REEXAMINING THE CARBON CYCLE DURING THE TOARCIAN OCEANIC ANOXIC EVENT

The Toarcian Oceanic Anoxic Event (T-OAE) of the Early Jurassic was a severe environmental perturbation associated with significant changes in global climate and biogeochemical cycling. Among these changes was a major perturbation to the global carbon cycle that is expressed as a large (up to 7‰), negative carbon isotope excursion (CIE) found in sedimentary successions worldwide. Conceptual models have linked this carbon cycle perturbation to the massive injection of greenhouse gases into the ocean-atmosphere system from volcanism associated with the Karoo-Ferrar Large Igneous Province (LIP). High-resolution carbon isotope records also display higher order CIEs superimposed on the falling limb of the broader Toarcian CIE. These small-scale CIEs have been suggested to reflect methane clathrate destabilization events or the pulsed release of methane sourced from contact metamorphism associated with the Karoo-Ferrar LIP. However, uncertainty surrounds efficacy of these proposed drivers in explaining the Toarcian carbon isotope record.

We will present a numerical modeling study of the Toarcian carbon cycle, which evaluates the various scenarios for the generation of the CIEs associated with T-OAE. Previous numerical modeling efforts that explored the Toarcian carbon cycle were hampered by uncertainty surrounding various aspects of carbon isotope record, namely the duration and magnitude of the overall CIE. Fortunately, work over the past decade has placed better constraints on these aspects of the Toarcian carbon isotope record, and our new modeling study leverages these new data. With our model, we also test the role of a previously unconsidered feedback within carbon cycle: terrestrial methanogenesis. Based on our modeling exercises, we propose that the production and release of methane from terrestrial environments (wetlands, lakes, estuaries, etc.) as a viable alternative scenario for the generation of the observed high-frequency CIEs. Importantly, the inclusion of terrestrial methanogenesis in the Toarcian carbon cycle lessens the amount of carbon needed from other potential carbon sources in order to generate the overall CIE. More broadly, our study suggests enhanced wetland methanogenesis may be an important and underappreciated climatic feedback during other Oceanic Anoxic Events.