Oxygen isotope fractionation between crocodilian phosphate and water

Oxygen isotope compositions of phosphate (δ18Op) were measured in tooth enamel from captive and wild individuals of 8 crocodilian species. A rough linear correlation is observed between the δ18Op of all the studied species and the oxygen isotope composition of ambient water (δ18Ow). Differences in mean air temperature, diet and physiology could contribute significantly to the large scatter of δ18Op values. The combination of these parameters results in a fractionation equation for which the slope (0.82) is lower than that expected (≥ 1) from predictive model equations that assume temperature and diet as fixed parameters. Taking into account large uncertainties, the observed oxygen isotope fractionation between phosphate and ambient water does not statistically differ from that formerly established for aquatic turtles. Case studies show that δ18Op values of fossil crocodile tooth enamel can be used to discriminate between marine and freshwater living environments within a precision of about ± 2‰ only.