Oxygen isotope composition of vertebrate phosphates from Cherves-de-Cognac (Berriasian, France): Environmental and ecological significance

The European record of continental vertebrates during the Earliest Cretaceous (Berriasian) is usually based on accumulation assemblages in littoral localities where faunas of various living environments and ecologies are mixed. A recently excavated site located in the southwest of France (Cherves-de-Cognac) corresponds to a hyper-saline lagoonal environment that has yielded a rich vertebrate fauna including brackish and freshwater fish, amphibious crocodilians and turtles, as well as terrestrial dinosaurs and mammals. While some of these taxa were most likely parautochthonous, others may have been transported from distant environments. In order to decipher the respective living environments of these vertebrates, and better understand the local climatic conditions, apatite phosphates of their teeth or bones have been analyzed for their oxygen isotope compositions. From these preliminary measurements, differences in phosphate δ18Op values observed between taxa most likely reflect differences in sources of ingested waters that are compatible with their assumed ecologies. Reconstructed δ18Ow values of environmental waters using PO4–water fractionation equations show that marine and brackish waters were submitted to enough evaporation for generating a hypersaline lagoon, as reflected by δ18Ow values ranging from − 1.7‰ to + 1.7‰, whereas the more inland freshwaters have δ18Ow values ranging from − 4.1‰ to − 1.8‰. The lowest water δ18Ow value of about − 4‰ estimated from the δ18Op value of terrestrial mammals, which is assumed to be the composition of local meteoric waters, leads to estimate a mean air temperature of about 20 ± 3 °C at Cherves-de-Cognac during the Berriasian (30–35°N). Such temperatures are recorded today at similar latitudes; however, a weak equator to pole thermal gradient is evidenced by the latitudinal distribution of thermophilic taxa such as crocodilians.