Strontium isotope mapping of elephant enamel supports a micro-sampling and modeling workflow to reconstruct past migrations

Strontium isotope ratios (87Sr/86Sr) of incrementally grown tissues have been used to study animal movement/migration. Despite growing interest in this tool, questions remain regarding how different dental tissues preserve the same 87Sr/86Sr input signal, and how different biological processes and sampling methods influence measured enamel 87Sr/86Sr. We collected 87Sr/86Sr data from tusk dentine and molar enamel of Misha, a zoo elephant with known movement history, using in situ laser ablation (LA-ICP-MS) mapping, conventional drilling, and micromilling techniques. We compared 87Sr/86Sr data from different substrates to evaluate the influence of turnover, tissue type, and sampling methods. Our LA-ICP-MS analysis showed a remarkable 87Sr/86Sr geometry consistent with enamel apposition. Enamel maturation affects 87Sr/86Sr primarily in the outer enamel. Conventional drilling and micromilling produced attenuated 87Sr/86Sr records due to enamel maturation and sample averaging. Caution should be taken when interpreting 87Sr/86Sr data from conventionally drilled enamel, which may bias reconstructions of movement/migration. Our results show that LA-ICP-MS of the innermost enamel with associated dental growth measurements can best recover the primary 87Sr/86Sr turnover history. However, biological turnover of 87Sr/86Sr is still the primary source of input signal attenuation and should be accounted for. We recommend a micro-sampling and inverse modeling workflow which we demonstrate using Misha’s tusk and enamel data. While a broader application of this approach would benefit from more experiments, it can facilitate quantitative interpretations of movement/migration histories and help answer questions of individual life history such as human/animal seasonal subsistence and reproductive patterns.