Permian-Triassic extinction event

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The Permian-Triassic (P-Tr) 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 ^[1] and 70 percent of terrestrial vertebrate species becoming extinct.

Contents 1 What became extinct and what survived 1.1 Completely extinct 1.2 Survivors 2 Duration of event 3 After the extinction event 3.1 Very slow recovery 3.2 Changes in marine ecosystems 3.3 Fungal spike 3.4 Land vertebrates 4 Explanatory theories 4.1 The supercontinent Pangaea 4.2 Impact event 4.3 Supernova 4.4 Volcanism 4.5 Anoxic oceans 4.6 Atmospheric hydrogen sulfide buildup 4.7 Methane hydrate gasification 4.8 A combination 5 References 6 External links 7 Other resources

[edit] What became extinct and what survived

[edit] Completely extinct

Some of the groups which became extinct were in decline, while others were apparently flourishing until the catastrophe struck them down.

• Plankton: fusiline foraminifera (almost extinct before the catastrophe),
• Marine invertebrates: tabulate and rugose corals (the reef-builders of the Paleozoic - their extinction devastated marine ecosystems); blastoid echinoderms (may have become extinct shortly before the P-Tr boundary); graptolites; the 2 remaining trilobite genera (trilobites had been in decline since the early Devonian).
• Fish: placoderms; acanthodians.
• Land plants: cordaites.
• Land vertebrates: all Permian anapsid reptiles except the procolophonids (testudines have anapsid skulls but are thought to have evolved later, from diapsid ancestors); the remaining pelycosaurs.^{[citation needed]}

[edit] Survivors

The groups that survived suffered very heavy losses, and some very nearly became extinct at the end-Permian. Some of the survivors did not last for long, but some of those which barely survived produced diverse and long-lasting lineages.

"Dead clades walking" which became extinct in the Triassic include: many bryozoa; Orthocerida; the Goniatitida and Prolecanitida orders of ammonites; procolophonids (the last of the Permian anapsid reptiles).

Articulate brachiopods (those with a hinge) have declined slowly ever since the P-Tr extinction.

Groups which very nearly became extinct but later became abundant and diverse include: the Cerititida order of ammonites; crinoids ("sea lilies" - apparently only the genus Isocrinus survived, and it only started to diversify again in the Jurassic).^{[citation needed]}

[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 over the same relatively short period, showing that the extinction was global and sudden, not gradual or localized.

New evidence from strata in Greenland shows evidence of a double extinction, with a separate, less dramatic extinction occurring 9M years before the P-Tr 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.^{[citation needed]}

[edit] After the extinction event

[edit] Very slow recovery

"Normal" levels of biodiversity do not appear until about 6 million years after the end of the Permian, and in fact recovery was extremely slow for the first 5 million years. This pattern is seen in land plants, marine invertebrates and land vertebrates. (Palaeos). The early Triassic shows two well-known signs of how long the recovery took:

• The coal gap - throughout the early Triassic (8M years) there were insufficient large plants to form coal deposits, and hence little food for large animals.
• Each major segment of the ecosystem - plant and animal, marine and terrestrial - was dominated by a small number of genera, which appeared virtually world-wide. A healthy ecosystem has a much larger number of genera, each living in a few preferred types of habitat.

[edit] Changes in marine ecosystems

Before the extinction about 67% of marine animals were sessile, but during the Mesozoic only about 50% were sessile. 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 in more complex mobile species such as snails, urchins and crabs.

Bivalves were fairly rare before the P-Tr extinction but became numerous and diverse in the Triassic. One group, the rudist clams, became the Mesozoic's main reef-builders, but their type of reef was very different from that built by the extinct tabulate and rugose corals - mud collected and solidified round the rudists' rather long stalks.

Scleractinian corals, which build modern reefs, appeared in the Triassic but were rare until the Cretaceous-Tertiary extinction event wiped out the rudists.

Stromatolites, which had been restricted to very saline and other marginal environments since the Ordovician, spread into many "normal" marine environments during the early Triassic and possibly facilitated the recovery of marine ecosystems by providing a food source.

Complex ecosystems in which species interacted with one another became much more common after the P-Tr extinction.^[2][3]

[edit] Fungal spike

For some time after the P-Tr extinction, 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.^[4] Fungi flourish where there are large amounts of dead organic matter.

However, some researchers argue that fungal species did not dominate terrestrial life, even though their remains have only been found in shallow marine deposits.^[5] Alternatively, others argue that fungal hyphae are simply better suited for preservation and survival in the environment, creating an inaccurate representation of certain species in the fossil record.^[6]

[edit] Land vertebrates

Before the extinction, mammal-like reptiles were the dominant terrestrial vertebrates.

Lystrosaurus (a herbivorous mammal-like reptile) was the only large land animal to survive the event, becoming the most populous land animal on the planet for a time.^[7]

Some temnospondyl amphibians also made a relatively quick recovery after being nearly exterminated - capitosauria and trematosauria were the main aquatic and semi-aquatic predators for most of the Triassic, some specializing to prey on tetrapods and other on fish.

Early in the Triassic, archosaurs became the dominant terrestrial vertebrates, until they were overtaken by their descendants the dinosaurs. Archosaurs quickly took over all the ecological niches previously occupied by mammal-like reptiles (including the lystrosaurs' vegetarian niche), and mammal-like reptiles could only survive as small insectivores.

[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.

[edit] The supercontinent Pangaea

About half way through the Permian (in the Kungurian age of the Permian's Cisuralian epoch) all the continents joined to form the super-continent Pangaea, surrounded by the super-ocean Panthalassa[2]

This configuration radically decreased the extent of shallow aquatic environments and exposed formerly isolated organisms of the rich continental shelves to competition from invaders. Pangaea's formation would also have altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons near the coasts and an arid climate in the vast continental interior.

Marine life suffered very high but not catastrophic rates of extinction after the formation of Pangaea (see the diagram "Marine genus biodiversity" at the top of this article) - almost as high as in some of the "Big Five" mass extinctions. The formation of Pangaea seems not to have caused a significant rise in extinction levels on land, and in fact most of the advance of mammal-like reptiles and increase in their diversity seems to have occurred after the formation of Pangaea.

So it seems likely that Pangaea initiated a long period of severe marine extinctions but was not directly responsible for the "Great Dying" and the end of the Permian.

[edit] Impact event

Evidence that an impact event caused the Cretaceous-Tertiary extinction event has led naturally to the speculation that impact may have been the cause of other extinction events, including the P-Tr extinction, and the consequent search for evidence of impact at other extinction horizons and for large impact craters of the appropriate age.

Reported evidence for an impact event from the P-Tr boundary level include rare grains of shocked quartz in Australia and Antarctica,^[8][9] fullerenes trapping extraterrestrial noble gases,^[10] meteorite fragments in Antarctica,^[11] and Fe-Ni-Si–rich grains of possible impact origin.^[12] However, the veracity of most these claims has been challenged.^{[13][14][15][16]} The supposed shocked quartz from Graphite Peak in Antarctica has recently been reexamined by optical and transmission electron microscopy which showed that the observed features are not due to shock, but rather to plastic deformation, consistent with formation in a tectonic environment.^[17]

Several putative impact craters have been suggested as possible causes of the P-Tr extinction, including the Bedout structure off the northwest coast of Australia,^[9] and the so-called Wilkes Land crater of East Antarctica.^[18] In all cases an impact origin has yet to be demonstrated, and has been widely criticized, and in the case of Wilkes Land, the age of this sub-ice geophysical feature is very poorly constrained.

If impact is the cause of the P-Tr extinction, it is possible, if not likely, that the crater no longer exists because most of the Earth’s oceanic crust, which is more extensive than continental crust, dating from this time has been destroyed by subduction. It has also been speculated that in the case of very large impacts, the crater may be masked by extensive lava flooding from below.^[19]

[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^{[citation needed]}. 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^{[citation needed]}. 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^{[citation needed]}.

There appears to be no independent evidence that a supernova occurred near the earth at the right time. Also, the extinction distribution (96% marine, 70% terrestrial) is inconsistent with the stated results of the supernova theory.

[edit] Volcanism

The flood basalt eruption which produced the Siberian Traps was the largest known volcanic event on Earth and covered over 200,000 square kilometers (77,000 mi²) with lava [3]. The eruption was formerly thought to have lasted for millions of years, but recent research dates the eruptions to a period of million years immediately before the end of the Permian.

The eruptions took place in an area which was rich in coal, and the heating of this coal would have released vast amounts of carbon dioxide and methane into the air, causing severe global warming. Ward reports a massive increase in atmospheric carbon dioxide immediately before the "Great Dying".

The direct effects of the Siberian Traps eruptions would have been[4]:

• Dust clouds and sulfuric acid aerosols which would have stopped photosynthesis both on land and in the upper layers of the seas, causing food chains to collapse.
• Immediate severe global warming because the eruptions occurred in coal beds. This is an additional hazard which was apparently unique to the Siberian Traps eruptions. Massive volcanism more usually causes short-term cooling because dust clouds and aerosols block the sun.
• Acid rain when the sulfuric aerosols washed out of the atmosphere. These would have killed land plants and mollusks and planktonic organisms which build calcium carbonate shells.
• Further global warming when all of the dust clouds and aerosols washed out of the atmosphere but the excess carbon dioxide remained.

Severe global warming can cause anoxic events in the oceans by disrupting the thermohaline circulation and causing convective overturn of the oceans, which would bring anoxic deep-sea water to the surface. There is evidence that this happened at the end of the Permian - see below.

[edit] Anoxic oceans

There is good evidence that the oceans became anoxic (almost totally lacking in oxygen) at the very end of the Permian:

• Wignall and Twitchett (2002) report "a rapid onset of anoxic deposition ... in latest Permian time" in marine sediments around East Greenland.
• The uranium/thorium ratios of late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction^{[citation needed]}.

This would have been devastating for marine life, except for anaerobic bacteria in the sea-bottom mud. There is also evidence that anoxic events can cause catastrophic hydrogen sulfide emissions for the sea floor - see below.

The sequence of events leading to the anoxic oceans would have been^{[citation needed]}:

• Global warming reduced the temperature gradient between the equator and the poles.
• The reduction in the temperature gradient slowed or perhaps stopped the thermohaline circulation.
• The slow-down or stoppage of the thermohaline circulation prevented the dispersal of nutrients washed from the land to the sea, causing eutrophication (excessive growth of algae), which reduced the oxygen level in the sea.
• The slow-down or stoppage of the thermohaline circulation also caused oceanic overturn - surface water sank (it has more salinity than deep water because of evaporation caused by the sun) and was replaced by anoxic deep water.

The most likely causes of the global warming which drove the anoxic event were^{[citation needed]}:

• The Siberian Traps eruptions, which certainly happened in a coal-rich area.
• A meteorite impact, if one can be shown to have happened and to have struck an area from which a large quantity of carbon would have been released.

[edit] Atmospheric hydrogen sulfide buildup

Kump, Pavlov and Arthur (2005) suggested that a severe anoxic event at the end of the Permian could have made sulfate-reducing bacteria the dominant force in oceanic ecosystems, causing massive emissions of hydrogen sulfide which:

• poisoned plant and animal life on both land and sea.
• severely weakened the ozone layer, exposing much of the life that remained to fatal levels of UV radiation.

This theory has the advantage of explaining the mass extinction of plants, which would otherwise have thrived in an atmosphere with a high level of carbon dioxide.

The evidence in favour of this theory includes:

• Fossil spores from the end-Permian show deformities that could have been caused by ultraviolet radiation, which would have been more intense after hydrogen sulfide emissions weakened the ozone layer.
• Grice et al (2005) reported evidence of anerobic photosynthesis by Chlorobiaceae (green sulfur bacteria) from the end-Permian into the early Triassic, which would have produced hydrogen sulfide emissions. The fact that this activiy persisted into the early Triassic is consistent with fossil evidence that the recovery from the Permian-Triassic extinction was remarkably slow.

[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.^[20]

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; this and the binding of oxygen in the oceans caused the catastrophic global ocean anoxia.

[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" (ed: this link is now dead and a search reveals nothing matching the title...)
• 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
• The History Files: Permian Extinction Event BBC News extract
• Grice et al (2005) "Photic Zone Euxinia During the Permian-Triassic Superanoxic Event" (Science Vol. 307. no. 5710) - abstract
• Kump, L.R., Pavlov, A., and Arthur, M.A. (2005). "Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia". Geology v. 33, p.397–400
• Ward, P.D. (2006) "Impact from the Deep". Scientific American October 2006.
• Wignall, P.B., and Twitchett, R.J. (2002) Permiam-Triassic sedimentology of Jameson Land, East Greenland: Incised submarine channels in an anoxic basin (Journal of the Geological Society, Nov 2002)

[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.
• Erwin D, 2006. Extinction - How life on earth nearly ended 250 million years ago. Princeton University Press, Princeton, New Jersey.
  Summarised by National Geographic
• 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)