Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 81–92
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Palaeogeography, Palaeoclimatology, Palaeoecology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Vegetation response to glacial–interglacial climate variability near Lake Malawi in
the southern African tropics
Kristina R.M. Beuning a,⁎, Kurt A. Zimmerman a, Sarah J. Ivory a, Andrew S. Cohen b
a
b
Department of Biology, University of Wisconsin-Eau Claire, Eau Claire, WI 54701, United States
Department of Geosciences, University of Arizona, Tucson, AZ 85721, United States
a r t i c l e
i n f o
Article history:
Received 27 March 2008
Received in revised form 29 November 2009
Accepted 16 January 2010
Available online 25 January 2010
Keywords:
Pollen
Vegetation
Pleistocene
East Africa
Paleoclimate
a b s t r a c t
Pollen records from Lake Malawi, Africa spanning the last 135 kyr show substantial and abrupt vegetation
response to multiple episodes of extreme aridity between 135 and 75 ka. Peaks in both the relative
abundance and total production of Podocarpus pollen define the first two of these drought episodes. From
135 to 127 and again from 117 to 105 kyr BP, Podocarpus percentages remain above 16% with peak values as
high as 38% indicating a period marked by a cool climate resulting in expansion of montane forest taxa to
lower elevations. Marine palynological records from the Angola Margin and Congo Fan show similar peak
Podocarpus percentages at this time (oxygen isotope stage 5d) indicating a similar climate across the
African continent at this latitude. From 105 to 90 ka, continuing drought resulted in total pollen
accumulation rates in Lake Malawi to fall to less than 300 grains/cm2/yr of predominately grass pollen.
This episode in African history was severe enough to cause the disappearance of pteridophytes and forest
taxa such as Uapaca and Brachystegia as well as montane taxa (Podocarpus, Olea spp. and Ericaceae) within
the pollen source area of Lake Malawi. These taxa all remain nearly absent from the surrounding vegetation
for the next 18,000 years. The resultant semi-desert vegetation would have been inhospitable for early
humans living within or traveling through the Lake Malawi region. Increasing moisture following these arid
intervals allowed expansion, creation and maintenance of a more diverse landscape vegetation mosaic
around Lake Malawi including Zambezian miombo woodland, humid evergreen woodland and afromontane
forests. The relative abundance of each fluctuated in response to either cooling (i.e. afromontane expansion
from 60 to 56 ka) or moisture balance (i.e. increasing humid evergreen woodland between 75 and 65 ka).
Notably there was no significant change in vegetation composition during the Last Glacial Maximum (LGM)
(30–15 ka) as compared to the previous 20,000 years.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
In February 2005, seven continuous drill cores were recovered
from two sites in Lake Malawi in southeast Africa. These cores total
623 m of continuous core extending back at least 300,000 years
(Scholz et al., 2006; Scholz et al., 2007). The location of Lake Malawi
along the southernmost position of the Intertropical Convergence
Zone (ITCZ) makes these records vitally important for our understanding of continental climate change in the tropics through the
Pleistocene and allows us to test questions of the magnitude, timing
and interconnectivity of tropical climate change with high-latitude
fluctuations throughout the Ice Ages.
Through the use of sedimentological, geochemical and paleoecological data, the Malawi research team has begun reconstructing the
environmental history of the region and integrating this history into
⁎ Corresponding author.
E-mail address: beuninkr@uwec.edu (K.R.M. Beuning).
0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2010.01.025
our understanding of global change during the Pleistocene (Cohen
et al., 2007; Scholz et al., 2007; numerous papers this volume). Our
component of the project is paleoecological, utilizing pollen and
charcoal preserved in the sediments to reconstruct vegetation history
surrounding the basin. Using our modern understanding of the
climatic constraints of specific plant species (e.g. volumes in Flora
Zambesiaca and the Flora of Tropical East Africa) and independent
climate records obtained from other indicators within the sediments
(Johnson et al., 2011-this issue; Park and Cohen, 2009; Scholz et al.,
2011-this issue; Stone et al., 2011-this issue), we hope to elucidate the
timing, magnitude and community composition response of tropical
vegetation to changes in temperature and moisture availability
through the middle-late Pleistocene.
This paper presents results from core MAL 1C, a continuous
145 kyr record from the central basin. Prior to acquisition of this
sediment core, very few continuous lacustrine records from the
African continent reaching beyond 40 kyr BP were available, primarily
due to desiccation and subsequent deflation of lake basin sediments
or simple gaps in the sedimentary sequence especially during the Last
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K.R.M. Beuning et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 81–92
Glacial Maximum (LGM) (25–15 kyr BP). As a result most studies of
Pleistocene terrestrial vegetation change in Africa come from marine
cores (e.g. Van Campo et al., 1982; Dupont et al., 2000; Zhao et al.,
2003). Although a valuable paleoecological record, marine pollen
assemblages show only a part of the palynoflora of the adjacent
continent due to the lack of preservation of many thin-walled types
during aerial, riverine and oceanic transport to the core sites (Dupont
et al., 2000). Thus, a lacustrine record, which often minimizes these
transport and preservation issues, is ideal for understanding terrestrial vegetation dynamics. The Malawi drill cores provide just such a
record integrating the pollen signal of a large source area while also
providing a robust record of regional vegetation change.
Earlier palynological studies of tropical East Africa have demonstrated substantial vegetation change during the LGM and the
transition into the Holocene (see review in Kiage and Liu, 2006 and
Van Zinderen Bakker and Coetzee, 1988). In most cases during the
LGM, grass-dominated communities covered the landscape in response to aridity caused by reduced global moisture availability due to
expansive high-latitude ice sheets. In fact, the LGM aridity has been
used as the archetype for tropical response to Northern Hemisphere
ice sheet expansion. As the arboreal component of the vegetation
began to recover during the transition to the current interglacial, some
records also show a reversal to the more herbaceous LGM-type
community during the Younger Dryas interval (e.g. Beuning et al,
1997; Ryner et al., 2006; Vincens et al., 1994). When observed with a
broad filter, Holocene vegetation of tropical Africa has shown a gradual
increase in plant species tolerant of reduced moisture availability in
response to the transition from early-Holocene humidity to lateHolocene aridity (e.g. Beuning, 1999; Umer et al., 2007; Van Zinderen
Bakker and Coetzee, 1988; Vincens et al., 1999).
Additional snapshots of African vegetation beyond the LGM and
into the last glacial period derive from marine records (e.g. Van
Campo et al., 1982; Dupont et al., 2000; Zhao et al., 2003), and lake
cores (Debusk, 1994; Gasse and Campo, 1998; Vincens et al., 2007;
Scott et al., 2008). The timing of vegetation changes demonstrated by
these authors often corresponds to the marine isotope stages 3–5d but
are all within the backdrop of more arid conditions than the current
interglacial. No proxy records of vegetation cover from the eastern
half of the continent extending through the penultimate interglacial
have been published.
Here we provide the first continuous, lacustrine record of tropical
vegetation change extending from beyond the penultimate interglacial to present. Previous analyses of the seismic stratigraphy, sediment
litho- and chemostratigraphy, and the ostracod and diatom assemblages of the MAL 1C core (Cohen et al., 2007; Scholz et al., 2007),
allowed delineation of two pronounced megadrought intervals
between 135 and 75 ka, from 135 to 127 ka and again from 115 to
95 ka. During these megadroughts, Lake Malawi, the 4th largest lake
in the world with regard to volume, dropped over 600 m in water
depth (Scholz et al., 2007). In this paper, we demonstrate substantial
vegetation response to these multiple periods of profound aridity. In
addition, within the last 75 ka we find vegetation change in response
to short-term cooling within MIS 4 (74–59 kyr BP) that is surprisingly
more pronounced than any response to aridity during the LGM or
increased moisture leading into the Holocene. This continuous record
of terrestrial vegetation change provides the setting for early
anatomically modern human evolution and range expansions and
contractions within and out of the African continent.
2. Study site and methodology
Lake Malawi, the southernmost lake in the West African Rift Valley,
is located between 9°30′S and 14°30′E. The present lake depth is
approximately 700 m with a surface area around 29,500 km2 making
it the third largest lake in Africa and the fourth deepest lake in the
world (Malawi Department of Surveys, 1983). As with most large
lakes, Lake Malawi contains only one naturally occurring outlet, the
Shire River, which is located at the southern end of the lake (Fig. 1).
Fig. 1. Physical map of equatorial south Africa modified from Lehner et al. (2008) showing Lake Malawi and cross-continent Ocean Drilling Program sites from the Congo Fan
(GeoB1008) and Angola margin (GeoB1016). Lake and river catchments are delineated by thicker, solid lines. Inset taken from http://africa.theworldatlas.net/.
K.R.M. Beuning et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 81–92
83
The one-meter annual variation in lake level is caused by
fluctuations in rainfall during the wet and dry seasons driven by
annual movements of the Intertropical Convergence Zone (ITCZ).
While the seasonal location and southward extent of the ITCZ is
driven by the strength of the northeast African monsoon and the
southeast trade winds, the amount of moisture brought by these
winds is influenced by the sea-surface temperatures of the Indian and
Pacific Oceans (Cane et al., 1994; Goddard and Graham, 1999; Tierney
et al., 2008). Presently, Lake Malawi lies at the southern end of the
ITCZ and its catchment receives highly seasonal and variable annual
rainfall ranging from approximately 800 to 2400 mm/yr depending on
location (Agnew and Stubbs, 1972). In all locales, maximum rainfall
occurs from November to May.
2.1. Modern topography and vegetation
Lake Malawi has an elevation of 472 masl (meters above sea level)
with a catchment of over 65,000 km2 (Agnew and Stubbs, 1972).
Surrounding the northern half of the basin are several mountainous
highlands and escarpments which rise to over 2400 m (Fig. 2), and to
the north are the Rungwe volcanic highlands. The Nyika Plateau lies to
the west and is characterized by a mosaic of undulating Afromontane
grasslands with intermittent patches of Afromontane evergreen
forests (see Afromontane undifferentiated vegetation in Fig. 2). The
distribution of these Afromontane vegetation constituents is driven by
the topography, with trees favoring the wide, level valley bottoms and
grasses dominating on the convex valley sides (Meadows, 1984).
Afromontane forest typically occurs between 1200 and 2500 masl
in regions with mean annual rainfall of between 1250 and 2500 mm.
In regions with more rainfall, a diverse, tall (25–45 m) arboreal
assemblage grows including members of the Sapotaceae, Ebenaceae,
Myrtaceae, Prunus africana, Olea capensis and Podocarpus latifolius.
When rainfall is less, a shorter stature forest develops with a
somewhat different composition that includes Apodytes, Ilex, Kiggelaria, Prunus africana and Podocarpus falcatus and latifolius (Polhill,
1958; White, 1983).
At lower elevation, the primary vegetation community is Zambezian miombo woodland (Fig. 2). Zambezian miombo woodlands are
dominated almost exclusively by Brachystegia with varying associated
species depending upon differential precipitation: wetter miombo
(>1000 mm/year rainfall) or drier miombo (<1000 mm year rainfall). With even less rain, Zambezian scrub woodland grows and is
characterized by diminishing arboreal height (3–7 m from 10 to
20 m) with a pronounced and severe dry season of greater than five
months from May–June to October (Carter and Radcliffe-Smith, 1988;
White, 1983). Species of Uapaca are usually present. Uapacadominated Zambezian scrub woodland can form an ecotone between
open grassland and the Brachystegia-dominated miombo woodland.
North Zambezian undifferentiated woodlands are those lacking the
dominant miombo or scrub woodland taxa yet still containing a
diverse arboreal community composition including Afzelia quanzensis,
Burkea africana, Dombeya rotundifolia, Pericopsis angolensis, Pseudolachnostylis maprouneifolia, Pterocarpus angolensis and Terminaolia
sericea (White, 1983). Today, many of these natural vegetation
assemblages are disturbed by anthropogenic manipulation of the
landscape.
Fig. 2. Natural vegetation types surrounding the Lake Malawi catchment (modified
from White (1983)). The dots within lake represent the 2005 Malawi drill sites
discussed in the paper. Data in this paper is from a core taken at Site 1.
2.2. Methods
Sixty-three 0.5 cm3 sediment samples from Malawi drill core 1C
were processed for pollen following standard methods (Faegri and
Iversen, 1989). Sediment types ranged from organic-rich gyjtta in
the upper parts of the core to several layers of calcareous silty clay
with minimal organic remains near the base (for more information
regarding lithostratigraphy of the 1C core see Scholz et al., 2009).
Sampling resolution represents on average every 2000 years, al-
though some temporal intervals, i.e. the megadroughts, have much
higher density analyses, while others, i.e. the Holocene, have fewer.
Calibrated micropolystyrene spike was added to each sample to allow
for calculation of pollen concentration and pollen accumulation rates
(PAR) using the depth–age model presented in Scholz et al. (2007).
For most samples, over 600 identifiable grains were counted. Grains
were identified using a reference collection of African pollen, atlases
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of pollen morphology, and imagery available through the African
Pollen Database. Percentages of all identified taxa were calculated as
percent of the total, identifiable grains. Unidentifiable grains (broken/
crumpled) were added to the pollen sum prior to calculation of their
percent. Several taxa were placed into broader vegetation groups
based on the habitat preference of the parent plants to facilitate
interpretation. The three groups presented in Fig. 3 include: Other
Montane Taxa (Apodytes, Ericaceae, Ilex, Juniperus, Kiggelaria, Myrtaceae, Olea africana, Olea capensis, and Syzygium-type), Evergreen
Trees (Celtis, Alchornea, Macaranga, Moraceae, Ixora, Myrica, Faurea,
and Trema) and Other Woodland Taxa (Acalypha, Acanthaceae,
Blighia, Combretaceae, Craterispermum, Commiphora, Lannea, Maclura, and Mimosaceae). The ‘Other’ category included all types
whose total abundance remained less than 0.5% in all samples
analyzed as well as types whose constituent taxa represented diverse
ecological preferences.
The geochronology of core 1C is based on accelerator mass
spectrometry radiocarbon dates on bulk organic matter, paleomagnetic and 10Be analyses and a few luminescence dates. Detailed
information regarding construction of the geochronology is presented
in Scholz et al. (2007).
3. Results and discussion (Figs. 3 and 4)
3.1. Zones 1–3 (135–105 kyr BP)
Within the interval 135–105 ka, primary vegetation response to
multiple arid periods (Zones 1 and 3) is defined by peaks in both the
relative abundance and total production of Podocarpus pollen. From
135 to 127 and again from 117 to 105 kyr BP, Podocarpus percentages
remain above 16% with peak values as high as 38%. These high
percentage values in combination with minor peaks in the Other
Montane Taxa strongly suggest that aridity associated with the
megadrought occurred in conjunction with an increased adiabatic
lapse rate, at least seasonally, allowing downslope expansion of
afromontane taxa in the highlands to the north and west of Lake
Malawi (Fig. 2). Samples from within the Podocarpus forest of the
Drakensburg Mountains, South Africa contain only ∼ 40% Podocarpus
pollen (Scott, 1989). Such high values in the Lake Malawi sediments
would seem to necessitate either abundant Podocarpus plants growing
in close proximity to the basin, or an expanded Podocarpus community at greater distance from the basin in conjunction with an
increase in the magnitude of northern or westerly winds in the annual
wind regime over the lake. Perhaps the latter is more likely as the
pollen percentages of lowland Zambezian miombo woodland indicators like Uapaca and Brachystegia follow a pattern similar to Podocarpus, with increases during the arid intervals and declines during
the intervening wetter interval from 127 to 117 ka. In fact, the relative
abundance of these lowland taxa during the arid intervals is
comparable to modern surface samples from Lake Malawi, which
currently has extensive miombo woodland in its catchment (Debusk,
1997). Also during Zones 1 and 3, Cyperaceae percentages increase
and Typha comprises as much as 6% of the total pollen assemblage, the
highest values of the entire 135 kyr record. The substantially-reduced
lake levels would have provided abundant near-shore habitat for
wetland taxa.
What is puzzling is the near absence of arboreal taxa, other than
Podocarpus, from 127 to 117 ka, the penultimate interglacial. West
African climate reconstructions (Dupont et al., 2001) and the
composite lake-level curve suggest a warmer and much wetter
environment at this time than during the megadroughts (Zones 1
and 3), although the lake was still at least 150 m below present levels
(Fig. 4). Although Podocarpus is present within this interval, the
relative abundance of this taxon is much lower than the preceding or
subsequent zones (Fig. 3). While a warmer climate could limit Podocarpus, which prefers the cooler environment of the high-elevation
afromontane habitat (Polhill, 1958; White, 1983), the increased
moisture would presumably allow lowland expansion of arboreal taxa
at least within a Zambezian miombo woodland community. In fact the
only major taxa represented in the 1C pollen record that increases in
relative abundance during the penultimate interglacial is Poaceae,
with a corresponding peak in charcoal production (Fig. 4). These
results suggest a past interglacial vegetation community quite different than the Holocene.
An open grassland in the lowlands surrounding the lake, as
implied by the data, seems incongruous with the rise in lake level,
which requires a more positive annual moisture balance. We can
hypothesize three scenarios under which such apparently incompatible boundary conditions would exist. First, the greater mean annual
precipitation did not occur seasonally, as it does today. If mean annual
rainfall increased but was distributed throughout the months rather
than in seasonal pulses, the net moisture balance would increase, but
there could be insufficient moisture during any one season, to allow
the establishment and maintenance of trees. Second, the opposite
could be true with increased seasonality in the regional climate. If
there was one shorter, intense humid season followed by a prolonged
severe dry season that would have provided enough drought stress to
limit the trees, despite increased annual precipitation overall, while
still allowing continuous grass cover. Third, the growth of trees was
not limited by climate but by a fire or grazing regime different from
the Holocene. Either increased fire or grazing (depending on grazer
preference) could have suppressed tree seedling growth.
3.2. Zones 4 and 5 (105–75 kyr BP)
Zones 4 and 5 are characterized by remarkably low total PARs of
less than 800 grains/cc/yr. The decline in PAR during this interval
inversely mirrors the terrigenous component (% sand) of the sediment (Cohen et al., 2008). Because sedimentation rates do not
increase along with the increase in sand, the decline in PAR at these
times is probably not due to dilution or grain degradation but instead
reflects a real change in pollen productivity and/or transport to the
coring site.
The most extreme of the megadrought intervals occurred within
this interval from between 105 and 93 ka (Zone 4). Although
increased aridity often results in an expansion of grasslands, this
drought was so severe that even grass-dominated plant communities
were limited. This is most strikingly recorded between 105 and 93 ka
when less than 300 total pollen grains/cm2/yr were deposited in core
1C in Lake Malawi. The grains deposited in this interval were
relatively well-preserved and showed no evidence of deterioration
due to re-working from exposed lake surfaces. Although these 300
grains were predominately grass, the grass vegetation from which this
pollen derived was so patchy and limited that fire could not be
sustained on the landscape, as indicated by the absence of charcoal in
the first 5000 years of this zone (Fig. 4, note log scale). The resultant
lowland vegetation within the Lake Malawi catchment may have been
desert-like, with grasses, Compositae and Amaranthaceae herbs
distributed intermittently on the landscape. At higher elevation, Podocarpus trees grew and may have flourished although their
dominance in the pollen spectrum may be due to their overrepresentation in the impoverished vegetation of this time interval. With
time the drought was apparently severe enough to decimate even the
high-elevation arboreal vegetation. At 105 ka, other montane taxa
drop to near zero and Podocarpus begins a decline from 38% to less
than 1% by 90 ka. Furthermore by 95 ka, Brachystegia, Uapaca and
even pteridophytes disappear. These taxa all remain nearly absent
from the surrounding vegetation for the next 18,000 years. Instead,
Poaceae becomes increasing predominant reaching peak values (as
high as 92%) at 90 ka (Fig. 3). Pollen spectra from the grass-dominated
Sudanian vegetation belt north of the equator contain similarly
high grass pollen abundance (80–90%) (Lezine and Edorh, 1991;
K.R.M. Beuning et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 81–92
pp 85–88
Fig. 3. Summary palynostratigraphy of core 1C, Lake Malawi, Africa. Total PAR (pollen accumulation rate) calculated as number of grains and spores/cm2/yr. Pollen types included in ‘Other Montane Taxa’ are: Apodytes, Ericaceae, Ilex, Juniperus, Kiggelaria, Myrtaceae, Olea africana, Olea capensis, and Syzygium-type. ‘Evergreen tree taxa’ include: Alchornea,
Celtis, Faurea, Ixora, Macaranga, Moraceae, Myrica, and Trema. ‘Other Woodland Taxa’ include: Acalypha, Acanthaceae, Blighia, Combretaceae, Craterispermum, Commiphora, Lannea, Maclura, and Mimosaceae. Pollen types included in the ‘Other’ grouping include: Adansonia, Aeschynomene, Aneilema, Annona, Anthocleista, Blepharis, Borassus, Bridelia,
Canthium, Cassia-type, Cephalosphorin, Cordia-type, Crassula, Craterostigma, Dioscorea, Diospyros, Drypetes, Elaeis, Elephantopus, Erythrococca, Eucalyptus, Euclea, Gallium, Gnidia, Hyalosepalum, Hyphaene, Isoberlinia, Khaya, Landolfia, Lindernia-type, Maesa, Mimusops, Neoboutonia, Nymphaea, Phoenix, Phyllanthus, Polygonum, Rapanea, Rhus, Saggittaria,
Strombosia, Striga, Toddelia, Trichilia, Zanthoxylum, and Zygophyllum. The criterion for inclusion in the ‘other’ group was 5 or less grains per sample (< 0.5%) at all analyzed depths in the MAL 1C core or diverse ecological preferences within the taxon group.
K.R.M. Beuning et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 81–92
Fig. 4. Total PAR (pollen accumulation rates, expressed in grains/cm2/yr) and number of
charcoal pieces from Malawi drill core 1C. Charcoal methodology described in and data
published in Cohen et al. (2007). Lake-level curve derived from stratigraphic evidence
of lake lowstands and is first published in Scholtz et al. (2007).
Saltzmann et al., 2002) and occur in regions when mean annual
precipitation is less than <800 mm/year.
During the recovery phase (90–75 ka) from this extreme drought,
abundant moisture availability, as evidenced by the 400 m rise in lake
level, allowed the growth of evergreen forest taxa (e.g. Moraceae,
Celtis and Macaranga; Fig. 3) characteristic of the semi-evergreen
lowland rainforests found today further to the North near Lake
Victoria. From 90 to 85 ka, the arboreal component of the vegetation
was limited to these evergreen components, which would require a
mean annual precipitation of at least 1200–1600 mm/yr (White,
1983). The rise in ‘Other Woodland Taxa’ during this 5000 year
interval is due primarily to increased number of Combretaceae pollen
grains, which could be lianas growing on the evergreen forest trees or
scrub trees growing on marginal, sandy soils within the lake
catchment (Fig. 3; White, 1983). By 85–80 kyr BP, miombo species
such as Uapaca and Brachystegia, reappear, possibly in response to a
decline in mean annual precipitation or a reduction in dry season
relative humidity at least in some portion of the Lake Malawi
catchment (White, 1983).
The virtual absence of montane taxa during this recovery phase
(90–75 ka) is probably the result of biogeography. If the geographic
extent of mortality of the montane taxa during the preceding
megadrought (115–95 ka) was widespread, a nearby seed source for
“restocking” the landscape would be absent. This would be especially
true for the isolate mountain highland “islands” that would rely on
regions far north (i.e. the Ruwenzori) or far south (i.e. S. Africa) as the
parent populations. In this case, the nearly 15,000 year delay in
recovery of montane vegetation could be partly due the slow
migration and chance distribution of “new” seeds to the region.
3.3. Zones 6–8 (75–30 kyr BP)
This interval appears to be one of instability both with regard to
lake level (Stone et al., 2011-this issue) and the vegetation surrounding Malawi (Fig. 3). At the start of Zone 6 (75–62 ka), total PAR rises to
over 2500 grains/cm2/yr, much of which is attributable to an apparent
expansion of Uapaca in the pollen source area. The percentage of
evergreen taxa declines, as does Brachystegia. However by 68 ka, a
89
brief, but rapid drop in total PAR to less than 500 grains/cm2/yr occurs
followed by a large peak in the relative abundance of Podocarpus
pollen and rise in total PAR leading into Zone 7 (62–52 ka).
Despite the riverine distribution of some Podocarpus species, Podocarpus pollen in African pollen diagrams is most frequently
interpreted as a montane indicator as most species are found at
high elevation (Polhill, 1958). Thus, increases in Podocarpus percentages purportedly represent expansion of the afromontane vegetation
community, presumably due to cooler conditions allowing downslope
movement (see explanation above in Zones 1–3). Yet, the patchwork
mosaic of grassland and forest in many afromontane communities
leaves some doubt as to whether increasing Podocarpus occurred
because of actual expansion of the vegetation type due to cooler
conditions, or merely a shifting grass-to-tree ratio within the highelevation environment (>2000 masl) perhaps as a result of succession or even moisture balance (Meadows, 1984). While this
possibility cannot be ruled out, the combined new TEX86 paleotemperature lake water data from Malawi core 2A (Woltering et al., 2009)
in conjunction with equatorial Indian ocean SST records (Bard et al.,
1997) strongly link Podocarpus expansion with direct air temperature
cooling.
Starting at around 75 ka, a sharp decline in SST occurs in the
equatorial Indian Ocean that culminated between 60 and 65 ka (Bard
et al., 1997). Furthermore, Woltering et al. (2009, 2011-this issue)
show a ∼ 4 °C drop in mean annual lake surface temperatures between
63 and 59 kyr BP representing the coldest lake water temperatures in
Lake Malawi between 75 ka and present. Presumably the decline in
lake temperature reflects surface cooling from surrounding colder air
temperatures and not enhanced deep-water upwelling during the
lake lowstand from 64 to 62 yr BP (Fig. 4). From 63 to 59 kyr BP, the
diatom assemblage is dominated by some shallower-water species
that are probably indicative of upwelling, however the modern
thermal gradient across the thermocline is only 0.5–1 °C during the
mixing season (Stone et al., 2009—volume). Thus without a much
colder hypolimnion than present, increased upwelling alone is
unlikely to result in a ∼ 4 °C sustained drop in water temperatures.
The increase in Podocarpus percentages began during this interval
of atmospheric cooling. Cooling, as indicated by the TEX86 data, began
at 75 ka coincident with a rise in Podocarpus percentages from near 0
to ∼ 5%. Furthermore, maximal cooling of ∼ 4 °C occurred between 60
and 59 kyr BP at the same time as a spike back to values between 26
and 32% Podocarpus, percentages similar to those during the
penultimate interglacial. At all other times during the last 75 ka, Podocarpus percentage values in the Lake Malawi sediment remained
below 15%.
Low sampling resolution within Zone 8 (52–30 ka) precludes
drawing any conclusions about the coupled or uncoupled changes in
vegetation and climate during a lake lowstand around 40 ka. Total
PAR does decline from its peak during Zone 7 and Cyperaceae pollen
percentages climb to and remain at between 12 and 18% consistent
with modern surface sample data from the lake basin (Debusk, 1997).
3.4. Zone 9 (30–15 kyr BP)
In Lake Malawi, the Last Glacial Maximum (LGM; 25–15 kyr BP) is
marked not by substantial change in plant community composition
but rather by low total pollen accumulation rates (PARs) that are
comparable to the recovery phase (90–75 ka) of the megadrought
period (Fig. 3). These results are in direct contrast to data from the
much smaller Lake Masoko, just north of Lake Malawi in the Rungwe
highlands (Figs. 2 and 3) (Garcin et al., 2006). At Lake Masoko,
beginning at 23 ka, an increase in more humid woodland taxa pollen
such as Macaranga, Trema and Celtis seemed to suggest increasing
moisture balance during the LGM as compared to the millennia prior.
The relative abundance of these evergreen tree taxa do not increase in
the Lake Malawi record during this interval (Fig. 3). Around Lake
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Malawi, the abundance of Poaceae, Cyperaceae and the drier
Zambezian woodland taxa (Brachystegia and Uapaca) during the
LGM remained similar to the prior interval, and in fact in the case of
the Zambezian woodland taxa are greater than during most of the
Holocene. This LGM grass-dominated landscape is consistent with
records from the equatorial rift lakes north of Lake Malawi and
suggests that despite a summer insolation maximum at 9°S the
regional climate of southern equatorial East Africa was synchronized
with Northern Hemisphere glacial boundary conditions. These results
do not preclude a southward shift in the ITCZ, which may have
increased rainfall over Lake Masoko. However, in this scenario not
only would a southward shift be necessary, but also a substantial
contraction of the ITCZ latitudinal range would need to occur so that it
did not extend much further south than northern Malawi. As Lake
Malawi integrates a much larger source area with a regional signal of
climate with a severe dry season, it is likely that the increase in moist
taxa during the LGM in Lake Masoko reflects, as proposed by the
authors of that study, a local signal of increased rainfall associated
with changes in local hydrology perhaps due to decreased evaporation caused by cooling (Garcin et al., 2006).
Despite the absence of landscape-scale change in vegetation
composition during the LGM near Lake Malawi, the LGM vegetation
did respond to climate conditions that differed from the previous
20,000 years as evidenced by a precipitous decline in total PAR
(Fig. 3). Sedimentation rate did not change during this interval so a
dilution effect is unlikely (Cohen et al., 2007). Instead, the decline in
PARs suggests “sub-optimal” conditions that may have resulted in
reduced resources for pollen production, yet with an overall moisture
balance not substantially different than during the late-Pleistocene.
The composite lake-level curve for Lake Malawi indicates that water
depths remained between near modern from 50,000 yr BP to the LGM
(Fig. 4). Perhaps the annual rainy season driven by the southward
movement of the ITCZ brought marginally less rain due to reduced
glacial sea-surface temperatures in the nearby Indian Ocean (Bard
et al., 1997). In contrast, equatorial regions to the north must have
experienced substantial moisture deficits due to southward compression of the ITCZ, reduced lake-effect rainfall (Lake Victoria) and/or
reduced cross-continent transport of oceanic-derived moisture (Lakes
Albert and Edward).
The lack of any substantial change in the relative abundance of
Podocarpus or other montane taxa in our record can also be used to
constrain or corroborate early work addressing temperature change
in tropical Africa during the LGM. Initial estimates of temperature
depression during the LGM in East Africa suggest a climate 5–8 °C
colder than modern based on changes in tree line elevation on East
African mountains (Van Zinderen Bakker and Coetzee, 1988) and
biogeography of montane taxa (Talbot, et al., 1984; Livingstone,
1993). Additional studies utilizing surface sample assemblages
throughout the eastern continent, Bonnefille et al. (1992) concluded
that a temperature decline of 2 ± 4 °C must have occurred at this time.
More recently, Powers et al. (2005) determined a 3.5 °C overall
warming since the LGM from the TEX86 paleotemperature proxy in
Lake Malawi. Our data from Lake Malawi is consistent with Powers
et al. (2005), and further show that any change that did occur must
have been less than a 4° reduction as this magnitude of temperature
change initiated a significant increase in Podocarpus or other montane
taxa as found during the latter half of MIS Stage 4.
Only four samples were counted within Zone 10, the Holocene
(Fig. 3). The results from these are consistent with the Holocene
vegetation around Malawi reported in Debusk (1998).
3.5. Comparison with West African Records
The paleovegetational history archived in the Malawi 1C pollen
record is remarkably congruent with pollen records from the Angola
Margin (Latitude: −11.7705; Longitude: 11.6819) and Congo Fan
(Latitude: − 6.5824 Longitude: 10.318) off the coast of equatorial
West Africa throughout the last 135,000 years (Fig. 5) (Shi and
Dupont, 1997; Jahns, 2000). While aridity on the African continent can
be controlled by changes in atmospheric moisture content modulated
by tropical sea-surface temperatures (SSTs) and the strength of the
African monsoon (Schefuβ et al., 2003), Bard et al. (1997) found no
significant change in SST during the periods of extreme aridity near
Lake Malawi. Thus, the consistent cross-continent moisture balance
Fig. 5. Podocarpus and Poaceae pollen percentages from the Angola Margin (GeoB1016) and Congo Fan (GeoB1008) overlain on Lake Malawi 1C data. Marine data from Shi and
Dupont (1997) and Jahns (2000).
K.R.M. Beuning et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 81–92
shown by this combined data implies large-scale vegetation changes
in Africa during the middle-late Pleistocene controlled not exclusively
by tropical SSTs, but perhaps instead by changes in Hadley circulation
or the extent of the ITCZ.
In addition to consistent monsoon dynamics, the parallel response
of Podocarpus in these three records strongly suggests climate forcing
that was coherent across the southern African tropics not only with
regard to moisture, but also air temperature. The slower response of
Podocarpus in the Angola Margin record as compared to Lake Malawi
and the Congo Fan to cooling following the penultimate interglacial is
surprising especially given the close proximity of the GeoB1016 drillsite to the central highlands of Angola. However, the Congo Fan record
is perhaps a more accurate representation of west-equatorial
vegetation dynamics due to the more direct riverine transport of
pollen and spores to the coring site in the suspended sediment load of
the Congo river, which drains a catchment area of 3.7 million km2
(Shahin, 2002). Long-distance aerial transport is limited in the Congo
Fan as surface winds blow landward or along the shore (Dupont et al.,
2000). In contrast, the Angola basin site derives much of its pollen
from the SE trades that flow across and draw pollen from 13 to 17°S,
an area south of the central Angola highlands (Dupont et al., 2000). As
a result, montane expansion in these highlands would have to be
southward before a substantial increase of taxa representative of this
vegetation community would be recorded in the Angola basin core.
3.6. Implications for early hominin habitats
Although scientific debate persists as to the geographic origin of
anatomically modern humans, much research supports the ‘out of
Africa’ hypothesis which contends that modern humans evolved in
Africa between 200 and 100 kyr BP and then expanded into Eurasia at
a later date (Stringer, 1990). Prior to their expansion out of Africa,
anatomically modern humans may have existed throughout the
eastern half of Africa, from South Africa to Eritrea (Jacobs et al., 2008;
Walter et al., 2000). A complex range of behavioral adaptation to
fluctuating climates and the resultant environmental stress has been
identified as one of the defining factors of modern human behavior
(Walter et al., 2000). Within this context, the results in this paper
provide important new information regarding the habitat in which
these early modern humans lived. The landscape vegetation provides
both food and shelter for these individuals. Our results demonstrate
substantial change in vegetation structure and community composition during what may have been a key period of modern human
evolution and migration (135 to 70 kyr BP). For example, the treeless
and thus potentially inhospitable landscape around Lake Malawi
during the penultimate interglacial lends credence to recent evidence
for enhanced use of coastal marine resources during this interval and
earlier (Fig. 3) (Marean et al., 2007; Walter et al., 2000). Furthermore,
increased moisture following the extreme arid conditions in tropical
Africa during MIS 5 and 6 allowed expansion and creation of a more
diverse landscape vegetation mosaic including Zambezian miombo
woodland, humid evergreen woodland and afromontane forests. This
diversity in resources would have provided excellent habitat for early
modern human populations expanding along and up the Nilotic
corridor to the north and eventually out of Africa.
Acknowledgements
We gratefully acknowledge all who helped in sediment acquisition, initial processing (at LacCore), and dating of the Lake Malawi
Scientific Drilling Project. We thank the government of Malawi for the
permission to undertake this research. Funding for the field and lab
work was provided by the U.S. National Science Foundation—Earth
System History Program (EAR 0602404 to K. Beuning; EAR 0602350 to
A. Cohen), the International Continental Scientific Drilling Program,
the University of Wisconsin-Eau Claire Office of Research and
91
Sponsored Programs, and the Smithsonian Institution (Evolution of
Terrestrial Ecosystems Program and Fellowship Program). Marine
pollen data and associated age models were accessed at http://www.
pangaea.de/. All pollen data presented in this paper are archived in the
African Pollen Database (APD) and are accessible at http://medias.
obs-mip.fr/apd/accueil.htm.
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