Exploring the variation of the δ18Op and δ18Oc relationship in enamel increments

Comparisons between the oxygen isotope ratios in the carbonate and phosphate fractions of bioapatite have been proposed and applied as an internal check for diagenesis because oxygen in the two compartments is believed to be incorporated simultaneously from the same body water. It has become clear, however, that the relationship is quite variable and for some time this variability was thought to be induced by a “species” effect, although it has never been clear why that should be the case. We studied sequential samples in enamel crowns of eight equid and cervid teeth from five different Upper Palaeolithic sites in peninsular Italy, encompassing an area with different geological units. Results suggest that δ18Oc values are slightly attenuated compared to the δ18Op values in their seasonal profiles. The observation is unexpected if both bioapatite mineral fractions precipitate simultaneously from body water. One explanation could be diagenetic effects on the carbonate fraction. The same pattern, however, has been observed in data comparing incremental δ18Oc and δ18Op in modern mammal enamel crowns. If confirmed, attenuation of intra-tooth δ18Oc sequences compared to δ18Op variability would imply that seasonal climate inferences from the two proxies are not completely equivalent. Our results may also go some way in explaining the extent of variability amongst different sets of data in the δ18Op/δ18Oc relationship and suggest that the classical approach in the bioapatite community of using a constant offset between δ18Op and δ18Oc values needs to be approached with caution.