Direct dating of human remains

To understand human evolution, archaeologists require precise chronologies to compare and contrast the fossil collection. While indirect dating techniques of human occupation sites usually applied on sediments are somehow more precise than direct dating techniques, the accuracy of indirect dating is frequently poor. Direct dating of human remains older than 50 to 60 ka (radiocarbon limit) is limited to U-series and ESR techniques. To minimize the impact of direct dating on valuable archaeological samples, non-destructive U-series and ESR analysis have to be carried out using specifically designed protocols. Both methods are seriously compromised by the fact that teeth accumulate large amounts of uranium following their deposition in sediments. During the three years of this PhD, a 2D mapping protocol has been developed on the isotope distributions and elemental concentrations of uranium and thorium in fossil teeth using laser ablation ICP-MS. Isotopic maps of enamel and dentine show complex patterns that imply that systematic mapping of fragments would provide accurate insight for U-series and ESR internal dose assessment. A fossil Neanderthal tooth from Payre (France) showed negligible U-migration through the external enamel surface compared to the internal migration from the dentine, with great implications for ESR dating. Non-destructive ESR analyses are carried out on enamel fragments instead of powders to minimize the impact of analysis on samples. Nevertheless, the ESR spectra of fragments have a high angular dependency which complicates their study and the establishment of experimental protocols. During this PhD, new measuring protocols and analytical decomposition of ESR spectra have allowed to gain new insight on the composite nature of the signal. The development of comprehensive model describing the influence of several oriented and non-oriented CO2- radicals in the spectra with complex kinetics and transfer processes has shown that major age underestimation can be expected for most fossil tooth enamel. The new model suggests that fossils such as the Irhoud specimen or Broken Hill had their age underestimated by around 30%, propelling the Irhoud specimen amongst the oldest anatomically modem humans in Africa