Rock surface and sand-sized sediment quartz dating using optically stimulated luminescence of a Middle-to-Upper Palaeolithic sequence at the Bordes-Fitte rock shelter (Les Roches d'Abilly, Central France)

In this study, we use optically stimulated luminescence (OSL) to establish a robust absolute chronology for the lower part of the stratigraphic record at "Les Roches d'Abilly", an important Palaeolithic site in Central France. There, lithic assemblages recording the Middle-to-Upper Palaeolithic transition and the behaviour of Neanderthal and Anatomically Modern Human populations are preserved. At the Bordes-Fitte rock shelter, part of "Les Roches d'Abilly", the archaeo-stratigraphic record has been sealed and preserved by the collapse of the shelter roof and this site has provided important data on the timing of human occupations and Middle-to-Upper Palaeolithic transition in this region.Here, we first obtain burial ages using standard multi-grain quartz techniques on sand-sized sediment grains and then investigate the information available in blue-stimulated luminescence-depth profiles into the surfaces of four quartz-rich cobbles found in the sedimentary record below the various collapses of the shelter roof. These profiles show qualitative evidence for past daylight exposure and a single burial event, allowing the data to be analysed quantitatively using a multiple-event mathematical model. Based on the results of this modelling, it is concluded that at least part of the cobble surfaces were well-bleached at burial and thus that estimated rock surface burial ages are unlikely to be significantly affected by incomplete bleaching. However, it appears that not all cobble surfaces were well-bleached, demonstrating the importance of only using surface doses derived from those cobble layers that can be shown to have been well-bleached at burial. The rock-surface burial ages of the most recent burial event are consistent with the quartz OSL ages derived from the surrounding sediments, giving burial ages of similar to 50 ka. The OSL ages are consistent with previously published 14C age control, although a single 14C age (uncalibrated age of 41, 900 +/- 1, 500 yr BP) of a lower unit may underestimate the OSL ages. This study provides a reliable absolute OSL chronology for the Bordes-Fitte rock shelter constraining the Middle-to-Upper Palaeolithic transition to occur between 44 +/- 2 ka and 48 +/- 3 ka ago (at the 68% confidence level).