PALEOSOL-BASED EVIDENCE FOR LATE PLEISTOCENE PALEOENVIRONMENTS OF THE LAKE VICTORIA REGION

Paleoenvironmental change during the Late Pleistocene is argued to have played an important role in the evolutionary history of early humans (Homo sapiens) in Africa, but few data are available from equatorial Africa prior to the Last Glacial Maximum. Recent work on Late Pleistocene sediments exposed along the eastern shores of Lake Victoria near Karungu, Kenya has focused on characterizing the effects of changing climate and environment when human populations dispersed throughout Africa and Eurasia. The generalized stratigraphy of Pleistocene deposits from Karungu and on nearby Rusinga and Mfangano Islands suggests that at ≤100 ka Homo sapiens in the Lake Victoria region were living on a fluvially dominated paleolandscape. Stratigraphic sections measured throughout Karungu reveal Pleistocene deposits that directly overlie Miocene paleotopography. Near topographic highs, freshwater tufas were deposited indicating a semi-permanent water source and are overlain by fluvial channel deposits. Above the fluvial deposits are paleosols whose development varies across the landscape. Most are paleo-Vertisols with pedogenic slickensides and smectitic clay mineralogy that indicate seasonal precipitation and a period of landscape stability. Higher in the section, the paleo-Vertisols disappear and are replaced by tuffaceous paleo-Inceptisols with few pedogenic structures suggesting a more dynamic, unstable landscape. Three chemically distinct tuffs that can be correlated throughout the region are intercalated throughout the deposits. The abundance of paleosols allows for paleoenvironmental reconstructions using bulk geochemistry. The proxies indicate that paleoprecipitation in the region was 700-900 mm/yr, significantly less than modern (~1200 mm/yr). Bulk geochemistry applied to pedotransfer functions to reconstruct soil properties such as fertility, genesis, and climate indicate that the paleo-Vertisol was fertile, but saline-sodic due to limited precipitation. Fossil assemblages within the deposits also suggest semi-arid conditions and indicate that the Lake Victoria region was dominated by open grassland vegetation. Together, the faunal and geologic evidence suggests that early modern humans were living on a seasonally dry, open grassland much different than the modern environment.