Luminescence dating at Border Cave: Attempts, questions, and new results

Abstract

Border Cave hosts a rare Middle and Early Later Stone Age sequence of deposits that extends as far back as ca. 250 thousand years (ka). The site's chronology has been built mainly on Electron Spin Resonance (ESR) ages obtained from teeth, conducted at the end of the 1990s, and on radiocarbon dating for the more recent layers. In order to refine the sequence's chronology, several materials were selected for luminescence dating, including 34 siliceous lithic artefacts, and quartz and feldspar grains extracted from 10 sediment samples. Since the radioisotopic contents of the cave sediments are abundant in the volcaniclastic host rock (about 2% K, 10 ppm 232Th, 2 ppm 238U in the sediments) and the ages for the lower layers are over 40 ka, high (>150 Gy) equivalent doses (De) were expected for most samples. The saturated thermoluminescence (TL) signals of the lithic artefact samples suggested either that they had not been heated in the past, or that they were already too old to be dated. The presence in the sediments of 40–63 μm quartz grains dominated by the fast Optically Stimulated Luminescence (OSL) component and with apparent high D0 values (200–1000 Gy) suggested, a priori, that they could be appropriate for measuring high Des. However, a systematic drop of the De values for these high D0 grains was observed and could not be fully understood and corrected accordingly. The recently proposed pIT protocol, together with a global growth curve (GGC) approach, was applied to 63–80 μm feldspar grains extracted from the sediment samples. The data suggest that the IR225 signal was likely not fully bleached at the time of deposition. Nonetheless, two age estimates, assumed to bracket the target age, have been calculated for each sample. These estimates are consistent with the previous ESR ages and the ESR/C14-based Bayesian models. Further improvements can be considered in the future with the pIT - GGC method, which would allow both increased accuracy and precision.