Bio- and chemostratigraphy of the Toarcian organic-rich deposits of some key successions of the Alpine Tethys

The Pliensbachian Toarcian (Jurassic) successions of the Alpine Tethys contain widespread organic -rich strata that may represent some of the deepest water records of the Toarcian Oceanic Anoxic Event (T-OAE). Most of these successions, however, have a poor ammonite record that precludes detailed correlation with coeval, ammonite -rich sites of NW Europe, a circumstance that casts doubt on their age and potential significance for the T-OAE. For instance, it has been repeatedly suggested that the main phase of organic -rich deposition in some of these basins started as early as the Late Pliensbachian, i.e. several hundreds kyrs before that seen in adjacent NW European epicontinental areas, possibly as a result of older climate events or contrasting paleoceanographic conditions. In this study, we present a reexamination of some published ammonite material and new biostratigraphic data based on nannofossils and ammonites from Bachental (Austria) and Urktlt (Hungary) in order to improve the age determination of two key sections of the Alpine Tethys. Our re-examination shows that the ammonite material previously used to support a Pliensbachian age for the organic -rich shale at Bachental was misidentified. These specimens, combined with new ammonite and nannofossil material from this locality, clearly point to an Early Toarcian age (basal Serpentinum ammonite Zone) for the organic-rich strata. The organic- and manganese -rich (but ammonite-lean) strata of trktit have yielded nannofossils diagnostic of the NJ6 nannofossil zone and are thus equivalent in age to T-OAE strata of NW European sections (uppermost Tenuicostatum basal Serpentinum ammonite zones). According to our revised biostratigraphy and carbon -isotope correlations, reducing conditions developed near synchronously in basins of the Alpine Tethys and those of NW Europe, while their termination was delayed by several thousands kyrs in NW Europe. Our results also show that many basins of the Alpine Tethys experienced unusually high rates of sediment accumulation and manganese burial, in close temporal correlation with the severe carbon cycle and climateperturbations characterizing the T-OAE interval. The improved dating of these poorly studied sections therefore sets the stage for future detailed studies to further characterize the response of relatively deep-water environments to the T-OAE climate perturbations