Uranium and thorium distributions in fossil bones from Olduvai Gorge, Tanzania and Kanam, Kenya

Abstract

The micro-distributions of uranium and thorium have been studied in fossil bone cortices in samples from Olduvai Gorge, Tanzania and Kanam, Kenya, and compared with modern bones collected from the surface at Olduvai Gorge. Concentrations in excess of 800 ppm U and 200 ppm Th are recorded in sections of some fossil bones, whereas concentrations do not exceed 20 ppm U or 1 ppm Th in the modern bones. In fossil bones, Th has a larger intra-bone variation than U. The distribution profiles of the two elements across the fossil bone cortices are quite distinct and reflect differences in their geochemical behaviour in the supergene environment. In particular, the two possible oxidation states of U, compared with one for Th, allow U to be more mobile than Th, but also more susceptible to variations in the redox potential of the depositional environment. In some fossil bone samples secondary leaching of U has occurred, whereas Th is retained in the bone cortex. U is incorporated into fossil bone earlier than Th.