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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
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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-
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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
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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
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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.
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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. Undergraduate students Dustin Grzesik, Scott Hadam, Carlene Heimiller, and
Kristen Przywara assisted in the collection and analysis of
new cores collected from Lake Victoria in June 2000 that
supported the chronology of core 64-2.
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