Mechanisms and Effects of the Toarcian Oceanwide Anoxic Event

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Geologic events help understanding geobiosphere evolution and serve predicting future development. The first critical Mesozoic geoevent in the Toarcian (T-OAE) is characterized by mass extinctions, oceanwide anoxia, and C-isotope excursions indicating perturbations of atmospheric and marine biochemical cycles. Trigger mechanisms are heavily disputed and the spatial extent of the T-OAE, either restricted to the NW-Tethys or of global nature is under debate. Key factors comprise release of gas hydrates, Karoo volcanism and anoxia caused by global transgression. Only in the Yorkshire Basin a tripartite isotope excursion linked to three extinction events could yet be proven. Here, molecular biomarker study, revealing relations between biotic changes, redox milieu and isotope shift, is limited by advanced thermal maturity. In the SW-German Basin, where detailed molecular geochemistry is feasible, a high-resolution C-isotope approach is excluded by methanogenesis affecting the C-isotope cycle. On the edge of the Paris Basin a section is available in Luxembourg, for high-resolution molecular biomarker and C-isotope dual study. Here, lag times between biotic or redox shifts and isotope response or vice versa can be studied at unprecedented resolution. Ocean acidification by massive CO2 emission upon the T-OAE seems to have caused coccolith corrosion. The global character of the T-OAE will be studied by molecular geochemistry via a section from the Pacific Queen Charlotte Island revealing a pronounced C-isotope excursion. This dual methodological and geospatial approach offers the ultimate potential of unraveling the nature of the T-OAE.