Siliceous islands in a carbonate sea: Modern and Pleistocene spring-fed wetlands in Ngorongoro Crater and Oldupai Gorge, Tanzania

A biogenic mechanism is proposed that produces a diagnostic geochemical sig nature of perennial spring-fed wetlands in arid basins, especially the sali ne and alkaline volcanic basins of East Africa, Respiration by vegetation a nd decay of organic matter within marshes in Ngorongoro Crater, Tanzania, l ocally increases aqueous P-CO2. The resulting lowered pH inhibits the preci pitation of calcite and decreases the rate of silica dissolution Steep geoc hemical gradients are produced, These are represented mineralogically by si liceous biomineralization within the perennial marsh and carbonate precipit ation in surrounding ephemeral wetlands, mudflats, and lacustrine environme nts, As a test of this model of geochemical sedimentation, the mineralogy o f the early Pleistocene Siliceous Earthy Claystone of Oldupai Gorge is cons istent with. its interpretation as a paleo-spring wetland deposit. Because such deposits are expressions of the paleo-water table, this approach has p otential applications in many studies of basin paleohydrology, paleoclimato logy, paleoecology, and archaeology.