Comprehensive stable isotope investigation of marine biogenic apatite from the late Cretaceous–early Eocene phosphate series of Morocco

Abstract

Fossil biogenic apatites were studied for their geochemical composition across the late Cretaceous–early Eocene Moroccan phosphate series in the Ouled Abdoun and Ganntour basins in Morocco in order to characterize paleoenvironmental conditions and to improve stratigraphy. The vertebrate remains show particularly good structural, mineralogical and chemical preservations, which relate to the favorable depositional environment of the phosphorite. The main studied fossils – shark tooth enameloid and dentine, and coprolites – show large range in δ13C values from −14 to +6‰, which can be coupled to different carbon sources. Enameloid yielded mostly positive δ13C isotopic compositions that are comparable with values reported from modern teeth. Coprolites have the lowest δ13C values that reflect burial conditions with intensive organic matter recycling.The large variation in δ18OPO4 values of the shark teeth can be related to ecological differences. However, the mean δ18OPO4 data reflect important temporal variation along the series, together with the corresponding average δ13C values. Comparisons with the global isotope records allow identifying the Early Eocene Climatic Optimum in the top of the Ouled Abdoun series (above Bed 0′). The isotope data further suggest a sedimentary gap during the latest Thanetian and the Paleocene Eocene Thermal Maximum. The top of the Paleocene series (Bed IIa) can be dated to late Selandian–early Thanetian, with the recognition of the Early Late Paleocene Event (ELPE). The Eritherium Bone Bed, that yielded the earliest known placental mammals from Africa, would be located below the ELPE and therefore, cannot be younger than late Selandian.The isotope data from the older Paleocene (Bed IIb) and Cretaceous (upper Bed III) beds in the Ouled Abdoun Basin can be correlated with the latest Danian–early Selandian and the latest Maastrichtian global isotope record, respectively. Based on the δ18OPO4 data, the Cretaceous layers of the Ganntour Basin cover most of the Maastrichtian period except the very early part. All these early Paleogene and Cretaceous chemostratigraphic ages, however, need further confirmations from other proxies. Yet, the interpretations are in general agreement with the biostratigraphy derived from the selachian fauna.