Palaeoenvironmental reconstructions by stable isotopes of Middle Miocene gastropods of the Central Paratethys

The stable isotopes of carbon and oxygen were used to provide information of mixing of marine and freshwater and subsequently used to determine palaeosalinities. In the Middle to Late Miocene St. Margarethen “Zollhaus” section (Upper Sarmatian “Mactra Zone”/Pannonian), a succession of three gastropod assemblages indicates changing environmental conditions from a limnic-fluviatile to littoral mudflat and a sublittoral marine environment. Several shells of the main representatives of the three assemblages [Potamides hartbergensis (Hilber), Granulolabium bicinctum (Brocchi), Potamides disjunctus (Sowerby)], and additionally shells of Lymnaea sp., a typical freshwater gastropod, and Pomatias conicus, a terrestrial gastropod, were taken for stable isotope analyses. Oxygen and carbon isotope data of the freshwater endmember are distinctively lower (δ18O∼−4.5‰/δ13C∼−9‰) than of the marine endmembers (δ18O∼−2.2‰/δ13C∼2.3‰). In contrast to the expected intermediate isotopic composition between the freshwater and marine environment, the shells of supposed brackish environments have higher δ18O values than the shells of the marine environment. These higher δ18O values are explained by evaporation, which affected the protected areas of riverine to lacustrine environments and small pools on mudflats, formerly considered to be the habitats of the brackish assemblages. Thus, the oxygen and carbon isotope data extend the already existing palaeoenvironmental information.