Intra-tooth study of modern rhinoceros enamel δ18O: Is the difference between phosphate and carbonate δ18O a sound diagenetic test?

The carbonate and phosphate oxygen isotopic difference (Δ18OC–P) has been suggested as a test for diagenetic alteration of isotope values in fossil apatite samples. The generally accepted Δ18OC–P value for well-preserved enamel is ~ 9‰. Only a few studies have focussed on the potential natural variability of this difference in modern and fossil mammal teeth. The aim of this study is to document the value of the carbonate-phosphate oxygen isotopic fractionation factor (αCO3–PO4) at an intra-tooth level. We measured the enamel phosphate and carbonate oxygen isotopic compositions of two upper molars (M1 and M2) of Rhinoceros unicornis following a sequence from the apex to the cervix of the crown. The average αCO3–PO4 is 1.0082 ± 0.0007. The mean value of the Δ18OC–P difference is 8.4 ± 0.7‰, close to the literature values given for other species. However, we also show that the values of the Δ18OC–P difference for R. unicornis upper molars can vary on a ~ 2‰ range in the same tooth. We develop a physiological model to explain the origin of inter-species and intra-tooth Δ18OC–P difference and to understand their implications on the status of Δ18OC–P as a tool for identifying diagenesis. The results imply that, by increasing the dataset of phosphate and carbonate oxygen isotopic composition for modern mammal species, we will be able to define a maximal and precise envelope corresponding to the well-preserved samples and to check a possible species dependence on the δ18OC/δ18OP relationship.