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 δ 13 C excursion of 3.5‰ at the Frasnian-Famennian (F-F) transition. Prior to this event, oceanic δ 13 C increased for a period of several million years. However, major perturbations of the carbon geochemical cycle, and corresponding sharp and strong negative spikes of δ 13 C, 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 δ 13 C values. Synchronous negative spikes of δ 18 O 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 δ 13 C 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 δ 13 C 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.

Abstract: The uppermost Middle to lowermost Upper Ordovician succession of interbedded radiolarian chert and shales exposed at Seal Creek in southeastern Australia contains a diverse, abundant, and well-preserved trace-fossil assemblage that inhabited pelagic, deep-oceanic sediments on an abandoned section of a submarine fan complex. Trace fossils occur throughout the measured section, but the types and abundance of the trace fossils vary from horizon to horizon. Planolites and Palaeophycus are common in all chert beds, while Zoophycos, Alcyonidiopsis, Compaginatichnus-like, and Teichichnus-like trace fossils are limited to the upper part of the succession. Shales interbedded with the cherts have a much less diverse and abundant ichnofauna than the siliceous layers. The appearance of Zoophycos on the deep-ocean floor in the earliest Late Ordovician may represent the first appearance of this ichnogenus in the pelagic realm, as benthic fauna migrated from shallow to deep water. The later emergence of Alcyonidiopsis at Seal Creek than in turbiditic mudstones elsewhere in the world perhaps reflects benthic migration from turbiditic to radiolarian chert facies. Similar chert facies in the Triassic-Jurassic succession of southwest Japan lack the common deep-sea ichnogenus Zoophycos but have peculiar zigzag-shaped burrows that are absent in the Seal Creek chert. This difference in ichnofauna may be an aftereffect of the mass extinction event at the Permian-Triassic boundary.