Vegetation Response to Climatic Variations in the southern African tropics during the Late- Pleistocene and Holocene

Pollen records from Lake Malawi, Africa spanning the last 135 kyr show substantial and abrupt vegetation response to multiple episodes of extreme aridity during the mega-drought period (130-90 ka). In contrast, vegetation composition and relative abundance remained fairly constant throughout the last 75 ka with no significant change during the Last Glacial Maxima (LGM) (35-15 ka). During the extremely arid mega-drought time period, fluctuations in pollen production define three distinct zones. The first zone, from 123-117 ka, is characterized by increasing amounts of grass, and decreasing amounts of both Podocarpus and evergreen forest taxa (i.e. Celtis, Ixora, Myrica, Macaranga), which, when matched with charcoal data, suggests a short period of extreme aridity. The disappearance of Brachystegia in this interval in conjunction with a peak in Amaranthaceae suggests conversion of the surrounding miombo woodland to an open grassland community probably caused by increased seasonality with a more prolonged and arid dry season. Peak amounts of Podocarpus (30-40%) along with diminishing levels of grass distinguish zone two (117-105 ka). This assemblage defines zone 2 as a period marked by a cool, dry 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 similar latitudinal climates across the African continent. Zone three (105-75 ka) shows the highest and most consistent levels of Poaceae. This evidence, along with markedly low levels of most other taxa, indicates that this period contained the most sustained long-lasting dry spells during the past 135 ka. This episode in African history was severe enough as to cause the disappearance of 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. The resultant semi-desert vegetation would have been inhospitable for early humans living within or traveling through the Lake Malawi region.