Intra-tooth study of modern rhinoceros enamel delta O-18: Is the difference between phosphate and carbonate delta O-18 a sound diagenetic test?
Résumé
The carbonate and phosphate oxygen isotopic difference (Delta O-18(C-P)) has been suggested as a test for diagenetic alteration of isotope values in fossil apatite samples. The generally accepted Delta O-18(C-P) value for well-preserved enamel is similar to 9 parts per thousand. 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 (alpha(CO3-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 alpha(CO3-PO4) is 1.0082 +/- 0.0007. The mean value of the Delta O-18(C-P) difference is 8.4 +/- 0.7 parts per thousand, close to the literature values given for other species. However, we also show that the values of the Delta O-18(C-P) difference for R. unicornis upper molars can vary on a similar to 2 parts per thousand range in the same tooth. We develop a physiological model to explain the origin of inter-species and intra-tooth Delta O-18(C-P) difference and to understand their implications on the status of Delta O-18(C-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 delta O-18(C)/delta O-18(P) relationship. (c) 2008 Elsevier B.V. All rights reserved.