A review of tempo and scale of the early Jurassic Toarcian OAE: implications for carbon cycle and sea level variations

During the Early Toarcian of the Jurassic Period (similar to 182 million years ago), Earth experienced a rapid and severe disturbance in the global carbon cycle associated with global warming, and widespread oceanic anoxia, known as the Toarcian Oceanic Anoxic Event (T-OAE). The carbon cycle perturbation is recorded by a pronounced negative (similar to-6%0) carbon isotope (delta C-13) excursion (CIE) in organic and inorganic marine sediments. Here we show that the CIE occurs as an abrupt and short-lived spike within a long-term delta C-13 cycling. The duration of the CIE has been compared in three key stratigraphic sections (Peniche, Sancerre and Yorkshire) and revised to be 300 kyr to 500 kyr, from which an obliquity origin may be inferred for strong delta C-13 cycles that occur in the initial, decreasing part of the CIE. The most popular hypothesis for the cause of CIE is rapid and massive injection of C-12 into the ocean and atmosphere/hydrosphere reservoirs from volcanism or methane hydrates. The well-expressed delta C-13 obliquity-scale cycles and their predominantly symmetric nature throughout the decreasing part of the CIE (nearly 240 kyr) favor the release of volcanic greenhouse gases. We posit that the dominance of the delta C-13 obliquity-scale cycles is the consequence of lengthened productivity seasons induced by the global warming from the Karoo-Ferrar LIP. The effect of Toarcian global warming on productivity and organic phenology bears similarity to present-day global warming and reports of a longer growing season worldwide. Cyclostratigraphic study of recently acquired detrital proxy data in the Sancerre section (Paris Basin) provides evidence for low sea-level conditions at the Pliensbachian Toarcian transition followed by high sea-level conditions at the initiation of the CIE. These results are similar to those obtained from other basins, hence pointing to the global nature of the sea-level change inferred from the Sancerre site. Finally, a prominent obliquity related detrital-input cyclicity was detected within the Pliensbachian Toarcian transition, in particular during the latest Pliensbachian, hinting at glacioeustatically driven sea-level change.