ULTRAFINE CLAY CHEMISTRY IN LAKE SEDIMENTS: A PALEOCLIMATE PROXY IN EAST AFRICA

Clay minerals form and are altered in nearly all lacustrine watershed environments, from mountain peaks to below the sediment-water interface. Elevated levels of lake salinity and alkalinity are generally required to form Mg-rich clays, although they may also form in association with weathering Mg-rich rocks. In outcrop or cores of ancient lake sediments, therefore, clay chemistry provides a potential proxy of paleochemistry, especially where siliceous or calcareous microfossils are not preserved. This can provide a powerful tool in East Africa, where investigations are underway into terrestrial records of Neogene climate change and links to hominid and vertebrate evolution. A new suite of 319 electron microprobe analyses of <0.1µm ultrafine clays from the Olorgesailie, Amboseli, Natron, Ngorongoro, Olduvai, and Eyasi basins in East Africa show that the octahedral cation compositions of these smectitic to illitic clays varies from Mg/(Al+Fe)=0.1 (upland soils) to Mg/(Al+Fe)>4 (saline, alkaline lake deposits). These results are consistent with the expectation that Al-rich clays occur in freshwater settings, while Mg-rich clays occur in saline, alkaline settings. Geographical variation in clay composition relates well to relative salinity and alkalinity of surface and groundwaters in the various basins. Using Mg/(Al+Fe) as an index of paleo-salinity/alkalinity, incrementally-sampled ultrafine clays from the center of paleo-lake Olduvai (~1.79 Ma) seem to record two lake expansions over ~18,000 years. The initial lake expansion correlates well with the marine record of increased African humidity, but the subsequent drying episode does not. This suggests that controls other than Milankovitch forcing may be at work, such as tectonism, volcanism, or some other paleo-hydrogeologic factor. The reliability of the ultrafine clay data depends on the degree of diagenetic alteration, which may be determined by continued work on other phases and replication of the time series at other exposures. Where diagenesis can be controlled, ultrafine clay chemistry is a potentially valuable indicator of lacustrine paleochemistry.