structural:scholar_edition, quality:correction
Methane clathrate gasification

Methane clathrates, also known as methane hydrates, consist of molecules of methane trapped in the crystal lattice of ice. This methane, produced by methanogen microbes, has a C ⁄ C isotope ratio about 6% below normal (δ C −6.0%). At the right combination of pressure and temperature, clathrates form near the surface of permafrost and in large quantities on continental shelves and nearby seabed at water depths of at least 300 m (980 ft), buried in sediments up to 2,000 m (6,600 ft) below the sea floor.

Massive release of methane from these clathrates may have contributed to the PTME, as scientists have found worldwide evidence of a swift decrease of about 1% in the C ⁄ C ratio in carbonate rocks from the end-Permian. This is the first, largest, and fastest of a series of excursions (decreases and increases) in the ratio, until it abruptly stabilised in the middle Triassic, followed soon afterwards by the recovery of calcifying shelled sealife. The seabed probably contained methane hydrate deposits, and the lava caused the deposits to dissociate, releasing vast quantities of methane. A vast release of methane might cause significant global warming since methane is a very powerful greenhouse gas. Strong evidence suggests the global temperatures increased by about 6 °C (10.8 °F) near the equator and therefore by more at higher latitudes: a sharp decrease in oxygen isotope ratios (O ⁄ O); the extinction of Glossopteris flora (Glossopteris and plants that grew in the same areas), which needed a cold climate, with its replacement by floras typical of lower paleolatitudes. It was also suggested that a large-scale release of methane and other greenhouse gases from the ocean into the atmosphere was connected to the anoxic events and euxinic (sulfidic) events at the time, with the exact mechanism compared to the 1986 Lake Nyos disaster.

The clathrate hypothesis seemed the only proposed mechanism sufficient to cause a global 1% reduction in the C ⁄ C ratio. While a variety of factors may have contributed to the ratio drop, a 2002 review found most of them insufficient to account for the observed amount:

However, the clathrate hypothesis has also been criticized. Carbon-cycle models which include consideration of roasting carbonate sediments by volcanism confirm that it would have had enough effect to produce the observed reduction. Also, the pattern of isotope shifts expected to result from a massive release of methane does not match the patterns seen throughout the Early Triassic. Not only would such a cause require the release of five times as much methane as postulated for the PETM, but would it also have to be reburied at an unrealistically high rate to account for the rapid increases in the C ⁄ C ratio (episodes of high positive δ C) throughout the early Triassic before it was released several times again. The latest research suggests that greenhouse gas release during the extinction event was dominated by volcanic carbon dioxide, and while methane release had to have contributed, isotopic signatures show that thermogenic methane released from the Siberian Traps had consistently played a larger role than methane from clathrates and any other biogenic sources such as wetlands during the event.

Adding to the evidence against methane clathrate release as the central driver of warming, the main rapid warming event is also associated with marine transgression rather than regression; the former would not normally have initiated methane release, which would have instead required a decrease in pressure, something that would be generated by a retreat of shallow seas. The configuration of the world's landmasses into one supercontinent would also mean that the global gas hydrate reservoir was lower than today, further damaging the case for methane clathrate dissolution as a major cause of the carbon cycle disruption.