Permian-Triassic extinction event Pt 1.

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We've all (well, maybe most) have heard of the great dinosaur extinction that defines the Cretaceous-Tertiary periods's (called the K-T boundary) in earths geological history.

But few know about an even bigger mass extinction event that occurred 250 million years ago at the end of the Permian Period. This extinction nearly wiped out all life on Earth. What survived evolved into the dinosaurs.

While the mass extinction at the K-T boundary can be attributed to extensive enviornmental stress's on the dinosaurs habitat [the supercontinent "Pangea" was splitting up and drifting away from the equatorial regions], coupled with a huge meteor impact.

The same cannot be said of the P-Tr extinction. Instead a multitude of events happened over the course of several million years. In effect, the worst possible geological events all happened at roughly the same time.


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.

Timing of the extinction

It used to be thought that rock sequences spanning the Permian-Triassic boundary were too few and contained too many gaps for scientists to estimate reliably when the extinction occurred, how long it took or whether it happened at the same time all over the world.[2] But newly discovered rock sequences in China and improvements in radiometric dating have made scientists confident that the end of the extinction can be dated to somewhere between 251.2 and 250.8MYA (millions of years ago).[3][4]

There is evidence worldwide of an abrupt and massive change in the ratio of carbon-13 to carbon-12. Scientists are confident that rocks which show this change were formed at the same time.[5][6][7][8]

It has also recently been discovered that many rocks of about the right age, both from continental shelf and from terrestrial environments (at the time), contain evidence of a "fungal spike", an enormous increase in the abundance of fungal spores.[9] Since fungi feed on the remains of dead organisms, especially plants, the fungal spike is interpreted as marking the time of the end-Permian extinction and the boundary between the Permian and the Triassic. This helps in dating rocks which are not suitable for radiometric dating.[10]

There is evidence that the extinction took a few million years but with a very sharp peak in the last 1 million years of the Permian (possibly in a period of under 60,000 years). This applies both to marine organisms (see the diagram "Marine Genus Biodiversity") and terrestrial organisms.[11][12] In fact many scientists believe: that there were two major extinction pulses 5M years apart, separated by a period of extinctions well above the background level; and that the final extinction killed off "only" about 80% of marine species alive at that time while the other losses occurred during the first pulse or the interval between pulses. According to this theory the first of these extinction pulses occurred at the end of the Guadalupian epoch of the Permian.

Terrestrial losses

It is harder to produce such detailed statistics for land, river, swamp and lake environments because good Permian-Triassic rock sequences from terrestrial environments are extremely rare (the Karoo is by far the best). Even so, 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).[21]
* The end-Permian is the only known mass extinction of insects.[22]
* Many land plants became extinct, including groups which had been very abundant such as Cordaites (gymnosperms) and Glossopteris (seed ferns).[23]

Notes:

* 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).

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 (a group of nautiloids); 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").

Paleontologists have found very few fossils from the Permian of archosaurs (or archosauriformes as some prefer to describe the Permian specimens), but in the Triassic the archosaurs took over all the medium to large terrestrial vertebrate niches, and were the ancestors of crocodilians, dinosaurs and birds.[24]

After the extinction event

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. [25]. The early Triassic shows 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.[26]
* 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, 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][29]
* "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).[30][31][32][33]
* River patterns in the Karoo changed from meandering to braided, indicating that vegetation there was very sparse for a long time.[34]

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.

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.[35][36]

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.[37]

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.[38] 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.[39] 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.[40]

Land vertebrates

Before the extinction, therapsids ("mammal-like reptiles") were the dominant terrestrial vertebrates.

Lystrosaurus (a herbivorous therapsid) was the only large land animal to survive the event, becoming the most populous land animal on the planet for a time.[41]

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 therapsids (including eventually the lystrosaurs' vegetarian niche), and therapsids and their mammaliform descendants could only survive as small insectivores[citation needed].

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.[42]

 

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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.

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.[43]

This configuration severely 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 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 severe marine extinctions but was not directly responsible for the "Great Dying" and the end of the Permian.

Massive volcanism

The final stages of the Permian saw two flood basalt events:

* A smaller 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.[44]
* 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. These eruptions were formerly thought to have lasted for millions of years, but recent research dates them to a period of a million years immediately before the end of the Permian.[45]

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. But it appears 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.[46]
* The basalt lava erupted or intruded into sediments which were in the process of forming large coal beds.[47]

The direct effects of the Emeishan and Siberian Traps eruptions would have been:

* 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.
* For the Emeishan eruptions, a cooling of the climate because dust clouds and aerosols blocked the sun.
* For the Siberian Traps eruptions, possibly immediate warming because of the carbon dioxide emitted as the lava heated the Siberian coal beds.
* Acid rain when the aerosols washed out of the atmosphere. This would have killed land plants and mollusks and planktonic organisms which build calcium carbonate shells.
* Further warming when all of the dust clouds and aerosols washed out of the atmosphere but the excess carbon dioxide remained.

But there is doubt about whether these eruptions were enough to cause directly a mass extinction as severe as the end-Permian:[48]

* 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 10ppt (parts per thousand) in the ratio of carbon-13 to carbon-12 (13C/12C) in end-Permian rocks and fossils all over the world.[49][50][51]

Most possible sources for such a reduction turn out to be insufficient:

* Gases from volcanic eruptions have a 13C/12C ratio about 5 to 8ppt below normal. But the amount required to produce a reduction of about 10ppt worldwide would require eruptions greater by orders of magnitude than any for which evidence has been found.[52]
* 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 13C/12C ratio. 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. But a study of a smaller drop of 3 to 4 ppt in 13C/12C 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.[53]
* Buried sedimentary organic matter has a 13C/12C ratio 20 to 25ppt below normal. Theoretically if the sea level fell sharply shallow marine sediments would be exposed to oxidization. But 6,500-8,400 gigatons (1 gigaton = 109 metric tons) 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 13C/12C ratio by 10ppt. This is not a realistic possibility.[54]

Only one sufficiently powerful cause has been proposed for the global 10ppt reduction in the 13C/12C ratio: the release of methane from methane clathrates.[55] 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 13C/12C ratio about 60ppt 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 (330 yards) deep. They can be found up to about 2000 meters (about 1.2 miles) below the seafloor, but usually only about 1100 meters (a little over 0.63 miles) below the seafloor. Estimates of the total amount of methane trapped in clathrates in to-day's oceans range from 3,000 to 20,000 gigatons.[56][57] 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.

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]

Methane is a greenhouse gas about 62 times as powerful as carbon dioxide. Its effect declines fairly quickly as it oxidizes readily, producing one molecule of carbon dioxide and two molecules of water per molecule of methane. A portion of the carbon dioxide stays in the atmosphere for centuries.[59][60]

There is strong evidence that global temperatures increased by about 6°C near the equator and therefore by more at higher latitudes:

* Oxygen isotope ratios (18O/16O) show a sharp decrease.[61]
* 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.[62][63][64]

The only mitigating feature in this scenario is that these processes took tens or hundreds of thousands of years, giving organisms time to adapt or migrate (migration would have been relatively easy, since most of the Earth's land was combined as Pangaea and most of the sea as Panthalassa).

This sudden release of methane hydrate is called the Clathrate gun and has also been proposed as a cause of the Paleocene-Eocene Thermal Maximum extinction event.[65]

 

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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.

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 anerobic photosynthesis persisted into the early Triassic is consistent with fossil evidence that the recovery from the Permian-Triassic extinction was remarkably slow.

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,[66][67] fullerenes trapping extraterrestrial noble gases,[68] meteorite fragments in Antarctica,[69] and Fe-Ni-Si–rich grains of possible impact origin.[70] However, the veracity of most these claims has been challenged.[71][72][73][74] 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.[75]

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,[67] and the so-called Wilkes Land crater of East Antarctica.[76] 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.[77]

It has been suggested that a large impact could trigger large-scale volcanism such as the Siberian Traps eruptions, but detailed analysis makes this appear unlikely.[78]

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. Estimates of the rate of supernova occurrence within 10 parsecs of the Earth vary from once every 100 million years[79] to once every one to ten billion years.[80]

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.

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.