PALEOENVIRONMENTAL CONTEXT OF PLEISTOCENE ARCHAEOLOGICAL AND PALEONTOLOGICAL SITES IN KENYA'S LAKE VICTORIA BASIN

Homo sapiens likely evolved in Equatorial Africa ~200 ka and by 40-80 ka dispersed throughout Africa and into Eurasia and Australia. There is considerable debate on the role of climate in these dispersals, in part because of the paucity and complexity of available pre-Last Glacial Maximum paleoenvironmental data from the region. We report here new evidence that bears on this debate from Kisaaka, Kenya, a Pleistocene site with abundant fossils and Middle Stone Age artifacts on the eastern margin of Lake Victoria. Geological evidence from Kisaaka shows a sequence of three paleosols intercalated with three tephra deposits that can be correlated over ~2 km. The oldest paleosol is clay-rich with pedogenic slickensides, wedge peds, and gilgai that identify the paleosol as a Vertisol. This evidence indicates precipitation seasonality, and that the region experienced a significant period of landscape stability of ~ 0.5-2 ka. This paleo-Vertisol is overlain by two poorly developed paleosols each separated by tephra deposits. These tuffaceous paleosols have weak or no horizonation with moderately developed angular blocky ped structure. The large and small mammal assemblage from Kisaaka collected in 2011-2012 (207 specimens from 24 taxa) is dominated by species indicating primarily open grassland vegetation (e.g., equids and alcelaphine antelopes). The recovery of arid-adapted species (e.g., Oryx) reflects an arid character to these grassland habitats. Rare black rhinoceros (Diceros bicornis) and bushbuck (Tragelaphus scriptus) indicates at least some dense/closed vegetation cover (e.g., shrubland), whereas hippopotamus (Hippopotamus sp.) indicates the presence of freestanding water. The faunal and geologic evidence suggests seasonally dry open grassland with an interval of landscape stability followed by instability perhaps related to volcanic eruptions interrupting pedogenesis. If Kisaaka is contemporaneous with Pleistocene deposits on the nearby Rusinga and Mfangano Islands, ~40 km to the north, this suggests a large expanse of arid, seasonal, open habitat in the Lake Victoria region, an environment quite different from that found in the area today.