Authigenic clays in East Africa: Regional trends and paleolimnology at the Plio-Pleistocene boundary, Olduvai Gorge, Tanzania

Ultrafine clays (< 0.1 μm) from six East African basins show wide chemical variability that can be conveniently described as the octahedral cation index (OCI), calculated as octahedral element ratio Mg/(Al+Fe). Values range from dioctahedral clays with OCI < 0.1 ( upland soils/detrital sources) to trioctahedral clays with OCI > 4 ( saline, alkaline lake deposits). The association of Mg-rich clays with saline, alkaline conditions provides a paleochemical proxy in sedimentary contexts. The octahedral chemistry of central-basin clays from paleolake Olduvai (similar to1.79 Ma) records two lake expansions, with an intervening lake contraction, over similar to 18,000 years. The initial lake expansion correlates well with the marine record of increased African humidity, but the subsequent drying episode does not, suggesting that some high-frequency environmental oscillations in East Africa may occur independent of obliquity and precession control. The clay record correlates well with the fossiliferous and archaeologically rich stratigraphy of the lake margin, and suggests that fossils and artifacts accumulated on the exposed lake margin over < 4000 years. The chemistry of clays is a potentially powerful tool to add to multi-proxy studies in basins with alkaline waters, particularly in stratigraphic intervals that lack pollen, calcareous, or siliceous microfossils.