Chemostratigraphy of Frasnian-Famennian transition: Possibility of methane hydrate dissociation leading to mass extinction
Published:January 01, 2007
Mohammad Hossein Mahmudy Gharaie, Ryo Matsumoto, Grzegorz Racki, Yoshitaka Kakuwa, 2007. "Chemostratigraphy of Frasnian-Famennian transition: Possibility of methane hydrate dissociation leading to mass extinction", Large Ecosystem Perturbations: Causes and Consequences, Simonetta Monechi, Rodolfo Coccioni, Michael Rampino
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Various scenarios have been proposed to explain the Late Devonian mass extinction, foremost among which are bolide impact and sea-level fall. We hereby propose a gas hydrate-induced model based on detailed geochemical and sedimentological data. The period of enhanced organic carbon burial in Iran, in south China, and in subpolar Urals corresponds to a brief negative δ13C excursion of 3.5‰ at the Frasnian-Famennian (F-F) transition. Prior to this event, oceanic δ13C increased for a period of several million years. However, major perturbations of the carbon geochemical cycle, and corresponding sharp and strong negative spikes of δ13C, which require a large input of isotopic light carbon into the ocean, also characterize the boundary horizons. Oxygen isotope ratios show negative excursions of 1.7‰ in south China and 4.1‰ in subpolar Urals that parallel the negative excursions in δ13C values. Synchronous negative spikes of δ18O are likely to imply a rapid increase of ocean temperature. We propose that the F-F boundary event was ultimately caused by voluminous and abrupt release of methane from marine gas hydrate into the ocean and atmosphere to trigger rapid global warming. Assuming that the total amount of inorganic carbon of the Devonian ocean was 40,000 gigatons (Gt) and δ13C of gas hydrate methane was −80‰, only 2600 Gt carbon from the total amount of 10,000 Gt gas hydrate carbon could have changed the oceanic δ13C values from +1‰ to −3‰, the observed magnitude of the F-F boundary excursion. Therefore only ∼26% of the gas hydrate could have triggered the boundary events. Widespread rift-related, basaltic volcanism along eastern Laurussia and northern Gondwana during the middle Late Devonian is believed to have contributed greatly to the global warming surrounding the F-F boundary, which in turn would have triggered massive dissociation of methane hydrate, especially if paired with intensive igneous and tectonic activity and rapid sea-level fall.