Disparity between Toarcian Oceanic Anoxic Event and Toarcian carbon isotope excursion

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE, Early Jurassic) is marked by widespread marine deoxygenation and deposition of organic carbon (OC)-rich strata. The genesis of the T-OAE is thought to be associated with environmental changes caused by the emission of ¹²C-enriched greenhouse gasses (CO₂, CH₄), manifested in a negative Toarcian carbon isotope excursion (nT-CIE). The nT-CIE is commonly used to stratigraphically define the T-OAE, and despite the complex interrelationship of the different environmental phenomena, both terms (nT-CIE and T-OAE) are commonly used interchangeable. We here demonstrate that occurrence of OC-rich strata is diachronous and not restricted to the nT-CIE, reflecting the interaction of global- and regional-scale processes. Thus, the interchangeable use of T-OAE and nT-CIE should be discarded. The nT-CIE, however, hosts the T-OAE climax, marked by the widest extent of OC-rich strata. Early Toarcian environmental changes, particularly sea level rise and rising temperatures, may have made marine areas more susceptible to develop oxygen deficient conditions, favoring OC-accumulation.

Graphical abstract