Mitigation of the diagenesis risk in biological apatite δ18O interpretation

The assessment of the potential blurring effect of diagenetic processes represents the greatest challenge for geochemists whose purpose is to reconstruct environmental parameters based on isotopic proxies. Numerous studies performed on phosphatic remains from terrestrial and marine sedimentary deposits have shown post-depositional isotopic modifications due to exchange reactions with surrounding waters. If diagenesis may be diagnosed through several lines of evidence such as mineralogical or chemical modifications of the pristine in vivo apatite after burial in soils and sediments, diagenesis does not prove that bioapatite phosphate exchanged oxygen isotopes with aqueous fluids. Therefore, we propose a new method of detection of diagenetic alteration of bioapatite phosphate based on a statistical approach designed to analyze sample collections. The approach requires that phosphatic remains belonged to animals living during a short time interval and within an environment characterized by steady air or water temperature and the δ18O of ambient water. We show that the distribution of bioapatite phosphate δ18O follows a normal distribution resulting from the combination of independent factors such as those related to the genetics, environment and analytical measurements. In the case of oxygen isotope exchange between the bioapatite phosphate and various amounts of circulating aqueous fluids, our numerical simulations reveal that the distributions of bioapatite phosphate δ18O are asymmetric with shapes depending on the temperature of reaction and the composition of the reacting aqueous fluid. This new statistical tool based on mass balance equations and Monte Carlo simulations is considered valuable to mitigate the diagenesis risk in the interpretation of bioapatite phosphate δ18O as paleoecological and paleoclimatic proxies.