Pulsed biogenic methane emissions coupled with episodic warming during the Toarcian Oceanic Anoxic Event

Reconstructing carbon release fluxes during ancient climatic warming events is important for improving predictions of carbon cycle and climate dynamics under future anthropogenic warming scenarios. We investigate the extent of biogenic methane release and its contribution to climate variability across the Toarcian Oceanic Anoxic Event (T-OAE) approximately 183 million years ago. To do this, we developed a global biogeochemical model and applied a Bayesian inversion using Markov Chain Monte Carlo (MCMC) simulations. Based on a high-resolution record of carbon isotope excursions from the Yorkshire section, our results indicate that a release of at least 4,700 Gt carbon from biogenic CH₄ (with a carbon isotopic composition, δ¹³C of -50 to -70‰) is necessary to accurately reproduce the pronounced pulsed shift in the δ¹³C, as well as the inferred changes in atmospheric pCO₂ and global temperature. This massive methane release may have led to a substantial increase in atmospheric pCH₄ and contributed to additional global surface warming, perhaps by more than 2 °C. We further elucidate that the liberation of methane may have been facilitated by an upsurge in methanogenesis alongside a concomitant decline in methane oxidation within organic-rich, sulfate-depleted marine environments. An active CH₄ cycle represents a positive feedback mechanism that exacerbates environmental deterioration during climatic warming events, ultimately contributing to mass extinction of marine life.