Paleoceanographic changes at the northern Tethyan margin during the Cenomanian–Turonian Oceanic Anoxic Event (OAE-2)

The late Cenomanian–early Turonian Oceanic Anoxic Event (OAE-2) represents major paleoceanographic and faunal perturbations. Samples from the northern Tethyan margin (Rehkogelgraben, Eastern Alps) were investigated in order to trace the paleoceanographic processes. Paleoecologic conditions were reconstructed by combining the results of assemblage counts of indicative microfossil groups (foraminifera, and radiolaria). Assemblages, size distributions and abundances show a tripartite subdivision for surface and bottom waters: 1) Oligotrophic surface conditions and oxic bottom waters with a reasonably high food supply for the late Cenomanian interval. 2) An OAE period with black shales characterized by very low numbers but high diversities and a lack of high-productivity indicators among planktic foraminifera. Low abundances of small sized benthic foraminifera indicate low oxic–dysoxic conditions at the seafloor. 3) Post-OAE assemblages are characterized by mesotrophic planktic species and benthic foraminifera suggest oxic bottom waters. It took about 300 ky to re-establish a pelagic carbonate-producing regime. The semi-enclosed basin situation of the Penninic Ocean is thought to be responsible for differences between the high productivity in the world ocean during the OAE-2 and the overall absence of high-productivity indicators and high foraminiferal diversities at Rehkogelgraben. The Penninic Ocean may have even served as a refuge during the environmental crisis.