Oxygen isotope evidence of Little Ice Age aridity on the Caribbean slope of the Cordillera Central, Dominican Republic

Quaternary Research 75 (2011) 461–470

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Oxygen isotope evidence of Little Ice Age aridity on the Caribbean slope of the Cordillera Central, Dominican Republic Chad S. Lane ⁎,1, Sally P. Horn, Kenneth H. Orvis, John M. Thomason Department of Geography, University of Tennessee, Knoxville, Tennessee, 37996, TN, USA

a r t i c l e

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Article history: Received 25 December 2009 Available online 12 February 2011 Keywords: Caribbean Dominican Republic Hispaniola Intertropical Convergence Zone Lake sediment Little Ice Age Ostracod Oxygen isotope

a b s t r a c t Climate change during the so-called Little Ice Age (LIA) of the 15th to 19th centuries was once thought to be limited to the high northern latitudes, but increasing evidence reflects significant climate change in the tropics. One of the hypothesized features of LIA climate in the low latitudes is a more southerly mean annual position of the Intertropical Convergence Zone (ITCZ), which produced more arid conditions through much of the northern tropics. High-resolution stable oxygen isotope data and other sedimentary evidence from Laguna de Felipe, located on the Caribbean slope of the Cordillera Central of the Dominican Republic, support the hypothesis that the mean annual position of the ITCZ was displaced significantly southward during much of the LIA. Placed within the context of regional paleoclimate and paleoceanographic records, and reconstructions of global LIA climate, this shift in mean annual ITCZ position appears to have been induced by lower solar insolation and internal dynamical responses of the global climate system. Our results from Hispaniola further emphasize the global nature of LIA climate change and the sensitivity of circum-Caribbean climate conditions to what are hypothesized to be relatively small variations in global energy budgets. © 2011 University of Washington. Published by Elsevier Inc. All rights reserved.

Introduction Many paleoclimate and historical records document a significant climate excursion between ~ A.D. 1400 and A.D. 1850, a period now commonly referred to as the Little Ice Age (LIA). Most records of LIA climate come from the high latitudes of the Northern Hemisphere, particularly the North Atlantic region and Europe (Mann et al., 2009) where both paleoclimate and historical records indicate significant decreases in regional air temperatures (e.g. Cronin et al., 2003, 2005; Moberg et al., 2005; Helama et al., 2009), decreased North Atlantic sea surface temperatures (Keigwin, 1996; Marchitto and deMenocal, 2003), increased meridional airflow (O'Brien et al., 1995), alpine glacial advances (Holzhauser et al., 2005), and increased North Atlantic sea ice (Broecker, 2000; Vare et al., 2009). The clear and copious evidence of LIA climate change from the high latitudes of the Northern Hemisphere led early investigators to regard this event as restricted to high-latitude locales. However, increasing evidence points to significant climate change in the tropics during the LIA. For example, oxygen isotope compositions of glacial ice in the tropical Andes indicate marked cooling during the LIA (Thompson et al., 2006), and geomorphological and

⁎ Corresponding author. Fax: +1 910 962 7077. E-mail address: [email protected] (C.S. Lane). 1 Current Address: Department of Geography and Geology, University of North Carolina-Wilmington, Wilmington, NC, 28403, USA.

sedimentary evidence document glacial advance in the Andean highlands during the LIA (Markgraf et al., 2000; Polissar et al., 2006; Rabatel et al., 2008). Several paleoclimate records from tropical Africa indicate increased aridity in western tropical Africa and increased precipitation in eastern tropical Africa during the LIA (Brown and Johnson, 2005), a pattern attributed to El Niño-Southern Oscillation (ENSO) variability and Intertropical Convergence Zone (ITCZ) dynamics (Russell and Johnson, 2007). The latter may be related to reductions in the intensity of the Asian monsoon, which have also been documented during the LIA in paleoclimate records from tropical and sub-tropical Asia (Thompson et al., 2000; Gupta et al., 2003). Some of the most detailed evidence of LIA climate change in the tropics has come from the circum-Caribbean region. In particular, the unique sedimentary conditions of the Cariaco Basin have produced highly resolved records of paleoceanographic and paleoclimate variability during the LIA (Haug et al., 2001; Black et al., 2004, 2007; Peterson and Haug, 2006). Of special interest is the sedimentary titanium (Ti) concentration record of the Cariaco Basin. The Cariaco Ti concentration record is thought to primarily reflect variations in terrigenous erosion in the watersheds of the Tuy, Unare, Neveri, and Manzanares rivers of northern South America. These variations in erosion, and in subsequent terrigenous sediment delivery to the Cariaco Basin by these river systems, are thought to be related to ITCZ precipitation in the region, with increased (decreased) Ti concentrations representing increased (decreased) precipitation due to a more northerly (southerly) mean annual position of the ITCZ (Peterson and Haug, 2006).

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C.S. Lane et al. / Quaternary Research 75 (2011) 461–470

Cariaco Basin sedimentary Ti concentrations reach some of the lowest values in the entirety of the Holocene during the LIA. If the Ti concentrations indeed reflect precipitation dynamics in northern South America, the implication is that the LIA was one of the driest periods of the last 10,000 yr in the circum-Caribbean region (Peterson and Haug, 2006), which is interesting because the LIA is generally considered to be a relatively minor climatic event relative to other Holocene climatic change events such as the 8.2 ka and 5.2 ka events (Mayewski et al., 2004). This increase in regional aridity has been attributed to a more southerly mean annual position of the ITCZ during the LIA. Such a dramatic change in climate would likely have had far reaching impacts in the region. In this study, we present high-resolution evidence of LIA precipitation variability reconstructed from the oxygen isotope composition of ostracod valves preserved in the sediments of a small mid-elevation lake on the Caribbean slope of the Cordillera Central, Dominican Republic. Precipitation variability along the Caribbean slope of the Cordillera Central is tightly linked to ITCZ dynamics, such that a southward displacement of the ITCZ during the LIA should have resulted in decreased precipitation at the site. We also place our paleoclimate proxy record in the context of other proxy records and modeling studies of LIA climate in an attempt to develop a more complete regional picture of LIA climate change in the circumCaribbean and of the mechanisms potentially responsible. Study site Laguna de Felipe (18°48′07″N, 70°52′47″W, 1005 m) is one of four small lakes located near the town of Las Lagunas on the Caribbean slope of the Cordillera Central of the Dominican Republic, on the island of Hispaniola (Fig. 1). Lane et al. (2009) provided detailed paleoenvironmental reconstructions for Laguna Castilla and Laguna de Salvador; this is our first report on proxies in the Felipe sediments. The land surrounding Laguna Felipe and all lakes of the area has been heavily modified by human activity and is dominated by

agricultural fields of corn, pigeon peas, and beans along with pasture. Small patches of forest near the lakes contain Hispaniolan pines (Pinus occidentalis Schwartz, Pinaceae), introduced rose apples (Syzygium jambos (L.) Alst., Myrtaceae), and other broadleaved species together with palms. When we cored Laguna Felipe in January 2004 it had a surface area of ~ 0.8 ha and a maximum water depth of 1.8 m. Unlike nearby Laguna Castilla and Laguna de Salvador (Fig. 1), Felipe was completely colonized by emergent aquatic plants, especially Typha domingensis Pers. (Typhaceae), with no open water. Felipe has significantly higher Ca2+ ion concentrations than the other Las Lagunas lakes, which has led to higher biogenic and authigenic carbonate concentrations in the sediments than in the neighboring lakes (Thomason, 2007; Lane et al., 2009). The temperature regime of the Las Lagunas area is typical of tropical mid-elevation locales, with a mean annual temperature of ~ 20°C and greater diurnal than seasonal temperature variation. Precipitation is primarily controlled by ITCZ dynamics. During the boreal winter, when the ITCZ at this longitude is primarily positioned over central South America, regional air pressures and trade wind intensities increase, inhibiting convective activity. While the trade winds do transport moisture onshore to some parts of Hispaniola, rainshadowed locations along the leeward slopes of Cordillera Central, including Las Lagunas, experience a distinct dry season (Bolay, 1997). Conversely, during the boreal summer when the ITCZ is located a few degrees south of Hispaniola and proximal-doldrum conditions dominate, convective activity increases. These conditions promote an increase in convective uplift along the Caribbean slope of the Cordillera Central, which is fed by sea-breeze moisture creating a distinct wet season around Las Lagunas. The island of Hispaniola is also located far enough north to be affected by polar air outbreaks from continental North America that can bring frontal precipitation to the island, but the position of Las Lagunas on the Caribbean (southern) slope of the Cordillera Central means it is largely rainshadowed from such precipitation events. Materials and methods

Figure 1. Location of the Las Lagunas study site (X) within the Dominican Republic (inset) and a topographic map of the Las Lagunas area. Laguna Felipe (italics) is the focus of this study; the paleoenvironmental history of nearby Laguna Castilla and Laguna de Salvador was reconstructed by Lane et al. (2009).

A 4.85 m long sediment core was recovered near the center of Laguna Felipe (18°48′07.2″N, 70°52′46.0″W) in ~ 1 m sections using a Colinvaux–Vohnaut locking piston corer (Colinvaux et al., 1999). Core sections were returned to the University of Tennessee in their original aluminum coring tubes and stored at 6°C. The aluminum core tubes were opened using a specialized router and the sediments were sliced lengthwise using a thin wire. Core sections were photographed upon opening and color (Munsell) and textural changes were recorded. The sediment core chronology is based on AMS radiocarbon dates on wood fragments and other plant macrofossils isolated from the sediments by wet sieving. Radiocarbon dates were converted to calibrated years BP (before A.D. 1950) using the CALIB 6.0.1 computer program (Stuiver and Reimer, 1993) and the dataset of Reimer et al. (2009). Sedimentation rates were calculated using the weighted means of the probability distributions of the calibrated ages (Telford et al., 2004), and ages were estimated using linear interpolation between the stratigraphic positions of the radiocarbon-dated materials. Ostracod valves, charophyte oospores and calcified stems, and gastropod shells were present throughout most of the Felipe sediment profile along with authigenic marls. Biogenic carbonate macrofossils were isolated from selected portions of the core using nested 500, 250, and 125 μm sieves at 1–4 cm sampling intervals, with higher sampling resolution around the time period encompassed by the LIA. Monospecific Cythridella boldii Purper ostracod valves were identified according to the description of Purper (1974) and with the assistance of Dr. Jonathan Holmes (University College London). Charophyte oospores were identified based on the descriptions of Wood and Imahori (1964) and Wood (1967).

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Isolated adult valves from the ostracod C. boldii were initially cleaned for isotopic analysis using a soft brush and distilled water. Any remaining organic matter was removed using a modified version of the methods of Lister (1988) and Diefendorf et al. (2006), in which carbonate fossils were roasted under vacuum at 375°C for 3 h. Similar treatment of multiple internal carbonate and dolomite standards indicated that no isotopic fractionation occurs with this cleaning method. The oxygen (δ18O) and carbon (δ13C) isotope compositions of the C. boldii valves were determined using an automated Finnigan CarboFlo system interfaced with a Finnigan MAT Delta-plus mass spectrometer. Biogenic carbonates were reacted with orthophosphoric acid at 120°C and the evolved CO2 was cryogenically purified on-line. Sample masses analyzed on the CarboFlo system averaged approximately 0.3 mg. All carbon and oxygen isotopic compositions have been temperature corrected to 25°C and are reported in standard δ-per mil notation relative to the Vienna-Pee Dee belemnite (V-PDB) marine-carbonate standard where:

463

The oxygen isotope compositions of fossil C. boldii valves (δ18OCyth) vary from −4.4‰ to 3.6‰. Maxima in the δ18OCyth record occur from ~150–100, ~450–250, ~1000–950 and ~1300–1150 cal yr BP, while minima occur from ~ 100–0, ~ 250–150, ~ 650–450, and ~ 1400– 1300 cal yr BP (Fig. 4). The carbon isotope compositions of fossil C. boldii valves (δ13CCyth) in the Felipe sedimentary record vary from −8.9‰ to −0.4‰. An overall trend exists of decreasing δ13CCyth values with time, with the exception of a large positive excursion ~100– 50 cal yr BP (Fig. 4). Linear regression analysis indicates no significant relationship between δ18OCyth and δ13CCyth values (r2 = 0.05). Isotopic analysis of duplicate samples at individual stratigraphic levels was limited by the availability of C. boldii valves in the sediment core. Where duplicate analyses were possible, the standard deviations of coeval δ18OCyth and δ13CCyth measurements were less than 0.7‰ and 0.4‰, respectively (n= 32). Discussion

h
 i δ13 C or δ18 Oðper milÞ = 1000 Rsample = Rstandard –1 ;

13

12

18

Isotope proxy interpretation

16

where R = C = C or O= O; respectively: Precision of the CarboFlo system was determined to be ±0.05‰ for δ13C and ±0.10‰ for δ18O using multiple internal and international standards. Results Radiocarbon analyses indicate that sediments began accumulating in Laguna Felipe approximately 1785 cal yr BP (Table 1; Figs. 2 and 3). Sedimentation rates vary from 0.13 cm/yr to 1.50 cm/yr (Fig. 3). The stratigraphy of the Felipe profile is somewhat complex (Fig. 2). Most of the profile consists of a matrix of organic silts and clays with intermixed marls, shells, and fine fibrous organics. The lowermost sediments of Felipe contain considerable fine sand, perhaps deposited during, and soon after, the slope failure event that created the lake basin (Lane et al., 2009). Two sections of mineral-rich clays are present from ~ 150–120 and ~250–190 cm sub-bottom. Ostracod valves are absent in the lower interval of mineral clay. The uppermost sediments consist of very fibrous organics, primarily partially decomposed remains of T. domingensis, and have poor carbonate preservation relative to the majority of the sediment record. Ostracod valves reach their highest abundances in the Felipe sediments between ~ 120 and 40 cm.

The oxygen isotope composition of lacustrine carbonates, including ostracod carapaces, in closed basin lakes depends primarily on the δ18O composition of the lake water in which they form and the temperature of the water at the time of formation (Craig, 1965; Fontes and Gonfiantini, 1967; Stuiver, 1970; Cohen, 2003). In closed basin lakes of the tropics, the δ18O value of lake water is primarily determined by the ratio of evaporation to precipitation (E/P). Temperature variability typically plays a very minor role in the oxygen isotope composition of tropical lacustrine carbonates (Covich and Stuiver, 1974; Gasse et al., 1990; Curtis and Hodell, 1993). During periods of increased (decreased) E/P ratios, the δ18O value of lake water will go up (down) as kinetic fractionation processes lead to an increase (decrease) in the relative concentrations of water molecules containing 18O compared to water molecules containing 16O. As ostracods assimilate their carbonate shells, they do so in isotopic equilibrium with the lake water, effectively sampling the oxygen isotope composition of the water at the time of carapace formation. The isotopic composition of ostracod carapaces can be influenced by factors other than lake water chemistry. For example, the δ18O and δ13C values of ostracod valves can be affected by species-specific fractionation differences, also called “vital effects,” (von Grafenstein et al., 1999; Keatings et al., 2002) and by variations in preferred microhabitat (Heaton et al., 1995; Ito et al., 2003). Furthermore, young ostracod instars can form carapaces with different isotopic

Table 1 Radiocarbon determinations and calibrations for Laguna Felipe. Lab numbera Depth (cm) δ13C (‰) Uncalibrated 14C age (14C yr BP) Calibrated age rangeb ± 2 σ (cal yr BP) Area under probability curve Weighted meanc (cal yr BP) β-233109

46

− 27.6

90 ± 40

β-233110

88

− 25.4

170 ± 40

β-228997

135

− 29.0

380 ± 40

β-281632 β-281633

159.5 364

− 22.9 − 29.4

620 ± 40 1490 ± 40

β-192643

457

− 25.5

1850 ± 40

a

− 1–−4 148–12 199–187 205–203 269–211 38–−3 118–64 231–124 295–243 407–316 508–421 662–545 1420–1302 1437–1434 1514–1461 1879–1701

0.013 0.693 0.017 0.003 0.275 0.182 0.135 0.488 0.194 0.419 0.581 1.000 0.895 0.002 0.103 1.000

128

156

420 603 1380

1785

Analyses were performed by Beta Analytic Laboratory. Samples β-233109 and β-192643 consisted of macroscopic wood fragments; samples β-233110, β-228997, β-281632, and β-281633 consisted of non-woody plant macrofossils. b Calibrations were calculated using Calib 6.01 (Stuiver and Reimer, 1993) and the dataset of Reimer et al. (2009). c Weighted mean of the probability distribution of the calibrated age.

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values as low as −100‰ (Cohen, 2003) and CO2 formed as a result of the oxidation of the methane is similarly depleted in 13C. Carbonate formation and preservation in the Las Lagunas sediment records The variable formation and preservation of authigenic and biogenic carbonates in the Felipe record appear to be linked to ion concentrations in the water column, particularly that of calcium. In cores from four lakes at Las Lagunas, we found biogenic carbonates only in the uppermost sediments of Laguna Felipe, whose waters also have the highest Ca2+ ion concentration. We interpret the presence of abundant carbonates in the upper Felipe sediment record as an indication of increased Ca2+ ion concentrations, likely controlled by changing lake levels, with decreased lake levels leading to an increase in Ca2+ ion concentration (Lane et al., 2009). The Laguna de Felipe δ18OCyth record

Figure 2. Sediment stratigraphy of the Laguna de Felipe sediment core. Radiocarbon dates are presented in radiocarbon years BP and calibrated years BP (parentheses).

compositions than adult ostracods of the same species in the same ambient conditions (Chivas et al., 1986; Engstrom and Nelson, 1991; Keatings et al., 2002). However, these complications can be minimized by analyzing only adult valves produced by a single species, when the goal is to determine relative temporal trends. The δ13C composition of lacustrine biogenic carbonates, including ostracod valves, is typically in equilibrium with the δ13C value of dissolved inorganic carbon (DIC) in the water at the time of formation (Lister, 1988). In productive freshwater lakes and wetlands, the DIC pool is primarily controlled by primary productivity and the breakdown of organic matter (Oana and Deevey, 1960; Cohen, 2003). Photosynthetic reactions in the water column preferentially utilize 12C from the DIC pool, thereby increasing δ13CDIC values. Thus, an increase in the δ13C value of biogenic carbonates is sometimes interpreted as an indication of increased primary productivity. The impact of primary productivity in the water column on δ13CDIC values can be slightly offset by high rates of organic matter oxidation in the water column and sediments, which can decrease δ13CDIC values by releasing the abundant 12C stored in photosynthetic tissues back into the DIC pool. Methanogenesis under anoxic conditions can also lead to very negative δ13CDIC values, as dissolved methane can have δ13C

1400–1000 cal yr BP We hypothesize that the positive isotope excursions in the Felipe δ18OCyth record at ~150–100, ~450–250, ~1000–950 and ~1300– 1150 cal yr BP indicate multiple periods of increased aridity (higher E/P ratios) in Laguna Felipe over the last two millennia. The first of these periods of inferred aridity from ~1300–1150 cal yr BP corresponds with other circum-Caribbean paleoclimate records that indicate aridity at this time. This is particularly true when error ranges of radiocarbon dates (typically≥40 14C yr) in paleoclimate records are taken into account. Sedimentary records from the Yucatan Peninsula indicate significant droughts at this time that have been linked with the Terminal Collapse of the Mayan civilization (Hodell et al., 1995, 2005a,b; Curtis et al., 1996). More specifically, this period of inferred aridity at Felipe and in the Yucatan coincides with what Hodell et al. (2005a) have termed the 'Early Terminal Classic Drought'. Holocene charcoal records from both high and low elevations in Costa Rica indicate increased biomass burning at this time, perhaps as a result of increased regional aridity (Horn and Sanford, 1992; Horn, 1993; Anchukaitis and Horn, 2005). Significant multi-decadal decreases in Cariaco Basin sedimentary Ti concentrations occur repeatedly between 1000 and 1200 cal yr BP (Haug et al., 2001, 2003; Peterson and Haug, 2006). The magnetic susceptibility of sediments off the coast of Puerto Rico increases during this time, likely as a result of increased deposition of hematite-rich dust from Saharan Africa due to intensified trade wind strength and a southern displacement of the ITCZ (Nyberg et al., 2001). While the Felipe δ18OCyth values generally decrease between 1200 and 1000 cal yr BP relative to those immediately before and after this time period (Fig. 4), the mere presence of abundant biogenic and authigenic carbonates in the sediment profile is likely indicative of somewhat arid conditions and lower lake levels during this time. Our interpretation of a generally shallow water environment in Felipe is further supported by the abundant fine fibrous organics throughout this section of the core, which indicate a shallow water environment rich in aquatic or emergent aquatic macrophytes (Fig. 2). An abundance of emergent and shallow water aquatic macrophytes during periods of high E/P ratios in Felipe might also explain the generally inverse relationship between δ18OCyth and δ13CCyth values during this period (Fig. 4). If high dissolved oxygen concentrations were maintained in the water column, which is likely considering the abundance of heterotrophic ostracods in the sedimentary record, high rates of emergent and aquatic plant productivity in the shallow water column may have led to a decrease in the δ13CDIC pool as increasing amounts of the vegetative organic matter were being oxidized at the sediment–water interface during times of high E/P. Conversely, deeper water conditions may have decreased the influx of fresh organic matter into the sediments and led to a greater fractionation of slightly more limited DIC in the water column, thereby driving up

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Figure 3. Age-depth diagram and sedimentation rates for the Laguna Felipe sediment core. Ages are the weighted means of the probability distributions of the calibrated radiocarbon ages (cal yr BP). The age probability distributions for each radiocarbon date are also presented (inset).

δ13CCyth values. Sedimentary records from Laguna Castilla and Laguna de Salvador, both located within 1 km of Laguna de Felipe, also contain abundant paleolimnological evidence of increased regional aridity around ~1200 cal yr BP including evidence of a major drop in the lake level, and probable desiccation, of Laguna de Salvador (Lane et al., 2009). All of the aforementioned study sites have precipitation regimes that are heavily influenced by ITCZ dynamics, thus the coherent evidence of increased aridity from these geographically diverse locales is consistent with a more southerly mean annual position of the ITCZ, and perhaps a more pervasive shift in the global climate system at this time (Mayewski et al., 2004). Hodell et al. (2001, 2005a) used the similar temporal cyclicity of solar cycles and spectral analyses of their oxygen isotope data from the Yucatan Peninsula to suggest that solar cycles are driving ITCZ migration dynamics during the late Holocene. Nyberg et al. (2002) hypothesized that strong, and perhaps more frequent, El Niño events in the tropical Pacific may have decreased Atlantic cross-equatorial sea-surface (SST) gradients, increased trade wind intensities in the Caribbean, and drawn the ITCZ into a more southerly mean annual position. This scenario would explain the counterintuitive pattern of increased circum-Caribbean aridity with higher reconstructed Caribbean SSTs (Nyberg et al., 2001, 2002). Caribbean SSTs do increase during El Niño events, but increased sea level pressures induced by the ‘zonal seesaw’ pattern can inhibit convective activity (Giannini et al., 2000, 2001a,b). Lane et al. (2009) also suggested that Pacific oceanic and atmospheric dynamics played a significant role in the aridity of the circum-Caribbean ca. 1200 cal yr BP based on differences in sediment characteristics and oxygen isotope values from Laguna Castilla and Laguna de Salvador for this time interval in comparison to the LIA. 1000–650 cal yr BP (including the Medieval Climate Anomaly) The increase in δ18OCyth values between ~ 1000 and 950 cal yr BP (Fig. 4), although only represented by two data points, is generally consistent with drought periods identified in high resolution paleoclimate records from the Cariaco Basin (Haug et al., 2003), and

may correspond to the ‘Late Phase’ of the Terminal Classic Drought detected in sediment records from the Yucatan Peninsula (Hodell et al., 2005a). In the Felipe sediment record the increase in δ18OCyth values is followed by a dearth of carbonates of any kind for almost 300 yr between 950 and 650 cal yr BP. Based on modern relationships between carbonate abundance and limnological conditions in the Las Lagunas lakes (Thomason, 2007; Lane et al., 2009), we interpret the near complete absence of carbonate material to represent a decrease in Ca2+ ion concentrations in the water column, most likely due to decreased E/P ratios, e.g., a wetter climate. A shift to mineral-rich clay sediments is also indicative of an open water, low energy depositional environment and perhaps an increase in erosion of mineral soils in the Felipe watershed as a result of increased precipitation (Fig. 2). A more northerly mean annual position of the ITCZ in the circum-Caribbean region has been hypothesized for this period (Haug et al., 2001; Peterson and Haug, 2006), which roughly coincides with the interval recognized as the Medieval Climate Anomaly (MCA). The mechanisms responsible for this northward shift in the mean annual position of the ITCZ remain in question, but a recent compilation of speleothem data from Scotland and tree ring data from Morocco indicate that a persistently positive phase of the North Atlantic Oscillation (NAO) may have played a role. Trouet et al. (2009) suggested that persistent La Niña conditions in the Pacific during the MCA (Mann et al., 2009) could have induced a positive phase of the NAO in the Atlantic, which would have increased the intensity of the westerlies. In turn, an increase in the intensity of the westerlies could have enhanced the Atlantic meridional overturning circulation (AMOC), increasing cross-equatorial temperature and salinity gradients in the Atlantic, and drawing the mean annual position of the ITCZ northward. Currently, a strong positive phase of the NAO is associated with decreased rainfall in the Caribbean as associated increases in trade wind intensity lead to enhanced heat loss from the tropical ocean and decreases in convection and precipitation (Malmgren et al., 1998; Giannini et al., 2001a). These relationships suggest that the inferred persistent La Niña conditions of the Pacific more heavily influenced Caribbean precipitation dynamics during the MCA than

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Figure 4. Oxygen and carbon isotope composition of Cythridella boldii ostracod valves in the Laguna de Felipe sediment core plotted vs. depth and calibrated radiocarbon age (cal yr BP). Black lines denote significant climate events described in the text. ‘Late TCD’ and ‘Early TCD’ refer to the late and early periods, respectively, of the Terminal Classic Drought as described and defined by Hodell et al. (2005a). ‘MCA’ refers to the Medieval Climate Anomaly and ‘LIA’ refers to the Little Ice Age.

they do today (Mann et al., 2009). It should be noted that persistent La Niña conditions may have also led to an increase in Caribbean tropical storm occurrence during the MCA and could explain decreased E/P ratios in Felipe if these storms regularly affected the Las Lagunas area (Tartaglione et al., 2003; Donnelly and Woodruff, 2007). 650 cal yr BP – present (including the Little Ice Age) Deposition of C. boldii ostracod valves and other carbonates resumes rather abruptly in the Felipe sediment record at ~650 cal yr BP (Figs. 2 and 4), signaling increased Ca2+ ion concentrations in the Felipe water column as a result of increased E/P ratios. The steady 5‰ increase in δ18OCyth values up until 300 cal yr BP also indicates a significant increase in E/P ratios during this period (Fig. 4). Sediments deposited from ~350– 300 cal yr BP (~125–110 cm depth) consist of coarse fibrous organics (Fig. 2) that may indicate a shallowing of Felipe and colonization by emergent or aquatic macrophytes. The steady decrease in δ13CCyth values may also indicate greater organic matter deposition and oxidation in the uppermost sediments of Felipe as water levels declined (Fig. 4). The variability in Felipe δ18OCyth values could be a result of factors other than changes in E/P ratios, but this seems unlikely. For example, a significant change in moisture source could cause a decrease in Felipe δ18OCyth values due to Rayleigh distillation effects. If meridional flow increased during the LIA, which is supported by the paleoclimate records from the general region (Nyberg et al. 2002; Lozano-Garcia et al., 2007), and moisture transport into Las Lagunas now came from

the north coast of Hispaniola as opposed to the south, Rayleigh distillation processes would decrease the δ18O value of precipitation feeding into Felipe as precipitation. However, the strong rainshadowing of the Cordillera Central largely prohibits this mode of precipitation delivery to Las Lagunas today, and there is no reason to think this was not the case during the LIA, even with more powerful and/or frequent polar air outbreaks. It is also possible that a decrease in lake temperature could cause an increase in Felipe δ18OCyth values as the oxygen isotope fractionation between calcite and water is temperature sensitive. But an increase in Felipe δ18OCyth values of 5‰ as observed between 650 and 300 cal yr BP due solely due to a drop in water temperature would require a huge and highly unlikely temperature decrease of ~15–20°C (Epstein et al., 1953; Kim and O'Neil, 1997). Such a large drop in water temperatures is inconsistent with the local pollen record, which shows no evidence of vegetation changes that would signal massive cooling (Lane et al., 2009), and with the much smaller estimates of LIA temperature decreases in reconstructions from high latitude locales (Mann et al., 2009). Additionally, a more southerly mean annual position of the ITCZ and increased trade wind intensities would have decreased atmospheric convection along the southern slope of the Cordillera Central and decreased cloud cover, thereby exposing Laguna Felipe to more solar radiation, which would have very likely increased lake temperatures. Thus, the most parsimonious explanation for the increased Felipe δ18OCyth values during the LIA is an increase in E/P ratios.

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The Felipe δ18OCyth record is very similar to the Cariaco Basin Ti concentration record (Haug et al., 2001) over the course of the LIA. The overall pattern of increasing Felipe δ18OCyth values between 650 and 300 cal yr BP (A.D. 1400–1650) resembles the pattern of decreasing Ti concentrations in the Cariaco sediments during the same period. Maxima in the Felipe δ18OCyth record at ~375 cal yr BP (~A.D. 1575) and ~300 cal yr BP (~A.D. 1650) match well with Ti concentration minima in the Cariaco record. Mineral-rich clay deposits, indicative of deeper water and low energy sediment deposition, and high amplitude decreases in δ18OCyth values in the Felipe record ~500–450 cal yr BP (~A.D. 1450–1500) and ~150–125 cal yr BP (~A.D. 1800–1825) correspond to spikes in Cariaco Ti concentrations (Fig. 5). Sedimentation rates are highest at Laguna Felipe between A.D. 1794 and A.D. 1822, perhaps because of increased erosion triggered by higher precipitation following an extended period of aridity (Fig. 3). The high Ti concentrations in the Cariaco record at ~200 cal yr BP (A.D. 1750) are not matched by high Felipe δ18OCyth values, which is puzzling given the close correspondence of the records before and after this time. There is no clear explanation for the absence of this apparent high amplitude drought event, or events, in the Felipe δ18OCyth record. The increasing abundance of coarse fibrous organics in the Felipe sediments combined with the steady increase in Felipe δ13CCyth values from ~ 650 cal yr BP to the present indicate a shallowing of Felipe consistent with sediment infilling. An increased abundance of decaying macrophytic plant tissues in the sediments of Felipe would explain the observed decrease in Felipe δ13CCyth values. It is possible that the suspected drought event in the Cariaco record ~ 200 cal yr BP was not detectable in the Felipe δ18OCyth record because Felipe became less drought sensitive with time with an increase in emergent macrophyte cover, but existing studies of the hydrology of open water vs. plant colonized lake surfaces indicate no clearly defined relationship between changing E/P ratios and emergent plant colonization (Lafleur, 1990; Price, 1994; Goulden et al., 2007).

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In any case, the close correspondence of the Felipe δ18OCyth record and the Cariaco Ti record through the majority of the period encompassed by the LIA, combined with the fact that the precipitation regimes of northern South America and the southern slope of the Cordillera Central are both controlled by ITCZ positioning, supports the hypothesis of a more southerly mean annual position of the ITCZ in the circum-Caribbean region during the LIA (Peterson and Haug, 2006). Subsequent to the LIA, Felipe δ18OCyth values hover around 0‰ and generally do not approach the consistently high δ18OCyth values of the LIA (Figs. 4 and 5). The Caribbean during the Little Ice Age Evidence is mounting that the circum-Caribbean region experienced marked changes in climate during the LIA, perhaps of a magnitude rivaling that of climate events earlier in the Holocene. However, the forcing mechanism(s) responsible for such changes remain elusive. Here we combine the findings of circum-Caribbean paleoclimate and paleoceanographic studies that have highly resolved archives of LIA climate variability with selected extra-regional records in an attempt to develop a more complete picture of LIA climate change in the circum-Caribbean and the potential mechanisms responsible (Fig. 6). The coincident increase in aridity in the geographically distinct locales of the Yucatan Peninsula (Hodell et al., 2005a), Panama (Linsley et al., 1994), northern South America (Haug et al., 2001; Peterson and Haug, 2006), Puerto Rico (Nyberg et al., 2001), and along the southern slope of the Cordillera Central of the Dominican Republic (Lane et al., 2009; this study) provides strong evidence that the ITCZ in the Caribbean was located at a more southerly mean annual position during the LIA. Recently, Sachs et al. (2009) used compoundspecific hydrogen isotope and microbial analyses from multiple Pacific island locales to show that the ITCZ was displaced southward over the

Figure 5. Comparison of the oxygen isotope composition of Cythridella boldii ostracod valves in the Laguna de Felipe sediment core with sedimentary titanium concentrations from the Cariaco Basin (ODP Site 1002; 10°42.73′N, 65°10.18′W; Haug et al., 2001, 2003). Increased Ti concentrations in the sediments of the Cariaco Basin are hypothesized to indicate increased terrigenous input from rivers draining northern South America as a result of increased regional precipitation. The age model for the Cariaco Basin data has been adjusted slightly for comparison to the Laguna de Felipe record. This adjustment is necessary because the Cariaco data are plotted such that BP refers to years before A.D. 2000, whereas the Laguna de Felipe age model follows the standard that BP refers to years before A.D. 1950.

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Figure 6. Paleoclimate and paleoceanographic conditions during the Little Ice Age in the circum-Caribbean and neighboring regions based on existing high resolution studies. Abbreviations: AMOC = Atlantic Meridional Overturning Circulation; ITCZ = Intertropical Convergence Zone; SST = Sea Surface Temperature.

Pacific as much as ~ 500 km relative to its modern position. While the Caribbean and tropical Atlantic ITCZ differ significantly in both form and behavior from the Pacific ITCZ, the inferred displacement of the Pacific ITCZ does indicate a significant change in tropical atmospheric dynamics during the LIA. Modern variations in ITCZ positioning are primarily controlled by oceanic–atmospheric interactions. More specifically, strong crossequatorial SST and sea surface salinity (SSS) gradients in the tropical Pacific and Atlantic Oceans produce strong differences in sea level pressure and induce more convective activity at higher northern latitudes (Chiang et al., 2002, 2008). Paleoceanographic records from the Caribbean and tropical North Atlantic indicate a decrease in SSTs during the LIA of as much as 2° to 3°C (Winter et al., 2000; Watanabe et al., 2001; Nyberg et al., 2002; Haase-Schramm et al., 2003; Black et al., 2007; Saenger et al., 2009). Such a significant decrease in SSTs not only decreases the cross-equatorial SST gradient of the tropical Atlantic and inhibits northward ITCZ migration, but it also decreases the latent heat and moisture content of the boundary layer, further inhibiting atmospheric convection in the region. This would be especially significant on the southern slope of the Cordillera Central of the Dominican Republic, where onshore transport and subsequent orographic uplift of moisture-laden air via sea breezes is the primary mode of cloud development and precipitation. Polar records of atmospheric transport indicate a strengthening of meridional airflow during the LIA (Kreutz et al., 1997; Fig. 6) and global reconstructions of air temperature support the hypothesis of intensified arctic air masses during the LIA (Mann et al., 2009). Nyberg et al. (2002) suggested that the decrease in Caribbean SSTs was seasonal, with the largest decreases occurring during winter most likely as a result of increased meridional flow from continental North America in the form of polar air outbreaks. Pollen data from eastern Mexico corroborate this interpretation and indicate much more frequent and/or intense polar air outbreaks from continental North America into the Gulf of Mexico and increases in associated precipitation during the LIA as compared to today (Lozano-Garcia et al., 2007). More frequent and/or intense polar air outbreaks into the Caribbean could explain the decrease in SSTs. Additionally, paleoceanographic records from the Florida Straits indicate a significant

decrease in outflow during the LIA that has been linked to a likely decrease in the AMOC (Lund and Curry, 2004; Lund et al., 2006). A decrease in heat advection into the Caribbean Sea from the tropical Atlantic via the North and South Equatorial Current systems as a result of decreased AMOC could have also caused a decrease in Caribbean SSTs and diminished the cross-equatorial Atlantic SST gradient during the LIA. Although the detailed records of circum-Caribbean and extraregional paleoclimate and paleoceanographic change during the LIA outlined above are comprehensive and informative, they do not provide us with definitive proof of the ultimate forcing mechanism(s) responsible for these changes. The general decrease in global temperatures during the LIA has been primarily attributed to decreased solar insolation caused by both the Maunder, Spörer, and Wolf solar activity minima (e.g. Stuiver and Braziunas, 1989) and increased atmospheric aerosol loading as a result of increased volcanic activity (e.g. Hegerl et al., 2003, 2006). Using multi-proxy networks on a global scale, Mann et al. (2009) provided further evidence that these two forcing mechanisms can explain the scale of the globally averaged temperature decrease of the LIA, but noted that internal dynamical responses to these forcings must be accounted for to explain the spatial heterogeneity of the global climate response. In particular, Mann et al. highlight the importance of dynamical responses with farreaching teleconnections, such as the Pacific 'thermostat' mechanism originally proposed by Clement et al. (1996). It seems plausible that a decrease in solar insolation during the LIA may have triggered dynamical responses in both the Pacific and Atlantic Oceans that would have affected precipitation in the circumCaribbean region, particularly at locations with precipitation regimes dominated by ITCZ dynamics. As discussed by Mann et al. (2009), the ‘thermostat’ response of the ENSO system (Clement et al., 1996) to the decreased solar insolation of the LIA may have led to a warming of the eastern Pacific and a more southerly mean annual position of the Pacific ITCZ as Pacific cross-equatorial SST gradients decreased (Sachs et al., 2009). Similarly, modeling studies indicate decreases in solar insolation can force the NAO into a negative phase (Shindell et al., 2001). Existing paleoclimate data provide direct evidence that the NAO was in a persistently negative phase during the LIA, which could

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have caused a decrease in the cross-equatorial SST and SSS gradients of the tropical Atlantic via changes in AMOC dynamics (Trouet et al., 2009). Combined with an increase in the frequency and/or intensity of polar air outbreaks from continental North America into the Gulf of Mexico and Caribbean Sea, this could have significantly increased sea level pressures and inhibited ITCZ development in the region. This dynamical response of the ENSO and NAO systems would largely explain the overall pattern of increased LIA aridity in locales with modern-day precipitation regimes dominated by ITCZ dynamics. Conclusion The abundant evidence of increased aridity from study sites with precipitation regimes dominated by ITCZ development in the northern tropics, including Laguna de Felipe, strongly indicates that the ITCZ was positioned at a more southerly mean annual position during much of the LIA. The existence of high-resolution paleoclimate records spanning the last millennium has made it possible to more accurately model regional patterns of LIA climate change and the potential mechanisms responsible. In the case of the circumCaribbean, it appears that atmospheric and oceanic responses to decreased solar insolation likely decreased tropical Atlantic and Caribbean SSTs and decreased the cross-equatorial SST gradient, thereby inhibiting ITCZ movement into the northern tropics. The significant variability apparent in paleoclimate records from the circum-Caribbean and neighboring regions emphasizes the sensitivity of regional and global climate dynamics to relatively small climatic forcings such as those hypothesized to be responsible for the LIA. Here as throughout the tropics (Hodell et al., 2005b; Stone, 2009), the potential impacts of LIA aridity on human society warrant further attention. Acknowledgments Funding was provided by grants to KHO and SPH from the National Geographic Society, and to SPH, KHO, and Claudia Mora from the National Science Foundation (BCS-0550382). During portions of this research CSL was supported by Hilton-Smith and Yates Dissertation Fellowships from the University of Tennessee and a Mead Witter Foundation Postdoctoral Fellowship from Lawrence University. JMT was partially supported by a future faculty grant awarded to C. Lane from the Academic Keys Foundation. We thank Andrés Ferrer and the Moscoso Puello Foundation for providing infrastructure for research in the Dominican Republic and helping us secure research permits, and Felipe Garcia and his family for allowing us to camp on their land near the lakes of Las Lagunas and for assisting us with field work. Additional field assistance was provided by Jeff Dahoda and Duane Cozadd. Zheng-Hua Li assisted with isotopic analyses and data interpretation. References Anchukaitis, K.J., Horn, S.P., 2005. A 2000-year reconstruction of forest disturbance from southern Pacific Costa Rica. Palaeogeography Palaeoclimatology Palaeoecology 221, 35–54. Black, D.E., Thunell, R.C., Kaplan, A., Peterson, L.C., Tappa, E.J., 2004. A 2000-year record of Caribbean and tropical North Atlantic hydrographic variability. Paleoceanography 19, PA2022. Black, D.E., Abahazi, M.A., Thunell, R.C., Kaplan, A., Tappa, E.J., Peterson, L.C., 2007. An 8-century tropical Atlantic SST record from the Cariaco Basin: baseline variability, twentieth-century warming, and Atlantic hurricane frequency. Paleoceanography 22, PA4204. Bolay, E., 1997. The Dominican Republic: a Country Between Rain Forest and Desert; Contributions to the Ecology of a Caribbean Island. Margraf Verlag, Weikersheim. Broecker, W.S., 2000. Was a change in thermohaline circulation responsible for the Little Ice Age? Proceedings of the National Academy of Sciences of the United States of America 97, 1339–1342. Brown, E.T., Johnson, T.C., 2005. Coherence between tropical East African and South American records of the Little Ice Age. Geochemistry Geophysics Geosystems 6, Q12005.

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