A 10,000-year high-resolution diatom record from Pilkington Bay, Lake Victoria, East Africa

Available online at www.sciencedirect.com R Quaternary Research 59 (2003) 172–181 www.elsevier.com/locate/yqres A 10,000-year high-resolution diatom record from Pilkington Bay, Lake Victoria, East Africa J. Curt Stager,a,* Brian F. Cumming,b and L. David Meekerc a b Natural Resources Division, Paul Smith’s College, Paul Smiths, NY 12970, USA P.E.A.R.L., Department of Biology, Queen’s University, Kingston, Ontario, Canada K7L 3N6 c Climate Change Research Center, University of New Hampshire, Durham, NH 03824, USA Received 7 March 2002 Abstract A new diatom record from Lake Victoria’s Pilkington Bay, subsampled at 21- to 25-year intervals and supported by 20 AMS dates, reveals a ⬃10,000 calendar year environmental history that is supported by published diatom and pollen data from two nearby sites. With their chronologies adjusted here to account for newly documented ancient carbon effects in the lake, these three records provide a coherent, finely resolved reconstruction of Holocene climate change in equatorial East Africa. After an insolation-induced rainfall maximum ca. 8800 – 8300 cal yr B.P., precipitation became more seasonal and decreased abruptly ca. 8200 and 5700 yr B.P. in apparent association with northern deglaciation events. Century-scale rainfall increases occurred ca. 8500, 7000, 5800, and 4000 yr B.P. Conditions after 2700 yr B.P. were generally similar to those of today, but major droughts occurred ca. 1200 – 600 yr B.P. during Europe’s Medieval Warm Period. © 2003 Elsevier Science (USA). All rights reserved. Keywords: Africa; Diatoms; Lake Victoria; Monsoons; Paleoclimate Introduction Lake Victoria’s Pilkington Bay, Uganda (Fig. 1), is a classic paleoecological reference site in tropical Africa. Pollen analysis of core P-2 provided some of the first evidence that tropical climates were dry during the last glacial and humid during the warm early Holocene (Kendall, 1969). A paleosol in P-2 first suggested that Lake Victoria may have dried out during the late Pleistocene, a hypothesis later confirmed by coring and seismic profiling (Johnson et al., 1996; Stager et al., 2002). However, causal mechanisms behind Lake Victoria’s climatic history remain less well understood than those affecting sites at higher latitudes. The Pilkington Bay pollen record, in conjunction with a fine-interval diatom record from nearby Damba Channel (Fig. 1; Stager, 1984; Stager et al., 1997), suggested that the Holocene history of Lake Victoria consisted of (1) an early Holocene maximum of rainfall and lake mixing, (2) a somewhat * Corresponding author. E-mail address: stagerj@paulsmiths.edu (J.C. Stager). less humid mid-Holocene phase of increased rainfall seasonality and relative water column stability, and (3) a late Holocene period of variable aridity and shallowing. The early/midHolocene climatic transition at Lake Victoria represented an abrupt global change in atmospheric circulation (Stager and Mayewski, 1997), and significant periodicities in the diatom record were linked to changes in monsoon activity, solar variability, and marine circulation (Stager et al., 1997). Here we present a ⬃10,000-yr diatom record from Pilkington Bay core 64-2 that yields new insights into the history of the Victoria basin. We focus on comparison with other Holocene microfossil records from Lake Victoria in order to refine the timing of major climate changes and to examine possible causal mechanisms behind those changes. This is the first time that the Pilkington Bay pollen and Damba Channel diatom series appear in adjusted form following new evidence that 14C dates from these cores are 330 years too old due to contamination with ancient carbon (Fig. 2; Stager and Cumming, unpublished data). This is also the first time that the chronology of Kendall’s (1969) much-cited pollen record has been presented in calendar year form. 0033-5894/03/$ – see front matter © 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0033-5894(03)00008-5 J.C. Stager et al. / Quaternary Research 59 (2003) 172–181 173 Fig. 1. Site maps (A) Pilkington Bay and vicinity. “P” marks 64-2 coring site. (B) Lake Victoria watershed. 1 ⫽ Pilkington Bay, 2 ⫽ Damba Channel. (C) Africa, with Lake Victoria watershed dotted. Study site Lake Victoria is the world’s most extensive tropical lake (ca. 69,000 km2) but is relatively shallow (ca. 80 m max. depth; Crul, 1995). Core 64-2 was collected near the mouth of Pilkington Bay, which occupies 40 km2 of the north end of Buvuma Island, Uganda (Fig. 1; 0°18⬘N, 33°20⬘E). The bay floor dips northward to a depth of 12 m and papyrus swamps fringe the shoreline. More than 90% of Lake Victoria’s water enters and leaves via the atmosphere with most rain falling around March–May (long rains) and October– December (short rains) with the arrival of the Intertropical Convergence Zone (ITCZ). Between rainy seasons, dry winds associated with the Afro-Asian monsoon system contribute to the breakdown of thermal stratification (Talling, 1966). Former high stands left raised strandlines ca. 18, 12, and 3 m above modern lake level. Only the lowest has been radiocarbon dated (⬃4000 B.P.; Temple, 1967), but all three probably formed after the lake recovered from desiccation ca. 15,000 years ago (Kendall, 1969; Talbot et al., 2000; Stager et al., 2002). The past 50 years have brought major changes to Lake Victoria, including eutrophication and persistent thermal stratification (Hecky, 1993). Native fishes have declined and the phytoplankton, formerly dominated by green algae and diatoms (Aulacoseira spp.) that require strong mixing, are 174 J.C. Stager et al. / Quaternary Research 59 (2003) 172–181 Chronology Fig. 2. (a) Composite age-depth regression for pipes 1–7, 9, and 10 from core 64-2, with calendar year ages determined after subtraction of 330 years from the 14C ages. The black dots are based on AMS dates. The open dots are dates estimated from the slope of the regression and other date(s) within the same pipe section. Pipe sections marked by dotted lines. (b) The basis for a 330-year correction. The age– depth curve from 137Cs and 210 Pb-dated gravity core P2K-4 from Pilkington Bay joins the regression line from AMS-dated piston core P2K-1 when 330 years are subtracted from the 14C dates, causing the diatom records of those cores to overlap. The same correction was also calculated from cores collected in adjacent Buvuma Channel (Stager and Cumming, unpublished data). now dominated by cyanobacteria and diatoms (Nitzschia spp.) that are typical of stable water columns (Ochumba and Kibaara, 1989; Hecky, 1993). Methods Core 64-2 was collected from a depth of 11 m in eleven 1-m sections with a modified Livingstone sampler by D. Livingstone and P. Weigl in 1964. The anchor point on the piston cable was lost after the seventh drive, so the starting position of the eighth drive was set as close as possible to that of the seventh. The last drive, 11, also duplicated much of drive 10, and these two overlaps are clearly apparent in the dated diatom stratigraphies. The core, stored under refrigeration at Duke University, was still moist and freshlooking when 1-cm3 subsamples were removed at 2-cm intervals in 1998. The 380 subsamples from nonoverlapping core sections were homogenized, digested in H2O2, and mounted with Permount. Between 300 and 500 diatom valves were counted per slide at 1000X with oil immersion. Taxonomy followed Stager (1984), Hustedt (1949), Patrick and Reimer (1966), and Gasse (1986). Photomicrographs of the dominant taxa are published in Stager (1984). Accelerator mass spectrometry (AMS) dates were obtained for 1-cm3 bulk sediment subsamples from 31 levels in the core (Table 1). Subsample ages were based on one to three dates from each individual core section and were converted to calendar years before 2000 A.D. with CALIB 4.3 (Stuiver and Reimer, 1993) after subtraction of 330 years from each 14C date due to ancient carbon contamination of organic matter in Victoria sediments (Figs. 2– 4; Stager and Cumming, unpublished data). Nineteen bulk dates from core P-2 (Kendall, 1969) and three from the Damba Channel core representing the last 10,000 years were treated in the same fashion. AMS and diatom data from sections 8 and 11 are omitted here because they represent duplicate coring drives and because the top of pipe 11 yielded an anomalously young age and diatom assemblages that suggest contamination by sediments pushed down the borehole during coring. Three outliers were also omitted from the remaining chronology, leaving 20 dates to calibrate the record of sections 1–7, 9, and 10 (Table 1). Due to disturbance during transport and storage, the uppermost 24 cm of the record were also omitted and the chronology of the diatom record of the 82- to 26-cm portion of section 1 was tuned to Pilkington Bay cores P2K-1 and P2K-4, which were extruded vertically in the field in June 2000 and dated by AMS, 210Pb, and 137Cs methods (Stager and Cumming, unpublished data; Fig. 2; Table 1). Diatom taxon habitats Planktonic diatom community patterns in Lake Victoria are very sensitive to wind-driven mixing. As is more general in East Africa’s large lakes, Nitzschia spp. tend to predominate over Aulacoseira spp. when water columns stabilize (Talling, 1966; Haberyan and Hecky, 1987; Ochumba and Kibaara, 1989; Owen and Crossley, 1992). Aulacoseira nyassensis var. victoriae (O. Müller) Simonsen, formerly common in Lake Victoria’s deep coastal channels, requires resuspension less frequently than Aulacoseira ambigua (Grun.) Simonsen, which is typical of shallow, well-mixed bays (Talling, 1966). “Long Nitzschia” in this article includes Nitzschia acicularis var. maior (O. Müller) and others that were not identified to species because of their fragmented condition. Although they were sometimes numerous during seasonal mixing periods of the mid-20th century (Talling, 1966), long, slender Nitzschia did not replace Aulacoseira spp. as Lake Victoria’s dominant diatom until stratification became more persistent in recent decades (Hecky, 1993). Nitzschia bacillum Hustedt and Nitzschia lacuum Lange-Bertalot, formerly listed as Nitschia fonticola Grunow (Stager, 1984; Stager et al., 1997), are often associated with cyanobacteria under highly productive and/or stratified conditions in African lakes (Kilham et al., 1986; Ochumba and Kibaara, 1989). “Nitzschia species A” was common in early/ middle Holocene sections of this core and the Damba Channel core (see photomicrograph in Stager, 1984) but not in midlake core V95-2P (Stager and Johnson, 2000); beyond this association with inshore waters, its ecological requirements are un- 175 J.C. Stager et al. / Quaternary Research 59 (2003) 172–181 Table 1 AMS dates from Pilkington Bay core 64-2 Sample no. pipe -(cm) Depth (cm) 1-(19–20) 1-(40–41) 1-(60–61) 1-(79–80) 2-(15–16) 2-(45–46) 2-(90–91) 3(15–16) 3-(45–46) 19.5 40.5 60.5 79.5 107.5 137.5 182.5 215.5 245.5 3-(90–91) 4-(20–21) 4-(81–82) 5-(11–12) 5-(85-86) 6-(46-47) 6-(80-81) 7-(10-11) 7-(44-45) 7-(83-84) 8-(10-11) 8-(52-53) 8-(76-77) 290.5 320.5 381.5 411.5 485.5 546.5 580.5 610.5 644.5 683.5 710.5 752.5 776.5 9-(10-11) 9-(51-52) 9-(76-77) 10-(12-13) 10-(89-90) 11-(10-11) 11-(30-31) 11-(41-42) 11-(81-82) 810.5 851.5 876.5 912.5 989.5 1010.5 1030.5 1041.5 1081.5 AMS date C yr B.P. 1-␴ cal yr range 385 ⫾ 60 485 ⫾ 55 825 ⫾ 65 1090 ⫾ 55 1580 ⫾ 60 1900 ⫾ 55 2310 ⫾ 55 2320 ⫾ 40 2640 ⫾ 65 2715 ⫾ 40 2775 ⫾ 55 2975 ⫾ 60 3445 ⫾ 60 3965 ⫾ 60 4420 ⫾ 50 5240 ⫾ 75 5885 ⫾ 55 6085 ⫾ 70 6375 ⫾ 80 7010 ⫾ 65 7300 ⫾ 120 6430 ⫾ 80 7005 ⫾ 65 7365 ⫾ 70 6760 ⫾ 60 7570 ⫾ 70 8040 ⫾ 75 7335 ⫾ 65 8520 ⫾ 75 8945 ⫾ 75 5550 ⫾ 65 8575 ⫾ 65 8595 ⫾ 75 9245 ⫾ 80 0–250 82–333 549–667 708–781 1115–1328 1404–1584 1884–2043 1931–2042 2233–2404 80 310 650 780 1255 1570 2043 1956 2290 2791–2892 3302–3487 3889–4132 4495–4857 5640–5777 6337–6457 6463–6713 6801–7055 7529–7662 7713–7989 6852–7207 7526–7660 7798–7990 2791 3410 4110 4750 5640 (6084) 6485 6940 (7353) 7840 7070 7593 7894 14 Cal yr chosen 8048–8215 8444–8639 7754–7986 9063–9220 9551–9727 5962–6218 9081–9466 9082–9476 9840–10258 8048 8563 (8871) 9220 (10,172) (9749) 9300 9439 9943 Yr/cm AMS sample no. x 10.9 17 6.7 10.5 10.5 10.5 11.1 11.1 AA-33752 AA-33753 AA-33754 AA-33755 AA-32504 AA-32503 AA-32502 Beta-153739 AA-32501 11.1 11.5 11.5 11.76 11.76 11.8 11.8 12.3 12.3 12.3 12.5 12.5 12.5 AA-33756 AA-33757 AA-33758 AA-32500 AA-33759 AA-32499 AA-33760 AA-33761 AA-32498 AA-33762 AA-33763 AA-32497 AA-33764 12.6 12.6 12.6 12.6 12.6 12.6 12.6 12.6 12.6 AA-33765 AA-32496 AA-33766 AA-32495 AA-33767 AA-32494 AA-33769 AA-33768 AA-32493 Note. Calendar age intervals were determined with CALIB 4.3 after subtraction of 330 years from the AMS dates. Pipe 2 had 8 cm missing from the top, but sample numbers started with 64-2-8. Duplicate pipes 8 and 11 were omitted from the final stratigraphy. Five dates were determined by interpolation after AMS ages were disregarded due to inconsistency with others in their respective pipe sections (in bold in parentheses). known. Taxonomy of Lake Victoria diatoms in the genus “Stephanodiscus” is unresolved and identifications have varied considerably (Hecky, 1993; Stager et al., 1997; Verschuren et al., 1998), but this has little impact on the 64-2 record because these diatoms are uncommon inshore and ecological requirements among species are similar (Gasse et al., 1995). The “littoral taxa” category includes Fragilaria spp. (mainly Fragilaria pinnata Ehr.) and benthic and epiphytic Achnanthes, Cocconeis, Epithemia, Gomphonema, Navicula, and Amphora spp. Higher percentages of these taxa reflect shallowing and greater proximity of littoral habitats to the coring site. diatom assemblages over the past 10,000 years. In the ordination, the species abundances were square root transformed, rare taxa were downweighted, and species scores were scaled to weighted averages of down-core sample scores (Ter Braak, 1990), yielding two principal axes, CAS1 and CAS2 (CAS ⫽ Correspondence Analysis, Square root transformed). Results Chronology Statistical analyses The 36 most abundant diatom taxa in the 64-2 core were subjected to correspondence analysis using the program CANOCO v. 4 (Ter Braak and Smilauer, 1998) in order to provide a summary of major changes in the Sediment accumulation rates determined for core 64-2 ranged between 0.8 and 1.0 mm/yr, somewhat lower than those in Kendall’s (1969) core P-2 from a slightly shallower site in Pilkington Bay (ca. 1.3–1.7 mm/yr) and higher than those in the deeper Damba Channel (0.6 – 0.7 mm/yr; Stager 176 J.C. Stager et al. / Quaternary Research 59 (2003) 172–181 Fig. 3. Major diatom taxa in core 64-2. Bold AMS dates indicate those not used in constructing the chronology. Core pipe sections are marked 1–11. Arrows indicate overlap sections 8 and 11 (see text). et al., 1997). Temporal gaps between pipes in 64-2, which resulted from incomplete recovery during coring, ranged between 80 and 480 years. Diatom stratigraphy Most diatoms belonged, in order of decreasing abundance, to the genera Nitzschia, Aulacoseira, Fragilaria, and Stephanodiscus. Nitzschia bacillum and N. lacuum dominated their genus in all samples, but long Nitzschia and “N. species A” were also common in the lower part of the core, especially between the 7- and 5-m levels (roughly 8000 – 6000 yr B.P.; Fig. 4). Shallow water diatoms were common at the start of the record but were rare by 9500 yr B.P. (Fig. 5a). Aulacoseira ambigua and Aulacoseira granulata varieties declined precipitously above the 8.3-m level (8300 yr B.P.), increasing again in the upper 3 m (ca. 3000 yr B.P.). Aulacoseira nyassensis was rare, but less so between the 7- and 3-m levels (7800 –3000 yr B.P.). Relative changes in Aulacoseira species percentages generally resembled those in the Damba Channel core, but a rise in A. ambigua percentages over the past 3000 years at Pilkington Bay is not registered at the deeper channel site (Figs. 5c and 5d). Littoral diatom abundances increased above the 3-m level and peaked in the lower half of the uppermost meter (1200 – 600 yr B.P.; Fig. 5). Stephanodiscus generally declined throughout the record, particularly with a rise in the percentages of Nitzschia spp. above the 6-m level (ca. 6800 yr B.P.) and then with a rise in A. ambigua and littoral taxa above the 3-m level (ca. 3000 yr B.P.). Correspondence analyses Our correspondence analysis ordination biplot arranges the main diatom taxa in the Pilkington Bay record along two principal axes, CAS1 and CAS2 (Fig. 6), which explain 37 and 52% of the cumulative variance in the diatom series. CAS1 mainly represents relative abundances of shallow and deep-water taxa and thus approx- J.C. Stager et al. / Quaternary Research 59 (2003) 172–181 177 Fig. 4. Selected diatom taxa in core 64-2. Bold AMS dates indicate those not used. Core pipe sections are marked 1–11. Arrows indicate overlap sections 8 and 11 (see text). imates a gradient of precipitation:evaporation ratios (P:E) and/or lake depth. CAS2 reflects the relative abundances of taxa that favor more or less stable stratification (Nitzschia spp. vs. Aulacoseira spp.) and thus approximates a gradient of water column stability. Principal correspondence analysis axes CA1 and CA2 from the Damba Channel diatom record were attributed to similar environmental factors and exhibit the general features of CAS2 and CAS1, respectively (Fig. 7; Stager et al., 1997). The major CA axes of both diatom records indicate that P:E/depth and mixing were greatly enhanced during the early Holocene. However, CAS1 displays less century-scale variability and does not distinguish an early Holocene humidity maximum from less humid mid-Holocene conditions as the Damba Channel’s CA2 does (Figs. 7a and 7d). The reasons for this discrepancy are unclear, but close coherence between the forest pollen curve, the Damba Channel’s CA2, and other records of Holocene rainfall regimes in equatorial East Africa (Richardson and Dussinger, 1986; Bonnefille and Chalié, 2000; Gasse, 2000) suggest that diatom assemblages in the Damba Channel were somehow more sensitive to short-term rainfall variability than were those in Pilkington Bay, which simply bracketed the entire humid period. CAS2 and CA1, however, are more consistent with each other, both showing that mixing was much reduced during the mid-Holocene (Figs. 7e and 7i). Paleoecological interpretation The following environmental history reflects our interpretation of the combined pollen and diatom data sets from Pilkington Bay and Damba Channel, using an updated chronology that places most events 300 – 400 calendar years earlier than has been reported previously (Stager and 178 J.C. Stager et al. / Quaternary Research 59 (2003) 172–181 Fig. 5. Comparisons of littoral taxa and Aulacoseira series from cores from Pilkington Bay (core 64-2) and Damba Channel (core Ibis-1). Vertical dotted lines mark major climate transitions from the (I) the early Holocene maximally humid phase, (II) the more seasonally humid phase, (III) the mid-Holocene drying phase, to (IV) the late Holocene variable phase. Percentages of littoral taxa in Pilkington Bay core P2K-1, to which the diatom record of core 64-2’s uppermost meter was tuned, appear between charts a and b. Mayewski, 1997; Stager et al., 1997). This reconstruction is consistent with offshore records (Stager and Johnson, 2000; Talbot and Laerdal, 2000), pollen data from the Burundi Highlands on the western rim of the Victoria basin (Bonnefille and Chalié, 2000), and lake level records from central Kenya to the east (Richardson and Dussinger, 1986). Early Holocene humid phase I (⬃9500 – 8300 yr B.P.) The P:E ratios and/or depth and lake mixing increased from the start of the record at 10,200 yr B.P. to ca. 9500 yr B.P. with maximal P:E/depth and mixing occurring ca. 8800 – 8300 yr B.P. (Fig. 7). Exceptionally rainy conditions prevailed throughout most of tropical Africa at that time (Nicholson and Flohn, 1980; Street-Perrott and Roberts, 1983; Gasse, 2000). Aulacoseira ambigua and A. granulata percentages were high in Pilkington Bay and Damba Channel ca. 9500 – 8300 yr B.P. (Figs. 5c–5f), due to a combination of water column instability and elevated silica inputs from soil weathering and erosion. Both northerly and southerly monsoon winds were stronger at that time, presumably as a result of high insolation contrast between boreal summer and winter (von Rad et al., 1999; Talbot and Laerdal, 2000) and would have contributed greatly to turbulence in the lake. J.C. Stager et al. / Quaternary Research 59 (2003) 172–181 179 outlet downcutting also contributed to the shallowing trend apparent in the CAS1, CA2, and littoral diatom series (Figs. 5 and 7). A minor P:E/lake level rise ca. 4000 yr B.P. (Fig. 7c and 7d) coincided with the formation of the 3-m beach deposits around the lake margin (Temple, 1967). Winddriven mixing was much reduced relative to the early Holocene (Figs. 7e and 7i). Late Holocene variable phase IV (⬃2700 –150 yr B.P.) Sedge and grass pollen percentages increased in conjunction with encroachment of marginal papyrus swamp due to shallowing, a regional expansion of grasslands, and anthropogenic deforestation (Kendall, 1969). Sediment accumulation and outlet downcutting are the most likely explanation for the long-term shallowing trend indicated by rising lit- Fig. 6. Correspondence analysis biplot showing the distribution of major diatom taxa in core 64-2 relative to principal axes CAS1 and CAS2. Early/mid-Holocene transition (⬃8300 –7800 yr B.P.) This abrupt climatic reorganization ended the early Holocene humidity maximum and left rainfall more seasonally restricted (Fig. 7; Kendall, 1969). Both the Pilkington Bay and Damba Channel diatom records (particularly CAS2, CA1, and Aulacoseira species; Figs. 5 and 7) also indicate a major reduction in the duration and/or intensity of water column mixing that contributed to a reduction of diatom sedimentation offshore (Stager and Johnson, 2000). Abrupt weakening of monsoon winds (Sirocko et al., 1996; Staubwasser, 1999) is a likely cause of the increased water column stability. Mid-Holocene seasonally humid phase II (⬃7800 –5800 yr B.P.) During this second phase of the “African Humid Period” (deMenocal et al., 2000) rainfall was more seasonally restricted and somewhat reduced relative to the early Holocene. Rains were apparently more torrential in nature (Maley, 1982), contributing to low ␦ 18O values offshore (Stager and Johnson, 2000). Moderate century-scale P:E increases were centered on ⬃7000 and 5800 yr B.P. (Figs. 7c and 7d). Mixing inshore and offshore (Figs. 7e and 7i; Talbot and Laerdal, 2000) remained less effective than it was during the early Holocene due to sustained weakening of dry monsoon winds. Mid-Holocene drying phase III (⬃5800 –2700 yr B.P.) Rainfall and inferred lake levels declined after 5800 yr B.P., as they did in most of tropical Africa (de Menocal et al., 2000; Gasse, 2000), but sediment accumulation and Fig. 7. Comparison of Holocene microfossil records from Lake Victoria, with all proxy values arranged to increase upward. (Upper box) Rainfall and lake depth proxies with boreal summer insolation. (Lower box) Mixing and seasonality proxies with rainy season insolation. (a) CAS1 from Pilkington Bay core, inverted ⫽ P:E and/or depth; (b) July insolation at equator; (c) moist forest pollen ⫽ P:E (Moraceae ⫹ Urticaceae ⫹ Alchornea ⫹ Macaranga ⫹ Trema; Kendall, 1969); (d) CA2 from Damba Channel core ⫽ P:E and/or depth; (e) CAS2 from Pilington Bay core ⫽ water column mixing; (f) seasonally dry forest pollen (Celtis ⫹ Holoptelia; Kendall, 1969); (g and h) October and April equatorial insolation, respectively; and (i) Damba Channel core CA1 ⫽ mixing. Vertical lines mark major climate transitions from (I) the early Holocene maximally humid phase, (II) the more seasonally humid phase, (III) the mid-Holocene drying phase, to (IV) the late Holocene variable phase. Numbers 1–5 mark century-scale P:E increases. 180 J.C. Stager et al. / Quaternary Research 59 (2003) 172–181 toral diatom percentages after 2700 yr B.P. (Figs. 5a, 5b, and 7), but abrupt lake level declines superimposed on the trend ca. 2700 –2500 and 1200– 600 yr B.P. were due to P:E reductions. A rise in A. ambigua percentages at Pilkington Bay ca. 2700– 2500 yr B.P. is also probably related to mixing enhanced by shallowing, but a concurrent drop in A. nyassensis percentages at Damba Channel suggests that a climatic change was behind the initial shallowing (Figs. 5c and 5h). Possible causal mechanisms Orbital insolation High northern summer insolation during the early Holocene increased tropical African rainfall by raising sea surface temperatures and strengthening monsoons (Street-Perrott and Roberts, 1983; Overpeck et al., 1996; Gasse, 2000). At the equator, the two seasons during which the ITCZ brings most rain to the Victoria watershed did not experience that orbital insolation increase (Figs. 7g and 7h). There, the P:E maximum most likely resulted from increased rainfall during what is now a dry, windy season as monsoonal rains over West Africa penetrated farther inland and stronger July insolation (Fig. 7b) enhanced local convective rainstorm activity. With the waning of boreal summer insolation during the mid-Holocene, the frequency of West African rain incursions and of convective rainstorms during June–August probably decreased in equatorial East Africa, leaving rainfall more seasonally restricted. In addition, austral summer warming drew the ITCZ farther south and held it there for longer periods (Tyson, 1999; Haug et al., 2001; Tyson et al., 2001), which would give it less time over the equator and reduce the duration of rainy seasons in the Victoria watershed. Mid-Holocene seasonality in the Victoria basin was likely enhanced yet further by maximal insolation contrast between the two equatorial rainy seasons (Figs. 7g and 7h). Deglaciation Two abrupt transitions that disrupted the gradual insolation-induced climate trends ca. 8200 and 5700 yr B.P. are likely related to major meltwater and ice rafting pulses in the North Atlantic (Bond et al., 1997). The abrupt drop in equatorial African P:E ca. 8200 yr B.P. apparently coincided with an abrupt cooling spike that is registered in polar ice cores (Alley et al., 1997), indicating the existence of strong teleconnections between high and low latitudes during the early to mid-Holocene. However, lingering uncertainty in the radiocarbon chronologies of these events prohibits firm conclusions regarding leads and lags between tropical and boreal sites. One could equally speculate that the final steps of northern deglaciation rearranged atmospheric circulation patterns on a global scale, that poleward water vapor and heat transport during tropical humidity maxima triggered the glacial melting pulses, or that external forcing such as solar variability caused simultaneous climatic changes in both regions. Solar variability The roughly 1400- to 1500-year spacing of century-scale P:E fluctuations at Lake Victoria, which is best resolved in the pollen and Damba Channel records (Figs. 7c and 7d), may be related to a ca. 1470-year periodicity in northern marine and ice core records that has been linked to solar variability (Bond et al., 1997; Mayewski et al., 1997). For most of the late Quaternary, high latitude warming and orbital insolation increases were accompanied by humidity in tropical Africa (Rossignol-Strick, 1983; Gasse, 2000; Tyson et al., 2001; Stager et al., 2002), so one might expect warming due to century-scale solar variability to increase P:E at Lake Victoria. The extended dry period ca. 2700–2400 yr B.P. (Fig. 7), for instance, coincided with a solar activity minimum that affected climates over much of the planet (van Geel et al., 2000), but this relationship was apparently reversed in equatorial East Africa ca. 1200– 600 yr B.P., when Europe and South Africa warmed (Tyson et al., 2000) while Kenya experienced century-scale droughts that were triggered by solar output increases (Verschuren et al., 2000). With our updated microfossil records, we can now show that those droughts also affected Lake Victoria (Figs. 5 and 7). We suggest that poleward ITCZ migrations were extended spatially and temporally by solar warming in both hemispheres, thus reducing ITCZ time at the equator more than would occur with orbital insolation effects on alternating hemispheres; increased evaporation and reduced cloud cover associated with solar maxima may also have amplified local lake level declines (Beer et al., 2000). However, it still remains to be shown how widespread this pattern was in the tropics, exactly what caused it, and how common it was during the late Quaternary. Acknowledgments We thank H. Doose-Rolinski, F. Gasse, D. Livingstone, T. Partridge, J. Richardson, M. Talbot, P. Tyson, D. Verschuren, and U. von Raad for valuable discussions and the International Center for Research in Agroforestry, the National Science Foundation (ATM-9808972), and Paul Smith’s College for financial support. 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