Thermoluminescence Dating and the Problem of Geochemical Evolution of Sediments — A Case Study: The Mousterian Levels at Hayonim

The thermoluminescence dating of burnt flints has become a major tool in the elaboration of the chronology of human settlements of the last half a million years. The reliability of an age estimate depends to a great extent on the accuracy with which the internal and external radiation dose rates prevailing during the duration of burial can be determined. While determining the internal dose rate is relatively straightforward, the same cannot be said of the external, particularly if the sediment surrounding the flints is mineralogically heterogeneous or has undergone diagenetic changes during prolonged burial. In this article we show some examples of the problems confronted when one attempts to determine the external dose rates experienced by flints buried in a relatively heterogeneous environment of sediments, such as those sampled at the Middle Paleolithic occupation levels at Hayonim (Israel) which show evidence of past mineralogical evolution. The impact of changes produced by leaching and chemical reactions between the components of hearth ashes on the radioisotopic composition of the sediment and consequently the environmental dose rates were examined by subjecting several distinct sediment layers to thorough mineralogical, radiochemical, and dosimetric analyses. The problems of external dose-rate determinations in mineralogically heterogeneous sediments are examined, particularly in those consisting of variable amounts of siliceous aggregates, apatite, and other phosphorus-rich minerals produced by diagenetic reactions with bone residues.