Stable isotope analyses of the carbonate component of bones and teeth: The need for method standardization

In addition to the collagen and phosphate components of bones and teeth, the stable carbon and oxygen isotope compositions of skeletal carbonate are frequently measured to determine diet, environmental conditions, and populational or individual migration in bioarchaeological research. A standardized method for the stable isotopic analysis of skeletal carbonate has not yet been established, despite the importance of these data. In this study, significant differences in the stable oxygen isotope values of bones and dental enamel were observed, which arose due to instrument- and methodology-based differences. The same target archaeological samples were analyzed in three laboratories, and the stable isotope analyses were supplemented by Fourier Transformation Infrared Spectroscopy (FTIR) measurements, which detected a strong OH- concentration-dependence in the stable oxygen isotopic shifts measured between the three laboratories. Laboratory experiments were conducted to quantify the effects of reaction temperature and phosphoric acid (H3PO4) concentration on the oxygen isotopic composition of carbonate, both of which resulted in significant, though smaller delta O-18 changes than the OH-related delta O-18 change. As such, we suggest that the delta O-18 values of bone structural carbonate should not be included in anthropological interpretation and that the analysis of tooth enamel should be conducted at 70 degrees C using 102% H3PO4, in order to make the results comparable. Furthermore, carbonated hydroxylapatite (CHAP) reference materials with varied isotopic compositions should be developed in the near future.