External dose rate determinations for ESR dating at Bau de l'Aubesier, Provence, France

At Bau de l'Aubesier, Provence, France, Mousterian artefacts and human teet h occur in inhomogeneous ("lumpy") sedimentary deposits that include bone a nd tooth fragments. Due to uranium (U) uptake by the bones and teeth, the e xternal dose rate experienced by the teeth has changed over time, Such chan ges must be considered when calculating thermoluminescence (TL) ages for bu rnt flint artefacts or electron spin resonance (ESR) ages for the teeth the rein. Here, we assess the external dose rates using three methods: in situ TL dosimetry gives the modern dose rate, whereas volumetrically averaged ge ochemical analysis is used to estimate time-averaged dose rates that consid er U uptake by U-rich sedimentary components (Blackwell and Blickstein, 200 0. Quarternary International 68-71, 329-343). In ESR isochron analysis (Bla ckwell and Schwarcz, 1993, Applied Radiation and Isotopes 44, 243-252), mul tiple subsamples with variable U concentrations measure the external dose r ate experienced by the tooth regardless of reworking, but do depend on the uptake model assumed. The methods each suggest significantly different dose rates. TL dosimetry for two combustion horizons measured external gamma do se rates at 0.143 +/- 0.012 mGray/y in Layer IV and 0.184 +/- 0.016 mGray/y in Layer H, Geochemical analysis when volumetrically averaged to account f or the influence from different sedimentary components indicates external g amma dose rates ranging from as low as 0.094 +/- 0.024mGray/y assuming RU f or U-rich sedimentary components to a maximum of 0.251 +/- 0.048 mGray/y as suming EU for the U-rich components. Isochron analyses suggest more variabl e dose rates, ranging from a minimum of 0.009 +/- 0.040 mGray/y (LU) in the lower layers to a high of 1.103 +/- 0.160 mGray/y (EU) in Layer H1. Three isochrons indicated that the teeth had experienced U leaching or a second u ptake event which would affect the accuracy of the dose rate determinations for those teeth. Other teeth may have experienced similar processes. Quate rnary researchers who want reliable dales in the age ranges for ESR or TL m ust carefully consider both the nature of the sediment, especially U-rich b one fragments, and its geometric arrangement around the samples to be dated , Corresponding results using more than one analytic method may be required for a reasonable confidence level. (C) 2000 Elsevier Science Ltd and INQUA . All rights reserved.