Effects of thermal treatment on TL and EPR of flints and their importance in TL-dating: application to French Mousterian sites of Les Forêts (Dordogne) and Jiboui (Drôme)

Abstract

This study of trapped charge phenomena in flint highlights the influence of laboratory thermal annealing conditions on TL properties and age evaluation. The effectiveness of thermal zeroing of geological, or weakly heated flint, is investigated by EPR and TL; the 380 °C TL peak (correlated to the Al centre) is totally erased after heating at 300 °C for 20 min and the 470 °C TL component (OHC centre) after heating at 400 °C for 20 min. We attempt to determine laboratory conditions of annealing that lead to a state of trap distribution and charge filling that is closest to that the archaeological material had after its ancient firing. Under these conditions, the original mode of TL growth with dose is expected to be more appropriate; consequently, more reliable TL ages should be obtained using the standard TL protocol. The way to determine appropriate annealing conditions is the result of a coupled TL–EPR study: EPR characterisation of organic radicals and evolution of the E ′ centre according to Toyoda et al.'s protocol (1993) together with the evolution of regenerated glow curve shapes and TL sensitivity with the thermal treatment. As an illustration, the following TL ages have been obtained with the assistance of EPR, on Mousterian flints from two French open air sites. The age of the Mousterian occupation at Les Forêts (Dordogne) 92.7 ± 4.5 ka , which fits into the palaeoenvironmental chronological frame, provides accurate information for archaeologists. The site of Les Forêts was the first open air Mousterian site found in south-west of France thanks to rescue excavations, in a region where cave and rock shelter sites essentially are known. The age of the Mousterian occupation at Jiboui Path (in the Alps, Drôme), 52.0 ± 2.6 ka , is consistent with possible seasonal occupation of altitude sites during warmer phases of the last Glacial period. As data for alpine area are rare, dating this type of site was of fundamental interest for the study of the (pre)history of populations.