ABSTRACT
The large lakes of the East African Rift Valley contain a unique record of past climatic change in the tropics, extending back several million years in time. Thus far sediment sampling has been restricted to piston coring or less effective methods for recovering long records, so our knowledge of past climate variability has been restricted to the last 20,000 years in most lakes, with glimpses of conditions as far back as 40,000 years in Lake Tanganyika and Lake Malawi. Patterns of sedimentation in the large lakes are complex, and acoustic remote sensing techniques must be used to identify promising coring targets that have not been disturbed by bottom currents, turbidity currents or mass wasting processes. All of the large lakes in the Rift Valley north of about 9° South Latitude exhibit a coherent pattern of lake level fluctuations over the past 20,000 years, with lowstands from 20 ka (thousand of years before present) to about 12 ka, followed by a rise to outlet levels between 12 and 10 ka, and highstands maintained throughout most of the Holocene. Where records extend further back in time, e.g., Lakes Tanganyika, Mobutu and Victoria, the onset of the lowstand appeared to have occured around 25 ka. Lake Malawi, by contrast, has a lake level history that appears to be exactly out of phase with the lakes to the north, and probably indicates the southern hemisphere response to orbital forcing on insolation and the millenial scale variability in the position of the Intertropical Convergence Zone. The paleoclimate records of the Rift Valley lakes frequently contain intriguing signals of high temporal resolution, including evidence for annual and decadal scale periodicities.
