Detailed mass spectrometric U-series analyses of two teeth from the archaeological site of Pech de l'Aze II: Implications for uranium migration and dating

We report Th/U ages obtained by thermal ionization mass spectrometry of cross-sections of two teeth from the archaeological site of Pech de l'Aze II. The aim of this study is to reconstruct the pathways and time dependency of uranium uptake into different constituents of the two teeth. A previous ESR dating study of the two teeth yielded age estimates in the range of about 130,000 to 200,000 years, which was in agreement with the geological assignment of Riss for layers from which the specimens were recovered. The closed system U-series ages of all analyses range from 48 to 137 ka, thus yielding age-estimates which are between 30 and 90 ka too young. The apparent closed system U-series ages of the dentine and cement fractions at the top of the teeth are the oldest, becoming gradually younger towards the interior of the two teeth. The difference between apparent U-series ages of dentine at the outer and inner parts of the two teeth is about 35 ka. The enamel sections show the same patterns in U concentration and progression of apparent Th/U ages as the adjacent dentine samples, but with generally younger ages. Our results indicate that uranium migrates both from the top and the base into the dentine of the teeth and from there into the adjacent enamel. Depending on the position of the sample, significant uranium uptake appears to have taken place over many tens of thousands of years. Modelling the U-uptake histories using a parametric model shows that, for the cement and dentine components, uptake is intermediate between linear uptake (LU) and early uptake (EU=closed system). The enamel, although often assumed to approximate more closely a closed system, shows evidence of more recent U uptake. These results also imply that it is not possible to use a simple, single parameter U-uptake model for all components of a given tooth, either for U-series or ESR dating.