Evaluation of analytical method for Sr isotopic ratio in phosphate fossils

Abstract

Sr isotopes (87Sr/86Sr) are useful tracers to reconstruct migration histories of extinct animals. When applied to fossil samples, it is essential to evaluate the effects of diagenesis. In some cases, acetic acid treatment, which is conventionally used, did not sufficiently remove the influence of diagenetic effects. In this study, as a preliminary survey to reconstruct the ecological history of fossil vertebrates using the 87Sr/86Sr ratio, we investigated the diagenetic process in phosphate fossil by combining acetic acid pretreatment with local composition analysis of elements (Si, Fe, Mn, and REE) as indicators of diagenetic effects. The results showed that the concentrations of Si, Fe, Mn, and REE in enamel-like tissue were about 1/5~1/10 of those in dentin. In the enamel-like tissues, REE/Ca was about 1000 times higher than that in the teeth of the dentin of modern animal, whereas Sr/Ca was about 2 times higher. Furthermore, acetic acid treatment reduced Sr/Ca in enamel-like tissues by about 30%. These results suggest that after acetic acid treatment, enamel-like tissues retain enough Sr isotopic information from the fossil animal's life.