High spatial resolution Sr isotope and trace element record of dental enamel mineralization in a woolly mammoth tooth: Implications for paleoecological reconstructions

The isotopic composition and abundance of trace elements in skeletal remains are often used for paleobiological and paleoecological reconstructions. Increasing attention is being paid to dental enamel as the most resilient tissue, but its pristine biogenic signature is obscured to varying degrees by secondary mineralization and diagenesis. We investigated these effects combining histomorphometry with the Sr isotope and trace element distribution in an Upper Paleolithic woolly mammoth molar using laser ablation (MC) ICPMS. We document a 12-year continuous record of seasonal changes in 87Sr/86Sr, Li, Pb and U whose distribution pattern follows incremental enamel growth features of the secretion stage of amelogenesis. Other elements are affected by enamel maturation proceeding along the Retzius lines with a decrease in uptake of Rb, Sr, Ba and/or Mg, an increase in uptake of Mn, and considerable enrichment of Zn. The Sr/Ba ratio is consistent with the seasonal pattern of the secretion stage, indicating a short time gap between the two mineralization phases, estimated to be no more than a few months. The maturation front determined by multi-element 2D mapping penetrates, locally, as far as the inner enamel but generally does not affect the region nearest the enamel-dentine junction, making this the optimal zone for obtaining accurate biogenic data. This allowed the preservation of seasonal fluctuations in Zn and Mg, absent in the regions affected by maturation. We interpret the seasonality as the result of changes in diet and metabolism during cold and warm periods that likely accompanied mammoth migration, as suggested by the parallel changes in the Sr isotopic composition. Diagenesis affected K, Mg and Mn, but only in a narrow zone at the contact with the cementum. The dentine is diagenetically altered but still preserves relics of seasonal compositional cycles, while the cementum is completely equilibrated with the depositional environment.