THINKING INSIDE THE SPHERE: CALCULATING EXTERNAL RADIATION DOSE RATES FOR ESR DATING AT ROC DE MARSAL, FRANCE

At Roc de Marsal (RdM) cave, Couche I in the lower layers yielded a Neanderthal infant skeleton that originally was thought to have been intentionally buried, although now appears not to have been. The site has also yielded a Quina Mousterian industry rich in scrapers overlying other Mousterian industries with high percentages of Levallois débitage but few scrapers. Preliminary ESR ages increased with depth, but thermoluminescence (TL) dates did not change significantly from top to bottom. Both methods rely on accurately calculating the external dose rate, D_ext(t), from the mineralogically inhomogeneous sediment, which constitutes the major dose contribution to the RdM ages. Large differences occurred in D_ext(t) derived from the volumetrically averaged sedimentary geochemical analyses used for the ESR and the in situ TL dosimetry used for the TL dates. Since D_ext(t) from éboulis is much lower than that from the fine-grained cave sediment, its contribution must be analyzed accurately, but sediment granulometry was never completed for the deposits. For all the ESR dates, total-station laser locations (“shots”) exist for all éboulis larger than ~5 cm in maximum diameter. While converting this linear data into volumetric éboulis contributions should be possible, it has never been attempted. Here, we derive a mathematical model to do so. In nearby caves hosted in similar limestone, éboulis frequency size distributions mimic an exponentially decreasing function. After testing several mathematical models to approximate the medium and large éboulis volumes, the tetrahedral and hemispheroid shapes provided more accurate estimates. A probability density function was developed that uses medium and large éboulis volumes to predict the small éboulis volume given the number of shots. Using a linear model for the frequency size distribution is not as reliable as using an exponential assumption for calculating both the TL and ESR ages.