PALEOENVIRONMENTAL AND PALEOECOLOGICAL CONDITIONS LEADING UP TO THE END-TRIASSIC MASS EXTINCTION FROM THE NORTHERN CALCAREOUS ALPS, AUSTRIA

Upper Rhaetian strata of the Northern Calcareous Alps (NCA) in Austria record one of the most complete marine sedimentary records in the world of paleoenvironmental and paleoecological conditions leading up to the end-Triassic mass extinction (ETE). We conducted petrographic (a total of 94 thin sections) and macrofaunal (a total of 43 bulk samples) analysis of five key sections spanning the upper Rhaetian (upper Kössen Formation, Eiberg Member) in the NCA. The Kuhjoch, Eiberg, Juifen, and Schlossengraben sections were deposited across the intraplatform Eiberg Basin with the Restentalgraben site deposited on the terrigenous-influenced northern side of a small carbonate platform (“Oberrhaet Limestones”), near the northwestern margin of the Eiberg Basin. At the Restentalgraben section, the bed right below the initial extinction horizon is characterized by various bivalve, echinoderm, coral, brachiopod and gastropod fragments indicating the presence of a robust marine ecosystem within the shallow carbonate platform right before the ETE. In the basin center, deeper water environments are characterized by a less complex trophic marine ecosystem, mainly dominated by bivalves and brachiopods. At the deepest Eiberg section, the bed 30 cm below the initial extinction horizon records a bivalve assemblage with 1 choristoceratid ammonoid and is characterized by the highest functional diversity (5 modes of life) observed in the section. Overall, this study demonstrates the persistence of a robust marine benthic community across different facies and water depths right before the onset of the ETE in the NCA.

In the Tethys, the combination of reduced salinity, episodic suboxic conditions and ocean acidification caused the sudden ecological crisis during which the pre-extinction marine ecosystem collapsed allowing opportunistic paleoecommunities to occupy vacant niches. During the initial extinction phase, eurytopic opportunistic paleocommunities flourished followed by their demise during the main phase of the extinction as environmental conditions worsened due to CAMP volcanic activity. Documented ecological shifts in the Tethys region reveal the sudden tempo of ecological changes in the Tethys during the ETE in contrast to the more gradual nature of ecological shifts documented in the Panthalassic basin.