Biostratigraphic constraints of the Early Toarcian Oceanic Anoxic Event: new data from calcareous nannofossil investigations of Boreal and Tethyan sections

Abstract

Calcareous nannofossil biostratigraphy was carried out in Upper Pliensbachian – Lower Toarcian sediments, which cover the Toarcian Oceanic Anoxic Event (T-OAE) interval. In particular, semiquantitative analyses were performed on a total of 156 samples in the composite Sogno Core (Lombardy Basin, Southern Alps) representing a pelagic Tethyan section. Quantitative investigations were applied to additional 168 samples across the Amaltheenton Fm. and Posidonienschiefer Fm., from two cores of the Boreal Realm (Lower Saxony Basin, northern Germany). Primary and secondary events of the Tethyan and Boreal zonations were recognized, allowing the identifications of the NJT5, NJT6 nannofossil Zones for the Sogno Core and the NJ5, NJ6, NJ7 Zones for the German sections, respectively. The sequence of nannofossil biohorizons is generally consistent with data available for various areas at lower and higher latitudes, confirming their reproducibility and reliability for intra and inter-regional correlations. Geochemistry evidences the presence of the negative C isotopic excursion across the “Fish Level” black shale interval expression of the T-OAE in the Sogno Core. The same anomaly is recorded in the German successions at the base of the Posidonia Shale witnessing the passage from well oxygenated to predominantly anoxic conditions. Our results show that the T-OAE C isotopic excursion recorded in the Sogno Core is excellently constrained by the first occurrence (FO) of Carinolithus superbus at the onset and the last occurrence (LO) of Mitrolithus jansae at the end. A significant decrease in abundance and size of Schizosphaerella punctulata (the “S. punctulata crisis”) and an abundance drop of M. jansae further characterise the T-OAE perturbation. Only S. punctulata shows a recovery at the end of the T-OAE, while M. jansae barely survived the palaeoenviromental stress and disappeared soon after its termination. The extreme rareness of S. punctulata and the absence of M. jansae in the Boreal Realm prevent the recognition of the “S. punctulata crisis” and the M. jansae decline. Our study reveals the LO of Biscutum finchii together with the FO of C. superbus as an additional event approximating the onset of the C isotopic excursion exclusively in the German successions. Further events, such as the LOs of Biscutum grandis, Crepidolithus granulatus and Parhabdolithus liasicus are detected within the C isotopic anomaly exclusively in the German sections. Nannofossil biostratigraphy permits the effective dating and correlating of Early Jurassic major palaeoceanographic events and particularly of the T-OAE which are of a great importance to derive a definitive model for the Posidonia Shale deposition.