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

Significant differences in the delta(18)O(p) value between teeth, and even within a single tooth were observed in a detailed study of the oxygen isotope composition of tooth enamel phosphate (delta(18)O(p)) of hypsodont teeth from bison and sheep jaws. The permanent molars and premolars of a fossil adult bison from eastern Wyoming (similar to 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 delta(18)O(p) for the bison compared to the sheep (5.6 parts per thousand and 3.5 parts per thousand, 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 parts per thousand) 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 delta(18)O(p) 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 delta(18)O(p) values, and the need to understand how they vary for a population of interest. If these variations are taken into account, the delta(18)O(p) values of single samples from small, late-forming teeth (e.g. premolars) can be used as a proxy for the delta(18)O 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.