Pollen records from Lake Malawi, Africa spanning the last 135kyr show substantial and abrupt vegetation response to multiple episodes of extreme aridity between 135 and 75ka. 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 105kyr 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 90ka, continuing drought resulted in total pollen accumulation rates in Lake Malawi to fall to less than 300grains/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 18,000years. 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 56ka) or moisture balance (i.e. increasing humid evergreen woodland between 75 and 65ka). Notably there was no significant change in vegetation composition during the Last Glacial Maximum (LGM) (30-15ka) as compared to the previous 20,000years.
Bibliographical noteFunding Information:
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 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 .
Copyright 2011 Elsevier B.V., All rights reserved.
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