Evidence for long-term averaging of strontium in bovine enamel using TIMS and LA-MC-ICP-MS strontium isotope intra-molar profiles

High spatial resolution micro-sampling of tooth enamel offers the possibility of high temporal resolution isotope data to reconstruct climate, environment, diet and mobility. Questions remain about the duration and pattern of the maturation phase of enamel and the existence and direction of chronological 'time-lines'. LA-MC-ICP-MS measurements of c. 400 μm craters and TIMS analyses of transverse enamel sections of an archaeological bovine third molar were undertaken to investigate the long-term averaging of incorporated strontium. The same gradually increasing isotope profile was obtained from both approaches, indicating that the large increase in spatial resolution did not change the response profile obtained. The results suggest that even at the microscopic scale, strontium is incorporated over a period in excess of 12 months. Averaging of the input signal may result from both long-term retention of strontium in the skeleton and recirculation in the body pool, or long-term maturation of enamel on a microscopic scale. Whichever mechanism is responsible, it may not be possible to recover strontium isotope ratios with a high time resolution from cattle molar enamel unless there is a large imbalance in the amount of strontium supplied by different sources. Consequently, strontium isotope profiles may not be synchronous with those of lighter isotope systems.