Putting South-West England on the (strontium isotope) map: A possible origin for highly radiogenic 87Sr/86Sr values from southern Britain

Reconstructions of ancient mobility based on strontium isotopes are only ever as reliable as estimates for baseline values of bioavailable strontium in the study area. Current biosphere mapping for Britain suggests that there are no sizeable areas hosting 87Sr/86Sr values above 0.714 south of Cumbria. As a result, archaeological humans or animals with such (for Britain) 'highly radiogenic' strontium isotope values are commonly interpreted as having moved either from Scotland or abroad. This paper presents the first dedicated strontium isotope map for SouthWest England based on 98 modern biosphere samples (including 68 new measurements). Numerous samples from the Cornubian granite (Dartmoor) have 87Sr/86Sr values above 0.714 (maximum 0.7287) and, based on their distribution, it is suggested that the previously elusive 'highly radiogenic' values are characteristic for areas where the soil has with high rubidium concentrations. These occur at lower elevations which are better suited for agriculture and permanent human settlement than the high moors. Previous interpretations of archaeological samples from southern Britain may need to be revised considering these new results, but they also highlight the continued need for biosphere sampling and the usefulness of geochemical maps as a routine part of strontium isotope investigations in archaeology.