Early Jurassic (Pliensbachian-Toarcian) extinction event

The early Jurassic Oceanic Anoxic Event (Pliensbachian-Toarcian is one of the most prominent and best recorded Mesozoic OAEs and witnessed dramatic changes in climate and ocean circulation together with an episode of mass extinction of various marine organisms. The event was characterized by the global accumulation of organic-rich sediments such as black shales along with evidence for warming, changes in the carbon dioxide levels, variations in seawater levels and periods of ocean anoxia. The generally accepted trigger for these major environmental perturbations is volcanic activity in the large igneous Karoo–Ferrar province and the characteristic carbon isotope anomaly, associated with the ocean anoxic event, is interpreted as the result of a massive injection of isotopically light carbon into the ocean/atmosphere either from methane gas hydrates or from the thermal metamorphism of sediments. Such a major increase in ocean/atmosphere CO2 also had the potential to cause changes in the pH of the ocean, and hence transient ocean acidification triggered by the emplacement of the large igneous province (Karoo-Ferrar) has also been argued for the Early Jurassic Toarcian anoxic event. Evidence for ocean acidification is based on widespread reef collapse that was particularly selective against corals and hypercalcifying sponges, a decrease in nannoplankton flux and geochemically distinctive fibrous calcite layers potentially reflecting an acidified water column. Given the number of major environmental perturbations associated with the Early Jurassic Toarcian Ocean Anoxic Event, it is however very difficult or even impossible to unambiguously attribute these biogeochemical changes to changes in the pH of the ocean. In contrast to the Permo-Triassic boundary, there are no B isotope data from Early Jurassic marine carbonates published and hence no direct seawater pH proxy information is to date available.