Calcareous nannoplankton changes across the early Toarcian oceanic anoxic event in the western Tethys

Abstract

Calcareous nannoplankton were profoundly affected by environmental perturbations coincident with the early Toarcian oceanic anoxic event (T‐OAE). We quantify the abundance of nannofossils across the T‐OAE at three locations in Western Europe, where the event is marked by a prominent negative carbon isotope excursion (CIE). Data were treated by statistical analysis, and the Shannon diversity index was applied in order to describe nannofossil assemblage changes related to paleoenvironmental evolution. In the basal Toarcian, before the T‐OAE, high proportions of taxa with an affinity for low temperature (e.g., P. liasicus, T. patulus, Bussonius) occurred. This observation is consistent with interpretations of published oxygen isotope records. During the T‐OAE, the lowest abundance of nannofossils is observed, but there is a peak of a coccolith (Calyculus) from an organism that probably thrived in low‐saline surface waters depleted of nitrate. At the end of the perturbation, the lowest diversities of nannofossils occurred, and assemblages are dominated by Crepidolithus crassus, a deep dweller. This interval corresponded to progressive reoxygenation of deep water and the reoccupation of the deep photic zone by nannoplankton. The highest abundance of nannofossils is recorded above the CIE and testifies to the recovery of the entire nannoplankton community. The T‐OAE was widespread (perhaps global) and probably linked to major changes in the atmosphere and hydrosphere. However, the record of this event varies in the basins surrounding the western Tethys, suggesting regional imprints on the global signal. The regional variability may attest to establishment of effective connections between the Arctic and Tethys oceans, which allowed cool, low‐saline water formed at high latitudes to stream toward the western Tethys.