SEDIMENTATION IN LAKE MALAWI (EAST-AFRICA) DURING THE PAST 10,000 YEARS - A CONTINUOUS PALEOCLIMATIC RECORD FROM THE SOUTHERN TROPICS

A suite of piston cores recovered from Lake Malawi (9-14-degrees-S, 34-35-degrees-E), east Africa in 1986 has been analyzed for major and minor elements, organic C and N. calcium carbonate and diatoms. An internally consistent stratigraphy was constructed from calcium carbonate abundance and variations in the two most abundant diatom genera, Stephanodiscus and Melosira, with age control obtained primarily from C-14 dating of the carbonate. Differences with time in Fe abundance in a transect of cores from different water depths have been interpreted to reflect changes in chemocline depth. The depth to the chemocline was on the order of 100 m shallower than present prior to 3500 yr B.P., indicating less seasonality. Carbonate production and preservation appears to be related to climatically induced changes in both salinity and chemical distributions in the water column. The carbonate, which precipitates from surface waters, is most abundant during the interval from about 10,000 to 6000 yr B.P. This micrite most likely represents periods of low lake level when salinity increased and carbonate precipitation was enhanced. Sedimentary evidence suggests that lake levels were 100-150 m lower than present during this period. This record is different from climatic trends in northern intertropical Africa, but appears to also be related to changes in insolation and monsoon circulation. This is the northernmost basin in Africa reported to exhibit a "southern hemisphere" response to the early Holocene northern hemisphere summer insolation maximum. The climatic hingeline north of Lake Malawi (approximately 9-degrees-S) implied by our results is significantly south of that indicated by general-circulation model simulations, however. The cores show evidence for periods of abrupt climate change during the interval or generally arid climate.