Permian–Triassic extinction event

The Permian–Triassic (P–Tr) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred 251.4 mya (million years ago),^[1]forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with up to 96 percent of all marine species^[2] and 70 percent of terrestrial vertebrate species becoming extinct. Because approximately 25% of species survived the event,^[3] the recovery of life on earth took significantly longer than after other extinction events.^[2] This event has been described as the "mother of all mass extinctions."^[4]

There are several proposed mechanisms for the extinction event, including both catastrophic and gradualistic processes, similar to those theorized for the Cretaceous–Tertiary extinction event. The former include large or multiple bolide impact events, increased vulcanism, or sudden release of methane hydrates from the sea floor. The latter include sea-level change, anoxia, and increasing aridity.^[3]

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Dating the extinction

With recently-discovered rock sequences in China and improvements in radiometric dating, the extinction is dated with a high degree of confidence;^[2] to 251.4 mya, with an elevated extinction rate remaining for some time thereafter.^[1] A large, abrupt global change in the ratio of ¹³C to ¹²C, denoted δ¹³C, coincides with this extinction,^[5][6][7][8]as does a marine and terrestrial "fungal spike", associated with the dying off of plants and animals fed upon by fungi. This spike in fungal spores in the fossil record is interpreted as the end-Permian extinction and the boundary between the Permian and the Triassic — which then becomes a useful correlator of beds unsuitable for radiometric dating.^[9]

Evidence that the extinction was spread out over a few million years, with a very sharp peak in the last 1 million years of the Permian,^[10][11] is often attributed to the Signor-Lipps effect, because the fossil record is incomplete and does not record the very last occurrence of a species; thorough statistical analysis of highly fossiliferous strata suggests that the main extinction was clustered around one peak, with tentative support for a second peak 250.6 mya.^[2][1] It adds no weight to the previously widely-held belief that there were two major extinction pulses five million years apart, separated by a period of extinctions well above the background level, with the final extinction killing off "only" about 80% of marine species alive at that time.^[12]

Extinction patterns

Marine organisms

Marine invertebrates suffered the greatest loses during the P–Tr extinction. In the intensively-sampled south China sections at the P-TR boundary, for instance, 280 out of 329 marine invertebrate genera disappear within the final 2 sedimentary zones containing conodonts from the Permian.^[1]

Statistical analysis of marine losses at the end of the Permian suggests that the decrease in diversity was caused by a sharp increase in extinctions instead of a decrease in speciation.^[14] The extinction rate of marine organisms was catastrophic.^{[15][4][1][16]}

It is more difficult to produce detailed statistics for land, river, swamp and lake environments, as rock strata from these environments are extremely rare — the Karoo basin in southern Africa is the most complete.

Marine invertebrate groups which survived including: articulate brachiopods (those with a hinge), which have suffered a slow decline in numbers since the P–Tr extinction; the Ceratitida order of ammonites; and crinoids ("sea lilies"), which very nearly became extinct but later became abundant and diverse.

Terrestrial invertebrates

The Permian had great diversity in insect and other invertebrate species, including the largest insects ever to have existed. The end-Permian is the only known mass extinction of insects,^[17] with eight or nine insect orders becoming extinct and ten more greatly reduced in diversity. Palaeodictyopteroids (insects with piercing and sucking mouthparts) began to decline during the mid-Permian; these extinctions have been linked to a change in flora. The greatest decline, however, occurred in the Late Permian and were probably not directly caused by weather-related floral transitions.^[4]

Most fossil insect groups which are found after the Permian–Triassic boundary differ significantly from those which lived prior to the P–Tr extinction. With the exception of the Glosselytrodea, Miomoptera, and Protorthoptera, Paleozoic insect groups have not been discovered in deposits dating to after the P–Tr boundary. The caloneurodeans, monurans, paleodictyopteroids, protelytropterans, and protodonates became extinct by the end of the Permian. In well-documented Late Triassic deposits, fossils overwhelmingly consist of modern fossil insect groups.^[18]

Terrestrial plants

The geologic record for terrestrial plants is sparse and based mostly on pollen and spore studies. At the P–Tr boundary, many groups of land plants went into abrupt decline, including very abundant taxa like the Cordaites (gymnosperms) and Glossopteris (seed ferns).^[19] The decline took 10 to 60 thousand years or longer for some groups.^[20] An analysis of fossil plants at the P–Tr boundary reveal that the dominant conifer genera were replaced post-boundary by Lycopodiophyta, the extant species being recolonizers of disturbed areas.^[21] As a consequence of the P–Tr plant extinction, there was a hiatus in early Triassic coal formation. Thick coal formations are found worldwide and date from the Carboniferous to the late Permian, and from the middle Triassic onwards. There are no coal seams that date from the early Triassic, dubbed the "coal gap".^[22] In eastern Australia a long active cold climate ecosystem, called the peat mire ecosystem, collapsed at the Permian–Triassic boundary with the extinction of close to 95% of peat-producing plants.^[23]

Palynological or pollen studies from East Greenland of sedimentary rock strata laid down during the extinction period indicate dense gymnosperm woodlands before the event. At the same time that marine invertebrate macrofauna are in decline these large woodlands die out and are followed by a rise in diversity of smaller herbaceous plants including Lycopodiophyta, both Selaginellales and Isoetales. Later on other groups of gymnosperms again become dominant but again suffer major die offs, these cyclical fauna shifts occur a few times over the course of the extinction period and afterwards. These fluctuations of the dominant flora between woody and herbaceous taxa indicate chronic environmental stress resulting in a loss of most large woodland plant species. The successions and extinctions of plant communities do not coincide with the shift in 13C values, but occurs many years after. ^[24]

Terrestrial vertebrates

The groups that survived suffered extremely 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. There is enough evidence to indicate that over two-thirds of terrestrial amphibian, sauropsid ("reptile") and therapsid ("mammal-like reptile") families became extinct. Large herbivores suffered the heaviest losses. All Permian anapsid reptiles died out except the procolophonids (testudines have anapsid skulls but are most often thought to have evolved later, from diapsid ancestors). Pelycosaurs died out before the end of the Permian. Too few Permian diapsid fossils have been found to support any conclusion about the effect of the Permian extinction on diapsids (the "reptile" group from which lizards, snakes, crocodilians, dinosaurs, and birds evolved).^[25][3]

Post-event biotic recovery

During the early Triassic (approximately eight million years after the event), the plant biomass was insufficient to form coal deposits, which indicates limited food for herbivores. The lack of coal deposits during this era is known as a coal gap.^[22] River patterns in the Karoo changed from meandering to braided, indicating that vegetation there was very sparse for a long time.^[26]

Each major segment of the early Triassic ecosystem — plant and animal, marine and terrestrial — was dominated by a small number of genera, which appeared virtually world-wide, for example: the herbivorous therapsid Lystrosaurus (which accounted for about 90% of early Triassic land verterbrates) and the bivalves Claraia, Eumorphotis, Unionites and Promylina. A healthy ecosystem has a much larger number of genera, each living in a few preferred types of habitat.^[27][28]

"Disaster taxa" (opportunist organisms) took advantage of the devastated ecosystem and enjoyed a temporary population boom and increase in their territory. For example: Lingula (a brachiopod); stromatolites, which had been confined to marginal environments since the Ordovician; Pleuromeia (a small, weedy plant); Dicrodium (a seed fern).^{[29][3][30][28]}

Changes in marine ecosystems

Prior to the extinction, approximately 67% of marine animals were sessile and attached to the sea floor, but during the Mesozoic only about half of the marine animals were sessile while the rest were free living. Analysis of marine fossils from the period indicated a decrease in the abundance of sessile epifaunal suspension feeders, such as brachiopods and sea lilies, and an increase in more complex mobile species such as snails, urchins and crabs.

Before the Permian mass extinction event some 251 million years ago, both complex and simple marine ecosystems were equally common, but after the recovery from the mass extinction the complex communities outnumbered the simple communities by nearly three to one.^[31]

Bivalves were fairly rare before the P–Tr extinction but became numerous and diverse in the Triassic and one group, the rudist clams, became the Mesozoic's main reef-builders. Some researchers think much of this change happened in the 5 million years between the two major extinction pulses.^[32]

Land vertebrates

Lystrosaurus, a pig-sized herbivorous dicynodont therapsid, constituted as much as 90% of some earliest Triassic land vertebrate faunas.^[3] Smaller carnivorous cynodont therapsids also survived, including the ancestors of mammals.

Archosaurs (which included the ancestors of crocodilians) were initially much rarer than therapsids, although they began to displace therapsids in the mid-Triassic.^[33]

Some temnospondyl amphibians 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 others on fish.^[34]

Fungal spike

After the P-Tr extinction there was an enormous increase in the abundance of fungi at marine and terrestrial sites all over the world, including Greenland, the southern Alps, Israel, Australia and South Africa; and fungi were the dominant form of terrestrial life some time after the extinction.^[35] This indicates a global devastation of land plants, since fungi flourish where there are large amounts of dead organic and especially plant matter. Because the fungal spike implies a spike in the ratio of dead to living non-fungal organic matter, it points to a sudden and huge extinction - if only a modest fraction of living biomass died per year, this would only modestly increase the dead-to-living ratio. So theories about the causes of the P-Tr extinction must explain why it was sudden and devastating.^[36]

Possible causes

There are several proposed mechanisms for the extinction event, including both catastrophic and gradualistic processes, similar to those theorized for the Cretaceous–Tertiary extinction event. The former include large or multiple bolide impact events, increased vulcanism, or sudden release of methane hydrates from the sea floor. The latter include sea-level change, anoxia, and increasing aridity.^[3]

Impact event

  Evidence that an impact event caused the Cretaceous–Tertiary extinction event has led to speculation that similar impacts may have been the cause of other extinction events, including the P–Tr extinction, and therefore to a search for evidence of impacts at the times of other extinctions and for large impact craters of the appropriate age.

Reported evidence for an impact event from the P–Tr boundary level includes: rare grains of shocked quartz in Australia and Antarctica;^[37][38] fullerenes trapping extraterrestrial noble gases;^[39] meteorite fragments in Antarctica;^[40] and grains rich in iron, nickel and silicon, which may have been created by an impact.^[41] However, the veracity of most these claims has been challenged.^{[42][43][44][45]} The shocked quartz from Graphite Peak in Antarctica has recently been reexamined by optical and transmission electron microscopy. It was concluded that the observed features were not due to shock, but rather to plastic deformation, consistent with formation in a tectonic environment such as volcanism.^[46]

Several possible impact craters have been proposed as possible causes of the P–Tr extinction, including the Bedout structure off the northwest coast of Australia,^[38] and the so-called Wilkes Land crater of East Antarctica.^[47] In each of these cases the idea that an impact was responsible has not been proven, and has been widely criticized. In the case of Wilkes Land, the age of this sub-ice geophysical feature is very uncertain - it may be later than the Permian–Triassic extinction.

If impact is a major cause of the P–Tr extinction, it is possible or even likely that the crater no longer exists. 70% of the Earth's surface is sea, so an asteroid or comet fragment is over twice as likely to hit sea as to hit land. But Earth has no ocean-floor crust over 200M years old, because the "conveyor belt" process of sea-floor spreading and subduction destroys it within that time. It has also been speculated that craters produced by very large impacts may be masked by extensive lava flooding from below after the crust is punctured or weakened.^[48]

It has been suggested that a large impact could trigger large-scale volcanism such as the Siberian Traps eruptions.^[49]

Vulcanism

The final stages of the Permian saw two flood basalt events. A small one centered at Emeishan in China. This occurred at the same time as the end-Guadalupian extinction pulse, in an area which was close to the equator at the time.^[50] The flood basalt eruptions which produced the Siberian Traps was one of the largest known volcanic events on Earth and covered over 200,000 square kilometers (77,000 square miles) with lava. The Siberian Traps eruptions were formerly thought to have lasted for millions of years, but recent research dates them to 251.2 ± 0.3 Ma — immediately before the end of the Permian.^[1][51]

The Emeishan and Siberian Traps eruptions may have caused dust clouds and acid aerosols which would have disrupted photosynthesis both on land and in the upper layers of the seas, causing food chains to collapse. These eruptions may also have caused acid rain when the aerosols washed out of the atmosphere. This may have killed land plants and mollusks and planktonic organisms which build calcium carbonate shells. The eruptions would also have emitted carbon dioxide, causing global warming. When all of the dust clouds and aerosols washed out of the atmosphere, the excess carbon dioxide would have remained.

The Siberian Traps had unusual features which made them even more dangerous. Pure flood basalts produce a lot of runny lava and do not hurl debris into the atmosphere. It appears, however, that 20% of the output of the Siberian Traps eruptions was pyroclastic, i.e. consisted of ash and other debris thrown high into the atmosphere, increasing the short-term cooling effect.^[52] The basalt lava erupted or intruded into sediments which were in the process of forming large coal beds, which would have emitted large amounts of carbon dioxide, leading to stronger global warming after the dust and aerosols settled.^[53]

There is doubt, however, about whether these eruptions were enough to directly cause a mass extinction as severe as the end-Permian:^[49]

• For dust and aerosols to affect life worldwide, the eruptions should be near the equator. But the much larger Siberian Traps eruption was near the Arctic Circle.
• The carbon dioxide emissions would have been more dangerous. If the Siberian Traps eruptions mostly occurred within a period of 200,000 years, they would have approximately doubled the atmosphere's carbon dioxide content, and recent climate models suggest that would have raised global temperatures by 1.5 to 4.5 °C. 200,000 years is near the short end of the range of estimates, and the warming would have been less if the eruptions were spread over a longer period.

Methane hydrate gasification

Scientists have found strong evidence of a swift decrease of about 10 ‰ (parts per thousand) in the ratio of Carbon-13 to Carbon-12 (δ¹³C) in end-Permian rocks and fossils all over the world.^[54][16]

Most proposed causes for such an isotope reduction were insufficient. Gases from volcanic eruptions have a δ¹³C about 5 to 8 ‰ below normal, but the amount required to produce a reduction of about 10 ‰ worldwide would require eruptions greater by orders of magnitude than any for which evidence has been found.^[55] A reduction in organic activity would extract ¹²C more slowly from the environment and leave more of it to be incorporated into sediments, thus reducing the δ¹³C. Biochemical processes use the lighter isotopes, since chemical reactions are ultimately driven by electromagnetic forces between atoms and lighter isotopes respond more quickly to these forces. However, a study of a smaller drop of 3 to 4 ‰ in δ¹³C at the Paleocene–Eocene Thermal Maximum concluded that even transferring all the organic carbon (in organisms, soils, and dissolved in the ocean) into sediments would be insufficient.^[55] Buried sedimentary organic matter has a δ¹³C 20 to 25 ‰ below normal. Theoretically if the sea level fell sharply shallow marine sediments would be exposed to oxidization. But 6,500–8,400 gigatons of organic carbon would have to be oxidized and returned to the ocean-atmosphere system within less than a few hundred thousand years to reduce the δ¹³C by 10 ‰, which would be an unrealistic timeframe.^[4]

Only one sufficiently powerful cause has been proposed for the global 10 ‰ reduction in the δ¹³C: the release of methane from methane clathrates.^[4] Methane clathrates, also known as methane hydrates, consist of methane molecules trapped in cages of water molecules. The methane is produced by methanogenic bacteria and archaea and has a <δ¹³C about 60 ‰ below normal. At the right combination of pressure and temperature it gets trapped in clathrates fairly close to the surface of permafrost and in much larger quantities at continental margins (continental shelves and the deeper seabed close to them). Oceanic methane hydrates are usually found buried in sediments where the seawater is at least 300 meters (984 ft) deep. They can be found up to about 2,000 meters (6,562 ft) below the seafloor, but usually only about 1,100 meters (3,609 ft) below the seafloor. Estimates of the total amount of methane trapped in clathrates in today's oceans range from 3,000 to 20,000 gigatons.^[56] Methane hydrates hold methane in an extremely compressed form and dissociate (break up), releasing the methane, if the temperature rises quickly or the pressure on them drops quickly.^[57]

The area covered by lava from the Siberian Traps eruptions is about twice as large as was originally thought, and most of the additional area was shallow sea at the time. It is very likely that the seabed contained methane hydrate deposits and that the lava caused the deposits to dissociate, releasing vast quantities of methane.^[58]

There is strong evidence that global temperatures increased by about 6°C near the equator and therefore by more at higher latitudes: a sharp decrease in oxygen isotope ratios (¹⁸O/¹⁶O);^[59] the extinction of Glossopteris flora (Glossopteris and plants which grew in the same areas), which needed a cold climate, and its replacement by floras typical of lower paleolatitudes.^[60][3]

Sea level fluctuations

Marine regression occurs when areas of submerged seafloor are exposed above sea level. This lowering of sea level causes a reduction in shallow marine habitats, leading to biotic turnover. Shallow marine habitats are productive areas for organisms at the bottom of the food chain, there lose increasing competition for food sources.^[61] There is some correlation between incidents of pronounced sea level regression and mass extinctions, but other evidence indicates there is no relationship; with regression itself creating new habitats.^[3] It has also been suggested that sea-level changes result in changes in sediment deposition rates and effects water temperature and salinity, resulting in a decline in marine diversity.^[62]

Anoxia

There is evidence that the oceans became anoxic towards the end of the Permian. There was a noticeable and rapid onset of anoxic deposition in marine sediments around East Greenland near the end of the Permian.^[63] The uranium/thorium ratios of several late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction.^[64]

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 from the sea floor - see below.

The sequence of events leading to the anoxic oceans would have been:^[64]

• Global warming reduced the temperature gradient between the equator and the poles.
• This reduction slowed or perhaps even stopped the thermohaline circulation.
• The slow-down or stoppage of the thermohaline circulation reduced the absorption and spreading of oxygen in the seas. It may also have 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 most likely causes of the global warming which drove the anoxic event were:

• The Siberian Traps eruptions, which certainly happened in a coal-rich area and emitted large amounts of carbon dioxide (see above).
• Methane hydrate gasification caused by the Siberian Traps eruptions - the change in the ration of carbon-13 to carbon-12 is strong evidence of methane hydrate gasification (see above).
• 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.

Hydrogen sulfide emissions

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, as well as severely weakening the ozone layer, exposing much of the life that remained to fatal levels of UV radiation.^[65] Indeed, anaerobic photosynthesis by Chlorobiaceae (green sulfur bacteria), and its accompanying hydrogen sulfide emissions, occurred from the end-Permian into the early Triassic. The fact that this anaerobic photosynthesis persisted into the early Triassic is consistent with fossil evidence that the recovery from the Permian–Triassic extinction was remarkably slow.^[66]

This theory has the advantage of explaining the mass extinction of plants, which ought otherwise to have thrived in an atmosphere with a high level of carbon dioxide. Fossil spores from the end-Permian further support the theory: many show deformities that could have been caused by ultraviolet radiation, which would have been more intense after hydrogen sulfide emissions weakened the ozone layer.

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 supercontinent Pangaea, surrounded by the superocean Panthalassa, although blocks which are now parts of Asia did not join the supercontinent until very late in the Permian.^[67] This configuration severely decreased the extent of shallow aquatic environments, the most productive part of the seas, 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 the Therapsids and increase in their diversity seems to have occurred in the late Permian, after Pangaea was almost complete. So it seems likely that Pangaea initiated a long period of increased marine extinctions but was not directly responsible for the "Great Dying" and the end of the Permian.

Combination of causes

The possible causes which are supported by strong evidence (see above) appear to describe a sequence of catastrophes, each one worse than the previous: the Siberian Traps eruptions were bad enough in their own right, but because they occurred near coal beds and the continental shelf, they also triggered very large releases of carbon dioxide and methane. The resultant global warming may have caused perhaps the most severe anoxic event in the oceans' history: according to this theory, the oceans became so anoxic that anaerobic sulfur-reducing organisms dominated the chemistry of the oceans and caused massive emissions of toxic hydrogen sulfide. However, one 2001 study indicated there are problems with this hypothesis; the researchers determined that the types of oceanic thermohaline circulation which may have existed at the end of the Permian are not likely to have supported deep-sea anoxia.^[68]

References

Further reading

• 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)