II 1280 GLAD9 PI06 6C 5H2 3.8 14.416 14312 II 1281 GLAD9 PI06 6C 5H2 13.8 14.516 14582 II 1282 GLAD9 PI06 6C 5H2 54.3 14.921 14805 II 1289 GLAD9 PI06 6B 6H1 129.3 15.99 15403 II 1290 GLAD9 PI06 6B 6H2 85.5 17.045 16113 II 1291 GLAD9 PI06 6C 6H2 2 17.43 16514 II 1292 GLAD9 PI06 6B 7H1 57.1 18 .27 17100 II 1293 GLAD9 PI06 6B 7H2 43.9 19.651 18053 II 1294 GLAD9 PI06 6C 7H1 143.9 20.351 18326 II 1301 GLAD9 PI06 6C 7H2 56.8 20.93 18520 II 1302 GLAD9 PI06 6B 8H1 104.7 21.747 19141 III 1303 GLAD9 PI06 6A 8H1 68.3 22.381 19749 III 1304 GLAD9 PI0 6 6A 8H1 131.8 23.016 20367 III 1305 GLAD9 PI06 6A 8H2 3.7 23.245 20589 III 1306 GLAD9 PI06 6A 8H2 51.2 23.72 21178 III 1307 GLAD9 PI06 6C 8H2 79 24.375 22927 III 1308 GLAD9 PI06 6B 9H1 134.3 25.087 23718 III 1309 GLAD9 PI06 6A 9H1 151.1 26.12 24355 I V 1310 GLAD9 PI06 6B 10H2 65.7 29.026 27831 IV 1311 GLAD9 PI06 6A 10H2 54.2 29.711 28880 IV 1312 GLAD9 PI06 6B 11H1 136.5 31.312 30820 IV 1314 GLAD9 PI06 6B 11H2 45.2 31.929 31291 IV 1315 GLAD9 PI06 6C 11H2 15.7 32.944 32605 IV 1316 GLAD9 PI06 6B 12H 1 111.7 34.12 34205 IV 1317 GLAD9 PI06 6A 12H1 99 34.838 35302 IV 1318 GLAD9 PI06 6A 13H1 85 37.729 37382 IV

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21 Table 3 1. Continued UF ID section ID depth in section ( cm ) mcd Peten Itza composite age model v. 1.0 (cal yr BP) Unit 1319 GLAD9 PI06 6B 14H1 145.1 40.622 39757 V 1320 GLAD9 PI06 6A 15H1 63 43.522 41435 V 1321 GLAD9 PI06 6B 16H1 113 46.426 nd V 1322 GLAD9 PI06 6B 17H1 114.9 49.345 nd V 1323 GLAD9 PI06 6B 18E1 122.1 52.341 nd VI 1324 GLAD9 PI06 6B 19E1 98.4 55.061 nd VI 1325 GLAD9 PI06 6C 2 0X1 69.5 57.739 nd VII 1326 GLAD9 PI06 6C 21X1 120.7 60.583 nd VII 1327 GLAD9 PI06 6C 22X2 83.9 63.263 nd VII 1328 GLAD9 PI06 6C 23X2 50.8 66.02 nd VII 1329 GLAD9 PI06 6A 23E2 142.2 68.903 nd VIII 1330 GLAD9 PI06 6A 24E2 132.5 71.825 nd VIII nd no d ata was collected Table 3 2. Bulk geochemical results for PI 6 UF ID %C (total) measured %N (total) measured atomic C org :N %C org (total C Inorganic C) calculated %CaCO 3 measured %Inorganic C (CaCO3/ 8.333) calculated 1263 5.2 0.12 37.1 1.9 27.6 3.3 126 4 5.3 0.17 26.7 2.2 25.8 3.1 1265 8.7 0.25 30.0 4.1 38.9 4.7 1266 11.5 0.45 22.0 8.4 26.4 3.2 1267 11.4 0.40 24.3 7.8 30.0 3.6 1268 2.2 0.03 65.3 1.1 9.1 1.1 1269 3.2 0.03 90.0 1.5 13.5 1.6 1270 1.3 0.02 54.4 0.6 5.9 0.7 1277 1.3 0.09 12.8 0.6 5.8 0 .7 1278 1.5 0.02 63.9 0.6 7.8 0.9 1279 6.23 0.11 48.7 2.1 35.0 4.2 1280 8.7 0.21 35.5 3.9 40.3 4.8 1281 8.0 0.18 38.1 2.9 42.4 5.1 1282 2.6 0.06 36.4 1.9 5.7 0.7 1289 1.3 0.07 16.2 1.0 3.0 0.4 1290 1.5 0.03 44.0 0.7 6.7 0.8 1291 1.3 0.06 18.7 0.8 4 .1 0.5

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22 Table 3 2. Continued UF ID %C (total) measured %N (total) measured atomic C org :N %C org (total C Inorganic C) calculated %CaCO 3 measured %Inorganic C (CaCO3/ 8.333) calculated 1292 1.4 0.02 60.9 0.78 5. 4 0.7 1293 1.8 0.02 76.7 0.6 9.6 1.2 1294 1.4 0.20 5.8 0.7 5.3 0.6 1301 3.1 0.03 87.4 0.3 23.2 2.7 1302 6.0 0.25 20.6 2.6 28.1 3.4 1303 5.7 0.26 18.9 2.9 24.0 2.9 1304 5.3 0.22 20.8 2.2 26.2 3.1 1305 7.4 0.34 18.8 4.1 28.2 3.4 1306 5.6 0.27 17.7 2.9 22.7 2.7 1307 5.8 0.32 15.5 3.4 20.0 2.4 1308 2.8 0.08 30. 1 1.7 9.2 1.1 1309 8.0 0.34 20.0 4.8 25.9 3.1 1310 4.1 0.17 20.8 2.4 14.4 1.7 1311 3.9 0.08 41.4 nd 37.9 4.6 1312 1.3 0.00 nd 0.1 9.9 1.2 1314 6.3 0.12 45.1 1.7 38.8 4.7 1315 3.6 0.09 30.0 1.4 14.5 1.8 1316 1.5 0.03 42.0 0.8 5.5 0 .7 1317 9.3 0.34 23.6 4.3 41.7 5.0 1318 2.9 0.10 24.9 1.4 12.5 1.5 1319 9.8 0.24 34.9 3.3 54.1 6.5 1320 4.3 0.09 41.1 1.0 27.9 3.4 1321 8.2 0.28 23.0 3.9 35.7 4.3 1322 1.4 0.03 41.1 0.7 5.9 0.7 1323 6.7 0.17 33.7 1.1 46.3 5.6 1324 4.9 0.23 18.4 1.3 30.1 3.6 1325 6.8 0.19 30.5 1.2 46.1 5.5 1326 7.8 0.28 23.9 3.9 32.8 3.9 1327 7.9 0.31 21.7 4.4 29.2 3.5 1328 7.2 0.16 38.6 1.3 48.9 5.9 1329 6.2 0.25 21.3 2.6 30.0 3.6 1330 10.9 0.05 187. 4 1.5 78.7 9.4 nd no data w ere collected

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23 Table 3 3. Bulk and compound specific carbon results UF ID 15 N air) 13 C TOC VPDB) 13 C 29 n VPDB) 13 C 31 n VPDB) 13 C 33 n VPDB) Predom inant chain length a CPI b ACL c OEP d 1263 3.7 25.7 nd nd nd nd nd nd nd 1264 2.2 24.1 30.1 29. 1 26.6 31 2.51 26.79 4.13 1265 0. 9 27.8 32.5 33.9 32.4 29 2.59 28.62 4.71 1266 1.4 28.9 32.0 33.5 33.6 29 4.97 29.19 9.34 1267 2.5 28.8 30.7 33.2 34.2 29 4.10 27.29 7.16 1268 8.7 18.5 30.4 29.7 27.0 29 1.40 26.58 2.05 1269 7.4 18.5 27.9 28.9 26.7 29 2.07 28.41 3.36 1270 8.2 18.5 30.6 30.9 29.3 23 3.52 25.27 2.50 1277 10.0 19.3 29.6 29.1 27.4 29 3.73 30.79 7.19 1278 7.6 19.8 29.7 28.9 25.8 31 3.66 31.74 8.12 1279 7.3 26.8 32.4 33.8 29.9 29 8.88 28.92 18.46 1280 5.6 26.7 32.0 33.9 30.9 31 8.69 29.37 18.19 1281 6.4 27.2 32. 2 34.2 31.3 31 12.25 28.97 30.38 1282 3.0 21.0 30.7 32.9 29.6 31 7.63 28.69 15.91 1289 3.6 19.1 25.5 30.0 28.0 33 7.03 30.16 13.57 1290 8.3 19.8 26.1 27.3 26.2 31 8.70 29. 66 15.16 1291 7.6 21.4 25.6 28.1 27.5 nd 6.58 30.22 12.11 1292 6.3 20.3 29.3 31.9 31.0 33 8.87 29.22 21.79 1293 6.6 22.5 29.5 31.3 29.7 31 7.27 30.43 14.01 1294 7.3 21.6 27.8 30.2 28.5 31 7.06 30.22 14.04 1301 5.5 23.3 31.0 31.2 28 .1 31 9.32 29.52 17.50 1302 5.0 26.1 32.1 32.2 30.6 31 8.50 30.19 17.78 1303 1.9 25.3 31.0 31.9 29.8 29 5.89 28.52 11.53 1304 1.2 26.0 30.9 31.5 29.5 31 5.45 28.98 10.94 1305 0.7 25.8 30.9 30.9 29.6 31 5.72 28.62 11.34 1306 3.4 25.8 32.6 33.1 30.5 31 8.18 30.13 18.35 1307 2.5 25.7 30.9 31.9 30.3 31 6.34 29.34 13.13 1308 7.6 19.9 nd nd nd 31 nd nd nd 1309 2.7 25.7 31.3 31.9 29.2 31 6.32 27.93 12.40 1310 7.7 21.9 33.2 33.2 30.4 31 9.75 29.95 22.36 1311 5.5 23.7 32 .5 33.4 31.3 31 9.85 30.50 22.36 1312 6.0 21.8 32.4 32.6 30.0 31 8.00 30.30 16.57 1314 5.5 24.2 33.1 33.5 32.1 31 9.79 29.78 20.99 1315 8. 0 24.1 32.3 32.4 29.3 31 9.20 30.12 20.15 1316 7.6 24.4 33.3 33.6 32.0 31 10.46 30.30 23.32 131 7 2.8 25.3 32.5 32.6 28.5 31 6.66 28.57 14.55

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24 Table 3 3. Continued UF ID 15 N air) 13 C TOC B) 13 C 29 n VPDB) 13 C 31 n VPDB) 13 C 33 n VPDB) Predom inant chain length a CPI b ACL c OEP d 1318 nd 23.7 32.3 32.6 29.6 31 7.84 29.99 18.76 1319 4.5 25.4 32.1 32.1 31.0 31 9.05 29.80 21.34 1320 7.8 nd 29.7 30.4 25.9 31 9.07 29.45 20.55 1321 1.2 26.8 32.3 33.3 32.2 31 6.74 27.98 16.64 1322 6.8 23.2 32.7 30.9 31.8 31 10.35 29.74 21.26 1323 2.9 23.2 30.4 29.2 26.8 31 6.27 29.78 14.11 1324 4.4 2 2.8 30.4 28.9 26.0 31 6.16 29.73 13.30 1325 3.1 27.2 34.9 34.0 32.7 31 7.92 30.47 17.42 1326 1.6 29.4 33.6 33.9 32.9 31 6.62 27.46 14.85 1327 1.6 29.2 33.7 34.3 33.8 31 nd nd nd 1328 2.0 28.2 34.7 35.5 34.8 31 7.00 29.47 16.69 1329 2.9 29.4 35.4 35.4 33.9 3 1 6.54 27.29 15.02 1330 0.9 27.6 34.2 35.1 34.5 31 12.86 30.06 27.18 nd no data w ere collected a Carbon number of the homologue with highest abundance b CPI ( X i + X i +2 +..+ X n )/ ( X i 1 + X i +1 +..+ X n 1 )+0.5* ( X i + X i +2 +..+ X n X i +1 + X i +3 +..+ X n +1 ), with i =25, n =33 c Average chain length (i*X i )/ X i where X is abundance and i ranges from 25 to 35 d. OEP= [C 27 +6(C 29 +C 31 )]/(4xC 28 +4xC 30 )

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25 CHAPTER 4 DISCUSSION Carbon Isotopes Upcore by Unit The 13 C TOC record closely tracks shifts in the magnetic suscept ibility and 13 C ostracod records, variables that are linked to climate conditions (Hodell et al. 20 08; Escobar et al. 2012) (Fig. 4 1 ; Fig. 4 2 ). This strongly suggests that stratigraphic changes in 13 C TOC record the shifting relative abundance of the two main vegetation types (C 3 /C 4 ) and are associated with wet and dry climate periods reflected by clay and gy psum deposition, respectively. The 13 C values from long chain n alkanes generally track the 13 C TOC record closely, with a few exceptions. Unit s VI II, VII, and VI Eight samples were taken from the bottom ~20 m of composite core P I 6 (Units VIII, VII, and VI). Unit VIII consists of course carbonate sand and has relatively low TOC, low %N and high %CaCO 3 values. The 13 C TOC values average 13 C n alk range from 35.5 to Unit VII, an organic rich carbonate mud, has an average TOC of 7% and 13 C TOC and 13 C n alk remain generally unchanged, suggesting little change in C 3 /C 4 compositi on between Units VIII and VII. Sediment from 55 to 50 mcd ( Unit VI) consists of a gray, clay mud with lower C:N values, indicative of a higher proportion of algal material. The 13 C TOC and 13 C n alk indicating an increase in the proportion of C 4 plants in the region. Pollen data also indica te expansion of grasses at the expense of forest, with a core high of 45% Poaceae (grasses) at the transi tion from Unit VII to Unit VI. Carbon isotope data for ostracods are not yet available beyond 43 ka.

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26 Unit s V and IV Units V and IV date from ~43 to ~23 ka and consist of alternating bands of laminated clay mud and gypsum rich sand. The 13 samples collected from these units for carbon isotope analysis show 13 C TOC and 13 C n alk values vary little (1 averagin g The observed variation is associated with changes in core lithology, but there does not seem to be a correlation between high isotope values (i.e. high C 4 input) or hig h percentage Poaceae in t he gypsum layers, as expected. It is likely that the low numb er of isotope samples in these u nits preclude identification of short term climate shifts during a perio d of high climate variability. A higher resolution record of clima te events is seen in the oxygen and carbon isotopes from ostracods, permitting identification of Heinrich Events 4, 3 and 2 (Escobar et al. 2012). The relatively high ostracod 18 O values probably reflect both colder temperatures and low lake water levels, i.e. greater evaporation/precipitation ratios (Escobar et al. 2012; Hodell et al. 2012). Pre LGM Chronozone Just prior to the onset of the LGM chronozone at 24 ka, 13 C TOC val ues record a 4 p lants, and hence aridity (Fig. 4 4 ). Comparison with 13 C n alk values is not possible because there was insufficient organic carbon for n alkane isotope measurement. This t ime period coincides with Heinrich Stadial 2 (Hemming 2004), when gypsum deposition increased in Lake Petn Itz (Mueller et al. 2010; Hodell et a l. 2012; Escobar et al. 2012). My isotope based inference for grass (C 4 ) expansion is supported by a >10% incr ease in Poaceae pollen, from 5.9 to 16.5% (Correa Metrio et al. 2012).

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27 Unit III: LGM Chronozone Unit III, representing the LGM chronozone, is dated from ~24 to ~19 ka and records moderate 13 C values averaging 31.9%, and 13 C TOC 13 C 29 13 C 31, and 13 C 33 respectively. The 13 C TOC values chan ge throughout The 13 C n alk Previous polle n research on a core from nearby Lake Quexil (Leyden 1984; Leyden et al. 1993, 1994) suggested that the LGM environment was exceedingly dry and cold, however dating of the section was compromised by a lack of terrestrial organic matter and relied on a sing le measure on a gastropod shell. Correlation of the well dated pollen record from Lake Petn Itz with the Quexil record shows that the purported LGM deposits of the Quexil section are in fact of deglacial age. Lithologic (Hodell et al. 2008), palynologica l (Bush et al. 2009; Correa Metrio et al. 2012), and ostracod isotope data (Escobar et al. 2012) from Petn Itz core PI 6 indicate cold, but relatively wet conditions during the LGM. My 13 C TOC and 13 C n alk values support this interpretation for a relati vely moist LGM, as the isotope values are lower compared to during lat e MIS3 and the deglacial (Fig. 4 3 ). Unit II: Deglacial Unit II consists of alternating layers of gypsum sand and laminated c lays similar to Units V and IV. The 13 C TOC record shows a >7 26 to This large isotopic shift is indicative of increased aridity associated with Heinrich Stadial 1 (HS1) (Fig. 4 6 ). Compound specific 13 C values also show large positive excursions, with 13 C 29 13 C 31 and 13 C 33 respectively. The climate shift to colder and/or drier conditions at ~18 ka, identified by Escobar et al. (2012) using oxygen

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28 isotope measures in ostracods, and cold conditions documented by simultaneous measures of gypsum hydration water an d oxygen isotopes in gastropods and ostracods (Hodell et al. 2012), is also detected in my 13 C records, confirming a vegetation response to climate change at this time. The transition from HS1 to the Bolling Allerod at 15 ka is marked by a negative shift in 13 C values in the isotope records. A return to pre Bolling Allerod 13 C values occurs at the onset of the Younger Dryas at 13 ka. These large carbon isotope shifts, reflecting changes in vegetation type, are also detected in the pollen record, as chang es between mesic/dry forests and dry scrub and xeric shrublands (Correa Metrio et al. 2012). Deglacial/Holocene Transition The transition from the Pleistocene to the Holocene at 10.5 yr BP shows the largest excursion in the 13 C TOC record, a shift from 18 to The latter value is the lowest 13 C TOC measure recorded in core PI 6 and indicates an unprecedented increase in C 3 vegetation, and hence warmer and wetter conditions. This transition also marks the timing of the most extreme change in vegetation rec orded in the pollen record. Percent Poaceae falls from ~20% to ~3.5% (Correa Metrio et al. 2012). C:N values fall from 65.3 to 24.3, but still indicate a largely terrestrial organic matter signal. Unit I: Holocene Early Holocene (~8,550 yr BP) low 1 3 C TOC values reflect relatively warm, moist conditions, with abundant C 3 plants. This finding is consistent with previous research that indicated increased precipitation and high water levels in Lake Petn Itz (Curtis et al. 1998; Hillesheim et al. 2005). This period was characterized by enhanced seasonality in the northern hemisphere tropics and probably by extreme northward

PAGE 29

29 migration of the ITCZ during the summer rainy season (Hodell et al. 2001, 2005 ). By the middle Holocene, however, there is evidence for climate drying around Petn Itz, as well as from sites in northern South America (Haug et al. 2001) and northern hemisphere Africa (Mueller et al. 2009). Higher carbon isotope measures support this inference and indicate greater abundance of C 4 plan ts Comparison of Compound Specific a nd Bulk Carbon Isotope Values Although 13 C values for carbon chain lengths ranging from C 14 to C 35 were obtained from GC IRMS measurements, the three long chain n alkanes selected for analysis were C 29 C 31 and C 33 because they are found almost exclusively in terrestrial plants (Rieley et al 1991; Collister et al.1994). With few exceptions, the C 29 C 31 and C 33 13 C values trend very well together (Fig.4 5 ). The C 29 and C 31 13 C values agree within 1 33 13 C values. This may indicate another possible source to the C 33 record, such as certain algal spe cies (Lichtfouse et al.1994a). It may also simply be a consequence of higher variability in the C 33 record. This pattern is observed in other isotope research where C 29 and C 31 are generally the most depleted va lues of all long chain carbons whereas C 33 and C 35 are isotopically enriched relative to C 29 and C 31 (Schefuss et al. 2003). Preservation of n alkanes can be determined by c alculating carbon preference index (CPI) values. High CPI values indicate living plant matter, whereas lower values signify diagenetically degraded organic matter (Hedges and Prahl 1993; Collister et al. 1994). Leaf waxes have CPI values >5 (Eglinton and H a milton 1967 ). CPI values in PI 6 are >5 from the base of the core to 12.82 mcd/12, 79 0 yr BP during the Younger Dryas where the value drops to 3.66 (Fig. 4 8 ). Values continued to decrease throughout the

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30 remainder of the deglacial, indicative of an increa se in the abundance of mature, detrital input to the organic matter source, reaching a minimum of 1.40 just pri or to the Holocene transition. CPI can be used if the predominant source of org anic matter can be determined. In the Petn Itz core, the predomi nant n alkanes are long chain and come from lan d plants (Ficken et al. 2000). Gas chromatographs for the PI 6 core identified the dominant n alkane as C 31 for the bottom portion of the core, up to the late LGM at 22.38 mcd (19.75 yr BP), after which the C 2 9 chain length dominated (Fig. 4 8 ). The dominant n alkane returned to C 31 and varied between C 29 and C 33 t hroughout the early deglacial. These values are all well within the range of vascular land plants. The only deviation from these values occurs at ~10 ,800 yr BP, where a sharp decrease to C 23 is observed, indicating an increase in algae and macrophytes to the source material. This finding is in agreement with C:N values, which fall to ~12.8, indicative of a larger aquatic plant contribution to the orga nic matter pool (Fig. 4 7 ). These findings confirm that the 13 C bulk values reflect dominance of terrestrial vegetation (Chikaraishi and Nar aoka 2003; Ficken et al. 2000). The Holocene transition at ~10.33 mcd shows an unusual discrepancy in the three com pound specific 13 C values. A large shift in 13 C values from the gyps um sand in the late deglacial, indicating dry conditions, to the more organic deposits of the warmer wetter early Holocene, was expected. The C 33 record does reflect that with a negative 13 C value. The C 31 29 record shows no significant change in values. Throughout the length of core PI 6, compound specific and bulk 13 C records track well, with C 29 and C 31 generally 1 3 3 Large shifts in the

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31 deglacial correspond with more positive 13 C values for both compound specific and bulk 13 C records, reaching maximum values during arid periods when gypsum sand was deposited and there was maximum relative abundance of C 4 plants. U nit III, the LGM chronozone, displays relatively low variation in both the compound specific and bulk 13 C records, consistent with the fairly homogenous nature of the gray clay mud deposited at that time. Because the compound specific and bulk 13 C record s from Petn Itz yielded similar inferences about past vegetation, the additional effort and cost required to prepare samples for compound 13 C analysis may not be justified. This is probably the case for the Petn Itz core because sedimented or ganic carbon is overwhelmingly from terrestrial sources. In other situations, compound specific 13 C analyses may be necessary to di scern past vegetation changes. Initial studies, looking at the relative abundances of n alkane chain lengths provide insigh ts into whether such detailed isotope measures are necessary

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32 Figure 4 1 Composite core PI 6 magnetic susceptibility (red line) and 13 C TOC (green line with triangles) by m eter c omposite d epth (mcd ) show tightly coupled variation indicating an agreement between lithology and 13 C TOC values. Figure 4 2 Values of 13 C TOC (red line with triangles ) and 13 C ostracod (blue line ) through the deglacial are likely representing the same carbon source.

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33 Figure 4 3. PI 6 13 C TOC (green line with triangles) and 13 C 33 (orange line with circles) during the deglacial show general agreement indicating bulk and compound specific measurements show similar trends. Figure 4 4. Percent Poaceae (green line with circles at left) percent total organic carbon ( red line wit h triangles in center, note scale direction), and 13 C TOC (blue line with circles at right) by meter composite depth ( mcd) show agreement throughout length of core indicating positive relationship between amount of grass, amount of carbon and relative percentage of C4 plants in environment.

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34 Figure 4 5. Bulk and compound specific c arbon isotopes : 13 C TOC (green line with circles at left) 13 C 29 blue line with squares second from left) 13 C 31 red line with diamonds second from right) and 13 C 33 (purple line with triangles at right) by m eter c omposite d ep th (mcd ) values show good fit indicating possible use of 13 C TOC to demonstrate 13 C n alk trend. Figure 4 6. Bulk and compound specific carbon isotopes show largest shifts at climatic boundaries Values shown from 0 43 ka BP ; climatic events in grey bars : A=Holocene, B=Younger Dryas & Preboreal C=Bolling Allerod, D=Heinrich Stadial 1, E=LGM, F=Heinrich Stadial 2, G= MIS3

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35 Figure 4 7 This scatter plot delineates the source of organic matter using 13 C TO C and C:N (carbon to nitrogen ratio) The resulting plot shows a dominance of C 3 plants in the source material. Figure 4 8. The d ominant n alkane values and carbon preference index (C PI ) by meter composite depth (mcd) for PI 6 show largest variations during the deglacial just prior to Holocene transition

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36 CHAPTER 5 CONCLUSION Forty nine subsamples from a 85 k a sediment core were analyzed for carbon isotopes in bulk carbon and compound specific long chain n alkane s Changes in 13 C are closely associated with shifts in other climate proxy data, including magnetic susceptibility, lithology, pollen, and oxygen and carbo n isotopes in ostracod shells. The 13 C values in bulk carbon and long chain n alkanes serve as proxies for shift s in the relative abundance of C 3 and C 4 plants, which are indicative of past changes in precipitation. Periods of gypsum deposition in the lake, associated with arid conditions, correspond with higher 13 C values, the latter indicating an increase in C 4 p lants, i.e. grasses. The high degree of variation in the 13 C bulk record during Heinrich Stadial I is a result of extremes in E/P and other regional climate events, such as the Bolling Allerod, Younger Dryas (Hodell et a l. 2012; Escobar et al. 2012). Thes e results show that changes in vegetation type in the lowland Neotropics were probably affected by large shifts in both precipitation and temperature during the last glacial cycle. Because 13 C results from bulk organic carbon and specific n alkane compoun ds show similar trends and the bulk organic carbon source has been identified through C:N as predominantly allochthonous in Petn Itz sediments, 13 C TOC values are sufficient to infer climate driven shifts in vegetation type

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37 LIST OF REFERENCES Bender M M (1971) Variations in the 13C/12C ratios of plants in relation to the pathway of photosynthetic carbon dioxide fixation. Phytochemistry 10:1239 1244 Bush MB, Correa Metrio AY, Hodell DA, Brenner M, Anselmetti FS, Ariztegui D, Mueller A Curtis JH, Grzesik DA, Burton C, Gilli A (2009) Re evaluation of climate change in lowland Central America during the Last Glacial Maximum using new sediment cores from Lake Petn Itz, Guatemala. In: Vimeux F, Sylvestre F, Khodri M (eds) Past Climate Variability in South America And Surrounding Regions: From the Last Glacial Maximum to the Holocene. Developments in Paleoenvironmental Research 14, Springer, New York, pp 113 128 Castaneda IS, Werne JP, Johnson TC (2007) Wet and arid phases in the southeast African tropics si nce the Last Glacial Maximum. Geology 35:823 826 Cerling TE, Wang Y, Quade J (1993) Expansion of C4 ecosystems as an indicator of global ecological change in the late Miocene. Nature 361:344 345 Chikaraishi Y, Naraoka H (2003) Compound specific delta D d elta C 13 analyses of n alkanes extracted from terrestrial and aquatic plants. Phytochemistry 63:361 371 Collister JW, Rieley G, Stern B, Eglinton G, Fry B (1994) Compound specific (delta C 13) analyses of leaf lipids from differing carbon dioxide metaboli sms. Org Geochem 21:619 627 Correa Metrio A, Bush MB, Hodell DA, Brenner M, Escobar J, Guilderson T (2012) The influence of abrupt climate change on the ice age vegetation of the Central American lowlands. J Biogeogr 39:497 509 Eglinton G, Hamilton RJ (196 7) Leaf epicuticular waxes. Science 156:1322 1335 Escobar J, Hodell DA, Brenner M, Curtis JH, Gilli A, Mueller AD, Anselmetti FS, Ariztegui D, Grzesik DA, Perez L, Schwalb A, Guilderson TP (2012) Quat Sci Rev:IN PRESS ) On the relationship between carbon isotope discrimination and the intercellular carbon dioxide concentration in leaves. Aust J Plant Physiol 9:121 137 Ficken KJ, Li B, Sain DL, Eglinton G (2000) An n alkane proxy for the sedimentary input of submerged/fl oating freshwater aquatic macrophytes. Org Geochem 31:745 749 Gu B, Schelske CL, Brenner M (1996) Relationship between sediment and plankton isotope ratios (del13C and del15N) and primary productivity in Florida lakes. Can J Fish Aquat Sci 53:875 883

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38 Hemmi ng SR (2004) Heinrich events: massive late Pleistocene detritus layers of the North Atlantic and their global climate imprint. Rev Geophys 42:8755 Hillesheim MB, Hodell DA, Leyden BW, Brenner M, Curtis JH, Anselmetti FS, Ariztegui D, Buck DG, Guilderson TP Rosenmeier MF, Schnurrenberger DW (2005) Climate change in lowland Central America during the late Deglacial and early Holocene. J Quat Sci 20:363 376 Hodell DA, Brenner M, Curtis JH, Guilderson T (2001) Solar forcing of drought frequency in t he Maya Low lands. Science 292:1367 1370 Hodell DA, Brenner M, Curtis JH, Medina Gonzlez R, Ildefonso Chan Can E, Albornaz Pat A, Guilderson TP (2005) Climate change on the Yucatan Peninsula during the Littl e Ice Age. Quat Res 63:109 121 Hodell DA, Anselmetti FS, Ariztegui D, Brenner M, Curtis JH, Gilli A, Grzesik DA, Guilderson TJ, Mller AD, Bush MB, Correa Metrio A, Escobar J, Kutterolf S (2008) An 85 ka record of climate change in lowland Central America. Quat Sci Rev 27:1152 1165 Hode ll DA, Turchyn AV, Wiseman CJ, Escobar J, Curtis JH, Brenner M, Gilli A, Mueller AD, Anselmetti F, Ariztegui D, Brown ET (2012) Late Glacial temperature and precipitation changes in the lowland Neotropics by tandem measurement of delta O 18 in biogenic car bonate and gypsum hydration water. Geochim Cosmochim Acta 77:352 368 Huang Y, Street Perrott FA, Metcalfe SE, Brenner M, Moreland M, Freeman KH (2001) Climate change as the dominant control on glacial interglacial variations in C 3 and C 4 plant abundance Science 293:1647 1651 Hughen KA, Eglinton TI, Xu L, Makou M (2004) Abrupt tropical vegetation response to rapid climate changes. Science 304:1955 1959 Leyden BW (1984) Guatemalan forest synthesis after Pleistocene aridity.Proceedings of the National Acad emy of Sciences of the United States of America. Vol. 81:4856 4859 Leyden BW, Brenner M, Hodell DA, Curtis JH (1993) Late Pleistocene climate in the Central American lowlands. In: Climate change in continental isotopic records. Geophysical Monograph 78, Wa shington, D.C., American Geophysical Union, pp165 17 8 Leyden BW, Brenner M, Hodell DA, Curtis JH (1994) Orbital and internal forcing of climate on the Yucatan Peninsula for the past ca. 36 ka. Paleogeogr Paleoclimatol Paleoecol 109:193 210

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40 BIOGRAPHICAL SKETCH Jenn ifer Lyn Mays was born in 1971 in Joplin, Missouri She found early inspiration for natural science and geology through her fathe r, a math and science teacher. Jennifer grew up in several small towns in Kansas surrounded by her animals and rock collections She fulfilled her childhood dream of becoming a Jayhawk after enroll ing at the University of Kansas in 1998. She greatly enjoyed a year abroad studying archaeology a nd geology at the University of Leicester, England and returned in 2000 to graduat e from the University of Kansas with a B achelor of A rts in a nthropology. After spending several years as a field archaeologist in New Mexico and several more in Washington, D istrict of C olumbia at an environmental non profit, she returned to graduate school to w ork with David Hodell Mark Brenner and Jason Curtis at the University of Florida There, Jennifer welcomed the opportunity to work on an interdisciplinary research project that incorporated several of her interests in paleoclimat ology, geology, and archa eology. She will graduate with her M aster of S cience in g eo logy in 2012 and has accepted a position as Program Specia list with the United States C limate Variability and Predictability Research Program ( U.S. CLIVAR). S he is looking forward to a career that will enable her to continue to contribute to the field of climate science