Shifts in oxygen and carbon isotope signals in marine molluscs from the Central Paratethys (Europe) around the Lower/Middle Miocene transition

Stable isotope data of mollusc shells from two adjacent time slices (Karpatian/Badenian) from the Central Paratethys were investigated to detect differences in environmental parameters and to depict shifts in the isotope signature of the Paratethyan seawater around the Early/Middle Miocene transition. Altogether seventeen different gastropod species and one bivalve species from the Karpatian (Korneuburg Basin) and the Badenian (Northern Alpine Foreland Basin) were selected. Representatives of various ecological guilds yielded different isotope patterns, especially in δ13C, reflecting different original habitats or nutrition. Although molluscs precipitate their shells in oxygen isotopic equilibrium, we preferred to compare isotope data of shells of the same genus of the family Turritellidae from the Karpatian and Badenian. The Badenian Turritella shells from the Northern Alpine Foreland Basin yielded distinctly higher δ18O and δ13C values than the respective Karpatian shells from the Korneuburg Basin. As the main factors controlling δ18O values in gastropod shells are temperature and δ18O of the water during precipitation, the differences between the oxygen isotope data of the two time slices can only be explained by these factors. Good independent palaeotemperature proxies exist for the Early to Middle Miocene of Central Europe; they do not indicate a significant temperature variation. Therefore, different δ18O values of the seawater in the different basins of the Central Paratethys must be responsible for the contrasting δ18O values. When calculated with commonly assumed δ18O seawater values for the Miocene Badenian, Turritella shells indicate too cold temperatures. These data clearly document the danger of palaeotemperature interpretations based solely on stable isotopes. As smaller and marginal marine settings such as the Paratethys Sea can be seriously influenced by regional differences in the isotope composition of the seawater, only a thorough, consistent control by other climate proxies (e.g. palaeoecology of molluscs, as in this study) allows a sensible interpretation.