PALEO-LAKE OLDUVAI: FRESHER OR SALINE? NEW RESULTS FROM CLAY CHEMISTRY

The Olduvai Gorge of northern Tanzania contains an important record of early hominin fossils and artifacts dating from the Plio-Pleistocene. Situated on the western edge of the East African Rift, the Olduvai Gorge is carved into the Serengeti Plain of East Africa where late Cenozoic environmental fluctuations in East Africa are thought to influence the progression of terrestrial ecosystems, evolution of early hominins, and development of stone tool technology. Lacustrine authigenic clay minerals found in closed basins are important geoarchives of paleoclimate due to their hydrologic sensitivity. Fluctuations in lake chemistry can cause phyllosilicates, e.g. trioctahedral clays, to form in highly saline and alkaline conditions whereas detrital dioctahedral clays form upland in freshwater conditions. When an endorheic lake basin has elevated alkalinity and silica content, Mg enrichment of authigenic clays can occur, providing a proxy for climatic change in East Africa aiding in the reconstruction of lacustrine paleoenvironmental conditions that influenced hominin habitats in the Olduvai region. Samples were collected at depths of 150-230 meters below surface in the 2014 Olduvai Gorge Coring Project (OGCP) Core 2A, extending well below the previously-studied outcrop exposures in the central basin and below the Ngorongoro volcanics dated to ~2 Ma and previously thought to represent the bottom of the sedimentary succession. Here, the position of the 060 peak XRD scans from randomly oriented ultrafine clay extracts from well dated bulk samples are analyzed to better examine mixtures of end-member compositions (Deocampo, 2004). Al-rich samples have 060 peaks situated at ~61.4° 2θ (1.509Å), whereas Mg-rich samples have peaks at ~61.0° 2θ (1.517Å). Differences in the 060 peak positions are used to determine whether lake Olduvai was a fresher (Al-rich clays) or saline (Mg-rich clays). Preliminary results show that 060 peak measurements are generally close to 1.509Å, confirming the dominance of dioctahedral aluminum-rich phases. These results suggest the presence of a less saline paleo lake Olduvai >2 Ma ago, compared to the hypersaline, hyperalkaline lake known from younger units of Bed I and II.