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Keywords:

  • Belemnites;
  • carbon and oxygen stable isotopes;
  • Early Jurassic;
  • Oceanic Anoxic Event;
  • palaeoceanography;
  • Toarcian

Abstract

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 (δ13Cbel, δ13Ccarb) and oxygen (δ18Obel, δ18Ocarb) isotope records were obtained from three sections in the Grands Causses Basin (southern France). These data were then compared with similar records along a north–south 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 δ18Obel-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 °C is observed across the Pliensbachian–Toarcian boundary. A pronounced negative shift of at least −3‰ (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 δ13Ccarb and δ13Cbel exists, indicating well-ventilated bottom-waters and normal marine conditions. Instead, data for the Toarcian Oceanic Anoxic Event indicate the development of a strong north–south 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.