THE ACACIA PROJECT: ANCIENT CLIMATE AND THE AUTHIGENIC CLAY INDEX OF ARIDITY
In general, lacustrine clays are derived from weathering products generated in upland soils. In evaporative basins, Al- or Fe-rich soil-derived clays are transported basinward, where they may react with saline, alkaline waters. Depending on fluid chemistry, this may induce either precipitation of new Mg-rich silicates, or Mg enrichment in octahedral sheets of pre-existing clays, or some combination. Crystallography and geochemistry can reveal multiple authigenic phases of variable octahedral compositions, and crystal water may undergo isotopic exchange with evaporatively enriched waters. Ultimately, the bulk geochemical signal carried by the clays is the product of the abundance and Mg content of the authigenic clay mineral phases.
To better understand these processes, the ACACIA Project is studying the elemental and isotopic geochemistry, and mineralogy, of clays from Pleistocene deposits of the Olduvai Gorge and the Olorgesailie Basin, a number of modern depositional systems in East Africa, and core samples from several collaborating drilling projects. The project began in 2014, and has preliminary analyses of clays from the Magadi, Olorgesailie, Olduvai, Turkana, and Baringo Basins. One preliminary finding from modern Lake Magadi is that early loss of aqueous Mg when dilute inflow meets evolved brines may prevent the formation of such minerals in the basin center. Therefore authigenic silicate assemblages from the most extreme saline and alkaline lakes may lack Mg-rich silicates.