Wednesday 2 April 2014

The cause of the end-Permian extinction.

The Earth has been dominated by multi-cellular life forms (animals, plants etc.) for a little over half a billion years, but life has not had it easy throughout all of this time, it has been hit by a number of mass extinction events, during each of which a large proportion of the life-forms existent went suddenly extinct. The best known of these is the extinction event at the end of the Cretaceous, which included the extinction of the non-avian dinosaurs and other large Mesozoic animals, but the biggest extinction event occurred at the end of the Permian, when roughly 96% of all marine and 70% of all terrestrial life was wiped out. The end-Permian extinction is sometimes known as the Great Extinction or the Great Dying.

The cause of the end-Permian extinction has been for several years linked to extensive volcanism in the Siberian Traps, which is thought to have led to a runaway greenhouse effect, causing rapid global warming and ocean anoxia, which in turn led to the dying out of first the majority of marine life, and then, as the carbon cycle broke down, the extinction of the majority of life on land. However the amount of carbon dioxide likely to have been released by the Siberian Traps volcanism seems unlikely to have been sufficiently high to cause global warming on the scale seen at the end of the Permian, which has led scientists to seek another source of CO₂. To this end it has been proposed that extensive coal deposits existed beneath Siberia at the time of the volcanism, and that these were destroyed in a series of raging fires, further raising atmospheric CO₂ levels. However even this model struggles to create the level of global warming seen at the end of the Permian, and furthermore is difficult to see how such fires could have burned for the lengths of time over which the warming occurred.

The extent of the Siberian Traps volcanism. MIT.

In a paper published in the Proceedings of the National Academy of Sciences of the United States of America on 31 March 2014, a team of scientists led by Daniel Rothman of the Lorenz Center and Department of Earth, Atmospheric, and Planetary Sciences at the Massachusetts Institute of Technology describe the results of a study of carbon isotope data on two rock sequences dating from the end of the Permian, and propose a new hypothesis for the extinction based upon this.

Rothman et al. studies rocks from Meishan in Sichuan Province, China and Gartnerkofel in the Carnic Alps of Austria. In both places they found that the proportion of isotopically heavy 'inorganic' carbon rose sharply compared to the proportion of isotopically light 'organic' carbon in the rocks as the end of the Permian was reached (biological organisms preferentially use isotopically light carbon, hence such carbon is often referred to as 'organic', and sudden changes in the proportion of heavy and light carbon isotopes are generally assumed to be linked to the activities of such organisms). 

The sudden release of carbon from coal deposits in Siberia would be expected to produce mainly isotopically light carbon, as this would have been used preferentially by the plants which went on to form the coal, but instead the reverse seemed to be happening; as the rocks neared the end of the Permian, the proportion of isotopically light 'organic' carbon fell, requiring a different explanation for the origin of the carbon.

Rothman et al. further note that methanogenesis in Bacteria (a process by which some bacteria can respire carbon dioxide, producing methane as a waste product, thereby surviving without free oxygen) has recently been theorized to have arisen first in Clostridia-type Bacteria in the Middle Ordovician, and spread to the widespread Archean group Methanosarcina some time since then by horizontal gene transfer (a process by which distantly related bacterial groups are able to exchange genes, sometimes known as 'bacterial sex', but in many ways more akin to trade).

A colony of Methanosarcina-type Archeans. Daily Tech/KRLE

Rothman et al. carried out a phylogenetic analysis of Methanosarcina-type Archeans, from which they derived a molecular clock which suggested the group had undergone a major evolutionary radiation at the end of the Permian. They therefore suggest that this would have been the time at which they acquired the methanogenic genes.

In order to carry out methanogenesis, micro-organisms need a supply of nickel, which is used as a catalyst in the reaction. The rocks from Meishan show a spike in nickel levels towards the end of the Permian, which Rothman et al. suggest may have derived from the Siberian Traps volcanism.

They therefore theorize that towards the end of the Permian the Methanosarcina-type Archeans, acquired the genes for metabolizing carbon dioxide into methane, a far more powerful greenhouse gas. At the same time the Siberian Traps volcanism was supplying a ready supply of both carbon dioxide and nickel, allowing the group to undergo a major expansion. This led to a dramatic increase in the amount of methane in the atmosphere, leading to the runaway greenhouse effect that caused the mass extinction. Like all organisms, the Methanosarcina-type Archeans preferentially used isotopically lighter 'organic' carbon in their biological processes, leading to a reduction in the proportion of isotopically light CO₂ in the atmosphere, and therefore (eventually) the rock record (limestone is formed from a carbon dioxide dissolved in the sea, but not from methane).

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