Sr isotopes of the shells of the euryhaline gastropod Potamides lamarcki from the Oligocene of the French Massif Central and Paris Basin — A clue to its habitats

Euryhaline mollusks are adapted to large ranges of salinities in their life environment but are not reliable indicators of the marine or non-marine origin of dissolved ions. Many palaeogeographical controversies based on such ambiguous organisms are long lasting questions. This is the case for the some Rupelian deposits of the French Massif Central. In these deposits, the presence of a gastropod, Potamides lamarcki, has been considered to reflect various palaeoenvironments, from marine or marginal (lagoons or marine ingressions) to continental (brackish to saline lakes). As a marginal or marine fossil, it was used to reconstruct Oligocene seaways in the European Cenozoic rift system. The application of Sr isotope ratio of well preserved aragonitic fossils of P. lamarcki from Rupelian deposits in the Massif Central and in the Paris Basin is tested as a method of solving this problem.These data confirm the reliability of Sr isotopes in discriminating between euryhaline gastropods of marine origin in the Paris Basin and of continental origin in the French Massif Central. The Potamides lamarcki shells of the Paris Basin Rupelian gulf have 87Sr/86Sr ratios from 0.707744 ± 18 to 0.707949 ± 11, i.e. close to the Oligocene seawater range (0.70780–0.70820, McArthur et al., 2001). In contrast, the P. lamarcki shells collected in the Oligocene deposits of the French Massif Central have high 87Sr/86Sr ratios ranging from 0.713231 ± 16 in the Limagne basin to 0.715205 ± 13 in the Aurillac basin. These are very distinct from contemporaneous seawater and reflect the Sr isotopic compositions of local continental waters in these basins during the Rupelian.It is suggested that the Potamides lamarcki of the French Massif Central basins lived in intracontinental saline lakes rather than in lagoons with connection to seawater. Accordingly, the presence of P. lamarcki and of other euryhaline fossils cannot be taken as evidence of seawater influence in the distinct grabens of this region during the Oligocene and palaeogeographical reconstructions of the western part of the European Rift System should be reviewed.