It is hardly contested these days that the massive Siberian Traps – the largest known continental flood basalt province – had something to do with the mass extinction at the Permian/Triassic boundary. Yet what actually produced sufficient, planet-wide environmental stress to slaughter up to 90% of all previously living species has not been pinned down. It was probably a combination of direct and indirect outcomes of the volcanic outpourings and several mechanisms have been suggested, such as: acid rain produced by SO₂ emissions from the magma; global warming as a result of volcanic CO₂ having accumulated in the atmosphere; a marked fall in the oxygen content of the atmosphere (see New twist for end-Permian extinctions in the May 2005 issue of EPN); increased phosphate fertilization of the oceans leading to anoxia and release of hydrogen sulfide gas.

Interestingly, the part of Siberia where the basalt floods took place is rich in coal measures and carbon-rich shales. Their thermal metamorphism by an overlying pile of lavas could conceivably have added huge amounts of CO₂ and methane to the atmosphere, creating strong greenhouse conditions: gas release from combustion and baking would have been almost instantaneous as each major flow came into contact with carbonaceous sediments. Yet direct evidence of widespread carbon combustion at the P/Tr boundary has not yet been demonstrated, although there are abundant gas-release structures in Siberia of around that age (Svenson, H. et al. 2009. Siberian gas venting and the end-Permian environmental crisis. Earth and Planetary Science Letters, v. 277, p. 490-500).

From a study of a near-continuous section of deep water marine sediments, whose ages range from Late Carboniferous to Cretaceous, something surprising has emerged. Silica-rich shales that span the P/Tr boundary show a major shift in d¹³C that matches the C-isotopic signature of the boundary elsewhere, and two lesser anomalies before the boundary event. At each C-isotope anomaly the shales also contain fly ash (Grasby, S.E. et al. 2011. Catastrophic dispersion of coal fly ash into oceans during the latest Permian extinction. Nature Geoscience, v. 4, p. 104-107), which forms today only in the rapid high-temperature combustion of coal in thermal power stations. It does not form from natural fires in underground and exposed coal seams that are caused by spontaneous combustion, usually ignited by rapid oxidation of pyrite in coal. The ash particles are smaller than 50 µm, and like similar sized, but denser, volcanic ash could easily be carried large distances. The Canadian team suggests that the fly ash formed when Siberian Trap basalts burned coals and organic-rich sediments, explosive release or explosive phases of the volcanism injecting them high in the atmosphere. Coal fly ash is not identifiable by normal microscopy, and its absence from the geological record may reflect that fact. Using organic petrography routinely on rocks from occurrences of the P/Tr and other boundary sequences should settle the matter.

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• Coal-fired trigger of mass extinction (nature.com)