THE ENVIRONMENT OF DEPOSITION INDICATED BY THE DISTRIBUTION OF RARE-EARTH ELEMENTS IN FOSSIL BONES FROM OLDUVAI GORGE, TANZANIA

Fossil bones of Pleistocene age from Olduvai Gorge, Tanzania incorpora ted relatively high concentrations (up to several thousands ppm) of ra re earth elements during their fossilization. The data indicate that t hese elements were mobile during the diagenetic processes involved in the fossilization of the bones. It is suggested that the high concentr ations arose through interaction of the bone with groundwaters enriche d in these, and other elements, introduced by contemporaneous carbonat itic or alkalic volcanism. The distributions of the rare earth element s within both the fossil bone and enclosing sediment are heterogeneous but spatially systematic, with concentration profiles showing a maxim um in the bone at its sediment interface. Despite the relative porous nature of bone, the steepness and variability of the rare earth elemen t profiles indicate chemical equilibrium has not been reached. Of the individual rare earth elements, Ce behaved differently from the others . Systematic variations in, and magnitudes of, the Ce anomaly within t he bone cortices from different depositional environments, indicate th at the redox potential of the environment during fossilization was the principal factor affecting the distribution of this element. (C) 1997 Elsevier Science Ltd.