THE CENTRAL PARATETHYS – AN EXAMPLE FOR THE ISOTOPIC EVOLUTION OF AN EPICONTINENTAL SEA

The marine Miocene deposits of Central Europe are a heritage of the Paratethys Sea. During its maximum extent this sea spread from the Rhône Basin in the west towards Inner Asia in the east. It came into existence during the latest Eocene and Early Oligocene due to the rising Alpine island chains which acted as geographic barriers. The Central Paratethys existed throughout the Early and Middle Miocene. Already during the latest Middle Miocene, marine connections to adjacent seas were strongly narrowed. Finally, at 11.6 Ma the western part of that sea became isolated and Lake Pannon formed. In this fragile system, severe changes in the composition of the Paratethyan nearshore faunas are triggered by climatic and geodynamic developments indicated by repeated isolation-events with highly endemic faunas. To trace these changes an analysis of a mollusc-based meta-data-set of 859 d13C and d18O data-pairs of Miocene nearshore settings in the Central Paratethys Sea and its descendant Lake Pannon was carried out. The observed trends document a strong tie to geodynamics which are largely decoupled from Miocene open ocean isotope curves. Semi- to fully enclosed, initially marine water bodies such as the Paratethys Sea are prone to switching seawater isotope signatures due to their rapid response to changes in the evaporation/precipitation ratio. Geodynamically triggered trends are thus simultaneously strongly modulated by global climate. During the Middle Miocene two phases of positive deviations from the SMOW occurred, which both were initiated by tectonic constrictions of the seaways and became amplified by global warming and regionally decreasing precipitation. The final disintegration of the Paratethys, coinciding with a further separation of an even smaller water body in the Pannonian Basin system, caused the breakdown of marine isotope signatures in the Late Miocene. Instead, the observed isotope trends suggest a comparably simple system of an alkaline lake with steadily declining salinity. The “ocean-derived” Paratethys Sea may thus act as key for understanding isotope trends in past epicontinental seas.