Toarcian Oceanic Anoxic Event

The Toarcian extinction event, also called the Pliensbachian-Toarcian extinction event, the Early Toarcian mass extinction, the Early Toarcian palaeoenvironmental crisis, or the Jenkyns Event, was an extinction event that occurred during the early part of the Toarcian age, approximately 183 million years ago, during the Early Jurassic. The extinction event had two main pulses, the first being the Pliensbachian-Toarcian boundary event (PTo-E). The second, larger pulse, the Toarcian Oceanic Anoxic Event (TOAE), was a global oceanic anoxic event, representing possibly the most extreme case of widespread ocean deoxygenation in the entire Phanerozoic eon. In addition to the PTo-E and TOAE, there were multiple other, smaller extinction pulses within this span of time.

Occurring during the supergreenhouse climate of the Early Toarcian Thermal Maximum (ETTM), the Early Toarcian extinction was associated with large igneous province volcanism, considerable CO2 emissions which elevated global temperatures, acidified the oceans, increased continental weathering and nutrients fluxes (phosphate) towards the oceans, resulting in their eutrophication, which prompted the development of anoxia, leading to a severe biodiversity loss. The biogeochemical crisis is documented by a high amplitude negative carbon isotope excursions, as well as black shale deposition.

The subsurface at Gajum, in Canzo, Italy, is crucial for the study of the Toarcian age due to the extraordinary thickness (15.35 m) of the "fish level", a layer of dark, organic-rich sediments. Composed of pelagic marly limestones, marls, and marly mudstones rich in calcareous nannofossils, it records high-resolution climatic and cyclostratigraphic variations, making it possible to trace the crisis and recovery of marine ecosystems during this anoxic event.

Timing

The Early Toarcian extinction event occurred in two distinct pulses, with the first event being classified by some authors as its own event unrelated to the more extreme second event. The first, more recently identified pulse occurred during the mirabile subzone of the tenuicostatum ammonite zone, coinciding with a slight drop in oxygen concentrations and the beginning of warming following a late Pliensbachian cool period.