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The Toarcian Oceanic Anoxic Event (T-OAE), a major environmental upheaval occurring approximately 183 million years ago during the Mesozoic Era, stands as one of the most severe perturbations to Earth's carbon cycle in geological history.

The T-OAE was characterized by a rapid global temperature rise of roughly 6° C, widespread marine extinction, and a negative carbon isotope excursion exceeding 6%, reflecting a massive release of ¹²C-rich carbon—from sources including methane or CO₂ derived from organic matter—into Earth's atmosphere and oceans. Studying the T-OAE, scientists have long posed the critical question: What were the dominant carbon sources and feedback mechanisms that triggered it?

Two mechanisms have been widely cited as potential primary drivers: volcanic activity from large igneous provinces and the release of thermogenic methane via contact metamorphism. Yet a key limitation persists: These processes alone cannot fully explain the globally synchronous, pulsed negative carbon isotope excursions preserved in the stratigraphic record, leaving a gap in scientific understanding.

A study in Proceedings of the National Academy of Sciences now offers new insights into the T-OAE's origins.

Led by Prof. Jin Zhijun, a member of the Chinese Academy of Sciences (CAS), and Prof. Zhao Mingyu from the Institute of Geology and Geophysics of CAS, an international research team has developed a global biogeochemical inversion model. This innovative model explicitly links methane cycling across three critical reservoirs: sediments, the ocean, and the atmosphere.

By integrating multi-proxy geological datasets with a Bayesian inversion framework—a statistical tool that refines predictions by incorporating uncertainty—the researchers quantified both greenhouse gas fluxes and their isotopic signatures during the T-OAE period. The findings challenge previous assumptions about the event's drivers.

Notably, the model demonstrates that large-scale pulses of biogenic methane—methane produced by microbial activity—were essential to reproducing the pulsed carbon isotope anomalies observed in geological records. The scale of these pulses is striking: Each individual methane release exceeded the total amount of anthropogenic carbon emissions since the Industrial Revolution. Over the entire T-OAE, the total volume of carbon released during these methane pulses approached the amount of carbon currently stored in all the world's oil and gas reserves.

The study further identifies the conditions that fueled this massive microbial methanogenesis: reduced sulfate availability in Early Jurassic oceans and enhanced burial of organic matter. Both factors created an environment in which methane-producing microbes thrived, amplifying methane release.

This methane release had cascading effects on Earth's climate and ecosystems. Methane's potent radiative forcing—its ability to trap heat in the atmosphere—amplified global warming by more than 2° C during the event. This temperature spike in turn drove further ocean deoxygenation, a hallmark of the T-OAE. Significantly, these climatic and oceanographic changes coincided with pulses of marine biodiversity loss, indicating that episodic methane release acted as a critical driver of both climatic instability and extinctions during the event.

This study highlights pulsed biogenic methane emissions as the dominant factor controlling the timing and severity of the T-OAE. It underscores the Earth system's vulnerability to rapid carbon release and feedback-driven climate change, offering valuable context for understanding present and future climate risks, the researchers noted.

The study was conducted in collaboration with researchers from Peking University, the University of Leeds, the University of Waikato, and other institutions.