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On the nature of methane gas-hydrate dissociation during the Toarcian and Aptian Oceanic anoxic events

^* Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom
^** Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, United Kingdom

The magnitude and timing of a major rapid negative carbon-isotope excursion recorded in marine and terrestrial matter through the Early Toarcian (Early Jurassic) and Early Aptian (Early Cretaceous) oceanic anoxic events (OAEs) have been proposed to be the result of large methane gas-hydrate dissociation events. Here, we develop and evaluate a global carbon-isotope mass-balance approach for determining the responses of each component of the exogenic carbon cycle (terrestrial biosphere, atmosphere and ocean). The approach includes a dynamic response of the terrestrial carbon cycle to methane-related CO₂ increases and climatic warming. Our analyses support the idea that both the Early Toarcian and Early Aptian isotopic curves were indicative of large episodic methane releases (5000 and 3000 Gt respectively) promoting warm ‘greenhouse’ conditions in the Mesozoic. These events are calculated to have increased the atmospheric CO₂ concentration by 900 and 600 ppmv respectively and land surface temperatures by 2.5° to 3.0°C. However, we show that much of the methane released from oceanic sediments is rapidly sequestered by terrestrial and marine components in the global carbon cycle, and this effect strongly attenuated the potential for ancient methane gas-hydrate dissociation events to act as major amplifiers in global warming. An increase in oceanic carbon sequestration is consistent with the deposition of globally distributed black shales during these OAEs. Our analyses point to the urgent need for high-resolution marine and terrestrial carbon-isotope records to better characterize the nature of the Toarcian and Aptian events and improve our interpretation of their consequences for the global carbon cycle.

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