Phytoplankton evidence for the timing and correlation of palaeoceanographical changes during the early Toarcian oceanic anoxic event (Early Jurassic)

The causes and duration of the early Toarcian anoxic event are controversial. Integration of data from calcareous and organic phytoplankton provides a biochronological framework that allows precise correlations across the western Tethys. In particular, the Carinolithus superbus nannofossil Zone can be used to correlate the levels enriched in organic matter and the related δ¹³C negative excursion. Although a variable duration is given in the literature for this negative excursion, it is likely that it lasted between 520 and 650 ka. Increased atmospheric pCO₂, related to excess volcanic emissions (magmatic activity in the Karoo and Ferrar Provinces), had an impact on climate and ocean chemistry, and marked the inception of a biotic crisis affecting many organisms. The beginning of the crisis within shallow carbonate platforms, documented at southern latitudes, predates the levels enriched in organic matter. Dinoflagellate cysts experienced a decrease in abundance in the C. superbus Zone, until they temporarily disappeared. The nannoplankton crisis was twofold: a decrease in size and low calcified specimens are observed in addition to a drastic decrease in absolute abundance. The increased atmospheric pCO₂, as a result of the magmatic activity and temporarily amplified by transient methane release, could have been the trigger for the biocalcification crisis, which first affected the probably more reactive neritic system, and eventually the nannoplankton community.