The Permian-Triassic extinction event was an extinction event that occurred approximately 252 million years ago (mya), at the boundary of the Permian and Triassic periods. It was the Earth's most severe extinction event, with about 90 percent of all marine species and 70 percent of terrestrial vertebrate species going extinct. For some time after the event, fungal species were the dominant form of terrestrial life.
At one time, this die-off was assumed to have been a gradual reduction over several million years. Now, however, it is commonly accepted that the event lasted less than a million years - from 252.3 to 251.4 mya (both numbers ±300,000 years) - geologically speaking, a very brief period of time. Organisms throughout the world, regardless of habitat, suffered similar rates of extinction, suggesting that the cause of the event was a global, not local, occurrence, and that it was sudden event, not a gradual change.
Many theories have been presented for the cause of the extinction, including plate tectonics, an impact event, a supernova, and extreme volcanism.
Plate tectonics
At the time of the Permian extinction, the continents had recently joined to form the super-continent Pangea and the super-ocean Panthalassa. This configuration radically decreased the range of shallow aquatic environments; many marine ecosystems would not have survived those changes. Pangea's formation would alter both oceanic circulation and atmospheric weather patterns. However, very gradual changes like continental drift alone probably could not cause the sudden, simultaneous destruction of both terrestrial and oceanic life.
Impact event
When large bolides (asteroids or comets) impact Earth, the aftermath weakens or kills much of the life that thrived previously. Release of debris and carbon dioxide into the atmosphere reduces the productivity of life and causes both global warming and ozone depletion. Evidence of increased levels of atmospheric carbon dioxide exist in the fossil record. Material from the Earth's mantle released during volcanic eruption has been shown to contain iridium, an element associated with meteorites. However there has been no trail of iridium and 'shocked quartz' such as the K-T event left behind, to be associated with the Permian event.
If an extraterrestrial impact triggered the Permian extinction event, scientists ask, where is the impact crater? Or have plate tectonics erased it during the last 252 million years? Geologist John Gorter of Agip Petroleum has found evidence of a circular structure 200 kilometers in diameter called the Bedout off the northwestern shore of Australia. The geology of the area dates to the end of the Permian; whether the Bedout is an impact crater is unclear.
It has also been proposed that such a collision might heat up ocean waters enough to produce "hypercanes," gigantic storms with winds possibly exceeding the speed of sound. Although not impossible, this theory has little supporting evidence.
Adrian Jones, University College of London, models the effects of impacts on the Earth's geological crust. After an impact, the crust rebounds to form a large shallow crater. Jones suggests that in a truly massive impact, the combined heat of the impact and rebound is enough to melt the crust. Lava floods through and the crater disappears beneath new crust. If Jones is right, the Permian meteorite crater can't be found because it doesn't exist.
Supernova
A supernova occurring within ten parsecs of Earth would produce enough gamma radiation to destroy the ozone layer for several years. The resulting direct ultra-violet radiation from the sun would weaken or kill nearly all existing species. Only those deep in the oceans would be unaffected. Statistical frequency of supernovae suggests that one at the P-T boundary would not be unlikely. While some sedimentary rock samples contain what may be records of short-term ozone destruction (large amounts of NOx gases and C14), this theory has little other evidence either for or against it.
The acid rain, global cooling, and other weather effects associated with enormous eruptions could have globally threatened life. Could volcanic activity over such a long time alter the climate enough to kill off 95% of life on Earth? Volcanic activity affects the concentration of atmospheric gases directly, and, indirectly, the oceanic dissolved gases. Increases in carbon dioxide enhance the greenhouse effect and cause global warming, which would reduce the temperature gradient between the equator and the poles. As a result, thermo-haline circulation would slow and eventually stop. The oceans would stagnate, and nutrients would fail to disperse themselves. Many marine ecosystems rely on upwelling and circulation of nutrients, oxygen included; without the regular circulation, organisms would starve or suffocate. In addition, sulfur and particulates contribute to cooling, or volcanic winter, which usually lasts three to six months. Combinations of the two effects could produce a cooling cycle in which the climate alternatively warms then cools. Such temperature fluctuations could cause connective overturn of the oceans, bringing anoxic bottom waters to the surface; in an already oxygen-deprived envirnoment, this would be fatal to many forms of life.
Significant evidence supports this theory. Fluctuations in air and water temperature are evident in the fossil record, and the uranium/thorium ratios of late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction. Numerous indicators of volcanic activity at the P-T boundary are present, though they are similar to bolide impact indicators, including iridium deposits. The volcanism theory has the advantage over the bolide theory, though, in that it is certain that an eruption of the Siberian Traps -- the largest known eruption in the history of Earth -- occurred at this time, while no direct evidence of bolide impact has been located.
The Permian extinction is unequalled; it is obviously not easy to destroy almost all life on Earth. The difficulty in imagining a single cause of such an event has led to an explanation humorously termed the "Murder on the Orient Express" theory (Wikipedia contains spoilers): they all did it.
Continental drift created a non-fatal but precariously balanced global environment, a supernova weakened the ozone layer, and then a large meteor impact triggered the eruption of the Siberian Traps.
There is no way to calculate the odds of such a coincidence, but for it to have occurred once in the four thousand million year history of Earth is not unbelievable.
Geologist Gerry Dickens suggested that the increased carbon-12 could have been rapidly released by upwellings of frozen methane hydrate from the seabeds. Experiments to assess how large a rise in deep sea temperature would be required to gasify solid methane hydrate suggested that a rise of 5°C would be sufficient. Released from the pressures of the ocean depths, methan hydrate expands to creat huge volumes of methane gas, one of the most powerful of the 'greenhouse gases.' The resulting additional 5°C rise in average temperatures would have been sufficient to kill off most of the life on earth.Volcanism
The P-T boundary was marked with many volcanic eruptions. In the 'Siberian traps,' now a sub-Arctic wilderness, over 200,000 square kilometers were covered in torrents of lava. The Siberian 'flood basalt eruption', the biggest volcanic effect on Earth, lasted for millions of years. Methane hydrate gasification
In 2002, a BBC2 'Horizons' documentary, 'The Day the Earth Almost Died' summarized some recent findings and speculation concerning the Permian extinction event.Paul Wignall examined Permian strata in Greenland and ascertained that he extinction lasted merely 80,000 years and showed three distinctive phases in the plant and animal fossils they contained. The extinction appeared to kill land and marine life selectively at different times. Such a process seemed too long, however, to be accounted for by a meteorite strike. His best clue was the carbon isotope balance in the rock, which showed an increase in carbon-12 over time. The standard explanation for such a spike - rotting vegetation - seemed insufficient.