Inter- and intra-tooth variation in the oxygen isotope composition of mammalian tooth enamel phosphate: implications for palaeoclimatological and palaeobiological research

Abstract

Abstract Significant differences in the δ18Op value between teeth, and even within a single tooth were observed in a detailed study of the oxygen isotope composition of tooth enamel phosphate (δ18Op) of hypsodont teeth from bison and sheep jaws. The permanent molars and premolars of a fossil adult bison from eastern Wyoming (∼500 yr B.P.) and a modern sheep from California were analyzed. The bison is assumed to have been free-ranging with a variety of possible water sources, whereas the sheep was raised on a ranch. Inter-tooth variability in δ18Op for the bison compared to the sheep (5.6‰ and 3.5‰, respectively) may be a result of behavioral differences. Analyses of multiple samples from the m3 of both the bison and sheep vary to a similar degree (3.5‰) in a similar cyclic pattern down the length of the tooth, a pattern which is interpreted to be seasonal. When present, inter- and intra-tooth variations in δ18Op are controlled by the water and food ingested by the mammals during the period of enamel formation. In these localities, well waters, surface waters, and mother's milk have different isotopic compositions at different times of the year. The data underscore the role of biology and behavior in determining δ18Op values, and the need to understand how they vary for a population of interest. If these variations are taken into account, the δ18Op values of single samples from small, late-forming teeth (e.g. premolars) can be used as a proxy for the δ18O value of local meteoric water for long-term climate studies. Multiple samples from a single third molar may provide information on the duration and timing of enamel growth, seasonality, as well as long-term climate change.