Lake-Level History of Lake Tanganyika, East Africa, for the Past 2500 Years Based on Ostracode-Inferred Water-Depth Reconstruction

Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 www.elsevier.com/locate/palaeo

Lake-level history of Lake Tanganyika, East Africa, for the past 2500 years based on ostracode-inferred water-depth reconstruction

Simone R. Alin Ã, Andrew S. Cohen

Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA

Received 25 April 2002; accepted 28 May 2003

Abstract

Assemblages of ostracodes from sediment cores illuminate lake-level history at decadal to centennial timescales during the late Holocene at Lake Tanganyika, East Africa. The ostracode-based lake-level curves for several cores resemble both each other and the only previously published lake-level record of comparable resolution for Lake Tanganyika during this interval, successfully reconstructing known highstands, improving the chronology of known lowstands, and contributing new information on late Holocene lake-level variability at this important tropical African location. In agreement with other late Holocene records from East Africa, the surface level at Lake Tanganyika reflects predominantly arid conditions throughout this interval, interrupted by relatively brief episodes of higher precipitation and lake level. The most pronounced lowstand in the record occurs at V200^0 BC, with other significant lowstands dating to the intervals V200^500 AD, V700^850 AD, the Medieval WarmPeriod (MWP; V1050^1250 AD at Lake Tanganyika), and the latter part of the Little Ice Age (LIA; V1550^1850 AD). The most important wet intervals in the lake-level record are centered on V500 AD, V1500 AD, and V1870 AD. The highstands and lowstands reported here for Lake Tanganyika appear to be fairly coherent with other records of rainfall throughout East Africa during the MWP and the LIA. Prior to the MWP, paleoclimate records are apparently less coherent, although this may be a reflection of the resolution and abundance of recent paleoclimatic data available for this climatically complex region. ß 2003 Elsevier B.V. All rights reserved.

Keywords: paleoclimate; Lake Tanganyika; East Africa; ostracodes; paleolimnology; multivariate analysis

1. Introduction conditions in East Africa during the late Holocene has been hindered by the paucity of su⁄ciently Broad-scale reconstruction of paleoclimatic resolved records of climate change. Recent e¡orts have improved the coverage of paleoclimatic records for this region (cf. Johnson et al., 2001, * Corresponding author. Present address: Large Lakes 2002; Verschuren, 2001, in press; Verschuren et Observatory, University of Minnesota, 10 University Drive, Duluth, MN 55812, USA. Fax: +1-218-726-6979. al., 2000), although coherency among rainfall pat- E-mail address: [email protected] terns in di¡erent areas of East Africa is highly (S.R. Alin). variable at both short and long timescales, render-

PALAEO 3169 9-9-03 32 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 ing generalization of paleoclimatic reconstructions a temporal lag of a few months to decades, fromone site to another unreliable ( Nicholson, although the importance of this phenomenon 1996, 2000). In addition, many potential paleocli- will depend on the timescale of the analysis at matic records (e.g. pollen, stable isotopes, lacus- hand. trine micro£ora and microfauna) may bear signa- The species composition of paleoecological os- tures of anthropogenic overprinting of their tracode assemblages can be used to reconstruct respective climate signals for recent millennia in lake-level £uctuations through time at high reso- this cradle of human evolution. We o¡er here a lution in lacustrine ecosystems. Previous studies new record of the lake-level and paleoenviron- have relied on detailed information on depth mental history of Lake Tanganyika during the and salinity tolerances of constituent species late Holocene, based on ostracode assemblages (e.g. Forester et al., 1994), or on extensive collec- in shallow-water sediment cores collected from tions of species assemblages throughout a calibra- sparsely populated stretches of the lakeshore. tion set of lakes with variant modern depth and This contribution improves upon the temporal salinity regimes (e.g. Mourguiart and Carbonel, resolution of a previously published lake-level 1994), as a basis for paleolake-level reconstruc- curve for Lake Tanganyika, which relied on stro- tion. Typically, these investigators have relied on matolitic and stable isotopic evidence to recon- the ecological signi¢cance of the relative abundan- struct highstands between V1250 and 1550 AD ces of common species through time as an indica- and at 430 þ 110 AD and lowstands of 315^40 m tor of lake level, as interpreted through use of a (deviation fromcurrent average lake level) at simple water-depth index or through multivariate some time after the late 5th century AD and be- techniques. tween the late 16th and early 19th centuries AD In cases such as Lake Tanganyika, ecological (Cohen et al., 1997b). tolerances of individual ostracode species are un- Lake Tanganyika, East Africa, is a large known or di⁄cult to determine. Lake Tanganyika 2 (32 600 km ), deep (zmax = 1470 m), meromictic, boasts an estimated 200 species of microcrusta- tropical (3^9‡S) rift lake (Fig. 1). The surface level cean ostracodes, with levels of endemism on the of the lake £uctuates at a variety of time and order of 70% (Martens, 1994). Local endemism, depth scales in response to both climatic and geo- species richness, and spatiotemporal heterogeneity morphic factors. On an interannual to decadal in the distributions of the component species are scale, the surface of Lake Tanganyika has varied high, such that de¢ning a water-depth index based on the scale of a few meters during the 1900s on the abundance of a few select species would (range: 772.5^776.7 masl [metersabove sea level], not yield a widely applicable index of lake-level average[ þ S.D.] = 774.4 þ 0.8 masl; data for change throughout the lake basin. Detailed depth 1900^1999 AD from Evert, 1980; Birkett et al., tolerance data are available for very few Tanga- 1999). However, lake levels were substantially nyikan ostracode species, but most species do higher in the mid^late 1800s, experiencing a max- show peaks in abundance over ¢nite depth ranges imum highstand of 784 m asl in 1878 AD, as a (on the scale of meters to a few tens of meters). function of both higher rainfall and alluviumplus Thus, ostracode species assemblages en masse vegetation blocking the lake’s outlet. Finally, the should inherently contain basic ecological infor- surface level of Lake Tanganyika has undergone mation, such as relative depth preferences, that even more extreme variation, on the order of tens can be useful in constructing paleolake levels. to hundreds of meters, in response to climatic and All Tanganyikan ostracodes are e¡ectively tectonic forcing at timescales of millennia and benthic, with no truly pelagic species found in longer (cf. Cohen et al., 1997a; Haberyan and the lake, and are currently limited to inhabiting Hecky, 1987; Talbot, 2001). It is worthwhile not- the upper 100^200 mof the water column,where ing that because of the immense area of the lake dissolved oxygen is available. Thus, we expect and its catchment, changes in climatic conditions that ostracode assemblages will vary substantially may be manifested as a change in lake level after with relative water depth in Lake Tanganyika.

PALAEO 3169 9-9-03 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 33

Because ostracodes are preserved abundantly and in high quality in sediment cores from the lake, it should be possible to reconstruct relative lake level fromthe sedimentrecord of ostracode species composition.

2. Methods

2.1. Core collection andsampling

The 2500-year reconstruction of relative lake level described here for Lake Tanganyika comes fromcore LT-97-56V, a 356 cmvibracore collected in 56 mwater depth. The coring site is about 700 msoutheast of a smallriver outlet draining a watershed of 5.7 km2 (5‡46.33PS, 29‡56.03PE; Fig. 1). The coring site is near the village of Mgondozi, Tanzania, but human population density is low in this area of the Tanga- Fig. 1. Map of Lake Tanganyika with locations of all cores V 2 discussed in the text indicated. Inset box shows location of nyika basin ( 17 people/km ; President’s O⁄ce - Lake Tanganyika within Africa. Planning Commission, 1991). Visual inspection of the core at the time of collection and subsequent X-radiography indicated that it experienced min- thereafter. Every other sample was analyzed for imal disturbance during the coring process, and grain-size distribution, sedimentary organic mat- the core top appeared to be intact upon recovery. ter (SOM) and carbonate content, and ostracode The core was cut into 150-cmsections in the ¢eld, assemblage composition (i.e. every fourth centi- sealed, and shipped to the U.S. for further anal- meter was analyzed to 20 cm, every eighth centi- ysis. Subsequently, the core was split, X-radio- meter thereafter). A gas gap of 13 cm was found graphed, and sampled every other centimeter for in the core between 56 and 70 cm. This length was the upper 20 cm, and every fourth centimeter excised fromall core data sets such that all ¢gures

Table 1 Radiocarbon dates for core LT97-56V fromLake Tanganyika Sample Material Depth in Fraction 14C age 2c calendar Area under number dated core modern 14C age range probability curvea (cm) (yr BP) AA-43467 single leaf 0^1 0.9606 þ 0.0055 323 þ 46 483^297 1.000 AA-43468 single leaf 2^3 0.9834 þ 0.0047 134 þ 39 279^171 0.423 152^51 0.405 45^6 0.153 AA-28408 plant fragments 17^18 0.9615 þ 0.0055 315 þ 45 477^293 1.000 AA-43385 single leaf 50^51 0.9582 þ 0.0041 343 þ 34 480^310 1.000 AA-27665 plant fragments 92 0.9203 þ 0.0070 667 þ 61 693^540 1.000 AA-43469 single leaf 162^63 0.7825 þ 0.0077 1 970 þ 79 2 117^1 726 1.000 AA-43470 single leaf 198^99 0.7017 þ 0.0053 2 846 þ 61 3 082^2 840 0.869 AA-43471 single leaf 242^43 0.6067 þ 0.0070 4 014 þ 93 4 735^4 240 0.904 AA-43472 single leaf 298^99 0.5425 þ 0.0098 4 910 þ 140 5 928^5 438 0.916 AA-43473 single leaf 322^23 0.2836 þ 0.0048 10 120 þ 140 12 392^11 261 0.938 a All calibrated ages representing v 0.100 relative area under the probability distribution are reported.

PALAEO 3169 9-9-03 34 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 re£ect a total core length of 343 cmwith continuous records.

2.2. Geochronology andsedimentology

Ten radiocarbon dates for LT-97-56V were based on terrestrial plant fragments to avoid the problemof 14C-reservoir age for samples pro- duced within the lake (Table 1). Leaf and wood samples were prepared for radiometric analysis by acid^base^acid pretreatment to remove carbonates and diagenetic organic material. All radiocarbon analyses were made by the Arizona AMS Laboratory. Calibrated radiocarbon ages were assigned using CALIB 4.3 (Stuiver et al., 1998a,b). The Southern Hemisphere correction was not ap- plied, because the study site is equatorial. Existing data on interhemispheric concentration di¡erences in atmospheric 14C provide inconclusive evidence on the relevance of the Southern Hemisphere correction to southern equatorial lo- Fig. 2. Depth^age relationships for core LT-97-56V. (A) Ra- cations. It is possible that this relationship has diocarbon ages for dated samples with associated error bars. changed with the southern migration of the Inter- Sedimentary layers below the depositional hiatus are indicated by gray shading and wavy erosional surface. Gray lines tropical Convergence Zone (ITCZ) through the represent zones of similar sedimentation rates in the core but Holocene (Haug et al., 2001), but the magnitude do not represent an age model. (B) Calendar-year age model of any di¡erence in the resulting age model should for core LT-97-56V. Error bars on individual dates are indi- be trivial. cated. Location of the depositional hiatus is shown by For the purpose of establishing an age model dashed horizontal and vertical lines. for core LT-97-56V, midpoints of the most probable calibrated age ranges (2c) were used, except Remaining sediments were sieved using 1 mm, 106 for the sample from 2^3 cm (Fig. 2). For this Wm, and 63 Wmmeshsieves, with the ¢nest frac- sample, the second-most probable age provided tion of sediment ( 6 63 Wm) retained on What- an age model more consistent with our observa- man-1 qualitative ¢lter paper. All samples were tion of an intact core top and was of essentially dried at 40‡C. equivalent probability to the most probable calibrated age. Calendar ages were assigned to strati- 2.3. Paleoecology graphic intervals based on a second-order polynomial ¢t of the calibrated radiocarbon data. The Ostracodes fromthe s 1 mm sediment fraction uppermost and lowermost radiocarbon dates were were added to the 106 Wm^1 mm fraction before excluded fromthe age modelon the basis of ap- sediment splits of the 106 Wm^1 mm sediment parent reworking and a depositional hiatus, re- fraction were counted for ostracodes. For all sam- spectively. Calibrated radiocarbon ages are re- ples containing su⁄cient ostracodes, 500 individ- ported in calendar years AD or BC and/or in yr uals were identi¢ed to the species level follow- BP (yr BP = calendar years before present, where ing published descriptions to the extent possible ‘present’ is 1950 AD by radiocarbon convention). (Martens, 1985; Park and Martens, 2001; Rome, Concentrations of SOM and carbonate (CO3) 1962; Wouters, 1979, 1988; Wouters and Mar- were estimated by loss-on-ignition at 550 and tens, 1992, 1994, 1999, 2001; Wouters et al., 925‡C, respectively (Bengtsson and Enell, 1986). 1989). As roughly half of the estimated 200 Tan-

PALAEO 3169 9-9-03 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 35

Fig. 3. CCA of live ostracode database. (A) Geographic coverage of live ostracode assemblages included in analysis, with sample locations indicated by black diamonds (n = 86). Many diamonds, particularly in the northern basin of Lake Tanganyika, overlap each other. (B) Distribution of live ostracode samples, marked by black circles, in CCA ordination space. The solid arrow indicates the approximate location of the diagonal depth-related axis. The dashed arrow shows how the ‘depth’ axis was subsequently rotated. (C) A view of the CCA plot, with coordinates and axes rotated. Black circles represent live ostracode assemblages, open triangles indicate assemblages from core LT-97-56V. Gray arrows and numbers show the approximate water depths where samples in the ordination plot were collected. (D) The relationship between depth scores of live ostracode samples and the water depth at which the samples were collected. The gray line shows the linear regression of these data and is represented by the equa- tion above. Dashed lines are 95% con¢dence intervals around the regression. ganyikan ostracode species remain to be described samples from various sites, habitats, and depths (Martens, 1994), extensive reference collections at in Lake Tanganyika. Prior to analyzing the live the University of Arizona were employed to as- species-abundance data, all singletons and double- sign identities to specimens from undescribed spe- tons by occurrence (i.e. all species occurring in cies, using a system of informal numeric monikers only one or two samples) were excluded from to name taxa (species list in Appendix 1). the database, leaving 93 species in the calibration analysis (Appendix 1). This live calibration data- 2.4. Relative water-depth reconstruction base consists of 86 samples (Fig. 3A). Environ- mental variables included substrate type (rocks, Relative water depth, and therefore lake level, sand, mud), water depth (0.3^80 m), latitude was reconstructed based on the output of a ca- (3.35‡^8.48‡S), longitude (29.15‡^30.78‡E), water- nonical correspondence analysis (CCA) of ostra- shed disturbance level (low, moderate, high), and code species-abundance data for live-collected vegetation type (algae, macrophytes, stromato-

PALAEO 3169 9-9-03 36 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 lites, none). CCA was performed using CANOCO depth, 49 cmlength; LT-98-12M, 126 m,40 cm; 4(ter Braak and Smilauer, 1998). CCA Axis 1 data from Cohen et al., 1999), and core MIT-1 was signi¢cantly and strongly correlated with sub- was taken o¡shore fromGombeStreamNational strate type (r = +0.67) and weakly with longitude Park in northwestern Tanzania (MIT-1, 15 m (r = +0.22). CCA Axis 2 correlated signi¢cantly water depth, 18 cmlength; data from Alin, with latitude (r = +0.80), water depth (r = +0.36), 2001). Human population density in both nation- and substrate type (r = -0.19); CCA Axis 3 with al parks is 6 5 people/km2 (Cohen et al., 1999), water depth (r = +0.74) and disturbance level so anthropogenic impact on the water-depth ¢del- (r = +0.20); and CCA Axis 4 with vegetation ity of ostracode assemblages should not be a con- type (r = +0.42), disturbance level (r = -0.41), and cern. Chronologies for cores were established by substrate type (r = +0.20). The ¢rst four canonical radiocarbon-dating samples of terrestrial material axes cumulatively explain 7.0%, 11.2%, 14.6%, contained in horizons in the sediment cores (cores and 16.4% of the variance in the species-abun- LT-98-2M, LT-98-12M, and MIT-1 had four, dance data. two, and three 14C dates, respectively; Alin et Relative water depths were constructed as fol- al., 2002; Cohen et al., 1999). The chronology lows. First, samples were plotted in CCA Axis 2 for LT-98-2M is based on a third-order polyno- vs. Axis 3 ordination space to maximize disper- mial ¢t of the most probable calibrated ages for sion of samples according to water depth (Fig. the four 14C dates, which gives a core-top age of 3B). In ordination space, live samples were ar- V1790 AD. For the purposes of this paper, the rayed by depth along a diagonal axis. All ostra- calibrated radiocarbon dates used to generate the code assemblages from core samples were in- age models for cores LT-98-12M and MIT-1 were cluded in the CCA as ‘supplementary’ samples, not necessarily the most probable calibrated ages, meaning that their species-abundance data did but gave results most consistent with other phys- not contribute to the gradient analysis. Rather, ical and faunal evidence. Cores LT-98-2M and CANOCO assigned ordination scores to core- LT-98-12M had low sediment mass-accumulation sample assemblages in a post hoc fashion, allow- rates (MARs) throughout (average LT-98-2M ing themto be plotted in the sameordination MAR = 0.007 þ 0.005 g cm32 yr31 ; LT-98-12M space as the live ostracode samples. Second, all MARW0.03 g cm32 yr31) and were sampled coordinates were rotated, such that the diagonal every third centimeter for ostracode assemblages depth axis in Fig. 3B became the new horizontal (Cohen et al., 1999). MIT-1 appears to have been axis (CCA Axis 2P or ‘depth’ axis in Fig. 3C). For deposited entirely within the past 150 years. All all live samples, sample scores on the new depth 1-cmcore intervals were tallied for ostracodes in axis (hereafter referred to as ‘depth scores’) are MIT-1, making it the highest-resolution record. tightly correlated with the actual depth at which We have the highest con¢dence in the chronology samples were collected, suggesting that depth is a for core LT-97-56V, as it is anchored by 10 radio- su⁄ciently strong environmental parameter in carbon dates, with relatively little ambiguity structuring living ostracode assemblages that we about reworking through the interval containing may be able to reconstruct relative changes in ostracode assemblages. The chronology for core water depth using paleoecological ostracode as- LT-98-2M also appears to be robust, although semblages (Fig. 3D). of lower resolution because of low sediment accu- We compared the lake-level curve generated mulation rates. fromLT-97-56V with lake-level reconstructions Depth scores for core assemblages were consis- based on ostracode data fromthree additional tently o¡set fromthe depths at which the cores cores to assess the e¡ects of core location and were collected (see Fig. 3C and Section 4). To water depth on lake-level reconstruction. Cores facilitate comparison among water-depth curves LT-98-2M and LT-98-12M were collected o¡- generated for the various cores examined, we shore fromMahale Mountains National Park in standardized depth scores for all cores by sub- west central Tanzania (LT-98-2M, 110 mwater tracting the depth-score average for a core from

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Fig. 4. Geochronology and sedimentology data for core LT-97-56V. (A) Radiocarbon ages. (B) Midpoint of 2c calendar-age range used for generating the age model. Dates in italics were excluded from the age-model calculation. (C) Grain-size distribution. (D) SOM concentration. (E) Sedimentary CO3 concentration. (F) Ostracode abundance in number of individuals per gram. all depth-score values in that core to derive a 97-56V (Fig. 2A), the data were ¢t much better lake-level deviation curve. by a single polynomial than by separate linear regressions or polynomials for the two intervals (Fig. 2B). 3. Results Sediment MARs in the lower core ranged between 0.036 and 0.078 g cm32 yr31 (average 3.1. Geochronology andsedimentology 0.053 þ 0.011 g cm32 yr31 for 103^302 cm, V4000 BC^1120 AD). In the upper core, sedi- Ten radiocarbon dates for core LT-97-56V in- ment MARs increased to span a range from dicate that sedimentation rates increased fairly 0.082 to 0.207 g cm32 yr31 (average 0.13 þ 0.04 constantly throughout the core, although the for 0^103 cm, 1120^1997 AD). Linear sedimenta- slope increased more steeply in the upper meter tion rates for LT-97-56V (1.2 þ 0.4 mm yr31 for of the sediment core (Table 1, Figs. 2 and 4). The 0^103 cm, 0.4 þ 0.1 mm yr31 for 103^302 cm) in- N13C pro¢le of core LT-97-56V closely resembles dicate that sample resolution is on the order of 8^ N13C pro¢les in other cores with intact, 210Pb- 10 yr sample31 at 32^40-yr intervals for the upper dated core tops collected fromvarious sites in 20 cm, with resolution stable but gaps increasing the lake, con¢rming the presence of core-top sedi- to 56^70 yr from 20 to 103 cm. Below 103 cm, ments in LT-97-56V (Alin, 2001; O’Reilly, 2001). sample resolution drops to 25 yr sample31 spaced Despite the appearance that two intervals of rel- at 200-yr intervals. atively constant sedimentation exist in core LT- Calibrated radiocarbon ages imply a deposi-

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Fig. 5. Correspondence between the lake-level record of Cohen et al. (1997b) and ostracode-based water-depth reconstruction for cores LT-97-56V, LT-98-2M, and LT-98-12M for the past 2500 calendar years. Dashed lines in the Cohen et al. (1997b) lake-level curve indicate the interval during which the temporally unconstrained lowstand during the 16th^19th centuries occurred. Rela- tive water-depth changes are plotted as lake-level deviations (LLD, in unitless depth-score values) for cores LT-97-56V, LT-98- 2M, and LT-98-12M. Gray zones highlight prominent high- or lowstand features to facilitate comparison among cores. Asterisks on core LT-97-56V pro¢le mark samples comprised of fewer than 500 ostracodes. tional hiatus of 5000^6000 yr near the base of 1500 AD). Carbonate concentrations increase to core LT-97-56V, ending at V6400 yr BP (V312 a broad peak of 2.1 þ 0.1% from140 to 175 cm cmdepth) ( Figs. 2B and 4B). Grain size ¢nes (400 BC^325 AD), with an additional sharp peak sharply across the hiatus, fromdominationby in carbonate concentration (4.2%) at 54^55 cm very ¢ne to medium sands below to silts above, (V1580 yr BP) representing a thin shell hash in indicating that the overlying sediments were de- the sediment (Fig. 4E). posited at substantially greater water depth (Fig. 4C). The reason for the hiatus is not apparent, 3.2. Paleoecology but may represent erosion or an interval of non- deposition, as nearshore sedimentation in large Ostracode remains were absent below 179 cm lakes can be episodic. (V500 BC) (Fig. 4F). Above this core depth, os- Concurrent with the abrupt ¢ning of sediment tracode abundance ranges from7 to 5570 valves/ grain size at 312 cm and resumption of sediment g, with an average of 1360 valves/g. Three anom- deposition, concentrations of SOM and CO3 pre- alously low values correspond to the peaks in served in the sediment doubled, from 1.4 þ 0.6% SOM at 62^71 cm( V1390^1500 AD) and 126^ to 3.2 þ 0.2% SOM and from0.3 þ 0.1% to 127 cm( V580 AD) (Fig. 4D^F). High SOM con- 0.6 þ 0.04% CO3, in the four samples on either centrations may explain intervals of low ostracode side of the hiatus (Fig. 4D,E). SOM percentages abundance, as organic acids and carbon dioxide continue to increase almost continuously upcore, generated by SOM degradation can lower pore- with peaks in abundance at 126^127 cm(6.9%, water pH and dissolve sedimentary carbonates, V580 AD) and 62^71 cm(7.2^8.7%, V1390^ including ostracode valves (Dean, 1999).

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3.3. Relative water-depth reconstruction

On a millennial timescale, the reconstructed relative lake-level curves for cores LT-97-56V, LT- 98-2M, and LT-98-12M show two prominent peaks roughly corresponding to lake highstands reported by Cohen et al. (1997b) to have occurred between V1250 and 1550 AD and at 430 þ 110 AD (Fig. 5). Based on the better-resolved chronology of core LT-97-56V compared to core LT- 98-2M in this interval, we suggest that the ¢rst highstand in fact occurred at 580 þ 70 AD, and both records place the latter highstand at V1500 AD (LT-97-56V: 1495 þ 45 AD, LT-98- 2M: 1470 þ 55 AD). Lake level started to rise Fig. 6. Relationship between historical instrumental lake-level data since 1846 AD and reconstructed water-depth values for sometime between 1200 and 1300 AD, was inter- cores LT-97-56V and MIT-1. Instrumental lake-level data are rupted by a brief decline in lake level at 1400 þ 50 from Birkett et al. (1999) and Evert (1980). Dashed line in AD, and started to decline frompeak levels instrumental data plot represents mean lake level from 1900 shortly after 1500 AD. In addition, the lake-level to 1999 AD. Gray shading indicates proposed correlation curve delineates the timing of the previously de- zones between the cores and the instrumental record. Rela- tive water-depth changes are plotted as lake-level deviations scribed, but temporally unconstrained, lowstands (LLD, in unitless depth-score values) for cores LT-97-56V from Cohen et al. (1997b). Core LT-97-56V indi- and MIT-1. cates that the most signi¢cant lowstands in Lake Tanganyika’s late Holocene history were centered around 200 þ 90 BC, 25 þ 90 BC, 290 þ 75 AD, 1960s, late-1970s, and late-1980s, again within 440 þ 75 AD, 1090 þ 50 AD, 1200 þ 50 AD, the range of errors associated with the core’s 1730 þ 35 AD, and 1800 þ 30 AD (Fig. 5). Other age model. The sensitivity of this core record to potentially important intervals of falling lake level reconstructing such ¢ne-scale variations in water occurred at 720 þ 70 AD, 850 þ 65 AD, 1400 þ 50 depth may be explained by the fact that it was AD, and 1580 þ 45 AD. collected in 15 mwater depth, so smallchanges At a more recent and highly resolved timescale, in water depth represent a greater proportional the reconstructed lake-level records fromcores shift in depth than would be the case for the deep- MIT-1 and LT-97-56V also correlate quite well water cores. with the recent historical record of lake-level change since 1846 AD at Lake Tanganyika (lake-level data from Birkett et al., 1999; Evert, 4. Discussion 1980)(Fig. 6). Two relative highstands re£ected in the water-depth record fromcore LT-97-56V fall 4.1. Robustness of ostracode-based water-depth within the errors associated with the age model reconstruction and sampling resolution of known highstands in the 1870s AD and 1960s AD. However, the lake- The ostracode-based lake-level curves presented level curve fromcore MIT-1 bears a striking re- here accurately reconstructed known past changes semblance to the instrumental lake-level record, in lake level on timescales of decades to millennia. beginning with a pronounced fall in both recon- Mourguiart and Carbonel (1994; Mourguiart et structed and measured lake levels in the late al., 1998) experienced similar success in recon- 1800s. Thereafter, the MIT-1 record sensitively structing paleolake levels at Lake Titicaca, based re£ects meter-scale variations in lake level seen on a relatively large calibration data set of ostra- in the instrumental record in the 1910s, 1940s, code species distributions and ecological toleran-

PALAEO 3169 9-9-03 40 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 ces. As in Lake Tanganyika, many of the species record than another highstand of comparable Mourguiart and Carbonel (1994, 1998) relied on magnitude but longer duration, as the ostracode for their paleolake-level reconstructions had not assemblage representing the longer highstand will yet been described taxonomically. These examples have a greater proportion of individuals from indicate that incomplete taxonomic or ecological deepwater species. The sedimentation rate will knowledge of the systemmaynot necessarily hin- also a¡ect the extent of time averaging and thus der the successful use of ostracode species-abun- apparent relative water-depth changes. Third, dance data to generate paleolake-level reconstruc- substantial variability in the species-assemblage tions, provided that su⁄cient sampling of life data may be attributable to the remaining signi¢- assemblages and identi¢cation of specimens to cant environmental factors in CCA Axis 2 vs. stable taxonomic categories have occurred. Ostra- Axis 3 space: latitude and watershed disturbance code-based reconstruction of lake-level curves can level. The signi¢cance of latitude and longitude as potentially yield reliable, nearly continuous re- ‘environmental’ variables re£ects the degree of cords of relative lake-level change when the sedi- highly localized ostracode endemism within the ment record is sampled at su⁄ciently high resolu- lake. Species composition of assemblages changes tion and is anchored by a robust chronology. substantially along the lakeshore, which should However, all ostracode assemblages were lo- a¡ect the absolute values of depth scores among cated in ordination space at a substantial o¡set sites. Anthropogenic watershed disturbance may fromtheir true depth. Despite this, the relative also overprint the signal of lake-level change in lake-level reconstructions based on themappeared paleoecological ostracode assemblages. For the to be reliable. We would expect some o¡set be- purposes of this paper, we have included only tween water depths reconstructed in this manner species-assemblage data collected o¡shore from using paleoecological assemblages and the actual minimally disturbed watersheds, to eliminate the water depths where living assemblages were col- confounding signal of changing watershed distur- lected for several reasons, and for this reason we bance levels through time on our paleolake-level have limited our discussion here to relative reconstruction. The three longer-termlake-level changes in water depth. First, post-mortem pro- records presented here also come from a limited cesses of spatial and temporal averaging render stretch of the lakeshore, reducing the in£uence of paleoecological ostracode assemblages as compo- species endemism on the comparison. Finally, be- sites of living assemblages integrated over larger cause the ostracode-based method of inferring areas and through longer time intervals than the past water depth does not accurately reconstruct living assemblages used for the calibration data true lake level, it is preferable to use this method set (Alin, 2001; Park et al., 2003). Therefore, to re¢ne the timing of relative changes and use the paleoecological assemblages may comprise in- other physical evidence (e.g. winnowed shell dividuals fromspecies typically inhabiting a some- lags, depth distribution of former beachrocks what greater variety of substrate types and depths and stromatolites) to constrain the realized mag- than the muddy and silty sediments from which nitude of the lake-level change. cores are collected. Alin (2001) has shown that A further factor which may in£uence the qual- paleoecological assemblages in shallow water are ity of the ostracode paleoecological record is the o¡set in ordination space fromliving assemblages abundance of SOM in the sediment interval. As because of substrate di¡erences. In general, the discussed previously, high concentrations of SOM cores were also collected in somewhat deeper can dissolve ostracode valves, potentially biasing water than the majority of living assemblages. the remaining ostracode assemblage. Three core Second, the duration of a highstand (or lowstand) samples with high SOM also had low ostracode may a¡ect the reconstructed depth score for that valve abundances (asterisks in LT-97-56V pro¢le interval. Because ostracode assemblages represent in Fig. 5). However, we suggest that the simulta- time-averaged composites, a brief highstand may neous peaks in SOM and lake level and the cor- be more weakly registered in the paleoecological responding nadirs in ostracode abundance may be

PALAEO 3169 9-9-03 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 41 explained by covariation in water depth and SOM (1997b) appears to represent recurrent or persis- preservation, with rami¢cations for ostracode tent drought conditions between V1550 and 1850 preservation. Verschuren (2001) showed that AD, resulting in lake lowstands or rapid declines SOM and water depth covary signi¢cantly in a in lake level at 1580 þ 15 AD, 1730 þ 35 AD, and small, shallow-lake setting. Johnson and Ng’ang’a 1800 þ 30 AD (Fig. 5). Other signi¢cant intervals (1990) demonstrated that this relationship does of low lake levels occurred at 200 þ 90 BC, 25 þ 90 not pertain to deepwater milieus of large lakes. BC, 290 þ 75 AD, 440 þ 75 AD, 1090 þ 50 AD, We suggest that in shallow-water environments 1200 þ 50 AD, and lake levels fell during the in- within large lakes (i.e. 6 100 m), the positive cor- terval(s) 720 þ 60 AD and 850 þ 65 AD. These re- relation between water depth and organic-matter sults are consistent with the observation that lake preservation may still be valid because the de- levels were generally lower than the lake outlet creased intensity of winnowing with increasing level for most of the past 2500 years (Cohen et depths will allow greater accumulation of SOM al., 1997b). (cf. Verschuren, 2001). Based on the timing of lake-level increases and declines, it appears that Lake Tanganyika was 4.2. Late Holocene lake-level variation at Lake relatively low through the latter part of the Medi- Tanganyika eval WarmPeriod (MWP; V1050^1250 AD), rising or high during the ¢rst half of the Little Ice The lake-level curves reconstructed here based Age (LIA; V1250^1550 AD), and re£ecting dom- on ostracode species-abundance data indicate inantly arid conditions throughout the lake basin more substantial variation in lake level than pre- during the latter half of the LIA (V1550^1850 viously reported during the past 2500 years (Figs. AD). Charcoal-abundance data from Cohen et 5 and 6). These lake-level reconstructions accu- al. (1999) further corroborate the occurrence of rately replicated known lake-level changes in in- severe droughts and increased ¢re frequency in strumental data since V1850 AD (Birkett et al., the Lake Tanganyika basin between V1550 and 1999; Evert, 1980) and in stromatolitic and stable 1850 AD (Alin, 2001; Cohen et al., 1999). Char- isotopic records dating to V1000 BC (Cohen et coal abundance increased earlier (starting at al., 1997b). This is true despite the fact that not V1200 AD) than indications of lake-level decline all ostracode species in core samples were repre- in core LT-98-2M, indicating that the onset of sented in the live calibration database and that drought may have begun as early as the 13th cen- the depth range of samples in the live database tury AD in the central basin of Lake Tanganyika lies dominantly above the water depth of the (Cohen et al., 1999). The time lag between this cores examined here. In particular, the lake-level early terrestrial indicator of drought in the central curves reported here substantiate the late Holo- basin and the drop in lake level re£ected by os- cene (V1250^1550 AD, 430 þ 110 AD) high- tracode assemblages may be explained by the ob- stands reported by Cohen et al. (1997b), although servation that the highlands in the northern basin these new data re¢ne the timing of the maximum of the lake (in Burundi and the Democratic Re- highstands to 580 þ 70 AD and 1495 þ 45 AD. All public of Congo) serve as the major source of dates discussed here are fromcore LT-97-56V be- runo¡ to the lake and may not have been a¡ected cause of its higher sedimentation rate and resolu- by the increasing regional aridity as early as the tion, although dates fromcore LT-98-2M are lower elevation forests of the central basin (Cohen within 10^25 years of dates fromLT-97-56V for et al., 1997b). A stronger pulse in sedimentary all events in the period 1000^1600 AD. Lake level charcoal in£ux occurred simultaneously with in- appears to have begun rising sometime in the 13th dications of lowered lake levels fromthe mid- century toward this maximum at V1500 AD, 1500s AD to 1790 AD (core top) (Cohen et al., with a brief reversal of this rising trend at 1999). Peak carbonate abundance corresponding V1400 AD. In addition, the late 16th^early to the shelly hash layer at V54^55 cmalso co- 19th century lowstand discussed by Cohen et al. incided temporally with the onset of the 1550^

PALAEO 3169 9-9-03 42 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49

1850 AD droughts (shell hash age 1580 þ 15 AD). The presence of shell hash in water of this depth indicates that lake level was s 30 mlower than modern lake-surface elevation during deposition and further substantiates the ostracode-reconstructed drop in lake level at 1580 þ 15 AD. To- gether, these results suggest that regional aridi¢- cation began in the 1200s AD, increasing in extent and diminishing the source of runo¡ from the northern highlands by the mid-1500s AD, such that lake level experienced a substantial decline at that time. The aridi¢cation during the LIA comes in the context of a longer-termregional aridi¢cation in East Africa reported by many authors to have occurred during the last 1500^5500 years (e.g. Fig. 7. Paleoclimatic records of relative moisture balance Beuning et al., 1997; Russell et al., 2003; Stager fromvarious locales in East Africa. Records are arranged in and Johnson, 2000; Vincens, 1993). Cohen et al. latitudinal order fromnorth to south. Codes stand for: Nile, (1997b) demonstrate that Lake Tanganyika spent Nile River summer £ows (Herring, 1979); Turk, Lake Turka- much of the late Holocene as a closed basin, with na (Verschuren, in press); Vic, Lake Victoria (Nicholson, water levels just below the modern outlet eleva- 1998); Edw, Lake Edward (Russell et al., in press); Naiv, Lake Naivasha (Verschuren, 2001; Verschuren et al., 2000); tion (768 masl). In the central Lake Tanganyika Kili, Mount Kilimanjaro (ice-core record: Gasse, 2002; basin, the pollen record fromcore LT-98-2M Thompson et al., 2002); Tan1, Lake Tanganyika (stromato- shows substantial decreases in arboreal pollen lite record of Cohen et al., 1997b); Tan2, Lake Tanganyika from V20^30% representation starting at V400 (ostracode-based record of this paper); and Mal, Lake Mala- BC and declining to 6 10% after a second drop at wi (Johnson et al., 2001, 2002). Black and gray vertical bars indicate periods of severe and less severe drought conditions, around 500 AD (Cohen et al., 1999). Thus, the respectively, and/or highest (and high) diatomproductivity late Holocene history of Lake Tanganyika ap- for Lake Malawi. Solid white vertical bars with question pears to have been one of relatively low lake level marks denote lowstands with poor age control. Stippled punctuated by relatively short-lived episodes of white vertical bars represent periods of positive moisture bal- wetter conditions. ance, lake highstands, low diatomproductivity, etc. Horizon- tal gray zones represent approximate periods of lake-level lowstands in Lake Tanganyika, to facilitate comparison of 4.3. Agreement with other records of moisture timing among records. Typical timing assigned to the MWP balance in Africa (V900^1250 AD) and the LIA (V1250^1850 AD) is indicated in the upper right of the graph. The new information on the timing of lowstands presented here is fairly coherent with other Kivu experienced a prolonged lowstand, resulting climate records from East Africa during some in lake basin closure, between V3700 and 1400 yr time intervals, less so at other times. This is con- BP, although the resolution of the lake-level rec- sistent with the great variability in coherency ord at Kivu is insu⁄cient to compare the period among climate records at various timescales in of maximum aridity at Lake Kivu directly with this region (Nicholson, 1996, 2000). In the earliest the timing of the maximum lowstand at Tanga- part of the record, the most pronounced lowstand nyika (Haberyan and Hecky, 1987). During this re£ected in the lake-level curve fromcore LT-97- early part of the Lake Tanganyika record, no 56V (V200^25 BC) coincides with the maximum other regional records of su⁄cient resolution are late Holocene lowstand seen at Lake Edward at available for comparison. V2000 yr BP (Russell et al., 2003)(Fig. 7). Di- More regional records of moisture balance are rectly to the north of Lake Tanganyika, Lake available for comparison for much of the ¢rst

PALAEO 3169 9-9-03 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 43 millennium AD (Fig. 7). During the interval ords are available for more locales during the V200^500 AD (290 þ 75 AD, 440 þ 75 AD), Tan- ¢rst millennium AD, it will not be possible to ganyika again experienced relative lowstands. ascertain to what degree moisture-balance This contrasts with the stromatolite record of Co- changes are coherent among regions within East hen et al. (1997b), but as previously discussed, we Africa. suggest that their 430 þ 110 AD highstand oc- At the beginning of the second millennium AD, curred later (580 þ 70 AD). Within the same win- lake level at Lake Tanganyika fell and remained dow of time, lakes Turkana and Edward experi- relatively low during V1050^1250 AD, which enced lowstands, although of shorter duration corresponds to the timing of the MWP in many than that at Tanganyika (Russell et al., in press; locales, albeit with a later onset than in some Verschuren et al., 2000). The late 6th century areas (Fig. 7). Lakes Turkana, Edward, and Nai- highstand at Tanganyika is paralleled by a high- vasha all experienced pronounced and/or pro- stand at Lake Turkana and matches within the longed lowstands during this period (Russell et range of age-model errors with wet conditions al., in press; Verschuren, 2001, in press; Verschu- on Mount Kilimanjaro (Verschuren, in press; ren et al., 2000). Mt. Kilimanjaro’s N18O record Thompson et al., 2002). The N18O of ice in cores shows some of its most enriched values during the collected fromglaciers atop Kilimanjaro registers past 1500 years from V1040 to 1100 AD, sug- depleted signatures during approximately this in- gesting diminished rainfall in equatorial East Afri- terval (Thompson et al., 2002). Although Thomp- ca (Thompson et al., 2002). Nicholson (1998) col- son et al. (2002) interpret this record as a proxy lated information on severe droughts and famines for temperature, N18O records in the tropics are in East Africa based on oral histories and Nile more frequently interpreted to represent the pre- £ow records to infer changes in the level of cipitation ‘amount’ e¡ect and may therefore com- Lake Victoria prior to historical data. Nile River prise a rainfall rather than a temperature signal waters at their lowest annual £ow levels (May^ (Gasse, 2002). The Kilimanjaro N18O record bears June), measured at the Rodah Nilometer in a striking resemblance to the lake-level curve for Egypt, are dominantly comprised of runo¡ from nearby Lake Naivasha, supporting the interpreta- equatorial rainfall in the Lake Victoria region tion that times of depleted N18O correspond to (Herring, 1979). Thus, the Nile £ow and Lake wetter, rather than colder, intervals (Thompson Victoria records discussed here are not entirely et al., 2002; Verschuren, 2001; Verschuren et independent, although the Lake Victoria record al., 2000). Thus, the Kilimanjaro ice-core record comprises historical data on droughts and famines bears N18O signatures consistent with increased fromoral traditions as well. In contrast with the precipitation in the region coeval with the Tanga- preceding observations, Nile £ow records and oral nyika V500 AD highstand. The timing of the traditions suggest that rainfall in the Lake Victo- following lowstand at Lake Tanganyika is sug- ria watershed, directly west of Lake Naivasha, gested here to fall in the interval V700^850 was relatively high (Herring, 1979; Nicholson, AD, and Kilimanjaro appears to be relatively 1998). With the exception of Lake Victoria, a rea- dry during this interval (Thompson et al., 2002). sonably coherent picture thus unfolds of the Lake Tanganyika ended the ¢rst millennium with MWP in East Africa being a relatively arid peri- a relative highstand at V950^1000 AD, which od, although a more direct and highly resolved may correspond to a return of wetter conditions record of moisture balance conditions from the on Kilimanjaro (Thompson et al., 2002). Lake Victoria basin will be necessary to resolve Although certain features of the lake-level record this issue. for Tanganyika appear to overlap with other re- During the ¢rst half of the LIA (V1250^1550 gional indicators of moisture balance, these ap- AD), East Africa generally experienced a wetter parent connections are not yet su⁄ciently re- climate. Lake Tanganyika experienced its maxi- solved chronologically to convince us of their mum highstand at V1450^1550 AD, according temporal coincidence. Until higher-resolution rec- to ostracode water-depth reconstructions. The

PALAEO 3169 9-9-03 44 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 rise in lake level leading to this highstand started ity during the 1820s^1830s AD. Low Nile £ows sometime in the 13th century AD, reaching a rel- also characterize the period of the 1580s^1620s ative peak re£ected in all three ostracode lake- AD, while high Nile £ows dominated between level records at V1300 AD, followed by a brief 1360 and 1530 AD (Herring, 1979; Nicholson, decline at V1400 AD, and culminating in a max- 1998). Two arid intervals in the Lake Naivasha imum highstand at V1500 AD (Fig. 5). This record overlap substantially with the three 1550^ maximum highstand is coincident with one of 1850 AD Tanganyika lowstands (Verschuren et two LIA peak periods of cooling in the sea sur- al., 2000: 1560^1620 AD, 1760^1840 AD). Lower face temperatures (SSTs) o¡ the west coast of lake levels in Turkana and Edward are consistent Africa and in the Northern Hemisphere (deMeno- with the picture of regional drought during the cal et al., 2000). During the early LIA, lakes Tur- late LIA (Russell et al., in press; Verschuren, in kana, Victoria, and Naivasha had high lake levels, press). Kilimanjaro oxygen isotope values also ex- and Naivasha also experienced an arid interval at perience periods of relative enrichment during this V1400 AD (Nicholson, 1998; Verschuren, in time (Thompson et al., 2002). Finally, high pro- press; Verschuren et al., 2000). The centennial- ductivity in the north of Lake Malawi suggests average £ow conditions in the Nile were higher northerly winds (i.e. a more southerly ITCZ) than average through this time (Herring, 1979), and/or lower lake level in the basin (Johnson et and N18O values in the Kilimanjaro ice core al., 2001, 2002). Interestingly, a high-resolution were again depleted, consistent with higher rain- record of bulk titanium(Ti) content in a Cariaco fall in East Africa (Gasse, 2002; Thompson et al., Basin (north of Venezuela) sediment core reveals 2002). Biogenic silica concentrations in sediment general LIA aridity, with peak aridity at V1590 cores, which are used as an indicator of paleopro- AD, 1670^1690 AD, and 1750^1780 AD, match- ductivity levels, were relatively low in the north ing the timing of East African droughts fairly basin of Lake Malawi during the ¢rst half of the closely (Haug et al., 2001). Sedimentary Ti con- LIA (Johnson et al., 2001, 2002). Johnson et al. tent re£ects rainfall over northern South America, (2002) interpret times of high productivity to re- with arid intervals interpreted to re£ect southward £ect northerly winds driving upwelling in the migration of the ITCZ (Haug et al., 2001). Thus, north basin, which in turn would suggest a south- the latter half of the LIA appears to be charac- ward incursion of the ITCZ, although it has also terized by recurrent, pronounced droughts been suggested that the periods of higher produc- throughout East Africa, with teleconnections to tivity coincided with lowered lake levels at Lake other tropical regions, which may be related to Malawi. Low productivity in Lake Malawi’s a shift in ITCZ location. north basin during the early LIA may therefore Since the termination of the LIA, many sites in be consistent with the interpretation of relatively East Africa have instrumental records of rainfall high lake levels and the southerly winds that are and/or lake level, making it possible to assign ac- dominant there today. Thus, all available indica- curate elevations to lake-surface levels. At Lake tors suggest that the early LIA in East Africa had Tanganyika, a highstand of 784 masl occurred in a positive moisture balance. 1878 AD (Evert, 1980), and this is re£ected in the In contrast, the second half of the LIA lake-level curves of cores LT-97-56V and MIT-1. (V1550^1850 AD) was a period marked by re- This peak is contemporaneous with the second current or persistent drought conditions in many cooling peak in the West African SST record dis- parts of East Africa. Tanganyika saw signi¢cant cussed by deMenocal et al. (2000), and also co- declines or lowstands in lake level in the intervals incides with high lake levels in lakes Naivasha and 1580 þ 15 AD, 1730 þ 35 AD, and 1800 þ 30 AD possibly Malawi (Johnson et al., 2001; Verschu- (Fig. 5). Nicholson (1998) reports severe droughts ren, 2001). The ostracode record of core MIT-1, and famines during the 1580s AD and several and possibly that of core LT-97-56V, also re£ects during the 1720s^1760s AD in the Lake Victoria the increase in lake level resulting fromintense region, in addition to a period of continental arid- rainfall during 1961^1962 AD.

PALAEO 3169 9-9-03 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 45

5. Conclusions Acknowledgements

The broad congruence between previously We are grateful to the National Science Foun- published and new lake-level data presented here dation (Grants EAR-9627766 and ATM-9619458 for Lake Tanganyika indicates that paleoecologi- to A.S.C., and a Graduate Research Fellowship cal ostracode assemblages can provide reliable to S.R.A.), the UN/GEF Lake Tanganyika Bio- and sensitive reconstructions of past changes diversity Project, and the University of Arizona in lake level. En masse, an ostracode assemblage (small grants and a fellowship to S.R.A.) for re£ects the relative water depth of its habitat, funding and logistical assistance. We thank the in addition to the in£uence of other environ- Tanzanian Commission for Science and Technol- mental factors. These other factors make it im- ogy and the Tanzanian Immigration Agency for possible to quantitatively reconstruct lake-level the permits necessary for conducting this research. changes, but when used in tandemwith available The assistance of the crew of the R/V Explorer, physical evidence of lake highstand or lowstand C. Scholz, J. McGill, P. Cattaneo, and numerous levels, it should be possible to assess both the others was essential and greatly appreciated in timing and magnitude of past lake-level £uctua- collecting the cores. In addition, we thank D.L. tions. Dettman and O.K. Davis for assistance with ra- The ostracode-based lake-level reconstruction diocarbon dating and identi¢cation of radiocar- presented here contributes more highly resolved bon samples, respectively; C. Birkett at NASA information on the timing and direction of lake- for lake-level data fromsatellite altimetry; J.S. level £uctuations in Lake Tanganyika for the past Alin and G.W. Holtgrieve for assistance with 2500 years. In general, the late Holocene climate data analyses; T.C. Johnson, J.M. Russell, J.T. history of Lake Tanganyika is characterized by Overpeck, K.W. Flessa, P.N. Reinthal, R. Robi- aridity, punctuated by brief intervals of higher chaux, and C.M. O’Reilly for helpful discussions rainfall. The ostracode water-depth record re¢nes and/or comments on earlier versions of the manu- the timing of known but undated lowstands of script; and K. Martens and P. Mourguiart for 315^40 min the late Holocene to 580 þ 70 AD constructive reviews. This is International Decade and recurrent droughts during the second half of East African Lakes (IDEAL) contribution of the LIA (1580 þ 15 AD, 1730 þ 35 AD, and number 147. 1800 þ 30 AD). The MWP also appears to have been arid at Lake Tanganyika (V1050^ 1250 AD), although the intervening early part Appendix1 of the LIA was characterized by higher lake levels that may have been sustained longer List of species included in the live ostracode than any other highstand in this record (rising database for CCA. Undescribed species are given nearly continuously from V1250 AD to a peak a numerical moniker and refer to specimens in the at V1500 AD). Other occasions of marked arid- reference collection at the University of Arizona1. ity not resolved in previous records at Lake Tanganyika include the periods V200^0 BC, V200^500 AD, and V700^850 AD. Records of MWP and late LIA aridity and early LIA 1 The numbers of undescribed species in the University of wetness are reasonably coherent throughout East Arizona ostracode reference collection do not necessarily re- Africa. The sparsity of earlier paleoclimate re- £ect the overall diversity of undescribed species in their respec- cords in East Africa makes it impossible at tive genera. When numbered species are identi¢ed as either a this juncture to ascertain whether lake levels £uc- described species or lumped with another numbered species, their numbers are not recycled in order to avoid future taxo- tuated in synchrony in response to decade^cen- nomic confusion. Thus, the species numbers suggest arti¢cially tury scale climate changes in the preceding centu- high numbers of undescribed taxa in various Tanganyikan ries. ostracode genera.

PALAEO 3169 9-9-03 46 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49

SUPERFAMILY CYTHEROIDEA Romecytheridea tenuisculpta3 Family Cytherideidae Romecytheridea n.sp. 10 Archaeocyprideis tuberculata2;3 Romecytheridea n.sp. 14 Archaeocyprideis n.sp. 23 Romecytheridea n.sp. 15 Archaeocyprideis n.sp. 103 Romecytheridea n.sp. 18 Archaeocyprideis n.sp. 15 Tanganyikacythere burtonensis3 Cyprideis loricata2 Tanganyikacythere caljoni3 Cyprideis mastai3 Tanganyikacythere n.sp. 12 Cyprideis profunda2;3 Tanganyikacythere n.sp. 16 Cyprideis rumongensis Tanganyikacythere n.sp. 18 Cyprideis spatula3 Family Limnocytheridae Cyprideis torosa Gomphocythere alata3 Cyprideis n.sp. 20 Gomphocythere coheni2;3 Cyprideis n.sp. 23 Gomphocythere cristata3 Cyprideis n.sp. 243 Gomphocythere curta3 Kavalacythereis braconensis3 Gomphocythere downingi2;3 Mesocyprideis irsacae3 Gomphocythere wilsoni2;3 Mesocyprideis nitida3 Gomphocythere woutersi2;3 Mesocyprideis pila3 Gomphocythere n.sp. 18 Mesocyprideis n.sp. 2B3 Limnocythere n.sp. 83 Mesocyprideis n.sp. 93 Family Cytheridae Romecytheridea ampla3 cf. Elpidium n.sp. 1 Romecytheridea longior2;3 SUPERFAMILY CYPRIDOIDEA Family Cyclocyprididae Allocypria aberrans 3 2 Species that were published under other names in Alin et Allocypria humilis 2;3 al. (1999) or Wells et al. (1999) have the following synonomies Allocypria mucronata (old name = new name), based largely on subsequent formal Allocypria n.sp. 53 descriptions of these taxa in Park and Martens (2001) and Allocypria n.sp. 113 Wouters and Martens (1999, 2001): Cytheroidea ^ Archaeocy- Allocypria n.sp. 17 prideis sp. 1 = Archaeocyprideis tuberculata, Cyprideis n.sp. 1=Tanganyikacythere n.sp. 1, Cyprideis sp. 3 = Cyprideis pro- Allocypria n.sp. 18 funda, Gomphocythere n.sp. = Gomphocythere coheni, Gompho- Mecynocypria complanata cythere sp. 3 = Gomphocythere downingi, Gomphocythere sp. Mecynocypria conoidea3 13 = Gomphocythere wilsoni, Gomphocythere sp. 16 = Gompho- Mecynocypria declivis cythere woutersi, Gomphocythere sp. y = Gomphocythere wilsoni, Mecynocypria de£exa Gomphocythere sp. z = Gomphocythere downingi, Mesocyprideis 3 sp. 2A = Cyprideis loricata, and Romecytheridea n.sp. 13 = Ro- Mecynocypria emaciata 3 mecytheridea longior. Cypridoidea ^ Allocypria claviformis Mecynocypria opaca group = Allocypria mucronata. Mecynocypria subangulata Mecynocypria n.sp. 8 3 Species that also occurred in core LT97-56V. Additional Mecynocypria n.sp. 93 species that occurred in core LT97-56V but not in the live database: Cytheroidea ^ Gomphocythere n.sp. 11, Kavalacy- Mecynocypria n.sp. 13 thereis hystrix, Mesocyprideis n.sp. 4, Tanganyikacythere n.sp. Mecynocypria n.sp. 14 18. Cypridoidea ^ Allocypria claviformis, Allocypria humilis, Mecynocypria n.sp. 17 Allocypria cf. inclinata, Allocypria n.sp. 20, Candonopsis n.sp. Mecynocypria n.sp. 203 15, Cypridopsis n.sp. ‘¢vespur’, Cypridopsis n.sp. 21, Cypridop- Mecynocypria n.sp. 22 sis n.sp. 25, Mecynocypria obtusa, Mecynocypria parvula, Me- 3 cynocypria n.sp. 21, Mecynocypria n.sp. 33, Mecynocypria n.sp. Mecynocypria n.sp. 29 (opaca group) 3 36, Mecynocypria n.sp. 37, Mecynocypria n.sp. 40, and Tanga- Mecynocypria n.sp. 30 nyikacypridopsis n.sp. 1.

PALAEO 3169 9-9-03 S.R. Alin, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 31^49 47

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