Heated flint TL-dating for Gruta da Oliveira (Portugal): dosimetric challenges and comparison of chronometric data

The Mousterian stratigraphy of Gruta da Oliveira (Torres Novas, Portugal) contains one of the latest occupations by Neanderthals on the Iberian Peninsula. Thermoluminescence (TL) dates on heated flints were obtained for layers 13 and 14, and compared with the radiocarbon (charcoal and burnt bone) and U/Th (secondary carbonates) dating results available for those layers and the succession as a whole. As the internal dose rates are very low, the TL age estimates are very dependent on the external dose rates, which show significant variation, including indications of spatial patterning. Through close inspection of the radiation geometries of samples and dosemeters, the appropriate gamma dosimetry is identified and the weighted mean TL ages (1-σ) calculated accordingly are 55 ± 7 ka for layer 13 and 77 ± 8 ka for layer 14. These ages are stratigraphically consistent with the radiocarbon chronology for layers 9-11 and with the termini ante and post quem provided by U-series dating for the beginning and end of the accumulation of the deposits. The TL results corroborate that all radiocarbon dates below layer 11 are minimum ages only and, given the constraints derived from the proxy data available, place layer 13 and 14 post MIS 5. Sample as well as dosemeter numbers are low at Gruta da Oliveira, which limits the precision of TL-dating of a prehistoric human activity. Further refinement of the chronostratigraphy will be based on U-series dating of non-human-related, interstratified secondary carbonates.