Euxinic conditions and high sulfur burial near the European shelf margin (Pieniny Klippen Belt, Slovakia) during the Toarcian oceanic anoxic event

The Toarcian oceanic anoxic event (T-OAE; 183 Ma) was accompanied by severe biotic and geochemical perturbations that are considered as some of the most severe of the Mesozoic era. Nevertheless, paleoceanographic conditions during the T-OAE in the Tethyan domain remain poorly understood owing to the relatively low number of marine successions from this area investigated so far. In this study, we report new geochemical, palynological and pyrite framboid size data from Zazriva, northwestern Slovakia, a Lower Toarcian marine succession recording organic-rich sedimentation at the margin of the NW-European shelf. The lowermost Serpentinum ammonite Zone records high total organic carbon (TOC) contents (2 to 5 wt%) and very low organic carbon isotope values (from -30 to -32 parts per thousand) characteristic of the T-OAE negative carbon isotope excursion in coeval sites. The size of the pyrite framboids, the high S contents independent on TOC, low sulfur isotope values (delta S-34 < -15 parts per thousand) and predominance of amorphous fraction in palynofacies indicate that the deposition of this interval took place under sulphidic and anoxic conditions (euxinia) interrupted by brief events of improved oxygenation. The total sulfur contents are exceptionally high (4-12 wt%), a feature that can be explained by enhanced syngenetic pyrite formation due to the combination of euxinic conditions and high export of reactive iron from suboxic porewaters of adjacent shallower marginal sediments. These results suggest that basinal successions of the European shelf margin may have accumulated considerable amounts of sulfur and carbon during the T-OAE, and corroborate the view that small basins may have played a disproportionally large role in regulating biogeochemical cycles during past events of environmental perturbations.