U-Pb dating of fossil enamel from the Swartkrans Pleistocene hominid site, South Africa

We demonstrate that young fossil enamel older than the range of the U-series (similar to 300 ka) can be dated by the U-Pb methods using new models of U and Pb loss and uptake. Contrary to the current hypothesis of U uptake that only considers the adsorption/diffusion mechanism, we here develop a complete time-dependent model which takes gains and losses of the most critical nuclides (U-238, U-234, and Th-230) into account, both during chemical (dissolved U) and physical (Th and U alpha-recoil) processes. Lead is assumed to be a mixture between two components of common Ph and a radiogenic component; the proportions of these components are calculated from the Pb isotope abundances and U/Pb ratios. We apply this new U-Pb method to bovid enamel from the Swartkrans Cave (Gauteng Province, South Africa). This cave has yielded abundant early Pleistocene hominid remains attributed to Paranthropus and Homo as well as various associated archaeological vestiges. Biochronological comparisons with East Africa have provided age estimates ranging between 1.8 and 1.0 Ma, which, however, remain poorly constrained. After correction for initial U-234 disequilibrium and further U-238 loss, the U and Ph isotope data yield ages of 1.83 +/- 1.38, 1.36 +/- 0.29, and 0.83 +/- 0.21 Ma for the three stratigraphic units, Members 1, 2, and 3, respectively. We discuss the consequences of these radiometric results for hominid evolution in South Africa. (C) 2007 Elsevier B.V. All rights reserved.