An improved single grain OSL chronology for the sedimentary deposits from Diepkloof Rockshelter, Western Cape, South Africa
The single grain optically stimulated luminescence (OSL) chronology for the sedimentary deposit at Diepkloof Rockshelter, reported by Jacobs et al. (2008c), has recently been critiqued and several reasons proposed for why the OSL ages for the Intermediate and Early Howieson's Poort (HP) and Still Bay (SB) techno-complexes might be inaccurate. Tribolo et al. (2013) presented a series of OSL and thermoluminescence (TL) ages that were in agreement with each other, but, for some part of the sequence at least, were much older than the OSL chronology of Jacobs et al. (2008c). In this paper, we have tested the criticisms of Tribolo et al. (2013) and colleagues related to both the equivalent dose (De) estimates and the beta dose rates by performing a series of targeted experiments, combined with updates and re-assessments of our error calculations. We show that the De estimates are stable over a range of alternative measurement conditions and also over time. We also demonstrate the reproducibility of our measurement procedures for the beta dose rates, and their accuracy tested against a range of independently obtained estimates. We show that, for the stratigraphic units (SUs) where there are major discrepancies in age between Jacobs et al. (2008c) and Tribolo et al. (2013)-notably the Intermediate HP and Early HP-and for which both studies had single grain OSL ages, the estimation of potassium (K) in the sediment surrounding the dated grains is critical. We provide new and updated De and dose rate estimates, and final ages which we compare with our previous age estimates and those of Tribolo et al. (2013). The differences in the size of the errors associated with the ages reported in the two independent studies are also addressed. We can show that our ages are robust and consistent with the original chronology, but we cannot satisfactorily explain why the TL and OSL ages provided by Tribolo et al. (2013) might be wrong. So, the dating conundrum at Diepkloof Rockshelter remains. As a result, we caution against the development of HP and SB age models based on only one of the chronologies for this site. At this stage, extrapolation of the Tribolo et al. (2013) chronology to a re-interpretation of the southern African MSA would appear to be premature, especially as the ages do not differ systematically between the two studies and as differences between TL and OSL ages are not an issue at other sites in southern Africa where both dating methods have been applied. Further work is needed to resolve the question of the Diepkloof chronology.