A biogenic mechanism is proposed that produces a diagnostic geochemical signature of perennial spring-fed wetlands in arid basins, especially the saline and alkaline volcanic basins of East Africa. Respiration by vegetation and decay of organic matter within marshes in Ngorongoro Crater, Tanzania, locally increases aqueous P CO2 . The resulting lowered pH inhibits the precipitation of calcite and decreases the rate of silica dissolution. Steep geochemical gradients are produced. These are represented mineralogically by siliceous biomineralization within the perennial marsh and carbonate precipitation in surrounding ephemeral wetlands, mudflats, and lacustrine environments. As a test of this model of geochemical sedimentation, the mineralogy of the early Pleistocene Siliceous Earthy Claystone of Oldupai Gorge is consistent with its interpretation as a paleo-spring wetland deposit. Because such deposits are expressions of the paleo-water table, this approach has potential applications in many studies of basin paleohydrology, paleo-climatology, paleoecology, and archaeology.

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