Spatial variability of watermass conditions within the European Epicontinental Seaway during the Early Jurassic (PliensbachianToarcian)

In order to constrain spatial variability in watermass conditions within the European Epicontinental Seaway prior to, during and after the Toarcian Oceanic Anoxic Event, carbon (13Cbel, 13Ccarb) and oxygen (18Obel, 18Ocarb) isotope records were obtained from three sections in the Grands Causses Basin (southern France). These data were then compared with similar records along a northsouth transect across the European Epicontinental Seaway. As the conclusions reached here strongly depend on the reliability of belemnite calcites as archives of palaeoceanographic changes, an attempt was made to improve the understanding of isotope signals recorded in belemnite calcite. Intra-rostral carbon and oxygen-isotope data from six belemnite specimens belonging to the genus Passaloteuthis were collected. Intra-rostral carbon-isotopes are influenced by vital effects, whereas oxygen-isotopes reflect relative changes in temperature and salinity. Palaeotemperatures calculated from 18Obel-isotope records from the Grands Causses Basin confirm relatively low temperatures throughout the Late Pliensbachian. Similar cool water conditions have previously been shown in Germany, England, Spain and Portugal. A temperature increase of up to 6 degrees C is observed across the PliensbachianToarcian boundary. A pronounced negative shift of at least 3 parts per thousand (Vienna-Pee Dee Belemnite) is recorded in bulk carbonate carbon during the lower Harpoceras serpentinum zone, typical of the Toarcian Oceanic Anoxic Event. Before and after the Toarcian Oceanic Anoxic Event, a good correlation between 13Ccarb and 13Cbel exists, indicating well-ventilated bottom-waters and normal marine conditions. Instead, data for the Toarcian Oceanic Anoxic Event indicate the development of a strong northsouth gradient in salinity stratification and surface-water productivity for the Western Tethyan realm. This study thus lends further support to a pronounced regional overprint on carbon and oxygen-isotope records in epicontinental seaways.