Finding radiogenic Sr-isotope biospheres: can a home in Britain be found for people with high 87Sr/86Sr?

Abstract

With increasing regularity archaeological humans with 87Sr/86Sr >0.714 are being excavated in Britain and are difficult to explain or identify possible places of origin. The main aim of this thesis was to identify any geological or anthropological reasons for their high 87Sr/86Sr values. Therefore, plant 87Sr/86Sr data has been obtained from several Precambrian lithologies and igneous intrusions in Britain with the aim of identifying values >0.714. Only 20 of the 151 samples measured recorded values >0.714. It was found that biosphere 87Sr/86Sr values >0.714 represented only 10% of all known British Sr-isotope biosphere data and only ~2.8% of the area of Britain. They are thus numerically and spatially rare and can also be considered agriculturally marginal for archaeological populations.

Unexpectedly, plant samples collected from ancient woodlands were found to have elevated values by +0.002 compared to plant samples collected from unforested land on the sedimentary Triassic bedrock of central England. Further work is needed to replicate this result and to establish if the same trend is found on other bedrock lithologies. This finding could have major implications for how biosphere and human 87Sr/86Sr data are interpreted in archaeological migration and mobility studies, particularly in recently de-forested regions.

Three British quern or millstone rock types (Millstone Grit, Pennant Sandstone and granite) were investigated to establish if they could contribute bioaccessible high 87Sr/86Sr directly in the human stomach from rock grit accidentally ingested via grinding grain, or deliberately through pica or geophagy, using the Unified Bioaccessiblity Method (UBM: Hamilton et al., 2015). The results showed that Sr in ingested rock grit is bioaccessible to humans. However, unrealistic quantities of rock grit needs to be consumed to significantly alter skeletal 87Sr/86Sr (e.g. to change the value by ±0.001 or greater). The study thus provides reassurance that non-local or unusually high 87Sr/86Sr values cannot be explained by the direct ingestion of rock grit, clays or soils.