Pulsed biogenic methane emissions and episodic carbon cycle perturbations during the Toarcian Oceanic Anoxic Event

Reconstructing carbon release fluxes during extreme climatic events in Earth history—particularly quantifying the magnitude and climatic impacts of biogenic greenhouse-gas emissions—is crucial for building high-confidence “past–future” climate analog frameworks. In paleoclimate research, the Toarcian Oceanic Anoxic Event (T-OAE; ~183 Ma), one of the most prominent global warming episodes of the Mesozoic, still features key knowledge gaps regarding the coupled mechanisms linking its carbon-isotope excursions (CIEs) to greenhouse-gas release. Here we integrate multi-proxy constraints to develop a global coupled biogeochemical model that explicitly represents methane cycling across the sediment–ocean–atmosphere system, and we apply a Markov chain Monte Carlo (MCMC) Bayesian inversion to systematically quantify methane emission fluxes during the T-OAE for the first time. Model simulations indicate that reproducing the pulsed negative CIEs, the rise in atmospheric pCO₂, and the 4–6 °C global warming inferred from paleotemperature proxies requires at least ~4700 Gt (CO₂-equivalent) of sustained biogenic methane input to the Earth’s surface system. Notably, the inferred carbon-isotopic composition of the methane (δ¹³C = −50‰ to −70‰) closely matches the characteristic fractionation associated with methanogenic archaeal metabolisms. The model further suggests that methane release may have amplified methanogenesis and increased organic-matter input, while sulfate-depleted ocean conditions reduced methane oxidation, together establishing a positive feedback of “enhanced methane production–suppressed oxidation efficiency.” Sensitivity experiments show that methane emissions of this magnitude could drive an atmospheric pCH₄ increase of >5 ppm, producing additional radiative forcing sufficient to yield ≥2 °C extra surface warming. Moreover, oceanic methane release promotes a millennial-scale decline in dissolved oxygen, triggering systemic collapse of benthic habitats. This nonlinear coupling between biogeochemical cycling and ecosystem responses may have been a key driver of widespread marine biotic losses during the T-OAE.