Continental weathering and redox conditions during the early Toarcian Oceanic Anoxic Event in the northwestern Tethys: Insight from the Posidonia Shale section in the Swiss Jura Mountains

The geochemistry and mineralogy of organic-rich sediments of the Rietheim succession in northern Switzerland were studied to evaluate the main impacts of the early Toarcian Oceanic Anoxic Event (T-OAE; ~183Ma ago) on the depositional environment in the NW Tethys Ocean. The geochemical data obtained (notably, δ13Corg, δ13CCaCO3, Rock-Eval pyrolysis, TOCcff) support the hypothesis that the negative C-isotope excursion at the onset of the T-OAE mainly reflects a major perturbation in the global carbon cycle rather than local environmental changes (e.g., basinal restriction, productivity). Rock-Eval pyrolysis and δ13Corg-δ15Ntotal crossplots indicate that the organic matter was primarily of marine origin during the T-OAE, and deposited in an epicontinental sea setting. Detrital proxies [notably, detritus index, siliciclastic balance and Ln(Al2O3/Na2O)] suggest that the elevated pCO2 levels and greenhouse climate prevailing during the T-OAE induced significant acceleration of the hydrologic cycle and an increase in continental chemical weathering rates. Redox-sensitive proxies [relative pyrite contents, organic and sulfide balances, V/(V+Ni) ratios, trace elements enrichment factors, trace-element-TOC co-variation patterns and TOC/Ptotal molar ratios] indicate that the Rietheim Posidonia Shale sediments accumulated under oxic-to-dysoxic bottom-water conditions, with more reducing conditions during the T-OAE (anoxic and possibly euxinic). These changes in redox conditions were probably induced by substantial thermohaline stratification driven by enhanced freshwater input and the recurrent influx of brackish Arctic seawater into the NW Tethys through the Viking Corridor, in agreement with previous sedimentological and geochemical evidences from the NW Tethys Ocean. In the studied succession, such conditions were perhaps also responsible for sequestration of most of the regenerated P in the deeper water column by adsorption and/or co-precipitation into authigenic phases. Overall, the major palaeoenvironmental changes documented in this study provide a robust example of Earth’s possible responses to the rapid onset of extreme greenhouse conditions. •The T-OAE negative CIE reflects a 13C-depleted carbon injection to ocean–atmosphere.•Detrital proxies suggest an increase in continental weathering rates during the T-OAE.•Redox proxies suggest moderate anoxic bottom-water conditions during the T-OAE.•Dysoxic-to-anoxic conditions prevailed until at least to the bifrons