Otolith diagenesis comparing microstructural, mineralogical and geochemical data: application to Pliocene fossils from southeastern France

Microstructure, mineralogy and elemental and isotopic compositions of recen t and fossil otoliths have been investigated. Fossil otoliths come from mar ine Pliocene localities of southeastern France. The study of the different parameters show that diagenetic changes are weak. Microstructural features specific to otoliths, such as needle-like crystals and growth increments, a re usually well preserved. When present, the microstructural modifications remain limited inside each otolith. Except a few pyritic specimens, the onl y mineral found is aragonite like in recent specimens. The chemical composi tions of fossil and recent otoliths are not significantly different, and an y element is preferentially changed by diagenesis. The isotopic composition does not show strong alterations. However, the difference in O-18 content between recent fishes having different ecological habits is not found with the same magnitude in fossil specimens. Finally, when elemental and isotopi c compositions are compared, the specimens are sorted according to taxonomy and not according to their geologic age. This suggests that biogenic signa ls have been preserved.