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The Permian-Triassic (P-T or PT) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251 million years ago (mya), 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.

Contents 1 Duration of event 2 After the extinction event 3 Explanatory theories 3.1 Plate tectonics 3.2 Antarctic impact event 3.3 Supernova 3.4 Volcanism 3.5 Atmospheric hydrogen sulfide buildup 3.6 Methane hydrate gasification 3.7 A combination 4 References 5 External links 6 Other resources

[edit] Duration of event

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 Ma (both numbers ±300,000 years), a very brief period of time in geological terms. A detailed study of plutonium-to-lead decay in zircons in ash beds in China dates the extinction 252.6 ± 0.2 million years ago, synchronous with the Siberian flood volcanism, (Mundil 2004). Organisms throughout the world, regardless of habitat, suffered similar rates of extinction, showing that the event was a global, not local, occurrence, whatever its cause, and that it was a sudden event, not a gradual change. New evidence from strata in Greenland shows evidence of a double extinction, with a separate, less dramatic extinction occurring 9 Ma before the Permian-Triassic (P-T) boundary, at the end of the Guadalupian epoch. Confusion of these two events is likely to have influenced the early view that the extinction was extended.

[edit] After the extinction event

For some time after the event, fungal species were the dominant form of terrestrial life. Though they only made up approximately 10% of remains found before and just after the extinction horizon, fungal species subsequently grew rapidly to make up nearly 100% of the available fossil record.<ref name="eshet">Eshet, Y. et al. (1995) Fungal event and palynological record of ecological crisis and recovery across the Permian-Triassic boundary. Geology, 23, 967-970.</ref> However, some researchers argue that fungal species did not dominate terrestrial life, even though their remains have only been found in shallow marine deposits.<ref name="wignall">Wignall, P.B. et al. (1996) The timing of palaeoenvironmental changes at the Permo-Triassic (P/Tr) boundary using conodont biostratigraphy. Hist. Biol. 12, 39-62.</ref> Alternatively, others argue that fungal hypha are simply better suited for preservation and survival in the environment, creating an inaccurate representation of certain species in the fossil record.<ref name="erwin">Erwin, D.H. (1993) The Great Paleozoic Crisis: Life and Death in the Permian, Columbia University Press.</ref> The listrosaurus was the only large mammal to survive the event, becoming the most populous land animal on the planet for a time.<ref>Before the Dinosaurs, Discovery Channel</ref>

Analysis of a survey of marine fossils from the period showed a decrease in the abundance of sessile epifaunal suspension feeders, animals anchored to the ocean floor such as brachiopods and sea lilies, and an increase of more complex mobile species such as snails, urchins and crabs. Complex ecosystems in which species interacted with one another became much more common after the event. <ref>"Abundance Distributions Imply Elevated Complexity of Post-Paleozoic Marine Ecosystems" Peter J. Wagner, Matthew A. Kosnik, Scott Lidgard, Science (journal) 24 November 2006:Vol. 314. no. 5803, pp. 1289 - 1292DOI: 10.1126/science.1133795</ref> <ref>"Marine Life Leaped From Simple to Complex After Greatest Mass Extinction", Andrew C. Revkin, New York Times, November 28, 2006</ref>

[edit] Explanatory theories

Many theories have been presented for the cause of the extinction, including plate tectonics, an impact event, a supernova, extreme volcanism, the release of frozen methane hydrate from the ocean beds to cause a greenhouse effect, or some combination of factors. Recently, a group of scientists claimed they have discovered a 300-mile-wide (480 km) crater in the Wilkes Land region of East Antarctica, which they believe may be linked with the extinction.

[edit] Plate tectonics

At the time of the Permian extinction, all the continents had recently joined to form the super-continent Pangaea and the super-ocean Panthalassa. This configuration radically decreased the extent and range of shallow aquatic environments and exposed formerly isolated organisms of the rich continental shelves to competition from invaders. As the planet's epicontinental systems coalesced, many marine ecosystems, especially ones that evolved in isolation, would not have survived those changes. Pangaea's formation would have altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons. Pangaea seems to have formed millions of years before the great extinction, however, and very gradual changes like continental drift alone probably could not cause the sudden, simultaneous destruction of both terrestrial and oceanic life.

[edit] Antarctic impact event

In June of 2006, Dr. Ralph von Frese announced the discovery of the Wilkes Land crater in the Wilkes Land region of East Antarctica, which may mark the site of the impact that caused the Permian-Triassic extinction.<Ref>Britt R. R.: "Giant Crater Found: Tied to Worst Mass Extinction Ever", http://space.com/scienceastronomy/060601_big_crater.html, June 1 2006</ref> A 300-mile-wide crater more than a mile beneath the East Antarctic Ice Sheet was found using gravity fluctuations measured by NASA's GRACE satellites to peer beneath Antarctica's icy surface, imaging a 200-mile-wide (320 km) plug of mantle material—a mass concentration, or "mascon" in geological parlance—that occurs within the Earth's crust and appears to have been emplaced somewhere between 100 and 500 million years ago—a broad time span that brackets the specific age of the Permian-Triassic extinction.

When the scientists overlaid their gravity image with airborne radar images of the ground beneath the ice, they found the mascon perfectly centered inside a circular ridge some 500 km (300 mi) wide. The Wilkes Land crater is more than twice the size of the Chicxulub crater in the Yucatan peninsula, which marks the impact that may have ultimately killed the dinosaurs 65 million years ago. The Chicxulub impactor (most likely an asteroid) is thought to have been 6 miles (10 km) wide, while the Wilkes Land impactor (either an asteroid or perhaps a large comet nucleus) could have been up to 30 miles (50 km) wide—four or five times as wide.

The gravity measurements also suggest that it could have set the stage for the breakup of the ancient Gondwana supercontinent by creating the tectonic rift that later pushed Australia northward. Approximately 100 million years ago, Australia split from Gondwana and began drifting north, pushed away by the expansion of a rift valley into the eastern Indian Ocean.^{[citation needed]}

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. Analysis of the ratios of carbon and boron isotopes in the fossil record provides evidence of increased levels of atmospheric carbon dioxide. Material from the Earth's mantle released during volcanic eruption has also been shown to contain iridium, an element associated with meteorites. At present, there is only limited and disputed evidence of iridium and shocked quartz occurring with the Permian event, though such evidence has been very abundantly associated with an impact origin for the Cretaceous-Tertiary extinction event.^{[citation needed]}

If the estimated date of the Wilkes Land, Antarctica, event is not correct, and a different extraterrestrial impact triggered the Permian extinction, the crater record of such an event would most likely be erased because there is no Permian-age oceanic crust remaining; all of it has been subducted, so plate tectonics during the last 252 million years have erased any possible P-T seafloor crater.

Adrian Jones, at University College London, has modeled the effects of impacts on the Earth's geological crust and suggests that after an impact, the crust rebounds to form a large shallow crater. 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.<ref name="Jones">Jones, A. et al. (2002) Impact induced melting and the development of large igneous provinces. Earth and planetary science letters, 202, 551.</ref> If Jones is right, the Permian meteorite crater can't be found because it doesn't exist.

But in the past geologist John Gorter of Agip found evidence of a circular structure 200 kilometers (125 mi) in diameter called the Bedout, in currently submerged continental crust off the northwestern coast of Australia, and geologist Luann Becker, of the University of California, confirmed it, finding shocked quartz and brecciated mudstones.<ref name="beckeraustralia">Evidence of Meteor Impact Near Australia Linked to Largest Extinction in Earth's History (May 13 2004 Press Release, Univ. of Calif., Santa Barbara).</ref> The geology of the area of continental shelf dates to the end of the Permian. The Bedout impact crater is also associated in time with extreme volcanism and the break-up of Pangaea. "We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time," Dr. Becker explains. "This is what happened 65 million years ago at Chicxulub but was largely dismissed by scientists as merely a coincidence. With the discovery of Bedout, I don't think we can call such catastrophes occurring together a coincidence anymore," Dr. Becker added in a news release.<ref name="beckeraustralia"/>

[edit] Supernova

A supernova occurring within ten parsecs (or 32.6 light years) 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. A gamma ray burst (the most energetic explosions in the universe; believed to be caused by a very massive supernova or two objects as dense as neutron stars colliding) that occurred within ~6000 light years would produce the same effect.

[edit] 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 (77000 mi²) were covered in torrents of lava. The Siberian flood basalt eruption, the biggest volcanic event on Earth, lasted for millions of years.

The acid rain, brief initial global cooling with each of the bursts of volcanism, followed by longer-term global warming from released volcanic gases, and other weather effects associated with enormous eruptions could have globally threatened life. The theory is debated if volcanic activity, over such a long time, could alter the climate enough to kill off 95% of life on Earth. Volcanic activity directly affects the concentration of atmospheric gases and indirectly affects the oceanic dissolved gases. Increases in carbon dioxide enhance the greenhouse effect, leading to increased global temperatures and reducing the temperature gradient between the equator and the poles. As a result, thermohaline 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 alternately warms then cools. Such temperature fluctuations could cause convective overturn of the oceans, bringing anoxic bottom waters to the surface; in an already oxygen-deprived environment, 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 yet been confirmed to match the correct date.

[edit] Atmospheric hydrogen sulfide buildup

In 2005 Dr. Lee R. Kump, a geoscientist from Pennsylvania State University, published a theory explaining a cascade of events leading to the Great Extinction. Several massive volcanic eruptions in Siberian Traps, described above, started warming the atmosphere. The warming itself did not seem to be large enough to cause so massive an extinction event. However, it could have interfered with the ocean flow.

Cold water at the poles dissolves atmospheric oxygen, cools even more, and sinks to the bottom, slowly moving to the equator, carrying the dissolved oxygen. The warmer the water is, the less oxygen it can dissolve and the slower it circulates.

The resulting lack of supply of dissolved oxygen would lead to depletion of aerobic marine life. The oceans would then become a realm of bacteria metabolizing sulfates, and producing hydrogen sulfide, which would then get released into the water and the atmosphere, killing oceanic plants and terrestrial life. Once such process gets underway, the atmosphere turns into a mix of methane and hydrogen sulfide.

Terrestrial plants thrive on carbon dioxide, while hydrogen sulfide kills them. An increase in the concentration of carbon dioxide would not cause plants to become extinct, but according to the fossils, plants were massively affected as well. Hydrogen sulfide also damages the ozone layer, and fossil spores from the end-Permian era shown deformities that could have been caused by ultraviolet radiation.

Dr. Kump and his colleagues are now looking for biomarkers, indicating presence of green sulfur bacteria in the ocean sediments. Such bacteria indicate lack of oxygen in combination with available sunlight. Such biomarkers were recently found in appropriately dated shallow water sediments by Kliti Grace and her colleagues from Curtin University of Technology, Australia.

[edit] Methane hydrate gasification

In 2002 a BBC2 'Horizon' documentary, 'The Day the Earth Nearly Died,' summarized some recent findings and speculation concerning the Permian extinction event. Paul Wignall examined Permian strata in Greenland, where the rock layers devoid of marine life are tens of meters thick. With such an expanded scale, he could judge the timing of deposition more accurately and ascertained that the entire 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. Two periods of extinctions of terrestrial life were separated by a brief, sharp, almost total extinction of marine life. 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.

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 sublimate solid methane hydrate suggested that a rise of 5°C (10 F) would be sufficient. Released from the pressures of the ocean depths, methane hydrate expands to create 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.<ref>How to kill (almost) all life: the end-Permian extinction event , Michael J. Benton and Richard J. Twitchett, Department of Earth Sciences University of Bristol UK, TRENDS in Ecology and Evolution Vol.18 No.7 July 2003 [1], cited by 21 other articles.</ref>

This sudden release of methane hydrate is called the Clathrate gun and has also been hypothesized as a cause of the Paleocene-Eocene Thermal Maximum extinction event.

[edit] A combination

A combination involving some or all of the following is postulated: 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. The resultant global warming eventually was enough to melt the methane hydrate deposits on continental shelves of the world-ocean.

[edit] References

[edit] External links

• "The Permo-Triassic extinction" Introduction.
• "The Permo-Triassic extinction" A more detailed introduction. Bibliography.
• BBC2 'The Day the Earth Nearly Died' website.
• BBC: "The Extinction Files"
• PBS series Evolution: "Extinction!" video segment
• Luann Becker, "Exploring Antarctica: Understanding Life on Earth and Beyond": includes links to scientific papers
• SpaceRef: "Big Bang in Antarctica: Killer Crater Found Under Ice" Radar images courtesy of Ohio State University.
• Science Daily: Global warming led to atmospheric hydrogen sulfide and Permian extinction
• Science Daily: Big Bang In Antarctica: Killer Crater Found Under Ice
• Lee Siegel, "Rocks Reveal Details of Mass Extinction" Based on Peter D. Ward, David R. Montgomery, Roger Smith, "Altered River Morphology in South Africa Related to the Permian-Triassic Extinction", in Science 8 September 2000
• David Morrison, "Did an Impact Trigger the Permian-Triassic Extinction?"
• Gregory J. Retallack, John J. Veevers, and Ric Morante, "Global coal gap between Permian-Triassic extinction and Middle Triassic recovery of peat-forming plants" GSA Bulletin, 108/2 (February 1996) pp 195-207
• Giant Crater Found: Tied to Worst Mass Extinction Ever Robert Roy Britt (SPACE.com) 1 June 2006 06:07 p.m. ET
• Rocks Reveal Details of Mass Extinction Lee Siegel (SPACE.com) 02:44 p.m. ET 7 September 2000
• "Permian Extinction Event" BBC News extract on the History Files

[edit] Other resources

• Becker L, Poreda R J, Hunt A G, Bunch T E, Rampino M, "Impact Event at the Permian-Triassic Boundary: Evidence from Extraterrestrial Noble Gases in Fullerenes" Science (2001) 291 pp 1530-33.
• Benton M J (2003) When Life Nearly Died: The Greatest Mass Extinction of All Time, Thames & Hudson. Overview written for the layman.
• Mundil, Roland, Kenneth R. Ludwig, Ian Metcalfe, Paul R. Renne, 2004. "Age and Timing of the Permian Mass Extinctions: U/Pb Dating of Closed-System Zircons", Science Magazine, (17 September 2004) pp 1760-63. (On-line abstract)
• Over, Jess (editor), Understanding Late Devonian and Permian-Triassic Biotic and Climatic Events, (Volume 20 in series Developments in Palaeontology and Stratigraphy (2006). The state of the inquiry into the extinction events.
• Sweet, Walter C. (editor), Permo-Triassic Events in the Eastern Tethys : Stratigraphy Classification and Relations with the Western Tethys (in series World and Regional Geology) (2003)ca:Extinció del Permià-Triàsic

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