Upper Pliensbachian – Toarcian (Jurassic) palaeoenvironmental perturbations in a temporal and regional context: an extended 87Sr/86Sr, δ13C and δ18O belemnite isotope study from Bulgaria

The Upper Pliensbachian–Toarcian (Jurassic) sedimentological, palaeontological and geochemical (belemnite 87Sr/86Sr, δ13C and δ18O) record is examined in two Eastern Tethyan (Bulgarian) locations. This interval contains the well-known Early Toarcian ocean anoxic event (T-OAE) and its manifestation and temporal context is examined in Bulgaria. Many of the features characteristic for the SW European sections were identified: collapse of carbonate platform productivity at the Pliensbachian/Toarcian boundary, the T-OAE (a short pulse of anoxic deposition in the Falciferum ammonite Zone), an Early Toarcian rapid warming event seen in the belemnite δ18O record that peaked around the Falciferum/Bifrons ammonite zonal boundary. The long-recognized positive δ13C excursion in the late Falciferum ammonite Zone is also seen but a precursor, sharp δ13C negative excursion seen around the Tenuicostatum/Falciferum ammonite zonal boundary in many organic carbon records is not evident in the belemnite data, a curious absence noted from other belemnite records. Subsequent fluctuations of the 87Sr/86Sr, δ13C and δ18O suggest there may be a further perturbation of the global isotopic systems. On the other hand, belemnite Sr isotope values from Bulgaria are in accord with those seen in Western Europe and hence its value for chronostratigraphy.