LIMNOLOGY AND PALEOLIMNOL OGY OF HYPERSALINE LAGO ENRIQUILLO, DOMINICAN REPUBLIC By DAVID GRAY BUCK 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 2004
Copyright 2004 by David Gray Buck
iii ACKNOWLEDGMENTS This project would not have been possibl e without the support of the School of Natural Resources and Environment and Dr. Stephen R. Humphrey. In addition, the NSF-IGERT Working Forests in the Tropics program provi ded funding during the final semester of this project. My thesis work was carried out under the guidance of Dr. Mark Brenner. His never-ending pa tience, critical thinking, and encouragement were essential to this projectÂ’s completion. In addition, I would like to thank Dr . Michael W. Binford and Dr. Douglas S. Jones for serving on my committee. Dr. David A. Hodell provided guidance and insight throughout th e project. Dr. Jason H. Cu rtis provided instruction on lab techniques for the analysis of stable isotopes and sediment geochemistry and also assisted with data collection in the field. Dr . Carlos A. Alvarez Za rikian assisted with initial ostracod and foraminifer identificati on and Dr. Fred G. Thompson assisted with gastropod identification. Dr. Ma rk Pagani and Dr. Jonathan B. Martin assisted with hydrogen isotope and basic water chemistry an alysis, respectively. Dr. William M. Last provided mineralogy data. Research f unding was provided by a grant from the University of Florida College of Liberal Ar ts and Sciences to Dr. Mark Brenner in addition to student research grants from the Geological Soci ety of America and the Gulf Coast Association of Geological Societies. I am grateful for having been able to use the facilities of the Florida Institute of Paleoenvironmental Research in UFÂ’s Land Use and Environmental Change Institute.
iv Work in the Dominican Republic was fac ilitated by Dr. Andreas Schubert and the staff of the Programa Medioambiental Transf ronterizo. Dr. Ramon Sanchez, Franklin Reynoso, and Matilde Mota of the Subsecretar ia de Areas Protegidas y Biodiversidad granted permission for fieldwork. Domingo Morillo and Juan SaldaÃ±a of the Instituto Nacional de Recursos Hidrolicos (INDRHI) kindl y shared data on rainfall and lake level fluctuations. I am also grateful for th e help of HermÃ³genes Mendez, Angel Ferrera Benitez, Lidio MÃ©ndez Benitez, Jorge Trinidad Ferreras, and Dolores MÃ©ndez MÃ©ndez and the guards of Lago Enriquillo National Park. Finally, I am indebted to my parents for their consistent sup port and encouragement throughout my life, both academic and extracurr icular. And ultimately, the completion of this project would not have been possible wi thout the constant emotional, physical and mental support of my loving wife, Ellie Harrison-Buck.
v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iii LIST OF TABLES............................................................................................................vii LIST OF FIGURES.........................................................................................................viii ABSTRACT....................................................................................................................... ix CHAPTER 1 INTRODUCTION........................................................................................................1 Site Description............................................................................................................3 Previous Studies in Lago Enriquillo.............................................................................6 Origins of the Enriquillo Basin..............................................................................6 Holocene History of Enriquillo Valley..................................................................7 Limnological and Environmental Studies of Lago Enriquillo..............................8 Recent Developments in Lago Enriquillo...................................................................13 2 PHYSICAL AND CHEMICAL PROPERTIES OF HYPERSALINE LAGO ENRIQUILLO, DOMINICAN REPUBLIC..............................................................15 Introduction.................................................................................................................15 Study Site....................................................................................................................1 5 Methods......................................................................................................................17 Results and Discussion...............................................................................................18 Temperature and pH............................................................................................18 Salinity and Major Ion Concentrations................................................................18 Stable Isotopes.....................................................................................................20 Microbenthos.......................................................................................................22 Conclusions.................................................................................................................22 3 A LATE HOLOCENE RECORD OF CLIMATE VARIABILITY FROM HYPERSALINE LAGO ENRIQUI LLO, DOMINICAN REPUBLIC......................25 Introduction.................................................................................................................25 Materials and Methods...............................................................................................30 Results........................................................................................................................ .34
vi Chronology..........................................................................................................34 Lithoand Biostratigraphy..................................................................................35 GRA Bulk Density and Magnetic Susceptibility.................................................40 Stable isotopes.....................................................................................................40 Discussion...................................................................................................................44 Interpretation of Sediment Proxies......................................................................44 Changing Environments of the Lago Enriquillo Basin.......................................47 Open marine to brackish wate r environment (~400 BC Â– 170 AD)............47 High ecosystem variability (170 Â– 1000 AD)..............................................48 Freshening of Lago Enriquillo (1000 Â– 1350 AD).......................................49 Lago Enriquillo variability (1350 Â– present)................................................50 Regional and Global Climate Comparisons...............................................................50 Mechanisms for GRA Bulk Density a nd Magnetic Susceptibility Variability...51 Comparison to the Yucatan Peninsula.................................................................56 Regional Climate Linkages Â– Enriquillo and the Cariaco Basin.........................57 Extra Tropical Teleconnections Â– GISP2 Ice Core.............................................57 Mechanisms for Late Holocene Climate Variability...........................................59 Conclusions.................................................................................................................62 4 CONCLUSIONS........................................................................................................63 LIST OF REFERENCES...................................................................................................66 BIOGRAPHICAL SKETCH.............................................................................................74
vii LIST OF TABLES Table page 1.1 Holocene sea-level changes in the Enriquillo Valley.................................................8 2.1 Salinity, major ion concentrations and stable isotope ratios of Lago Enriquillo waters, spring waters entering the lake, and Caribbean Sea waters.........................20 3.1 Individual cores retrieved from coring location ENR 13-VI-01 used to develop the composite core. .................................................................................................31 3.2 Accelerator mass spectrometry radi ocarbon dates for samples from Lago Enriquillo, Dominican Republic..............................................................................31
viii LIST OF FIGURES Figure page 1.1 Map showing the location of HispaÃ±ol a within the Greater Antilles of the Caribbean Basin.........................................................................................................2 1.2 Bathymetric map of Lago Enriquillo show ing the two distinct basins at 2-m depth contours............................................................................................................4 1.3 Monthly rainfall and evaporation data fr om pluvimetric station Puerto Escondido located within the Lago Enriquillo watershed............................................................5 1.4 Elevation of Lago Enriquillo in meters below sea level from1950 to 2003............11 2.1 Map of Lago Enriquillo on the Cari bbean island of HispaÃ±ola. ............................16 2.2 Water column profiles from the north and south basin of Lago Enriquillo.............19 2.3 Changes in salinity and stable isotopes ( 18O, D) for Lago Enriquillo waters......21 2.4 Elevation of Lago Enriquillo (mBSL) from 1950 to 1963, and timing of hurricanes (H.) and tropical storms (T.S.) passing ne ar the Enriquillo basin..........23 3.1 Map of Lago Enriquillo including bathymetry........................................................26 3.2 Age-depth relationship for core ENR 13-VI-01.......................................................35 3.3 Sediment geochemistry for core ENR 13-VI-01......................................................38 3.4 Biostratigraphy in core ENR 13-VI-01....................................................................39 3.5 GEOTEK multisensor core logger data of lithologic variables for core ENR 13-VI-01..........................................................................................................42 3.6 Stable isotope ( 18O) stratigraphy from core ENR 13-VI-01...................................43 3.7 Comparison between sediment geoc hemistry and lithologic variables...................52 3.8 Changes in lithologic variables versus bulk mineralogy..........................................55 3.9 Regional comparisons of late Holocene climate variability.....................................58
ix Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science LIMNOLOGY AND PALEOLIM NOLOGY OF HYPERSALINE LAGO ENRIQUILLO, DOMINICAN REPUBLIC By David G. Buck December 2004 Chair: Mark Brenner Major Department: Interdisciplinary Ecology I report on the limnology and paleolimnology of hypersaline Lago Enriquillo, Dominican Republic. The modern lake system was studied during th e course of three field visits between June 2001 and Marc h 2003. A combination of salinity, pH, temperature, stable isotopes ( 18O and D), and major ion concentrations were measured during these visits. The water column was well mixed in the lake's two basins during sampling in June 2001, November 2002, and March 2003. Salinity increased from 80 to 106Â‰ between June 2001 and March 2003, illustrati ng the variability in lakewater ionic strength on short time scales. Co mparison of stable isotope ratios ( D and 18O) in freshwater input streams and open lake wate rs provides evidence of intense evaporation from this closed-basin lake. In addition microfossil assemb lages collected from surface sediment samples reveal that both halophili c and halophobic taxa inhabit the system. An understanding of the modern limnological vari ability in the system aids in better
x interpreting the paleolimnological record of late Holocene climate variability from Lago Enriquillo. The paleolimnology of hypersaline Lago Enri quillo, Dominican Republic is inferred from physical, lithologic, bi ostratigraphic and geochemical data in a 2.70 m sediment core representing the last ~2300 years of de position. Results suggest highly variable regional climate dynamics. Shifts in lake water salinity and associated hydrologic fluctuations reflect changes in evaporati on/precipitation ratio (E /P) and are revealed through biostratigraphy and stable isotope analysis ( 18O) of calcium carbonate shells of foraminifera, ostracods and gastropods. Vari ations in the paleolimnological record result from a dynamic Holocene history that included a period of connection with the Caribbean Sea prior to 270 BC, followed by a brackishestuarine period from 270 BC to 450 AD. Pronounced drying ca. 800 Â– 1000 AD is inferred from changes in the biostratigraphic record and is also suggested by positive excursions in 18O of ostracod and gastropod shells as well as deviations in sediment bulk density and magnetic susceptibility. The past ~1000 years (ca. 1000 AD Â– present) were characterized by high E/P variability as reflected by rapid changes in microfossil assemblages and 18O values. High variability in the Lago Enriquillo paleoclimate record is tied closely to regiona l patterns of E/P and tropical climatic regimes. The late Holocen e Lago Enriquillo record shows that droughts on the Yucatan Peninsula (Hodell et al., 2001) and northern South America (Haug et al., 2003) were pan-Caribbean events related to shifts in the position of the Intertropical Convergence Zone (ITCZ) and the North Atlant ic subtropical high-pr essure system that may also be correlated with solar forcing mechanisms.
1 CHAPTER 1 INTRODUCTION Inland saline water bodies are valuable fo r studying: the dynam ic nature of salt lakes and their aqueous geochemistry (H oldren & MontaÃ±o 2002); the presence of unique, salt-tolerant biota (W illiams 1998); economically important chemicals such as lithium, borax, sodium carbonates, or zeolites (Eugster & Hardie 1978); sedimentary archives of Holocene climate variability (D ix et al. 1999). According to Eugster and Hardie (1978), for saline lakes to develop and persist, three basic conditions must be present: 1) the body of water must sit in a hydrologically closed ba sin; 2) evaporation must exceed the sum of all inflows; and 3) th ese inflows must be sufficient to sustain the body of water. Lago Enriquillo, Dominican Republic is a pa rticularly interesting saline water body for study (Figure 1.1). Lago Enriquillo is hypers aline, with historic salinities ranging from 35Â‰ to over 100Â‰, depending on rainfall (Buck et al., in press ). It is a hydrologically closed basin with a number of groundwater-fed spri ngs and intermittent streams entering the basin, yet experiences extreme evaporative pressure. Its conservation is highly valued for future gene rations and it contains sedimentary archives useful for understanding late Holocene climate variability in the Caribbean basin. The purpose of the paper is to expand on the existing knowledge of this unique system. I used a combination of limnologica l and paleolimnologica l techniques to: 1) describe the current chemical and physical characteristics of Lago Enriquillo; and 2)
2 create a record of late Holocene climate va riability for Lago Enriquillo and the greater Caribbean Basin. Figure 1.1. Map showing the loca tion of HispaÃ±ola within th e Greater Antilles of the Caribbean Basin. The Dominican Repub lic (B) comprises the eastern portion of the island. A closer view (C) sh ows Lago Enriquillo and its proximity to Laguna Rincon and neighboring Etang Saumatre in Haiti. The introduction to Lago Enriquillo is based on several stud ies, originally published in Spanish, which contain valuable information on the lakeÂ’s biological and
3 chemical characteristics. These articles ar e summarized briefly to provide a background of information on the lake that brings toge ther what were originally disparate data sources. Following the introduc tion, I present water column a nd sediment data collected in three field excursions between 2001 and 2003. The first section is on the physical and chemical characteristics of Lago Enriquillo with a focus on the chemistry of the water column and 17 springs and 6 streams and canal s that discharge into the lake. Stable isotope analyses ( 18O and D) and major ion concentrations are used to illustrate the extreme evaporative pressure in Enriquill o. The relationship between salinity and 18O is reported in this section and will be e xpanded on later as part of the multi-proxy paleolimnological study. Chapte r three presents data obtaine d from a 270 cm sediment core collected from Lago Enriquillo in June 2001. A multi-proxy paleolimnological approach, including sediment geochemistry, biostratigraphy, and stable isotopes, was used to reconstruct late Holocene climate variab ility in the Enriquillo basin and the rest of the Caribbean basin. Site Description Lago Enriquillo is situated in the southw estern corner of the Dominican Republic approximately 4km from the borde r with Haiti (Figure 1.1). Th e lake is part of a larger geographic feature, the Neiba/Cul-de-Sac Va lley, which stretches from the Bahia de Neiba in the Dominican Republic to Port-au-Prin ce, Haiti. The valley, like the majority of HispaÃ±ola, is composed of Miocene age limestones that have been metamorphosed and structurally altered during th e movement of the Caribbean plate relative to the North American plate. This tectonic activity ha s caused much of the uplift and coincident depression in the Neiba Valley. Lago Enriquillo sits in one of thes e depressions and is
4 bordered on its north by the Sierra de Neiba and to the south by the Sierra de Bahoruco. It is separated from the Caribbean by a low 4m rise and separated from the neighboring basin of Etang Sumatre by a sill that is greater than 100m above sea level in some places (Mann et al. 1984). Figure 1.2. Bathymetric map of Lago Enriquillo showing the two dis tinct basins at 2-m depth contours (modified from Ar aguÃ¡s-AraguÃ¡s et al., 1993). Lago Enriquillo is the largest lake in the Caribbean, with a surface area of 200km2 and a mean water depth of 6.10m. Currently the lake level is 46m below sea level (mBSL). The lake is separated into two basins with the nort hern basin reaching depths of up to 22.5m and the southern basin reachi ng a maximum depth of 10.1m (Figure 1.2) (Schubert 2000a). The watershed of Lago Enriquillo is estimated to be 3500km2. The basin is closed, hydrologically. Water enters the lake via precipitation and several fresh water streams and springs that drain the surr ounding Sierras. Wate r is thought to leave the lake only via evaporation, although some outflow may occur by gr oundwater seepage.
5 Average Annual Precipitation versus Evaporation Lago Enriquillo Pluvimetric Station Puerto Escondido 0 50 100 150 200 250 JFMAMJJASOND(mm) rainfall evaporation Figure 1.3. Pluvimetric stati on Puerto Escondido is located at N18Â° 19Â’ 15Â”, W71Â° 34Â’20Â”, elevation = 400 mASL. Monthly ra infall values are averaged between 1968 and 1993. Average annual rainfall duri ng this interval totaled 618 mm. Monthly evaporation values were av eraged between 1970 and 1990. Average annual evaporation during this interval totale d 1887 mm. Months that did not have data were not included in the calculation. Data provided by INDRHI (Instituto Nacional de R ecursos Hidrolicos). Lago Enriquillo is a hypersaline lake with salinities ranging from 35Â‰ to 100Â‰, depending on rainfall. Annual precipitati on ranges from 508mm on the southeast shore to 729mm on the northwest shore. Mean daily temperature around the lake ranges between 22.3 C and 33.7 C. Estimated annual evaporation rates can be as great as 2000mm, almost three times that of precip itation (Figure 1.3) (Sc hubert, 2000a). Most weather affecting Lago Enriquillo enters from the east and follows the long axis of the valley. Although Lago Enriquillo is situated with in the rain shadow of the Sierra, there are two distinct rainy seasons. A short rai ny season runs from Ma rch to April with a longer one from July to September. Rainfall peaks in May and in July-August. During the dry season (December to mid-March) rain fall is less than 20mm/month (Schubert 2000a).
6 Previous Studies in Lago Enriquillo Origins of the Enriquillo Basin Lago Enriquillo and its surrounding watershed have been the focus of a variety of scientific investigations. In a survey of the geography and geology of the Enriquillo valley, Cucurullo (1949) describes Â“la hoya de EnriquilloÂ” as the lowest elevation on the island of HispaÃ±ola. He cite s lateral compression in a NNE-S SW direction as responsible for the mountain ranges that occur in series across the island oriented WNW-ESE. The study documents a series of faults oriented along this WNW-ESE axis that border the Sierra de Neiba and Sierra de Bahoruco. Cucurullo (1949) cites earlier geologic and geographic studies that disc uss processes involved in the evolution of the valley, including: a marine transgression during the Miocene and Pliocene; presence of elevated corals and continual uplift of these features; a possible inland seaway connecting the Neiba/Cul de Sac valley system; and the role that sedimentation and fluvial damming by the Yaque del Sur river played in the isol ation of Lago Enriquillo from the Caribbean Sea. Recent studies on the geological origins of the Enriquillo valley have focused on the well-preserved sub-aerial fringing coral reef deposits in the Enriquillo valley (Wilson et al. 2000; Curran and Greer 1998; Greer 1997; Stemann an d Johnson 1995). Wilson et al. (2000) studied paleoshore erosion associ ated with the coral reef and the oncesubmerged limestone cliffs in the valley by examining boring holes of bivalves, sponges and serpulid worms. These borings exhibit characteristics that suggest at least two extended intervals of relatively stable sea-leve l stands in the valley (Wilson et al. 2000). Curran and Greer (1998) document large serpu lid worm mounds associated with the Enriquillo reef system and Greer (1997) exam ined facies changes in the development and
7 eventual cessation of reef formation in the valley using U/Th dating and threedimensional mapping of the coral deposits. Ear lier studies on the corals of the Enriquillo valley provided a valuable chronology for coral development using U/Th dating and reveal an extended period of marine transg ression and consequentia l coral reef growth (Mann et al. 1984; Taylor et al. 1985). These dates provide a framework for the Holocene development of Lago Enriquillo as it was separated from the sea and became a closed-basin, inland, saline lake. Holocene History of Enriquillo Valley Subaerial coral reef deposits are found along the perime ter of the Enriquillo valley and on Isla de Cabritas, an island separating th e north and south basins of the lake. The presence of these corals suggests that a mari ne incursion occurred in the recent geologic past lasting long enough and maintaining ad equate circulation to allow for the development of coral beds. The most common species of coral within these deposits are Montastraea sp. and Siderastrea siderea . Stemann and Johnson (1992) state that the diversity and ecology of the Enri quillo coral reef de posits are similar to that of modern Caribbean reefs. A series of radiocarbon and 234U/230Th ages were obtained from these coral beds and provide a time scale for the ma rine incursion and development of the coral beds (Table 1.1). The 5710 90 yr BP beginning of sedimentation over the reef deposits is also believed to mark the beginning of the closing of the valley from the sea. Mann et al. (1984) suggest that the isolati on of Lago Enriquillo began with the damming of the valley caused by increased sediment delivery from the Rio Yaque del Sur that drains the Sierra
8 de Neiba to the north. The possibility of tectonic uplift along the southern banks of the lake could have further isolated Lago En riquillo from the sea (Taylor et al. 1985). Table 1.1. Holocene sea-level ch anges in the Enriquillo Valley 2820 40 yr BP to Present Lago Enriquillo is being evaporated and is currently 46 mBSL 5710 90 to 2820 40 yr BP late Holocene sedimentation over the reef with fossil bivalves recording a gradual shift from marine to brackish waters. Thrombolitic stromatolites near present sea level 6200 80 to 4760 40 yr BP mid-Holocene development of a Caribbean fringing reef in a protected environment 8990 60 to 6200 80 yr BP colonization of reef corals above oyster bed ( 34mBSL) 9760 100 to 8990 60 yr BP marine transgression and growth of oysters on an alluvial fan deposit ( 33m BSL) (adapted from Mann et al. 1984; Taylor et al. 1985) Limnological and Environmenta l Studies of Lago Enriquillo Between 1967 and 1977, the Center for Inves tigation of Marine Biology (CIBIMA) of the University of Santo Domingo organized a program of periodic expeditions to Lago Enriquillo with the goal of ev aluating current limnological condi tions. The investigations included studies of physico-chemical characte ristics of the lake water, and flora and fauna, both in the limnetic and littoral zone s (InchÃ¡ustegui et al. 1978). The earliest records of lake salinity published by CIBIMA (1967) document a lake water salinity of 40.6Â‰. Between 1967 and 1977 however, sali nity increased from 40.6Â‰ to 79Â‰. During the course of a six month inves tigation between January and June 1977, the salinity of Lago Enriquillo varied litt le, between 75 and 79Â‰. Dissolved oxygen concentrations in surface waters were ve ry low (0.46-1.68 mg/L) and pH was between 8.35 and 8.45 (InchÃ¡ustegui et al. 1978). Wind plays an important role in the ecology and hydrodynamics of Lago Enriquillo. In four of 11 Secchi disk measurements taken, the Secchi depth (zsd) was <1.0m with all values <1.80m. Margalef (1986) reported Secchi depths of ~ 1.0m.
9 Strong trade winds move across the valley and contribute to sediment resuspension and high turbidity. Examination of littoral deposit s suggested that aeolian transport is an important process for erosion and sedimentati on in Enriquillo (InchÃ¡ustegui et al. 1978). Biotic aspects of Lago Enriquillo were also documented by InchÃ¡ustegui and colleagues (1978). Most biotic activity was concentrated in and around areas of lower salinity (i.e. mouths of fres hwater springs and rivers), although they cite a personal communication regarding the presence of green algae ( Cladophora sp.) in the saline lake waters. The dominant green algae in the zones of lower salinity include Hormonthamnion enteromorphoides and Enteromorpha flexuosa . A monograph published in conjunction w ith the CIBIMA study focused on contamination and the presence of pathogenic bacteria in Lago Enri quillo (Perez de InchÃ¡u stegui et al. 1978). The authors report the presence of pathogenic bacteria, including Salmonella anatum and Alteromonas putrefaciens , which were sampled from hypersaline lake water as well as from waters adjacent to the Borbollones sp ring and La Azufrada spring. Under certain conditions these two bacteria produce H2S. This may contribute to the mild sulfur smell detected in 2002 and 2003 at the Borbollones Sulfur spring and La Azufrada spring, the latter of which is visited by the largest numbe r of tourists. Other pathogens isolated in the microbiology analysis included Escherichia coli , Psuedomonas aeruginosa , and others (Perez de Inch Ã¡ustegui et al. 1978). In a preliminary analysis of plankton to ws InchÃ¡ustegui et al. (1978) document a variety of zooplankton including copepods of the family Oncaedae and the rotifer Brachionus plicatilis . Also noted were the diatoms Pleurosygma strigosum , Amphipora alata , Nitzchia spp., two species of Navicula , and Amphora cymbifera . Three
10 cyanophytes were also reported: Microcoleus , Arthrospira and Anacystis (InchÃ¡ustegui et al. 1978). The principal fish species living in Lago Enriquillo is Tilapia mossambica , an African cichlid that was in troduced to the lake around 1950 (InchÃ¡ustegui et al. 1978; Schubert 2000b). Small fishes of the family Poeciliidae (live-beari ng fish) congregate in the near-shore environment as we ll as at the mouths of spri ngs and rivers that enter the lake (InchÃ¡ustegui et al. 1978). Lago Enriquillo is known for its population of the American crocodile, Crocodylus acutus . The population is considered to be the largest in the world, although the animals experience c onsiderable reproductive stress because of the very high lake water salinities (Schubert, 2000b). During the early study of the Enriquillo wate rshed, InchÃ¡ustegui et al. (1978) noted that channelization of small streams that once drained into Lago Enriquillo had routed fresh water away from the lake, to agricu ltural lands. They commented on the potential effect that water diversion could have. Inch Ã¡ustegui and colleague s (1978) cite an 1892 study that reports lake level was 0.61m above sea level. Following the CIBIMA study, lake level was recorded to be 41.95m BSL (Cruz & Duquela-Gonzalez, 2003). Margalef (1986) expressed some doubt about the re liability of the early measurement. Nevertheless, it is clear that the lake stage displays pronounced variability. This variability has been documented by the Na tional Institute of Hydrologic Resourses (INDRHI), in collaboration with other sc ientists from the Dominican Republic, International Atomic Energy Agency (IAEA), and US universities. Unpublished data compiled by INDRHI and the Programa Cultura del Agua under the gu idance of surveyor
11 Roberto A. Cruz (Cruz & Duquela Gonzalez, 2003) illu strate the dramatic fluctuations of lake level during the past 50 years (Figure 1.4). -50 -45 -40 -35 -30 -25 -20 195019601970198019902000YearElevation (mbsl) lake level Figure 1.4. Elevation of Lago Enriquillo in me ters below sea level (mBSL) from1950 to 2003. Lake level data provided by Ju an SaldaÃ±a of INDRHI and was surveyed by Roberto Cruz and Tesis Duquela-Gonzalez (2003). The Enriquillo Valley has also received attention from exploration geologists seeking petroleum deposits. Although no extr active efforts followed these explorations, important observations were made and record ed in a brief unpublished report presented to the Institute of Mining, Sant o Domingo, by Canadian Supe rior Oil, Ltd. (Canadian Superior Oil, 1979). This report states that the surface area of the lake in 1979 had been reduced by 35% in the previous decade. This loss in surface area correlated with a drop in lake level of almost 4.6m. The greates t reduction of surface area occurred in the SE basin where it was estimated the shorelin e had receded by as much as 5km when compared to older topographic maps (Canad ian Superior Oil, 1979). This dramatic response in the SE basin was, in part, a result of its bathymetry, which is characterized by little slope and a large area of relatively shallow water. Conversely, the NW basin is deeper, has a much steeper slope, and contains more water, so a drop in lake level does not expose large areas of the littoral zone (see Figure 1.1).
12 A visit to Lago Enriquillo in 1983 by Ra mon Margalef documented the dynamic nature of the lake (Margalef, 1986). Margal ef (1986) measured salinities between 35.6 and 36.9Â‰ in the deepest part of the north basin (zmax=23m). The water column was fairly well mixed at that time, with a cha nge in temperature from surface to bottom of only -2.2ÂºC and a corresponding salinity incr ease of 1.3Â‰ (Margalef, 1986). Dissolved oxygen in the surface water at the time of Ma rgalefÂ’s visit was 9.3 mg/L and the water column became anoxic at approximately 16m. Margalef compared relative concentrations of the major ions of Enriqui llo waters with seawater, and noted that Enriquillo waters are related to seawater, but also exhibit characteristics of the local spring waters. For example, elevated concentrations of Mg++ relative to sea water (2.62:1), when compared with the concentrations of Na+ and Cl(1.65:1 and 1.78:1, respectively) led Margalef to conclude that the waters of Lago Enriquillo are not derived simply from the concentration of seawater. Interaction between spring water (high Ca, low alkalinity) and the saline lake water (low Ca, high alkalinity) produces a lake water supersaturated with Ca++, reaching concentrations of 9.0 mM (Margalef, 1986). The high Mg++ concentration is related to dolomite deposits in the watershed and the low concentration of K+ is perhaps the result of ionic exchange (Margalef, 1986). In his examination of the flora and fauna of the lake, Margalef (1986) noted many of the same species that were documented by CIBIMA (InchÃ¡ustegui et al. 1978). In plankton samples from lake water, the phytoplankter Chromulina was found in high concentrations as were several species of dinofla gellates including Cyanomonas sp. and Oxyrrhis marina . In addition to the diatoms doc umented by CIBIMA, Margalef (1986) identified specimens from the genera Cyclotella and Mastogloia . He also remarked on
13 the presence of spherical-shaped isopods as well as abundant milio lid foraminfera, but did not identify them further. Lago Enriquillo was also the subject of a recent study by members of the IAEA and INDRHI that focused on the physio-chemical parameters of the lake as well as extensive oxygen-18, deuterium and tritium isotopic analys es of lake and spring waters (AraguÃ¡s AraguÃ¡s et al. 1993). Surface water conduc tivity ranged from 50.0 to 83.75 mS/cm. Values for pH ranged from 8.01 to 8.14. Majo r ion concentrations illustrate that Enriquillo is a Na-Cl dominated sy stem with concentrations of particular ions relative to those of sea water ranging from 1.22:1 for Ca++ to 2.17:1 for Na+ and Cland 2.88:1 for SO4-. Isotopic values of surface waters range from +3.34 to 4.10Â‰ for oxygen-18 ( 18O) to 17.3 to 21.9Â‰ for deuterium ( D). Anomalously low values for both 18O and D from an area in the northwest corner of th e northern basin are proba bly a consequence of nearby fresh water inputs from Boca de Cachon and Borbollones springs (AraguÃ¡s AraguÃ¡s et al. 1993). Based on these isotopi c analyses, AraguÃ¡s Ar aguÃ¡s et al. (1993) concluded that: 1) there is re stricted circulation between th e two basins, and 2) there is larger input of fresh water in the southe rn basin making it isotopically lighter. Recent Developments in Lago Enriquillo Extensive on-going research and conservati on efforts in the Enriquillo basin were officially recognized and honored in 2002 by the UNESCO Man and Biosphere program. UNESCO declared 476,700 h ectares, including Lago Enriquillo, the surrounding sierra, and adjacent marine area, the Jaragua-Bahoruco-Enriquillo Biosphere Reserve (UNESCO 2002). This area is home to a wide variety of mari ne birds and is also a resting and feeding location for many speci es of migratory birds as well as a
14 reproductive site for the pink flamingo ( Phoenicopterus ruber ) (Dinerstein et al. 1995). The lake and surrounding watershed are home to numerous plant and animal species that are endemic to the island of HispaÃ±ola and the ecoregion. The two studies that follow contribute to the growing literature on the ecology and limnology of Lago Enriquillo. The physical and chemical characteristics of Lago Enriquillo provide an additional data set doc umenting the water dynamics of the system. The paleolimnology of Lago Enriquillo illustra tes the usefulness of saline waters as archives of Holocene climate data and argue s for climatic teleconnections across the Caribbean and the North Atlantic.
15 CHAPTER 2 PHYSICAL AND CHEMICAL PROPERTIES OF HYPERSALINE LAGO ENRIQUILLO, DOMINICAN REPUBLIC Introduction Lago Enriquillo, Dominican Republic, (18Â°31.7' N, 71Â°42.91Â’ W) is a hypersaline lake of marine origin. During the middle Holocene, ca. 6000-4800 years before present (yr BP), Lago Enriquillo was an embaymen t of the Caribbean Sea and supported a fringing coral reef. The embayment was isol ated from the Caribb ean Sea between 5000 and 2800 yr BP by tectonic uplift and fl uvial damming by the Yaque del Sur River (Bond, 1935; Mann et al., 1984; Taylor et al., 1985). Today, Lago Enriquillo is a closed basin lake that is home to a unique flora and fauna, and whose sediment may serve as an excellent source of paleoenvi ronmental information. Here I present data on modern physico-chemical parameters and microfauna distributions that re flect the systemÂ’s sensitivity to variations in rainfall, evapor ation, and fresh water inputs. These data will aid in interpreting past envi ronmental changes as recorded by faunal and geochemical proxies in sediment cores retr ieved from Lago Enriquillo. Study Site Lago Enriquillo is the largest lake in the Caribbean. Currently its watershed encompasses ~3500 km2 and lake level is ~46 m below sea level. Its surface area is ~200 km2, and its estimated mean water depth is 6.10 m (Figure 2.1) (Schubert, 2000a). The lake has displayed historic salinities rangi ng from about 35Â‰ in 1983 (Margalef, 1986) to >100Â‰. Annual rainfall near Lago Enriquillo ra nges from 508 mm on the SE shore to
16 729 mm on the NW shore (Instituto Nacional de Recursos HidrÃ³licos, unpublished data). Rainfall maxima occur in late-March and August. Monthly precipitation in the dry season (December to early March) averages <20 mm. Annual rainfa ll is highly variable, however, and is influenced by tr opical depression and hurricane activity in the Caribbean. Mean daily temperatures around the lake vary between 22.3 C and 33.7 C, and annual evaporation can be as great as 2000 mm (Schubert, 2000a). Figure 2.1. Map of Lago Enriquillo on the Cari bbean island of HispaÃ±ola. Bathymetric map of Lago Enriquillo showing 2-m de pth contours (modified from AraguasAraguas et al., 1993). Letters along the shoreline indicate the approximate location of springs mentioned in the te xt: (A) La Azufrada , (B) Borbollones, (C) Boca de Cachon, (D) Caoba, (E) La Zurza, (F) El Guayabal.
17 Methods I report physical (temperature), chemical (pH, salinity, ion concentrations, stable oxygen and hydrogen isotopes) and surface sedi ment data, based on samples collected from Lago Enriquillo during three visits in June 2001, November 2002, and March 2003. Water samples for chemical analysis were ta ken from freshwater sp rings that discharge into the lake, from the lake surface, and fr om water-column profiles in the lakeÂ’s north and south basins. Water column temperature and pH were measured with a Horiba U-10 meter. Salinity was measured using a SPER Sc ientific refractometer. Major cations and anions were measured with a Dionex M odel DX 500 ion chromatograph. Chloride concentrations in hypersaline lake waters were determined by titration using 0.1M AgNO3 and K2CrO4 indicator solution (Mohr method). Total dissolved inorganic carbon (DIC) was measured with a UIC/Coul ometrics Model 5011 Coulometer and 5030 Carbonate Carbon Preparation System. Oxygen isotope ratios ( 18O) of 128 water samples were measured by CO2 equilibration using a Micromass Multiprep device interfaced to a VG Prism II mass spectrometer. A subset of 32 samples was measured for deuterium ( D) on a ThermoFinnigan MAT253 using an H-Devi ce reduction furnace and a CTC PAL autosampler. Data are reported in standard delta notation ( ) relative to Standard Mean Ocean Water (SMOW). In addition to collecting limnological data, I also took surface sediment samples at the mouths of fresh water springs as well as from open lake water sites to identify the benthic microfauna of Lago Enriquillo. Surf ace sediments were processed using standard procedures (Murray, 1991). Samples were washed through a 63 m sieve, oven-dried
18 and then dry sieved to separate the sample into >212 m and >63 m fractions for identification of microbenthos. Organisms identified included foraminifera, ostracods, and gastropods. Results and Discussion Temperature and pH Surface water temperature in the north basin ranged from 28.6 C to 30.5 C, with the lowest temperature recorded in Marc h 2003 (Figure 2.2a). A subtle temperature difference of < 1 C between the upper and lower waters was observed. The south basin was isothermal at the time of sampling in 2002 and 2003, with temperatures ranging from 27.9 C (March 2003) to 29.7 C (November 2002). Among the three sampling dates, November was the time of greatest heat stor age in the water column. The pH of Lago Enriquillo waters varied little with time, sampling location, or depth (Figure 2.2b). Water column temperature and pH resu lts suggest that each basin of Lago Enriquillo is, separately, well mixed. Strong wi nds and a large fetch, particularly in the south basin, probably contribute to polymixis. Although the north ba sin exhibits subtle thermal stratification, bottom waters are only 1 C cooler than surface waters. Seasonal differences between the two basins are comparable. Salinity and Major Ion Concentrations Lakewater salinity in both basins increased by ~23Â‰ be tween June 2001 and March 2003 (Table 2.1). Salinity in the south basin was consistently ~2Â‰ higher than in the north basin. Changes in salinity were accompanied by proportional shifts in individual ion concentrations. Na+, K+, Mg++, and Ca++ consistently comprised 80%, 1.5%, 16.5%, and 2% of cation milli-equivalents L-1, respectively. Chloride and sulfate
19 had relative concentrations of 87.5% and 12.25%, respectively. Bicarbonate (HCO3 -), measured on a subset of water samples, re presented only ~0.03% of anion charges. 0 2 4 6 8 10 12 14 16 18 20 7.888.28.4 pHDepth (m) 0 2 4 6 8 10 12 14 16 18 20 26283032Temperature (ÂºC)Depth (m) June '01 N basin Nov '02 N basin Mar '03 N basin Nov '02 S basin Mar '03 S basin AB Figure 2.2. Water column profiles from the north and south basin of Lago Enriquillo. Temperature (A) and pH (B) profiles we re measured in June 2001, November 2002, and March 2003. Spring waters had salinities < 1Â‰, but di splayed differences in relative ion concentrations. Guayabal spring containe d the lowest total ionic charge with Ca++ representing 83% of the total cation millie quivalents (Table 2.1). Boca de Cachon had the highest total ionic charge dominated by Na+ and Clions. Borbollones and Zurza are sulfur springs and emit H2S.
20 Table 2.1. Salinity, major ion concentrations a nd stable isotope rati os of Lago Enriquillo waters, spring waters entering the la ke, and Caribbean Sea waters. * The concentration factor compares the ioni c concentrations of Lago Enriquillo waters to local Caribbean Sea waters (Bahia de Neiba). Because of EnriquilloÂ’s marine origin, this comp arison reveals information about the ionic development of the lakewaters since its isolation from the sea. Isotopes Cations Anions salinity 18O D Na K Mg Ca Cl SO4 HCO3 sample ID ppt (meq/L) (meq/L) (meq/L ) (meq/L) (meq/L) (meq/L) (meq/L) Enriquillo lake waters June, 2001 North basin 80.0 4.4 20.9 1077. 7 19.3 220.8 26.5 1237.2 175.8 n/d South basin 82.0 4.5 n/d 1086. 8 19.7 222.5 26.7 1220.3 166.7 n/d Nov, 2002 North basin 102.0 4.5 22.7 1379. 9 26.5 283.0 34.6 1515.4 211.6 5.0 South basin 104.2 4.7 22.6 1306. 2 25.1 277.2 35.5 1584.3 222.0 4.4 March, 2003 North basin 104.0 4.8 22.9 1482. 6 27.3 302.4 35.4 1632.2 216.0 4.9 South basin 105.9 4.7 23.3 1433. 6 26.2 291.4 34.2 1588.7 220.6 4.3 Enriquillo spring waters Borbollones 0.0 -4.1 n/d 1. 0 0.1 1.3 3.4 1.0 0.1 4.7 La Zurza-sulfur 1.0 -5.7 n/ d 3.5 0.1 2.4 2.6 5.7 0.3 4.4 Guayabal 0.0 -4.1 -17.1 0. 2 0.0 0.5 3.5 1.8 0.1 n/d Caoba 0.0 -5.5 -30.5 2.4 0.1 2.1 2.7 3.4 0.2 4.2 Boca de Cachon 1.0 -3.6 -14. 7 5.6 0.2 2.5 2.6 9.1 0.3 n/d Caribbean Sea Bahia de Neiba 37.5 1.2 6.9 479.0 9.9 112.3 20.5 550.1 58.9 n/d Conc. Factor* 2.8 3.9 3.3 3.1 2.8 2.7 1.7 3.0 3.7 Stable Isotopes In evaporating water bodies, heavier isotopes of water (H2HO and H2 18O) are Â“enrichedÂ” in lake water because of the pr eferential evaporation of lighter water molecules (1H2 16O). Enrichment of 18O and 2H (deuterium, or D) provides information on changes in the ratio of precipitation to evapor ation in a basin. Ther efore, the ratios of 18O to 16O ( 18O) and 2H to H ( D) in waters provide information about the evaporative
21 history of a water body and can al so be used to identify diffe rent input sources in water balance studies. -20 0 20 40 60 80 100 120 -8-6-4-2024618OASalinity = 11.2( 18O) + 46.865 r2 = 0.95 -40 -20 0 20 40 60 -8-6-4-2024618OGMWL (Craig, 1961) D = 8( 18O) + 10 Evaporation Line D = 4.7 18O) + 0.41 B springs lake waters mixing zone rain Bahia de Neiba Figure 2.3. Changes in salinity and stable isotopes ( 18O, D) for Lago Enriquillo waters. (A) Oxygen isotopes ( 18O) versus salinity for Lago Enriquillo input waters (rainfall and springs), lake water, and Caribbean sea water. The regression between salinity and 18O is highly significant (r2 = 0.95). Enrichment of 18O and salinity in lake water reflects in tense evaporation from the basin. (B) Solid line is the Global Meteoric Wa ter Line (GMWL), which defines the relation between 18O and D in meteoric waters worldwide ( D = 8( 18O) + 10 (Craig, 1961)). Lago Enriquillo wa ters (dashed line) deviate from the GMWL because intense evaporati on causes non-equilibrium isotopic fractionation. D Salinit y
22 In Lago Enriquillo waters, salinity and 18O are strongly correlated (r2 = 0.95) (Fig. 2.3a). Covariance of salinity and 18O illustrates the enrichment of 18O in Enriquillo waters due to evaporation. The relationship between D and 18O in meteoric waters is referred to as the Global Meteoric Water Li ne (GMWL) and is expressed by the equation D = 8( 18O) + 10 (Craig, 1961) (Fig. 2.3b). Lago Enriquillo waters deviate from the GMWL because of evaporation and specifica lly, non-equilibrium isot opic fractionation at the air-water boundary between the atmosphere and Lago Enriquillo waters (Gat, 1980). Microbenthos The foraminfera assemblage of Lago Enriquillo is dominated by milliolid species of the genus Quinqueloculina . The rotallid Ammonia beccarii is also present. The foraminifera assemblage near freshwater springs contains more taxa, including the rotallid Criboelphidium poeyanum , the common saline species Ammotium salsum , as well as a less abundant Rosalina sp. An unknown species, possibly of the genus Amphistegina , was also encountered. Its presence may be attributed to erosion of Holocene coral deposits within EnriquilloÂ’s watershed. Ostracods in Lago Enriquillo include Perissocytheridea rugata , P. bicelliforma , and Cyprideis sp. The gastropod Heleobops clytus (Thompson & Hershler, 1991) was obs erved adjacent to springs. Conclusions Lago Enriquillo exhibits only minor spatial and temporal differences with respect to temperature and pH. The measured chemical differences (i.e. pH, major ions, salinity, stable isotopes) suggest that the two basins are not mixed completely and may have been hydrologically separated at lower lake stage. The range of salinities in Lago Enriquillo enables both halophilic and ha lophobic taxa to inhabit the system. Freshwater hydrobiid
23 snails ( Heleobops clytus ) occupy spring discharge areas, whereas salt-tolerant foraminifera (eg., Quinqueloculina spp.) are found in the open lake. The modern Lago Enriquillo is a dynamic syst em that is susceptible to short-term hydrologic variations. Both basins are prone to pronounced shifts in salinity, chemical composition, and isotopic concentration on short time scales (i.e., seasonal/annual). Furthermore, long-term changes in Lago Enri quilloÂ’s salinity appear to be related to climate-driven shifts in freshwater input that are expressed in lake level changes. During the past 55 years, lake stage has been positively correlated with the passage of hurricanes and tropical storms in the area (Figure 2.4). -50 -45 -40 -35 -30 -25 -20 195019601970198019902000YearElevation (mbsl) lake level 1958 T.S. Gerd a 1961 H. Frances 1964 H. Cle o 1966 H. Ine z 1997 H. Georges 1975 H. Elois e 1979 H. David; H. Frederi c 1987 H. Emil y 1993 T.S. Cind y Figure 2.4. Elevation of Lago Enriquillo (mBSL) from 1950 to 1963, and timing of hurricanes (H.) and tropical storms (T.S.) passing near the Enriquillo basin. Lake level data were provided by R oberto Cruz, Tesis Duquela-Gonzalez, and Juan SaldaÃ±a of INDRHI. Hurricane a nd tropical storm data are from the National Oceanic and Atmospheri c Administration (NOAA) National Hurricane Center ( http://www.nhc.noaa.gov/ ) Future research will focus on reconstructing decade to century-scale variability in lake hydrology during the Holocene through anal ysis of sedimentary archives retrieved from Lago Enriqullo. Changes in the abundance of the gastropod H. clytus will serve as a
24 valuable proxy for changes in freshwater i nput into the system. Past variations in foraminferal assemblages will aid in interpreti ng past changes in lake water salinity. In addition, oxygen isotope analysis of the carbona te shells of aquatic invertebrates (e.g. foraminifera, ostracoda, gastropoda) collected from sediment cores will aid in the reconstruction of past moisture conditions (Covich and Stuiver, 1974). Understanding the ecology and distribution of microfauna and the modern physico-chem ical properties in Lago Enriquillo will aid in interpreting past changes in the hydrologic budget as observed in sediment cores.
25 CHAPTER 3 A LATE HOLOCENE RECORD OF CLIMATE VARIABILITY FROM HYPERSALINE LAGO ENRIQUI LLO, DOMINICAN REPUBLIC Introduction Saline lakes present a unique opportunity to study Holocene paleoclimate because they are sensitive to variations in regional precipitation (Last & Vance, 1997; Eugster & Hardie, 1978). Lago Enriquillo is a large, closed-basin, hypersaline lake of marine origin located in the Dominican Republic, on the island of HispaÃ±ola (Figure 3.1). Limnological investigations of the lake over the last ~20 years have illustrated this sensitivity with regard to changes in lake water chemistry and lake levels. Published salinity values and associated lake water ion concentrations in Lago Enriquillo range from close to sea water [i.e. ~35Â‰] (Margalef, 1986) to as high as ~3.0 times that of sea water (Buck et al., in press ). In addition, stable isotope studies of meteoric waters, ground waters, and lake waters illustrate the high degree of evaporative loss from the system (AraguÃ¡s AraguÃ¡s et al., 1993; Buck et al., in press ). A 50-year record of lake level changes documented Lago EnriquilloÂ’s st age variability on decadal time scales (Cruz & Duquela-Gonzalez, 2003). Lake stag e is positively correlated with tropical storm frequency in the basin (Buck et al., in press ). Documented response of the lake to recen t climate variability suggested that a sediment record from Lago Enriquillo might preserve an archive of longer-term, Holocene climate change in the region. Clim ate in the Caribbean Basin is dominated by the seasonal migration of the Intertropical Convergence Zone (ITCZ) and the North
26 Atlantic subtropical high-pre ssure zone (also referred to as the Azores-Bermuda high) (Hastenrath, 1984; Malmgren et al., 1998; Giannini et al., 2 000; 2001). Variations in the latitudinal location of these two air masses cr eate the wet/dry seasonal pattern. During the period when the ITCZ and the North Atla ntic high migrate northward (from May to November), heavy rains occur across the northern hemisphere tropical belt, carried by the prevailing trade winds. Figure 3.1. Map of Lago Enriquillo including bathymetry. Lago Enriquillo has a surface area of ~200 km2 and is currently 46m BSL. Me an water depth in the basin is 6.1m with the north basin reaching a ma ximum depth of 22.5 and the southern basin reaching 10.1m. It is the larg est lake in the Caribbean and has a watershed of ~3500 km2. Lake waters are hypersaline, ranging from 35 to >100Â‰, depending on rainfall variability.
27 From November to April, the ITCZ shifts to a southerly location and the North Atlantic high plays a more dominant role in weather patterns in the Caribbean basin bringing drier conditions to the region (Gray, 1993). Deglacial and Holocene vari ations in this regional climate dynamic have been examined employing paleoclimate archives fr om marine and lacustrine sediments. Marine records from the Cariaco Basin, near Venezuela, suggest that the glacialinterglacial transition was characterized by a period of intensified trade wind activity and subsequent increased upwelling along the southern margin of the basin as a result of meltwater pulses entering the Gulf of Mexico during the retreat of the Laurentide Ice Sheet (Peterson et al., 1991). The Y ounger Dryas Chronozone (~12.5 Â– 11.5 kyr BP) brought a brief return to cool er and drier conditions. Regi onal climate grew warmer and wetter following this period, probably due to a northward migration of the ITCZ (Peterson et al., 1991). Warm er, wetter conditions persiste d from ~10.5 to 5.4 kyr BP, a period referred to as the Holocene Â“thermal maximumÂ” (Curtis and Hodell, 1993; Haug et al., 2001). This moist period corresponds to a period when the seasonal difference in solar insolation in the Caribbean region was at a maximum (~8 kyr BP). This probably caused an enhancement of seasonal weather patterns in the northern hemisphere Neotropics (Hodell et al., 2001). The Holocene Â“thermal maximumÂ” was followed by a period of prolonged drying beginni ng about 3.5 kyr BP that is documented in lacustrine and marine sequences. Midto late Holocene drying is thought to ha ve been a response to a reduction in the seasonal difference in in solation, and a general s outhward shift in the location of the ITCZ (Haug et al., 2001).
28 A ~10,500 yr record from Lake Miragoane, Ha iti, registers a shift toward aridity beginning ~3200 yr BP. Drying is inferred from a +1.25Â‰ shift in the oxygen isotope composition of ostracod carapaces (Curtis a nd Hodell, 1993). Palynological data from Lake Miragoane also suggest greater aridity, with the expansion of xeric forest taxa Celtis , Bursera , Curatella and Cordia (Higuera-Gundy et al., 1999). The geochemical stratigraphy of a sediment core from Lake Valencia, Venezuela, suggests gradually increasing lake water salinity beginning approximately 3000 yr BP (Bradbury et al., 1981). High-resolution paleoclimate records from lakes on the Yucatan peninsula suggest a highly variable late-Hol ocene climate punctuated by periods of pronounced drought conditions (Fritz et al., 2001). A period of prolonge d drought between ~800 and 1000 AD is inferred from oxygen isotopic ratios ( 18O) in both ostracods and gastropods and sediment geochemistry (Hodell et al., 1995; Curtis et al., 1996; Hodell et al., 2001; Hodell et al., submitted). Droughts occurred in this region on a frequency of ~208 and 50 year intervals and may be rela ted to variations in solar ac tivity (Hodell et al., 2001). The Terminal Classic Drought in the Maya area occurred between 770 and 1100 AD and was relatively strong. Its widespread nature is evident by its detecti on in a high-resolution record of titanium (Ti) content in marine sediments from the Cariaco Basin, north of Venezuela (Haug et al., 2003). Records of late Holocene climate dynamics in the northern Caribbean Basin are less consistent, however, and suggest a co mbination of locally controlled climate variability and lake level dynamics. In southwest Jamaica, changes in ostracod assemblages record hydrologic variability in Wallywash Great Pond during the last ~10
29 kyr (Holmes, 1998). An increase in faunal di versity around 1.2 kyr BP is interpreted as a period of increased hydrologic input into th e system that is followed by a return to assemblages similar to those of the modern system (Holmes, 1998). Although increases in 18O of ostracod valves after ~1000 yr BP reflect a transition to slightly drier conditions, the inferred change in climate is not represented by changes in the ostracod community assemblage (Holmes, 1998). Nyberg et al. (2001) used sediment magnetic susceptibility, changes in foraminifer assemblages, and sediment geochemistry in a 2000-year marine sediment record taken near Puerto Rico, to explain climate variabil ity in the northeast Caribbean Sea. Time series analysis of magnetic susceptibility da ta exhibits similar periodicities to those measured from 18O and % sulfur (i.e. precipitated gypsum) measured in lake sediment cores from the Yucatan peninsula (Curtis et al., 1996; Hodell et al., 2001). A positive excursion in the marine core magnetic sus ceptibility centered at ~1000 AD, however, is thought to represent increased transport of terrigenous, ferromagnetic material and a transition to more humid condi tions (Nyberg et al., 2001). Marine saline ponds on Lee Stocking Isla nd, Bahamas, contain a 1500-year record that includes both marine and lacustrine se diment (Dix et al., 1999). A shift in foraminifer assemblages at ~1000 yr BP is interpreted to represent a phase of rapid marine transgression that may have persis ted until ~720 yr BP at which time the embayment closed. In addition, inputs of marine-derived sedi ment are preserved in these ponds and suggest a 300-400 year cycle sea level change re lated to either hurricane activity or high-order eustatic va riation (Dix et al., 1999).
30 Paleolimnological data from Lake Miragoane, Haiti, (Curtis and Hodell, 1993) represent the most complete Holocene pale oclimate record for the northern Caribbean basin. Its high resolution oxygen isotope profil e of ostracod valves illustrates changes in the ratio of evaporation/precipitation (E/P) in the basin and serves as a proxy for changes in local/regional climate. Chronological control for this record, however, was compromised by a scarcity of terr estrial organic material for AMS 14C dating. Radiocarbon dates on ostracod shells were ad justed for possible hard-water lake error (Deevey and Stuiver, 1964). Additional climate records from this region are needed to better understand the changing dyna mics of circum-Caribbean climate dynamics in the late Holocene. Lago Enriquillo, Dominican Re public, offered an opportunity to explore late Holocene Caribbean clim ate using a multi-proxy paleolim nological approach. Here I present the sediment geochemistry and lith ology, biostratigraphy, and stable isotope records from a sediment core that illustrates the dynamic nature of Caribbean climate change during the past ~2300 years. Materials and Methods On 13 June 2001, a 270-cm sediment core (ENR 13-VI-01) was retrieved from the north basin of Lago Enriquillo in approximately 10.5 m of water (Table 3.1). The 71 cmlong mud-water interface core (MWI-1a) was retrieved using a piston corer (Fisher et al., 1992) and was sampled at 1.0 cm intervals in the field and stored in whirlpak bags. Deeper sediments were retrieved using a modi fied square-rod piston corer (Wright et al., 1984). Retrieved sediment was transported to the University of Florida and stored at 4 C.
31 Table 3.1. Individual cores retrieved from coring location ENR 13-VI-01 used to develop the composite core. The com posite core stratigraphy was developed using magnetic susceptibilt y, bulk density, and %CaCO3 data from each individual core to identif y stratigraphic horizons where cores could be spliced together. Core I.D. Nominal Depth Composite Depth ENR MWI-1a 0 74 cm 0 71 cm ENR MWI-1b 71 193 cm 72 194 cm ENR Lex Overlap 130 228 cm 172 270 cm ENR Lex 2 180 257 cm 194 270 cm Table 3.2. Accelerator mass spectrometry radiocarbon dates for samples from Lago Enriquillo, Dominican Republic Sample Name Composite Depth 14C age Â± Calibrated Age Calendar Age Â± probability and depth (cm) (yrBP) (AD/BC) % ENR13-VI-01 MWI1A 24-25# 24cm 740 100 690 1260 AD 150 88.5% ENR13-VI-01 MWI1A 41-42# 41cm 760 100 735 1215 AD 185 95.4% ENR13-VI-01 MWI1B 86-87* 86cm 400 60 420 1530 AD 110 95.4% ENR13-VI-01 MWI1B 102-103* 102cm 900 60 810 1140 AD 120 95.4% ENR13-VI-01 MWI1B 146* 146cm 1070 45 990 960 AD 70 95.4% ENR13-VI-01Lex2 201* 214cm 1750 100 1715 235 AD 305 95.4% ENR13-VI-01 Lex2 248* 261cm 2195 35 2220 270 BC 110 95.4% # Denotes sample of possible aquatic origin * Denotes samples used in age model Radiocarbon dates for ENR 13-VI-01 we re determined by accelerator mass spectrometry (AMS) at Lawrence Livermore Na tional Laboratories. Samples consisted of terrestrial organic material (wood, char coal), a seed, and one sample of unknown origin (Table 3.2). Radiocarbon ages were calibrated and converted to calendar ages using OxCal Program version 3.9 (Bronk Ramsey, 2001; Bronk Ramsey, 1995) with a 100-year moving average of the tree-ring ca libration data set (S tuiver et al., 1998).
32 Intact sediment core sections were measured for bulk density and magnetic susceptibility using a GEOTEK Multisensor Core Logger (MSCL). Bulk density was measured by gamma-ray attenuation (GRA ) of the sediment using a sealed 137Cs source. Magnetic susceptibility was measured using a Bartington loop sensor and is reported in the cgs (centimeter, gram, second) scale. Both measurements were taken simultaneously at 0.5 cm intervals along the core length allo wing for stratigraphic correlation between the two data sets. Elemental analyses of sediments included total carbon (TC), to tal nitrogen (TN) and total inorganic carbon (TIC). Samples for elemental analysis were collected at 1 cm intervals and oven dried for 24-36 hours at 40 C. Samples were then ground with a mortar and pestle. TC and TN were measured using a Carlo Erba NA 1500 CNS elemental analyzer with autosampler. TIC was measured by coulometric titration (Engleman et al., 1985) with a UIC/Coulometrics Model 5011 coulometer coupled with a UIC CM5240-TIC preparation de vice. Analytical precisi on of TIC is approximately Â±0.5% based on repeated analysis of a calcium carbonate (CaCO3) internal standard. Organic carbon (OC) was estimated by the difference between TC and TIC. Oxygen isotope ratios ( 18O) were measured on shells of the foraminiferan Ammonia beccarii , valves of the ostracod Perissocytheridea rugata and shells of the gastropod Heleobops clytus . Sediment samples were first disaggregated in a weak (~2%) H2O2 solution and then washed through a 63Âµm sieve. After drying, specimens were picked from the 63Âµm (foraminifera), 150Âµm (ostracods) and 212Âµm (gastropods) fraction under a binocular microscope. Speci mens collected for isotopic analysis were then soaked in 15% H2O2 to remove any adhering organic material, cleaned and sonicated
33 in deionized water, and rinsed with metha nol before drying. For gastropod samples, shells were broken following the first cleani ng and the procedure was repeated a second time to ensure clean shell material. Sample weights for isotopic analysis ranged between ~250 and 600Âµg. Approximately 35 foraminifera tests were required for each analysis. Multiple ostracod valves from a sample were ground into a fine powder using a methanol-cleaned stainless steel rod and a fraction of this powder was used for each measurement. Gastropod shells were treated similarly. Measurement of multiple individuals in a single sedime nt horizon reduces variance inhe rent in measuring single, short-lived organisms (Heaton et al., 1995). Isotope samples were reacted in 100% orthophosphoric acid at 70 C using a Finnigan MAT Kiel III automated preparation device. The evolved CO2 gas was cryogenically purified before being measur ed online with a Finnigan MAT 252 mass spectrometer. Isotope ratios were compared to an internal gas sta ndard and expressed in conventional delta ( ) notation as a per mil (Â‰) deviation from Vienna PeeDee Belemnite. Precision for 18O samples was Â±0.09Â‰. Biostratigraphic analysis included fora minfera/ostracod assemblages and gastropod assemblages in core ENR 13-VI-01. Microfossil samples were examined at every 1-cm interval in conjunction with isotope samp ling. Changes in foraminfera and ostracod assemblages were noted on a presence/a bsence basis. Gastropod abundance was determined by counting individuals from the >212Âµm sediment fraction. Only complete gastropod shells were include d in the abundance counts. Mineralogy from core ENR 13-VI-01 was conducted on a subsample of material from the lower composite cores (Lex OL and Lex 2, see Table 3.1). Samples were
34 shipped to the University of Manitoba , Department of Geological Sciences Crystallography Lab. Samples analyzed fo r mineralogy were air-dried at room temperature, disaggregated in a mortar a nd pestle, and passed through a 62.5 mm sieve. Bulk mineralogy and detailed carbonate and ev aporite mineralogy were determined using a Philips PW-1710 powder diffractometer. An automated computer program designed to identify and match minerals was used to aid in mineral identificati on. Percentages of the various minerals were estimated from the bulk mineral diffractograms using the intensity of the strongest peak for each mineral as outlined by Last (2001). Results Chronology Seven AMS 14C dates were obtained from core ENR 13-VI-01 (Table 3.2). The two uppermost samples from 24 cm and 41 cm depth yielded the same age of 740Â±100 and 760Â±100 14C years BP, respectively. These dates seem old for such shallow samples. They may represent material that remained in the watershed for some time before being transported to the lake. It is also possible that these two samples were of aquatic origin and subject to hard water lake error (HWLE) . HWLE is associated with basins that receive 14C-deficient dissolved inorga nic carbon from the dissolution of ancient limestone (Deevey & Stuiver, 1964). Based on date s from paired ostracod and wood samples, HWLE in Lake Miragoane, Haiti was estimat ed to be ~1000 years (Curtis & Hodell, 1993). In Lake Punta Laguna, Mexico, HWLE was estimated to be about 1250 years (Curtis et al., 1996). The age-depth relation in core ENR 13-VI-01 was estimated assuming a linear sedimentation rate between five of the se ven dated horizons (Fi gure 3.3, Table 3.2). The basal age of the core is estimated to be 356 calendar years (cal yr) BC based on
35 extrapolation of the linear sedimentation ra te between the two oldest dated horizons. Dates were assigned to sediments above 86 cm depth (1530 AD) assuming linear sedimentation between 86 cm and the surface (2001 AD). Mean sampling resolution was on the order of ~11 years per 1 cm sample. -500 0 500 1000 1500 2000 050100150200250300Calendar age (AD/BC)depth Figure 3.2 Age-depth relationship for core EN R 13-VI-01. Ages for each sample were determined by linear interpolation between dated horizons (Table 3.2) including a value of 2000 calendar year AD at the surface. Ages for sediments below the 261cm sample ( 270 cal yr BC) were determined by extrapolation between the two basal dates. Lithoand Biostratigraphy Inorganic carbon is assumed to be associated primarily with CaCO3 in Enriquillo sediments and CaCO3 content was estimated as 8.33 x TIC. CaCO3 concentrations in Lago Enriquillo sediments are highest (>70%) below ~250 BC. Concentrations decrease upward in two steps, until reachi ng a mean value of ~50% CaCO3 between 200 Â– 1500
36 AD. Concentrations decline again from 1500 2000 AD to a minimum value of ~30% CaCO3 (Figure 3.3a). Percent organic carbon: percent nitrogen (OC/N) ratios have a mean value of 6.28 in the lower part of the core (400 Â– 145 BC). The mean value increases to 9.81 between 120 BC and 1100 AD. A brief period of decreased values (mean = 6.88) occurs between 1100 and 1400 AD. OC/N values then experience two brief periods of increased values between 1600 and 1750 AD and again around 1800 AD before decreasing towards to surface (Figur e 3.3b). Organic carbon (OC) concentrations in sediments are lowest in the bottom 25 cm (~400 Â– 80 cal yr BC). Concentrations then become highly variable between 80 BC and 1100 AD with a mean concentration of 1.68%. Concentrations increase towards the surface with a maximum concentration of 4.31% (Figure 3.3c). Stratigraphic variations in the content of CaCO3 and OC are also evident through a visual analysis of the sediment core (Fi gure 3.3d). The lower ~15 cm (~360 Â– 180 BC) of core ENR 13-VI-01 are composed almost entirely of CaCO3-rich sands. A large piece of wood was located within this interval and provided the basa l date for core ENR 13-VI01. Above this sand layer, CaCO3 concentrations decline coinci dent with an increase in OC. A series of spikes in the CaCO3 concentration of the sediments between 238 and 222 cm (~0 Â– 200 AD) are coincident with a visibly high concentra tion of gastropod and bivalve shells. Between 215 and 140 cm (~275 Â– 985 AD), alternating horizons of carbonate mud and more organic-rich layers become more regular with at least seven prominent layers occurring w ithin this ~700 year period. The dominant sedimentary facies between 140 and 20 cm (985 Â– 1890 AD), is a CaCO3-rich mud that is interspersed with horizons of darker, OC-rich layers that represent a ~0.5% increase in OC
37 concentrations. The upper 14 cm (~1920 AD to pres ent) of the core consist of alternating OC-rich and CaCO3-rich sediments that are represente d as darker and lighter bands of sediment, respectively (Figure 3.3d). The predominant species of ostracod, P. rugata, experiences dramatic fluctuations in concentration through out the sediment core (Figure 3.4a ). It is present in only two distinct sediment intervals in the lower half of the core, oc curring between 176 and 161 cm (640 Â– 800 AD) and 120 and197 cm (170 Â– 416 AD). Between 136 and 58 cm (~960 Â– 1684 AD), P. rugata is consistently present. It o ccurs only sporadical ly between 15 cm and 58 cm, and is absent from the upper 15 cm of the sediment record. Foraminifer assemblages are dominated by miliolid species (e.g. Quinqueloculina spp., Triloculina spp., Milliamina fusca ) and the rotallid, Ammonia beccarii . A. beccarii has a well-defined salinity to lerance of <67Â‰ (Bradshaw, 19 57), and its presence and/or absence reveals data on salinity variability in the lake water (Figure 3.4b). The bottommost interval in the sediment core (betw een 355 Â– 110 BC) contains two short intervals where A. beccarii is present (345 Â– 320 BC and 215 Â– 200 BC). Also present in this bottom interval is the planktonic foraminfer, Globigerina bulloides . A. beccarii is again present in the microfossil record between 234 and 213 cm (~55 Â– 300 AD). It is then absent from the record for the next 120 cm (~1100 cal years) with only one 2 cm interval containing the foraminifer. Between 91 and 25 cm (~1408 Â– 1863 AD), A. beccarii , in conjunction with the miliolid assemblage, is consistently present. A.beccarii is absent from the record in the upper 15 cm (1915 to present). Gastropod biostratigraphy in the sediment core contains two di stinct assemblages (Figure 3.4c). The first assemblage is char acterized by a brackish water snail of the
38 genus Battilaria . The second assemblage is defined by the presence of Heleobops clytus , a freshwater, epiphytic gastropod that was first described in samples from the Enriquillo basin (Thompson and Hershler 1991). 3040506070 -500 0 500 1000 1500 2000%CaCO3 2468101214OC/N 012345 -500 0 500 1000 1500 2000%C(org) Figure 3.3. Sediment geochemistry for co re ENR 13-VI-01. (a) Percent calcium carbonate, (b) organic carbon:nitrogen, and (c) percent organic carbon for ENR 13-VI-01. (d) Coarse lithologic va riations in the core also mirror the sediment geochemistry. Sand Wood CaCO3 shell Fine organic Or g anic-rich CaCO3 mud abc d Calendar Age AD(+)/BC(-)
39 A. beccariib -500 0 500 1000 1500 20000 5 10 15 050100150 Battilaria sp. H. clytusc 2000 1500 1000 500 0 -500aP. rugata Figure 3.4. Biostratigraphy in core ENR 13-VI-01. (a) The halophilic ostracod P. rugata , (b) the traditionally marine foraminifer A. beccarii , and (c) two gastropods, H. clytus whose habitat is adjacent to fresh water springs, and Battilaria sp., a common brackish water organism. The transition between these two assembla ges creates a well-defined boundary in the microfossil record that is centered between 215 and 208 cm (~275 Â– 352 AD). Battilaria sp. occurs below the 160 AD horizon with abundances greater than 5 individuals occurring at 106 and 42 AD, as well as 55 BC. H.clytus is present in the microfossil Calendar Age Â– AD(+)/BC(-) absence absence presence presence
40 record in varying abundances between 0 Â– 208 cm. Peaks in abundance (> 50 individuals) occur at 170, 655, 1115, 1517, 1557, 1609, and 1694 AD. GRA Bulk Density and Magnetic Susceptibility Bulk density and magnetic susceptibility were plotted relative to age (calendar years AD/BC) in core ENR 13-VI-01. GRA bul k density ranges from a high of ~1.9 in the oldest part of the co re to a low of ~1.16 gm/cm3 in the upper-most section of the core (Figure 3.5a). Sediments exhibit regular variability between 355 BC and 985 AD, with 11 periods of peak bulk density values greater than 1.6 gm/cm3 in this interval. Bulk density values then exhibit a gradual decline towards the top of the core, displaying only two periods with values greater than 1.6 gm/cm3 in the last ~1000 years. The magnetic susceptibility record of Enriquillo sediment s displays two pronounced periods of high susceptibility (Figure 3.5b). The first in terval occurs between ~355 and 150 BC, and shows two peaks with values of 8.61 cgs and 12.37 cgs. A second period of high values occurs between 810 and 985 AD, with a maxi mum of 14.55 cgs. Six additional peaks, albeit of lesser magnitude, occur in the in terval from 355 BC to 810 AD. After 985 AD, magnetic susceptibility values decrease upwards in the core, with only two values in the last millennium > 3.0 cgs. Stable isotopes P. rugata valves (Figure 3.6a) first appear in the record at ~220 cal yr AD. From 220 to 470 AD, 18O values average -0.49Â‰. The next ~ 200 years are devoid of ostracod valves. Ostracods return to th e sediment record ~665 cal yr AD and 18O values are highly variable for the next 300 year s before reaching maximum values of 1.58Â‰, 1.45Â‰, 1.51Â‰, and 1.24Â‰ centered at 974, 993, 1018, a nd 1065 cal yr AD, respectively.
41 Following this interval, oxygen isotope values decline to a minimum of -0.35Â‰. Values then return to an average of 0.58Â‰ before decreasing abruptly between 1750 -1825 cal yr AD to a value of -0.79Â‰. Oxygen isotope concentrations of P. rugata valves then increase towards the top of the sediment core. Ammonia beccarii (Figure 3.6b) displays irregular presence over the length of the sediment record. A. beccarii first appears between ~200 a nd 385 cal yr BC, during which time 18O values averaged -0.67Â‰. A. beccarii is absent from the microfossil record for ~280 cal yr and then returns to the reco rd around 80 cal yr AD. Oxygen isotope concentrations generally vary about a mean of -1.22Â‰ for the next 360 cal yr, but the period is also characterized by an abrupt shift to a mini mum value of -3.05Â‰ at 310 cal yr AD. A. beccarii is absent from the microfoss il record between 450 and 1480 cal yr AD. When it returns, oxygen isot ope values vary about a mean 18O value of -0.04Â‰. During the period between 1480 and 1960 cal yr AD, values vary between a maximum value of 0.81Â‰ at 1600 cal yr AD and minima of -1.93Â‰ and -0.69Â‰ centered at about 1690 and 1880 cal yr AD, respectively. The lowest 20 cm of core ENR 13-VI01 contains no fossil remains of the gastropod Heleobops clytus . H. clytus (Figure 3.6c) first appe ars in the microfossil record at 93 cal yr BC with a 18O value of 0.92Â‰. Oxygen isotope values vary about a mean value of 1.29Â‰ for the entire record. The record is marked by four distinct minima at 310, 700, 933 and 1335 cal yr AD. Follo wing the minium at 1335 cal yr AD, 18O values increase gradually towards the top before the gastropod disappears from the microfossil record in ~1960 cal yr A.D (15.0 cm sediment depth).
42 -500 0 500 1000 1500 2000 126.96.36.199.8 GRA bulk density (gm/cm3)a 510 magnetic susceptibility (cgs)b Figure 3.5. GEOTEK multisensor core logger data of lithologic variables for core ENR 13-VI-01. GRA bulk density (a) and magnetic susceptibi lity (b) for core ENR 13-VI-01. Bulk density and magnetic susceptibility are measured simultaneously on a GEOTEK multisensor core logger. Calendar Age Â– AD(+)/BC(-)
43 -500 0 500 1000 1500 2000-1.5-1-0.500.511.5 18O (Â‰, PDB)a -3-2-1018O (Â‰, PDB)b -2-10123 050100150-500 0 500 1000 1500 2000c# of gastropods18O (Â‰, PDB) Figure 3.6. Stable isotope ( 18O) stratigraphy from core ENR 13-VI-01. Carbonate shell material from the ostracod Perrisocytherridea rugata (a), the foraminifer Ammonia beccarii (b), and the gastropod Heleobops clytus (c). The 18O of H. clytus is also plotted versus its ab solute abundance (see Figure 3.4c). Records are smoothed with a 3-point r unning mean to illustrate long-term trends in local E/P ratios and regional climate dynamics. Calendar A g e Â– AD(+)/BC(-)
44 Discussion Interpretation of Sediment Proxies Changes in the geochemical composition of lacustrine sediments serve as a proxy for changes in the type and mode of mate rial transported with in a basin. Climate variability can affect materi al transport processes thr ough influencing physical and chemical weathering within a basin. In addition, climate has a strong influence on vegetation cover, vegetation ty pe, and soil erosion. The re lationship between vegetation cover, vegetation type and soil erosion aff ects the composition and amount of inorganic and organic material transported into basins (Rosenmeier et al., 2002; Binford et al., 1987; Engstrom & Wright, 1984). Changes in the composition of bulk organic matter, expressed as OC/N, provide information on changes in the proportion of terrestrial versus aquatic organic matter deposited in the lake basin. Low OC:N valu es are associated with organisms that are easily decomposed such as phytoplankton wh ile recalcitrant woody debris and other terrestrial organic material typically have high OC:N ratios. Poor preservation and diagenetic loss of nitrogen may lead to elevated OC:N ratios (Cohen, 2003). Carbonate deposited in lakes can be derived from either autochthonous or allochthonous sources. Autochthonous sources include the burial of shelled organisms (e.g. ostracods or foraminifera) or bio-induced precipitation during CO2 removal in photosynthesis. Alternatively, allochthonous sources of carb onate can be delivered via erosion and deposition of material from the lakeÂ’s watershed. Changes in carbonate concentrations of sediments can elucidate changes in productivity, water chemistry and regional climate factors.
45 Microfossil assemblages can serve as biot ic proxies of environmental change. Many organisms have well constrained habitat preferences and environmental requirements that aid in the determination of past conditions. Salin ity is considered a primary determining factor in organismal distributions, but other biotic and abiotic factors can also exert strong influence on their abundance (e.g food availability, competition, predation, temperature, disso lved oxygen, pH) (Murray, 1991; Herbst, 2001). Paleolimnological reconstructions are best served when multiple invertebrate groups are used (Crisman, 1978). Physical properties of sediment s (e.g. GRA bulk density and magnetic susceptibility) serve as an indirect measure of litho-facies characteri stics. Bulk density provides information on changes in lithol ogy and porosity. Magne tic susceptibility represents the degree to which a material can be Â“magnetizedÂ” by an external magnetic field and is a function of the composition and concentration of magnetic minerals in the sediment. Changes in these two physical properties can be related to both internal processes such as chemical redox reactions that result in magnetic mineral formation (Cohen, 2003), or climatic processes such as increased terrigenous input resulting from either wetter conditions (Nyberg, 2001) or increased Aeolian transport of terrigenous material during arid periods (Street-Perrott et al., 2000). Changes in sediment bulk density and magnetic susceptibility can reveal important information about internal and external processes within the ca tchment (Oldfield et al., 1983). Stable isotope analysis of calcite-secreti ng organisms has been used extensively as an effective proxy for inferring past environm ental change during the Holocene (Holmes, 1998; Curtis et al 1996; Curtis and Hodell, 1993; Covich and Stuive r, 1974). The oxygen
46 isotopic composition of authigen ic calcite and aragonite (e.g. foraminifera, ostracod and gastropod shells) is dependent on the temperature and oxygen isotopic ratio (18O/16O) of the host (i.e. lake) water (Cra ig, 1965). It is believed that temperature during the Holocene has been relatively stable (Brenne r el al. 2002) and long-term temperature variations in the tropics have been relatively small (Curtis and Hodell, 1993). Therefore, assuming that carbonate-secreting organisms produ ce their shells in e quilibrium with lake water, the changing isotopic signature of the ca licite or aragonite reflects the changes in the isotopic signature of th e host water (Stuiver, 1970). Isotope fractionation in the host water occurs because the lighter isotope, H2 16O, has a higher vapor pressure than H2 18O and is preferentially inco rporated into the gaseous phase during evaporative events, leaving the water enriched in 18O (Fontes and Gonfiantini, 1967; Curtis and H odell, 1993). The isotopic si gnature of the host water is thus controlled by climate conditions, name ly the ratio between evaporation and precipitation (E/P) (Curtis and Hodell, 1993). In addition, it ha s been shown that changes in salinity can affect 18O of carbonate shells (Chivas et al., 1985; 1986; Curtis and Hodell, 1993). Discerning the effects of salinity versus changing E/P ratios on the 18O of carbonate shells requires further informa tion on chemical concen trations within the carbonate shell material (i.e. Sr/Ca and Mg/Ca ratios) (Chiva s et al., 1986). Studies of Lago Enriquillo waters have shown that 18O and salinity covary up to salinities of 108Â‰ (r2 = 0.95) (Buck et al., in press). Changes in the 18O of shell material in Enriquillo probably represent changes in both the 18O and salinity of lake water.
47 Changing Environments of the Lago Enriquillo Basin Open marine to brackish wat er environment (~400 BC Â– 170 AD) The base of core ENR 13-VI-01 (~385 Â– 270 BC) contains a high concentration of CaCO3 (~70%) (Figure 3.4b). This section of th e core also contains the foraminifer, G. bulloides . Isotope values obtained from A. beccarii are relatively light (average 18O = 0.67Â‰). G. bulloides is a planktonic foraminifer that is most abundant in cool upwelling environments with salinities between 33 and 36Â‰ (Hilbrecht, 1996). The presence of G. bulloides , together with the high CaCO3 content of the sediments and the isotopically light signature of A. beccarii , indicate an open marine environment with salinities near modern Caribbean values (i.e. ~35Â‰). Studies of Holocene corals deposited in the Enriquillo basin during a marine transgression yielded upper UTh dates of ~3000 yr BP (~1050 cal yr BC) (Mann et al., 1984; Taylor et al., 1985). If basal sedime nts of core ENR 13-V I-01 contain a hiatus, part of the record spanning the transition fr om an open marine to a brackish environment may be missing. It has been suggested that the transition from an open marine to a brackish water environment was the result of fluvial dammi ng caused by the Yaque del Sur River that flows southeast into the Neiba/Cul de Sac Valley (Mann et al., 1984). Although a precise mechanism for basin isolation cannot be inferr ed from the sediment record, the transition is recorded in the sediment geochemistry a nd associated microfossil assemblages near the base of the core, about 270 Â±110 cal yr BC. Organic carbon content (Figure 3.4c) increases rapidly above this ba sal date, from < 1% to ~ 3%. Declines in bulk density and magnetic susceptibility mark the horizon where wood was sampled for the basal AMS 14C date.
48 Presence of the gastropod Battilaria sp. between 310 BC and 160 AD (Figure 5c) indicates a period of reduced marine influence and a transition to estuarine/brackish water conditions. The ha lophobic gastropod H. clytus is present in this interval, but in very small numbers. The 18O values from this interval sugge st a period of gradual freshening of the waters and a reducti on in E/P (Figure 3.7b, 3.7c). High ecosystem variability (170 Â– 1000 AD) Shifts in the microfossil assemblage around ~170 AD suggest a transition from brackish waters to an environment more cons istent with the hype rsaline modern lake ecosystem. The modern lake environment is characterized by hypersaline lake waters that receive fresh water inputs fr om a series of springs and streams. This combination of saline and fresh waters enables both halophobic and halophilic taxa to reside in the lake. In addition to the previously noted foraminife ra and ostracods, the freshwater gastropod H. clytus also characterizes the carbonate microfo ssil assemblage (Buck et al., in press). A pronounced shift in the gastropod asse mblage around ~170 AD marks a hydrologic transition from brackish waters to an envir onment more consistent with the hypersaline modern lake ecosystem. At this time, H. clytus comes to dominate the >212Âµm microfossil assemblage, and the gastropod Battilaria sp. all but disappea rs (Figure 3.5c). This transition is also marked by 18O minima for all three species measured (Figure 3.7). Following this transition from an estuarine to a hypersaline lacustrine environment, paleo-proxies indicate periods of cha nging lake water salinities that affect microinvertebrate assemblages and sediment lithology. Following the isotopic minimum at ~170 AD, 18O values of the ostracod P. rugata increase quickly by >1.0Â‰. This increase could not be verified isotopically with foraminifera because of their scarcity in
49 the record (Figure 3.5b). The period be tween ~170 and 400 AD has only four samples containing A. beccarii . A. beccarii has a limited salinity range in which it can reproduce (16-67%) (Bradshaw, 1957). Its near absence during this interval s uggests that salinities increased to values >67%. The 18O values of H. clytus also indicate increased E/P and salinity (Figure 3.7c). E/P and salinity continued to increase until ~670 AD when pronounced freshening occurred. This fresheni ng event is represented by a peak in H. clytus abundance (>50 individuals ) that is coincident with a decrease in 18O of >2.0Â‰ (Figure 3.5c). 18O values for P. rugata also decline between 650 and 720 AD. Carbonate shell material in the sediment co re appears to be alte red in the interval between ~700 and 1000 AD. Carbonate shells of both ostracods and gastropods showed pockets of erosion on the outer shell surface. Ostracods also appeared bleached. Few levels within this sediment in terval contained carbonate shell material that did not appear diagenically altered. However, in the few intervals that di d contain unaltered samples, the 18O records of P. rugata and H. clytus suggest three excursions to drier, more saline conditions in Lago Enriquillo between 700 and 1000 AD. The three excursions are centered between 725 and 775 AD, around 830 AD, and 966 Â– 999 AD. Freshening of Lago Enriquillo (1000 Â– 1350 AD) Between 1000 and 1300 AD, Lago Enriquillo experienced a prolonged period of wet, less saline conditions. This is inferred primarily from the decreasing 18O of P. rugata and H. clytus . Oxygen isotope ratios of P. rugata declined by nearly 1.5Â‰ and H. clytus declined by almost 3.0Â‰ (Figure 3.7a, 3.7c). From ~1000 to 1100 AD there was also an increase in the sedimentation rate in Lago Enriquillo that co incided with a subtle decline in the TOC/TN ratio. This d ecline suggests a great er contribution of
50 autochthonous carbon (i.e. phytoplankton) to the sediment organic matter, which may have been a consequence of greater al gal production under less saline conditions. The decline in 18O values continued from 1100 to 1350 AD. This period was characterized by core lithology >50% CaCO3 and declining organic carbon content (Figure 3.4). The isotope minimum m easured in the shell material of H. clytus at ~ 1350 AD coincides with a slight in crease in the abundance of th is gastropod (Figure 3.5c). Lago Enriquillo variability (1350 Â– present) Following the isotopic minimum at 1350 AD, Lago Enriquillo transitioned into a mode of consistent, yet damped variability w ith regard to sediment geochemistry, lake water salinity, and inferred E/P. Bulk density and magnetic susceptibi lity variations are reduced during this interval. A return to lake water salinities at or slightly higher than seawater is marked by the return of the foraminifer A. beccarii in the biostratigraphic record. Organic carbon concentrations and OC/N ratios remained high between 1500 and 1850 AD, suggesting increased c ontribution of terrestrially-derived organic matter. A decline in OC/N ratios after 1850 AD s uggests an increased contribution of phytoplankton to sediment organic ma terial. In addition, the gastropod H. clytus becomes more abundant in this interval . Between 1350 and 2000 AD, a total of 1377 individuals were counted compared to onl y 984 individuals counted from the entire sediment core below this interval (130 BC Â– 1350 AD). Four of seven peaks in abundance (>50 individuals/sample) occur in this time interval. This suggests a greater and more consistent contribution of fresh water into Lago Enriquillo. Regional and Global Climate Comparisons The geochemical, biostratigraphic, and isotope records from Lago Enriquillo illustrate this systemÂ’s sensitivity to local climate variations within its watershed. The
51 GRA bulk density and magnetic su sceptibility records from this unique inland saline lake may also reflect Holocene climate records. Comparisons between these two sediment variables from Lago Enriquillo and other la custrine and marine proxy climate records within the Caribbean basin suggest consistent responses to changes in larger regional climate regimes. Mechanisms for GRA Bulk Density and Magnetic Susceptibility Variability Bulk density measurements reflect the lithology and porosity of the sediments Magnetic susceptibility is an indicator of lithologic change, revealing changes in the magnetic character of sediments and also respond s to changes in sediment particle size. Comparison of these two vari ables with other sediment geochemical data helped elucidate potential large-scale drivers of environmental change in the Lago Enriquillo watershed and the larger Caribbean. A comparison between bulk density, ma gnetic susceptibility, organic carbon concentrations and CaCO3 concentrations suggests a se ries of complex relationships between these lithologic and geochemical variables in ENR 13-VI-01. Figure 3.7 compares these four proxies. The shaded gray bands (numbered 1 through 5) correspond with approximately 200 year time interval s surrounding periods of higher bulk density and magnetic susceptibility. Placement of these bands captures the two greatest excursions in bulk density and magnetic susceptibility cen tered around ~ 300 BC and ~900 AD. Each band is separated from ot her bands by ~200 years (Figure 3.7).
52 1234%C(org)b 3040506070%CaCO3c -500 0 500 1000 1500 2000 188.8.131.52.8 GRA bulk density (gm/cm3)a 510 -500 0 500 1000 1500 2000 magnetic susceptibility (cgs)d Figure 3.7. Comparison between sediment geoc hemistry and lithologic variables. This figure includes a comparison betwee n (a) GRA bulk density, (b) organic carbon concentration, (c) CaCO3 concentration, and (d) magnetic susceptibility. In addition, 5 separa te periods of high bulk density and magnetic susceptibility are highlighted. These bands represent ~200 years of the sediment record and are separate d by approximately 200 years as well. Within these periods of high bulk dens ity and, to a lesser extent, magnetic susceptibility the relationships between OC and CaCO3 can be illustrated. Calendar Age AD ( + ) /BC ( ) 2 1 3 4 5
53 During periods of high bulk density a nd high magnetic susceptibilty, organic carbon (OC) decreases implying a reduction in the input of organi c matter during these periods (Figure 3.7a,b,d). Declines in OC concentrations are co incident with declines in OC:N ratio (see Figure 3.3), suggesting a decrea se in terrestrial i nputs to the organic carbon pool. In addition, CaCO3 concentrations closely parallel the variability in bulk density suggesting increased de position of fine grained carb onate clay material during these intervals. This increase in CaCO3 is most likely not relate d to increased input from erosion in the watershed because of th e out of phase relationship between CaCO3 and organic carbon (Figure 3.7b,c). The alternating dark, orga nic-rich and grey, CaCO3-rich sediments (Figure 3.7b, c) may indicate reduced conditions at the sediment-w ater interface. It is possible for sulfate reduction to occur in saline environments (e .g. estuarine and marine environments) that have abundant sulfate (SO4 -2) in the water column (Berne r, 1980; Passier et el., 2001; Otero & Macias, 2003). Sulfate reduction occurs under anoxic conditions and is mediated by obligate anaerobic sulfate reducing bacteria. During sulfate re duction, two byproducts are produced: 2CH2O + SO4 -2 ---> H2S + 2HCO3 The two end products are then available for further reactions at the sediment-water interface. The bicarbonate ion can disso ciate further to a carbonate ion (CO3 -2) and become available for bonding with divale nt cations in the water column (e.g. Ca+2 and Mg+2). Sulfide ions, produced via further reduction of H2S, can also bond with cations in the water column. In saline environments under anoxic conditions, it is likely that these
54 sulfide ions will bond wi th reduced iron (Fe+2) in the pore waters and anoxic water column. Pyrite (FeS2) is ultimately formed although intermediate compounds can also be present (e.g. FeS). In marine sediments w ith alternating organicrich and organic-poor horizons, the transitions between horizons can contain minerals of high chemical remnant magnetization (CRM) (Garming et al., 2004). A connection between pyrite formation and magnetization may also exist in the Enriquillo sediments (Figure 3.8c). Microscopic inspection of sediments from intervals with high bulk density did confirm the presence of pyrite minerals. A lthough mineralogical data for the entire ENR 13-VI-01 core are not available, the a ssociation between pyrite and bulk density parameters in the interval between 356 BC and 630 AD (Figure 3.8a) suggests the two variables are correlated positively. Mineralogical data from the same sediment interval also suggest a relationship between quartz (SiO2) content and both bulk density and magnetic susceptibility (Figure 3.8b, d). No quartz-bearing deposits, however, are recorded in the watershed. Consequently, I attribute the presence of quartz in these deposits to two possible mechanisms, both of which suggest arid conditions. Intervals with high bulk density and low carbonate microfossil preservation contained high concentrations of siliceous sponge spicules. Lack of carbonate in these intervals, in conjunction with the presence of recalcitrant sponge spicules, s uggests reducing, low-pH conditions which would dissolve biogenic carb onate. High concentrations of detrital quartz may be attributed to periods of increas ed aeolian transport. Significant amounts of dust from North Africa have been documented in the Caribbean over the past 40 years (Prospero & Lamb, 1997). These dust plumes are inversely correlated with regional rainfall (Prospero & Lamb, 1997) and may also be correlated with the North Atlantic
55 Oscillation (Moulin et al., 1997). Long-term re cords of aeolian transport have also been documented for the Holocene and are correlated with periods of high E/P, i.e. drier conditions in West Africa (Str eet-Perrott et al., 2000). -400 -200 0 200 400 600 048 184.108.40.206.8Pyrite (%)a 0246810 220.127.116.11.8Quartz (%)b 048 24681012Pyrite (%)c 246810 4681012 -400 -200 0 200 400 600Quartz (%)d Figure 3.8. Changes in litholog ic variables versus bulk mine ralogy. Variations in the bulk density record are rela ted to pyrite (a) and quartz content (b). Magnetic susceptibility is also related to variati ons in sediment pyrite formation (c) and the quartz fraction (d). Pyrite forma tion is a proxy for anoxic bottom waters and increased salinity in the basin. Quartz is a proxy for Aeolian dust transport. (Pyrite and quartz are plotted as dashed lines.) GRA bulk density (gm/cm3) magnetic susceptibility (cgs) Calendar Age Â– AD(+)/BC(-)
56 Variations in bulk density a nd magnetic susceptibility ar e related to both pyrite and quartz concentrations in the mineral fracti on of sediments from Lago Enriquillo. The combination of these proxies suggests that high bulk density reflects periods of anoxia and sulfur reduction in the wa ter column caused by high salini ty and possibly periods of increased aeolian dust trans port resulting from arid condi tions across the equatorial Atlantic. Assuming this to be the case, the r ecord of bulk density can be used to relate the paleoclimate record from Lago Enriquillo to other Caribbean records to investigate the regional dynamics of climate variability. Comparison to the Yucatan Peninsula Paleolimnological data from the Yucatan Peninsula, Mexico, revealed several phases of drought during the last 2600 years (Curtis et al., 1996; Hodell et al., 2001; in press). The periodicity of gypsum precipita tion in a core from Lake Chichancanab suggests a strong linkage between solar forcing mechanisms and climate variability in the region (Hodell et al., 2001). In addition, the oxygen isotope record from Punta Laguna in the northeast Yucatan is in cl ose agreement with the gypsum record from Chichancanab (Curtis et al., 1996). The bulk density record from Lago Enriquillo closely mirrors both the percent sulfur record from Chichancanab (Figure 3.9a) and the 18O record from Punta Laguna (Figure 3.9b) suggesting similar forcing mechanisms. In particular, two periods of high gypsum content (Chich ancanab), positive excursions in 18O (Punta Laguna), and increases in bulk density (Enr iquillo) occurred between 475 and 250 BC and 750 and 1050 AD. The cyclicity of dr oughts observed in the Yucatan (~208 and 50 year) is apparent in the Enriquillo bulk density record as well.
57 Regional Climate Linkages Â– Enriquillo and the Cariaco Basin A marine record from the Cariaco Basi n off Venezuela reveals a high-resolution record of climate variability that is closely correlated with the records from the Yucatan and Enriquillo (Figure 3.9c) (Haug et al., 2003). The Cariaco record in fers periods of dry and moist climate via titanium (Ti) concentratio ns in the marine sediments. Titanium is used as a proxy for fluvial transport of te rrestrial material during moist periods. Consequently, Ti concentrations and rainfall are thought to be positively correlated in the Cariaco record. However, the Cariaco record is dated relative to an assumed correlation with the Medieval Warm Period (between 1000 and 1300 AD) and consequently Â“floats in timeÂ”. Although the exact timing of drought in the Cariaco record differs slightly from the Yucatan records and the Enriquillo recor d, the frequency and scale of drought events are comparable. Extra Tropical Teleconnections Â– GISP2 Ice Core The Lago Enriquillo record also compares with extra-tropical records from the North Atlantic. The 18O record from the Greenland GI SP2 ice core is a >3000 m proxy record of climate variability. The upper ~500 m of the GISP2 record correspond to the past ~2500 years and reveal close correlati on with the Lago Enriquillo bulk density record between ~700 and 2000 AD (Figure 3.9d). Variations in the oxygen isotopic ratio of the Greenland ice record are positively co rrelated with variations in atmospheric temperature (i.e. Temp = 18O) (Stuiver et al., 1995).
58 -500 0 500 1000 1500 2000 18.104.22.168.8 051015 % S GRA bulk density (cm/gm3)a 22.214.171.124 -2-101 GRA bulk density (cm/gm3) 18O (Â‰.PDB) Cytheridella ilosvayib 126.96.36.199.8 0.1 0.15 0.2 Titanium (%) GRA bulk density (cm/gm3)c -500 0 500 1000 1500 2000 11.41.8 -36-35-34 GISP2 18O (Â‰, PDB) GRA bulk density (cm/gm3)d Figure 3.9. Regional comparisons of late Ho locene climate variability. A comparison between Lago Enriquillo bulk density and: (a) % sulfur from Lake Chichancanab, Mexico (H odell, et al., 2001); (b) 18O record from Punta Laguna, Mexico (Curtis et al., 1996); (c) the % tit anium record from the Cariaco Basin in the southern Caribbean Sea (Haug et al., 2003); and (d) the 18O record from the GISP2 ice core (Stu iver et al., 1997). Panels a through c illustrate the strong regional climatic similarities between the Yucatan, Lago Enriquillo, and the Cariaco Basin. Pane l d illustrates periods of climatic teleconnectivity between the Caribbean Basin and high northern latitudes. Calendar Age Â– AD(+)/BC(-)
59 Examination of the post-818 AD interval re vealed a relation between solar activity (determined through variations in the production of cosmogenic 14C) and the GISP2 18O record (Stuiver et al., 1997). Peri ods of low solar activity (i.e. high 14C) are correlated with low 18O values (i.e. cooler temperatures) in the Greenland ice core that include a 20-40 year lag due to ocean-atmosphere inter actions (Stuiver et al., 1997, see figure 4). The comparison between the GISP2 and En riquillo records sugge sts that circumCaribbean and North Atlantic climate was a ffected by solar variability during the last millennium. Mechanisms for Late Holocene Climate Variability The mechanisms by which solar activity mi ght affect both circum-Caribbean and polar climate variability ar e poorly understood. Modeli ng experiments using a 0.25% reduction in solar activity s uggest a low latitude cooli ng response (Rind & Overpeck, 1995). Low latitude cooling can lead to a reduction in the inte nsity of Hadley cell circulation across the tropics, thus reducing atmospheric moistu re in the tropics and heat transport to higher latitudes. In addition, it has been hypothesized that increases in 14C may result in minor increases in precipita tion in the North Atlantic (Magny, 1993 Â– as cited in Stuiver & Braziunus, 1993). The co mbination of reduced heat transport and increased precipitation in the North Atlantic causes perturbations to the North Atlantic salt oscillator (Bro ecker et al., 1990) and ultimately to North Atlantic Deep Water (NADW) production and regional climate dyna mics including a cooler polar region (Stuiver & Braziunus, 1993). Additional mechanisms linking 14C to climate variability that have been suggested include changes in the amount of ultraviolet light reaching the EarthÂ’s upper atmosphere
60 and cascading effects on ozone production, upp er-atmospheric temperatures, and possibly cloud formation (Haigh, 1994; Van Geel et al., 1999). The relationship between 14C and Holocene climate becomes more tenuous on larger timescales. Between ~500 and 2000 AD, the 18O and gypsum concentration records fr om Yucatan lakes and the isotope record from Greenland are in phase with 14C. However, prior to 500 AD, the variability in both solar activity ( 14C) and these paleo-proxies are completely out of phase with one another. (Stuiver et al., 1997; Hodell et al., 2001). Alt hough correlations between longterm paleoclimate records from the circum -Caribbean and North Atlantic have been established using both terrestrial (Hillesheim et al., submitted) and marine archives (Peterson et al., 2000), establis hing that they respond to sola r variability has proven more difficult. Comparisons between lo ng-term proxy climate records and 14C will be better served by taking into account both solar-at mospheric and ocean-atmospheric production and diffusion of 14C. (Stuiver & Braziunus, 1993). Solar forcing mechanisms and their potential control on moisture and heat transport from the tropics also affects the dominant tr opical weather pa ttern in the Intertropical Convergence Zone (ITCZ). Variations in ocean circulation patte rns from the tropical Atlantic into the Gulf of Mexico, that are linked with the seasonal migration of the ITCZ, exhibit cycles similar to century-scale solar variability (Poore et al., 2004). Changes in salinity and associated foraminifer communities in the Cariaco basin are also related to the migration of the ITCZ that is in turn corr elated to solar variabili ty (Black et al., 2004). During the later Holocene (ca. 3500 BP), it is thought that a general shift southward of the ITCZ caused drier conditions in th e Caribbean basin in response to changing seasonal differences in insolation (Haug et al., 2001; Curtis & Hodell, 1993). This
61 orbitally-driven shift in insolation caused increased seasonality in the Southern Hemisphere and a corresponding decrease in se asonality in the Northern Hemisphere. The lower seasonality in the Northern Hemisphere decreased the likelihood of Â“pullingÂ” the ITCZ northward off of the equator (Haug et al., 2001). The migration of the ITCZ is also affected by SST in the equatorial Pacific. As SST increases in the eastern Pacific (during an ENSO year, for example) sea level pressure (SLP) also decreases. This changes the pattern of surface flow in the Caribbean and results in increased SLP over the Caribb ean. Increased trade wind activity follows the pressure gradient between the tropical At lantic and the equatorial Pacific and results in a decrease in SST (Giannini et al., 2000) . ENSO events effectively normalize temperatures across the equator that reinforc es the southward shift of the ITCZ, bringing dry conditions to the circum-Caribbean (Maslin & Burns, 2000; Haug et al., 2001). An additional driver of Holocene climate va riability in the Caribbean basin is the North Atlantic Oscillation SST tripole. The NAO SST tripole includes the North Atlantic dipole and the tropical Atlant ic pressure system (aka Bermuda-Azores high) located between 0Â° and 15Â° N (Visbeck et al., 2003) . These two SST/SLP phenomena can have competing and sometimes complementing effect s on rainfall in the Caribbean (Giannini et al., 2000). Rainfall in the north Caribbean and the NAO dipole are inversely correlated (Malmgren et al., 1998). Changes in the tr opical Atlantic high pressure system are closely correlated with ENSO (Latif & Gr Ã¶tzner, 2000; Giannini et al., 2000). An increase in SLP in the tropical Atlantic re sults in increased trade wind activity, reduced convective activity, and drying in the Caribbea n. The role of ENSO in determining SLP in the tropical Atlantic is complicated by changing SST across the NAO dipole.
62 Variability in Caribbean climate regimes is the result of complex interactions between large scale ocean-atmospheric connections that may also be sensitive to variations in solar activity. The agreement between lacust rine, marine, and glacial ice records suggests that climate variability duri ng the past ~2300 years occurred in response to similar forcing mechanisms that included periods of large-scal e excursions in both atmospheric, oceanic, and potentially solar fields. Conclusions Lago EnriquilloÂ’s hypersaline lake water chemis try is highly sensitive to changes in local and regional climate. The ~2300 y ear sediment record from Lago Enriquillo provides a high resolution, multi-proxy record of climate variability in the Caribbean basin. The Enriquillo record is consistent with Holocene climate records from the Yucatan peninsula (Curtis et al., 1996; Hodell et al., 2001; in press) and the Cariaco basin (Haug et al., 2003). The agreement between lacustrine and marine records from the Caribbean suggests that these c limatic excursions were region al in nature. In addition, comparisons between the Enriquillo record and Greenland GISP2 ice core record argue for teleconnections between the Caribbean and the North Atlantic. The shift in these records at ~500 AD from being out of phase to in phase with one another suggests dynamic and changing interactions between the pol ar and tropical region s of the Atlantic. Forcing mechanisms for this climate variabil ity include solar vari ability, ENSO, and the NAO SST tripole that result in shifts in the location of the ITCZ.
63 CHAPTER 4 CONCLUSIONS Lago Enriquillo is a unique and dynamic inla nd saline lake. With this study, I have documented the lakeÂ’s variability over bot h short and long time periods. Short-term variability is observed in lake water salinity, major ion concen trations and stable isotopes and is controlled largely by changes in E/P. Lake water salinity increased from 80 to 106ppt between June 2001 and March 2003 while ion concentrations increased by almost 20 percent. In addition, lake level decreased by ~1.8m during the same interval of time. Lake level has varied significantly during the past 50 years (Cruz and Duquela-Gonzalez, 2003) and is closely correlated with tropica l storm and hurricane frequency across the basin. Isotopic analysis ( 18O and D) of Lago Enriquillo lake waters illustrated the effects of intense evaporation in the system. Ev aporation of lake waters also resulted in a positive linear response between salinity and 18O. The microbenthic community of Lago Enriquillo contains both halophili c and halophobic organisms, including foraminifera, ostracods, and gastropods. Ha bitat preference of these organisms, in combination with the other modern physical and chemical characteristics ultimately allowed for better interpretation of the pale olimnological record re trieved from Lago Enriquillo. The paleolimnological record from core ENR 13-VI-01 spans the past ~2300 years and reveals the lakeÂ’s sensitivity to clim ate variability over long time periods. A combination of sediment physical, mine ralogical and geochemical properties, biostratigraphy, and stable isotope an alysis was employed in this multi-proxy
64 paleolimnological study. During the mi ddle Holocene, Lago Enriquillo was an embayment of the Caribbean Sea and supporte d a fringing coral reef. The embayment was cut off from the sea and transitioned to an environment synonymous with estuarine conditions. The lake experien ced a prolonged period of high variability between 170 and 1000 AD. This period of variability was punc tuated by periods of high salinity and E/P as inferred from the biostratigraphic and stab le isotope records. The lake experienced a brief freshening period between 1000 and 1350 AD that coincides with the Medieval Warm Period that is recorded in other reco rds from the circum-Cari bbean (Curtis et al., 1993; Hodell et al., 2001; Haug et al., 2003). Following this brief period of increased moisture availability, the lake was again influenced by the arid climate conditions that persist today. The Lago Enriquillo record is also well correlated with othe r regional climate records. Together these records present a late Holocene climate history that is characterized by pan-Caribbean episodes of hi gh E/P that occur with a ~208 and 50 year cyclicity. Mechanisms for this pan-Caribbean climate pattern focus on the migration of the ITCZ and how its position acr oss the equatorial Atlantic belt can be affected by larger regional climate signals including ENSO, the NAO tripole, and solar variability. Future research in the region will seek to construct additional paleolimnological records from the neighboring basins of La guna Rincon and Etang Sa umatre. In addition, a remote sensing project designed to monitor la ke level fluctuations over time will benefit the existing survey of lake levels and better st rengthen the proposed connection between tropical storm frequency and La go Enriquillo lake level. Fi nally, greater attention must be focused on the role that current watershe d management practices play in the ecology
65 and conservation of Lago Enriquillo. Curre ntly, extensive irrigation canals and water storage facilities are under c onstruction in the watershed that may ultimately upset the lakeÂ’s water balance. A complete survey of water control structures and volume estimates for water diverted from the lake wi ll benefit future natu ral resource managers and residents in the basin.
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74 BIOGRAPHICAL SKETCH David Buck was born January 8, 1974, in Hartford, Connecticut. His family moved to Charlotte, North Carolina, where much of his extended family still lives. David graduated from West Charlotte High School in June of 1992. He attended the University of North Carolina at Chapel Hill from 1992 to 1996 and graduated with a bachelorÂ’s degree in Latin American Studies and a mi nor in geology. Following college, David lived and worked in Prescott, Arizona, for tw o years before getting a job that would take him to Belize, Central America. He worked as the field station manager for Monkey Bay Wildlife Sanctuary for two years during which time he was responsib le for facilitating international study abroad programs that focuse d on the natural and cu ltural history of the Maya region. It was in Belize that David met his wife, Ellie, an archaeologist from Boston University. David began the masterÂ’s program in interd isciplinary ecology at the University of Florida in the fall of 2001. His research at UF has focused on acquatic ecology and late Holocene climate variability. While at UF , David has had the opportunity to work on projects in the Dominican Republic, Guatemala, and Florida. In addition to his academic work, David still maintains an active interest in ecology and conser vation issues in the Neotropics.