TERRESTRIAL EVIDENCE FOR LATE QUATERNARY LAKE LEVEL CHANGES FROM THE LAKE VICTORIA BASIN, EQUATORIAL AFRICA

Lake Victoria spans equatorial East Africa and is the largest African lake (66,400 km²). With water input determined primarily by rainfall over its surface, its lake level is highly sensitive to changes in moisture availability. Geological, geochemical, and paleontological data from deposits on Rusinga and Mfangano islands (Lake Victoria, Kenya) indicate dramatic late Quaternary lake level fluctuations (~40m), with important implications for past climate dynamics in the region. The Wasiriya Beds of Rusinga Island (>38 ka) yield Middle Stone Age artifacts, with sediment lithologies and fossil fauna indicating local, complex, cut-and-fill fluvial successions within an arid grassland setting. In particular, the presence of arid-adapted grazing species like oryx and Grevy’s zebra suggest much more arid conditions than today (<500 mm rainfall), implying substantially reduced lake levels. Isotopic analyses of pedogenic carbonates, soil organic matter, and herbivore tooth enamel support this interpretation. Deposits from the Waware Beds of nearby Mfangano Island (>35 ka) include Middle/Later Stone Age artifacts and similar fauna and sediment lithologies to Rusinga, also implying an arid grassland environment. Given its small size (<100 km²) and rugged topography, Mfangano Island could not support viable populations of large gregarious grazers. Based on modern analogues of herbivore densities it is likely that Mfangano was connected to mainland Kenya when the Waware Beds were deposited, requiring a lake level decline of ≥25m, which indicates substantially reduced lake levels. Lake-margin deposits ~18 m above present lake level on Mfangano Island represent an interval of markedly increased rainfall and lake size. Corbicula shells from these deposits date to 38 ka, broadly coincident with the fluvial deposits on Rusinga and Mfangano islands. These data provide the first dated terrestrial evidence for a period of reduced rainfall and expansion of arid grasslands and an interval of dramatic lake level rise and humid conditions in the Lake Victoria region. Studies from other regions in Africa also indicate a transition from arid to more humid climate ~38 ka, perhaps coinciding with Heinrich Event 4, suggesting the lake level change recorded in Lake Victoria may be the result of regional or global climate forcing.