The Permian-Triassic extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 252 million years ago, forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with about 96 percent of all marine species and 70 percent of terrestrial vertebrate species becoming extinct.
Scientists are still arguing why it happened, and the Siberian Traps have been one of the main suspect - although the causal mechanisms remain disputed.
Svensen et al. have come up with a new hypothesis in a paper titled “Siberian gas venting and the end-Permian environmental crisis”
, published in Earth and Planetary Science Letters
of 30 January 2009.
The hypothesis can be summarised in 3 phases:
1) Emplacement of magmatic intrusions (sills) into organic rich sediments and evaporites (252 million years ago)
2) Contact metamorphism lead to gas generation (transformation of organic carbon to gas like CO2
Cl) and over-pressure.
3) The gas was released to the atmosphere through enormous vertical pipes. The gas gave rise to global warming and destruction/depletion of the (life protecting) ozone layer.
* The original coverage with flood basalt may have been as high as 7 million km². The original volume of lava has been estimated to range from 1 to 4 million km³.
* Enormous volumes of Cambrian evaporites are present in the Tunguska Basin, with up to 2.5 km thick sequences of halite-rich strata, anhydrite, and carbonates. Furthermore the basin was petroleum-bearing prior to the end-Permian sill emplacement - with a large carbon gas production potential.
* More than 500 basaltic diatreme-like pipes are known in the northern parts of the Tunguska Basin, and the authors estimate that there is about a total of 6400 pipes in the basin. (A diatreme is a breccia filled volcanic pipe formed by a gaseous explosion)
The authors argue that the composition of the heated sedimentary rocks below the flood basalts is the most important factor in controlling whether a Large Igneous Province (like the Siberian Traps) causes an environmental crisis or not. They propose that a similar mechanism could have been responsible for the Triassic-Jurassic (~200 Ma) global warming and mass extinction, based on the presence of thick sill intrusions in the evaporite deposits of the Amazon Basin in Brazil. The end of Triassic mass extinction (about 200 million years ago) is the second largest known mass extinction event. The flood basalts of the Central Atlantic Magmatic Province (that might have released sufficient volumes of gasses for a mass extinction 200 million years ago) are difficult to explore with most of the clues hidden under the Amazon rain forest.
The second biggest known extinction at the Triassic–Jurassic was contemporaneous with the formation of the Central Atlantic Magmatic Province in the eastern US, South America, and western Africa - during the breakup of Pangaea. Thick sills are common in this province, emplaced in thick Permian evaporite deposits in the Amazon Basin in Brazil. Here too the authors propose that evaporite metamorphism caused generation and venting of carbon gases and halocarbons to the atmosphere, leading to global warming and atmospheric ozone depletion. The Amazon Basin needs to be investigated in detail to lend support to this hypothesis. To the authors the fact that the two biggest mass extinctions the last 252 million years occurred at the same time as two major sill emplacement events into evaporite basins, seems to be too much of a coincidence.Reference
Henrik Svensen et al.Siberian gas venting and the end-Permian environmental crisis
Earth and Planetary Science Letters 277 (2009) 490–500.