Heavy breathing in the Permian

Isotopic evidence suggests global warming may be the key to understanding mass extinctions, reports Sarah Day

Geoscientist Online Thursday 11 September 2008

Carbon isotope fractionation provides vital evidence in understanding what brought about the end-Permian mass extinction, according to research presented yesterday at the BA by Professor Paul Wignall (University of Leeds).

The end-Permian mass extinction took place 250 million years ago and wiped out 95% of the species on the planet. The extinctions took place at about the same time as the eruption of the Siberian Traps, during which an estimated 3-5 million cubic kilometres of basalt were extruded in a relatively short time.

Isotope signatures of Carbon 12 and 13 (¹²C, ¹³C) show wild fluctuations at this time, with the ratios in limestones (taken as a proxy for oceanic chemistry) becoming considerably “lighter”, i.e., rich in ¹²C. The high ratios of ¹²C can be explained in two ways. Either, due to the mass extinction, the biosphere had become greatly reduced, meaning that there were fewer organisms extracting the lighter isotopes, or there was a worldwide regression that exposed preserved organic carbon (also rich in the light isotope) to weathering and erosion.

Professor Wignall told the BA that it was already well established that sea levels rose, rather than fell, at the end of the Permian.

Both the widespread transgression and the mass extinction suggested, said Professor Wignall, may therefore have been a consequence of global warming. Firstly, he argues, multiple methane release events from the oceans into the atmosphere would have exacerbated the negative shift. Second, the eruption of the Siberian Traps would also have released significant amounts of light CO₂ into the atmosphere.

Both mechanisms reinforce the model of an end-Permian mass extinction that was closely related to the release of greenhouse gases. Such a link, said Professor Wignall, provided a mechanism for explaining the frequent coincidence in the geological record of mass extinction events and the eruption of Large Igneous Provinces.

Professor Wignall was speaking at the session “Isotopes – silent witnesses to Earth history” co-organised by the Society.