‘Why so high?’ Examining discrepancies between the Sr biosphere map and archaeological tooth data from the Peak District, England

The analysis of 87Sr/86Sr isotope ratios in human and nonhuman tooth enamel is used worldwide for archaeological and forensic purposes to establish if an individual is likely to have grown up in the area from which their remains were excavated. The English Peak District has produced an unusually high proportion of archaeological humans who, based on Sr isotope ratios, appear to have come from elsewhere. We have used modern plant samples from the Peak to show that the current understanding of Sr isotope ratios for this area is incomplete – we found many plant samples growing on gritstone sediments had higher Sr values than would be expected based on the current literature. In addition we demonstrated that the taxonomy of the plant does not appear to affect the Sr isotope values (we also found that mycorrhizal type did not determine Sr isotope values in these plants), rather it is the substrate on which it is growing that is important. In terms of human movement, our work suggests it is likely that many archaeological individuals found in the Peak District are indeed local, rather than migrants. It is also possible that the expansion of blanket peat in the Peak has over time reduced the amount of Sr entering the food chain from mineral soils, reducing the radiogenic Sr isotope values in more recent teeth. While our case study is the Peak District, our findings have implications for anomalously high archaeological 87Sr/86Sr isotope values in other upland regions with similar geologies and blanket peats.