The Permian-Triassic mass extinction event
The Permian–Triassic (P–T) mass extinction, also known as the Permian extinction, informally known as the Great Dying, was a mass extinction that occurred about 251.4 million years ago, acting as a limit between the Permian and Triassic geological period, or between the Primary Era (Paleozoic) and the Secondary Era (Mesozoic), just like that of the KT (Cretaceous Tertiary) marks the passage between the Cretaceous period and the Paleocene period, i.e. between the Secondary Era (Mesozoic) and the Tertiary Era (Cenozoic).
The Permian-Triassic extinction event was the most severe mass extinction event ever to occur on Earth, with the disappearance of 96% of marine species and 70% of terrestrial vertebrate species; it was the only known mass extinction of insects. It is estimated that 57% of all families and 83% of all genera died out. Because so much biodiversity was lost, the recovery of life on Earth was a much longer process than other mass extinction events. This event has been described as the "mother of all mass extinctions". The pattern of the extinction is still under debate, as different studies suggest one to three different stages.
A variety of mechanisms have been proposed to explain the extinctions; the first peak was probably due to a gradual environmental change, while the subsequent peak(s) were probably due to a catastrophic event. Some possible scenarios for these successive peaks include collisions with astronomical objects, an increase in volcanic activity, or the sudden release of methane hydrates from the seafloor; Gradual changes include sea level change, anoxia, increased aridity, and changes in the circulation of ocean currents as a result of climate change.
DATING THE EXTINCTION
Until recently, rock sequences with stratigraphic continuity that included the Permian-Triassic were thought to be too few and contain too many breaks in the chronostratigraphic sequence for scientists to reliably estimate when the extinction occurred, how long it lasted, or whether it occurred at the same time in all parts of the world. However, uranium-lead isotope ratio studies of zircons found in rock sequences near Meishan, Zhejiang Province, China have dated the extinction at 251.4 ± 0.03 million years ago, with a continuous high extinction rate that lasted for quite some time. A large (-9 per thousand) abrupt global change in the ratio of carbon-13 to carbon-12, called the C13 delta, coincides with this extinction, and is sometimes used to identify the Permian-Triassic boundary in rocks that are not suitable to radiometric dating.
It has been hypothesized that the Permian-Triassic boundary was associated with a marked increase in the abundance of marine and terrestrial fungi, and this would have been caused by a corresponding sharp increase in dead plant and animal remains. For a time this "fungal peak" was used by some paleontologists to identify the boundary between the two geological periods in stratifications that were not suitable to radiometric dating or that they had no fossil evidence, but even the proponents of this theory have pointed out that the "fungal spikes" could be a frequent phenomenon created by the post-extinction ecosystem in the Early Triassic. The idea of the mycotic spike has been criticized in several places, among which: Reduviasporonites, the most common hypothesized "mycotic spores", were actually fossil algae; the peak did not occur worldwide and in many places did not coincide with the Permian-Triassic boundary. Algae that had been mistaken for fungal spores may represent a transition to a lake world in the Triassic rather than a phase of death and decay. However, new chemical evidence points to a fungal origin of Reduviasporonites, diluting these criticisms.
Uncertainty persists about the total duration of extinction and about the timing of extinction of various groups within the overall process. Some evidence suggests that the extinction took place within a few million years, with a sharp peak in the last million years of the Permian. Statistical analyzes of a few fossil-rich strata at Meishan in southern China suggest that the main extinction clustered around a peak. Some research shows that different groups died out at different times; for example, although it remains difficult to date absolutely, the extinction of ostracods (crustaceans) and that of brachiopods (bivalve shelled marine animals) were separated by 0.72-1.22 million years. In one well-preserved sequence in eastern Greenland, animal declines are concentrated over a period of 10,000 to 60,000 years, while plants took several hundred thousand years to show the full impact of the event. An earlier theory, still supported in some documents, holds that there were two major extinction phases separated by a period of 5 million years, in which the level of extinctions was well above that of the two peaks, and whose the last extinction "only" killed 80% of the marine species that survived the previous period, with the other losses occurring during the first peak or in the interval between the two.
According to this theory, the first extinction peak occurred at the end of the Guadalupian epoch of the Permian. For example, all but one of the living genera of dinocephalians (Reptiles Synapsids Therapsids) became extinct at the end of the Guadalupian, as did Verbeekinidae, a family of large fusulinid foraminifera. The impact of the extinction at the end of the Guadeloupe on marine organisms appears to have varied across locations and taxonomic groups – brachiopods and corals suffered heavy losses.
PATTERNS OF EXTINCTION
The extinction had such a profound effect on Earth's ecosystems that traces of it can be found about a quarter of a billion years later. In the Late Permian many kinds of reptiles and amphibians were found on earth, together with many plants, especially ferns, but also conifers and many kinds of ginkgo. There were also complicated coral ecological systems at sea. In this period the continents were reunited in the supercontinent Pangea and animals could move freely. There were lush jungles, deserts and oceanic environments. After the extinction, one genus of vertebrate found itself in the dominant position: a medium-sized herbivore called Listrosaurus.
Also in the marine environment there was the prevalence of only one kind of organism: a brachiopod called Lingula. Finally, other genera and species reappeared - the so- called "taxa Lazarus", so called in reference to the biblical character who rose from the dead. It is clear that they somehow survived extinction, but in very small numbers. Like the extinction at the end of the Ordovician, it appears to have been divided into two phases, separated by an interval of about 10 million years, with the second being more severe than the first. There was the extinction of several groups including brachiopods, ammonites, corals, as well as gastropods and, unusually, insects. It took about 50 million years for life to fully recover its biodiversity. Nothing resembling a coral reef reappeared on the face of the Earth until 10 million years after the extinction, and the complete recovery of marine life took about 100 million years.
Marine organisms
Marine invertebrates suffered the greatest losses during the Permian-Triassic extinction. In South China's widely represented sections of the PT limit, for example, more than 280 out of 329 genera of marine invertebrates disappeared in the last two sedimentary zones that contained Permian conodonts.
Some statistical analyzes of losses to sea at the end of the Permian suggest that the decline in diversity was caused by a sharp increase in extinctions, rather than a decrease in speciation. The extinction mostly affected organisms with skeletons composed of calcium carbonate, especially those dependent on ambient levels of CO2 to produce their skeletons.
Among benthic organisms, the extinction event multiplied extinction rates and therefore caused the most damage to taxonomic groups of any previous normal rate of extinction. The rate of extinction of marine organisms was catastrophic.
Groups of marine invertebrates that survived included: jointed brachiopods (those with a pivot), which had undergone a slow decline in numbers until the P–T extinction; the ammonite order Ceratitids and the crinoids ("sea lilies"), which were nearly extinct, but later became abundant and diversified.
The groups with the highest survival rates generally had active circulation control, elaborate gas exchange mechanisms, and mild calcification; the most heavily calcified organisms and with simple breathing apparatuses were the most affected.
At least in the case of brachiopods, the surviving taxa were usually small, rare members of a diverse community.
Ammonites, which had been in a long decline for the previous 30 million years since the Roadian (Middle Permian) plateau, experienced a highly selective peak extinction at the end of the Guadalupian. This extinction greatly reduced morphological diversity, suggesting that environmental factors were responsible for this extinction.
Species variety and morphological diversity were further reduced to the PT limit; the extinction here was not selective, compatible with a catastrophic, short-lived, global-impact cause. During the Triassic, species diversity increased rapidly, while morphological diversity remained low. The space occupied by the ammonites became more restricted during the Permian period. Just a few million years after the start of the Triassic, the original landform space was reoccupied, but shared differently between clades.
Terrestrial Invertebrates
The Permian had a great diversity of insect and other invertebrate species, including the largest insects that ever lived. That at the end of the Permian was the only known mass extinction of insects, with the extinction of eight or nine orders of insects and ten others greatly reduced in diversity. THE palaeodictyopterans (insects with biting and sucking mouth parts) went into decline during the Middle Permian; these extinctions have been linked to a change in flora. The greatest decline occurred, however, during the Late Permian and was not directly caused by transitions in the flora due to climate change.
Most fossil insect groups that have been found after the Permian-Triassic boundary differ significantly from those that lived before the PT extinction. With the exception of Glosselitrods, Myoptera and Protortoptera, Paleozoic insect groups have not been found in deposits dated after the PT boundary. The caloneurods, monurans, palaeodictyopteroids, protelytropters and protodonates became extinct at the end of the Permian. In the well- documented Late Triassic deposits the fossils consist mostly of modern groups of insects.
Terrestrial plants
The testimonies of terrestrial plants are scarce, based above all on the study of pollen and spores. Interestingly, plants were relatively immune to the mass extinction, and the impact of all major extinctions was "insignificant" at the family level. Even the observed reduction in species diversity (by 50%) may be due mostly to taphonomic processes. However, there was a massive reorganization of the ecosystem, with a profound change in the abundance and distribution of plants.
At the P–T boundary, the dominant groups of flora changed and many land plant groups went into sudden decline, such as Cordaites (gymnosperms) and Glossopteris (seed ferns). The dominant genera of gymnosperms were replaced by lycophytes, which colonized the areas left empty.
Palynological (pollen) studies in East Greenland of rock strata settled during the period of the extinction indicate dense gymnosperm forests prior to the extinction event. At the same time as marine invertebrate microfauna went into decline these large forests disappeared and were followed by a growth in diversity of small herbaceous plants, including lycophytes, both Selaginellales and Isoetales. Subsequently other groups of gymnosperms returned to be the dominant flora to enter again into decline; these cyclical changes in the flora happened a few times during the extinction period and even afterwards. These fluctuations between shrub and herbaceous taxa indicate environmental stress resulting in the loss of larger shrub species.
The succession and extinction of plant communities did not coincide with the change in delta C13 values, but occurred several years later. Recovery of gymnosperm forests took about 4-5 million years.
The Coal Gap
No Early Triassic coal deposits are known, and the Middle Triassic deposits are thin and of low quality. This "coal gap" has been explained in a variety of ways. It was speculated that new, more aggressive fungi, insects and vertebrates had evolved that would kill large numbers of trees. However, these decomposers also suffered heavy losses of species during the extinction and are not considered a plausible cause for the interval. One possible hypothesis is that all plants that would later form coal died out in the Permian-Triassic extinction, and that it took 10 million years for a new set of plants to adapt to the humid, acidic conditions of the marshes of peat. On the other hand, other abiotic factors (not caused by living organisms) may be attributable, such as a decrease in precipitation or a higher contribution of clastic sediments. Finally it is equally true that there are few sediments of any type known from the Early Triassic and the lack of coal may simply reflect this scarcity. This gives the possibility that ecosystems typical of coal production may have reacted to the changed conditions by moving, probably in areas of which no Lower Triassic sedimentary traces remain. For example, in eastern Australia a cold climate was the norm for a long time, with a wetland ecosystem specialized for these conditions. Approximately 95% of these peat-producing plants became locally extinct at the PT boundary. It remains interesting that the coal deposits in Australia and Antarctica disappeared significantly earlier than the PT limit.
Terrestrial vertebrates
Even the groups that managed to survive suffered heavy losses in the number of species, and some vertebrate groups went almost to total extinction at the end of the Permian. However, some of the surviving groups did not last long beyond the period of the mass extinction, while others that barely survived managed to produce different types of descendant groups that continued to evolve. There is sufficient evidence to indicate that over two-thirds of the terrestrial amphibian, sauropsid (reptiles), and therapsid ("mammal-like reptiles") families went extinct.
Large herbivores suffered the greatest losses. All Permian anapsid reptiles disappeared, with the exception of the procolophonids (tortoises have anapsid skulls, but are now commonly believed to have evolved later, from diapsid ancestors). Pelycosaurs became extinct shortly before the end of the Permian. Too few fossils of Permian diapsid reptiles have been found to support any conclusions about the effects of mass extinction on diapsids (the group of reptiles from which lizards, snakes, crocodiles and dinosaurs, from which birds evolved).
Possible explanations for these patterns
The most vulnerable marine organisms were those that produced solid parts of limestone (i.e. from calcium carbonate) and had low metabolic rates and weak respiratory systems - mainly calcareous sponges, tabulated and rugose corals, calcareous brachiopods, bryozoans and echinoderms; about 81% of these genera became extinct. Related organisms that did not produce solid parts in limestone suffered minor losses, such as anemones, from which modern corals evolved. Animals possessing high metabolic rates, well-developed respiratory systems, and non-calcareous solids suffered insignificant losses – with the exception of conodonts, of which 33% of genera disappeared.
Biological Recovery
Early analyzes indicate that life on Earth recovered quickly after the Permian extinctions, but mostly in the form of pioneer organisms, or post-disaster taxonomic groups, such as the hardy Listrosaurus. Research indicates that specialized animals that formed complex ecosystems, with high biodiversity, complex feeding networks, and a wide variety of niches, also took much longer to recover. It is believed that this long recovery was due to successive waves of extinction that inhibited it, as well as to the prolonged environmental stresses that the organisms underwent, which continued in the Early Triassic. Other research indicates that full recovery did not begin until the Middle Triassic, 4 to 6 million years after the extinction; and some authors estimate that recovery was not complete until at least 30 million years after the extinction, i.e. in the Upper Triassic.
During the Early Triassic (4-6 mda after the PT extinction), plant biomass was not sufficient to form coal deposits, implying limited food resources for herbivores. River courses in the Karoo region of South Africa changed from a meandering to a braided flow structure, suggesting that vegetation was very sparse over a long period of time.
Every major sector of the Triassic ecosystem -vegetable and animal, marine and terrestrial- was dominated by a small number of genera, apparently spread over the whole face of the Earth, for example: the herbivorous therapsid Listrosaurus (which made up 90% of the vertebrates on land in the Early Triassic) and the bivalves Claraia, Eumorphotis, Unionites and Promylina. A healthy ecosystem has a large number of genera, each residing in a few preferred habitat types.
Post-disaster taxa (opportunistic organisms) took advantage of the devastated ecosystem and enjoyed a temporary population boom and increase in available land. For example: the Lingula (a brachiopod); the stromatolites, which had been confined to marginal environments since the Ordovician; the Pleuromeia (a small thin plant); the Dicroidium (a fern).
Recent (2012) studies on 15,000 conodont fossils recovered in the rocks of southern China have allowed an accurate reconstruction of the trend of the sea surface temperature at the beginning of the Triassic which highlighted how it was so high (between 50 and 60 °C) as to make the survival of marine organisms extremely difficult in the intertropical regions of the planet. The determination of the temperatures was made possible by the study of the ratio of the oxygen isotopes (O18/O16) present in the fossil skeleton of the conodonts. The extreme temperatures caused a disruption in the carbon cycle, further increasing carbon dioxide in the atmosphere and prolonging the period necessary for the biological recovery of the intertropical zones by 5 million years.
Changes in marine ecosystems
Before the extinction, about 67% of marine animals were attached to the seabed, but during the Mesozoic only half were sessile, while the remainder moved freely. Some analyzes of the marine fossils of the period indicate a decrease in the number of sessile organisms that fed by filtering, such as brachiopods and sea lilies, and an increase in mobile species such as snails, sea urchins and crabs. Before the Permian extinction, simple and complex marine ecosystems were both common; after the recovery from the mass extinction, complex communities outnumbered simple communities nearly three to one, and increased hunting pressure led to the Mesozoic Marine Revolution.
Bivalves were somewhat rare before the Permian-Triassic extinction event, but became very numerous and diverse during the Triassic, and one group, the rudists, became major components of Mesozoic reefs. Some researchers believe that most of these changes occurred in the five million years between the two major extinction peaks.
Crinoids ("sea lilies") underwent selective extinction, resulting in a decrease in the variety of forms in which they grew. Their subsequent adaptive radiation was rapid, resulting in flexible limb structures becoming widespread; mobility, which was mostly a response to increased predation, became a prevalent trait.
Terrestrial Vertebrates
Lystrosaurus, a herbivorous dicynodont therapsid the size of a pig, constituted 90% of the terrestrial vertebrate fauna of the Early Lower Triassic. Other small carnivorous cynodont therapsids also survived, including mammalian ancestors. In the Karoo region of South Africa, the therocephalians Tetracynodon, Moschorhinus and Ictidosuchoides survived, but they do not seem to have had a significant development in the Triassic.
Archosaurs (which included the ancestors of dinosaurs and crocodilians) were initially less common than therapsids, but began to dominate therapsids in the Middle Triassic. Between the Middle and Late Triassic dinosaurs evolved from a group of archosaurs and became the dominant group of terrestrial ecosystems for the remainder of the Mesozoic. This "Triassic revolution" may have contributed to the evolution of mammals by forcing therapsids and their mammaliform successors to live mainly as small nocturnal insectivores; it is probable that the nocturnal lifestyle has pushed at least the mammaliforms to develop fur and high metabolic rates.
Some temnospondyl amphibians made a relatively quick recovery, instead of going extinct. Mastodontosaurus and trematosaurs were the main aquatic and semiaquatic predators during most of the Triassic, some of tetrapods and some of fish. Land vertebrates took an unusually long time to recover from the Permian mass extinction; one author estimates that the recovery was not complete until 30 million years after the extinction, i.e. until the Late Triassic, in which archosaurs (dinosaurs, pterosaurs and crocodiles), amphibians and mammals abounded.
CAUSES OF THE EXTINCTION
Various mechanisms have been proposed to explain the causes of the mass extinction, including catastrophic events and gradual processes, similar to those theorized for the Cretaceous–Tertiary mass extinction.
Catastrophic events include collisions (even multiple) with celestial bodies, an increase in volcanism, or the sudden release of methane hydrates. Gradual processes include sea level fluctuations, anoxia, or an increase in aridity. Any hypothesis regarding the causes must explain the selectivity of the event, which mainly affected organisms with calcium carbonate skeletons.
Collision
Evidence that an astronomical impact caused the Cretaceous-Tertiary mass extinction has led to speculation that similar impacts may have caused other mass extinctions, including the Permian-Triassic extinction, and thus the search for evidence of collisions dating to the age of this event and of impact craters dating back to the period.
Evidence supporting an impact during the P–T boundary includes rare grains of lamellar quartz in Australia and Antarctica, fullerenes that had trapped noble gases of extraterrestrial origin, fragments of meteorite in Antarctica and grains rich in iron, nickel and silicon, which may have been created in an impact. However, the veracity of these finds has been questioned.
The flake quartz from Graphite Peak in Antarctica was re-examined in 2005 with more advanced technologies and it was concluded that the observed characteristics are not due to a shock, but rather to a plastic deformation, compatible with the formation in a tectonic environment such as a volcanic one.
Several impact craters have been proposed as possible causes of the PT extinction, including the Bedout facility off the northwest coast of Australia, the so-called Wilkes Earth Crater in East Antarctica, or, even more speculatively, the Gulf of Mexico. In each of these cases the hypothesis that an impact was the responsible event has not yet been proven, and often also widely criticized. In the case of Wilkes Land, the age of the geophysical structure beneath the ice sheet is very uncertain - it may be after the Permian-Triassic extinction event.
If the primary cause of the PT extinction was an impact, the crater is likely no longer in existence. Given that 70% of our planet's surface is covered by water, the chance that an asteroid or comet fragment will hit the ocean is twice as high as the chance that it will hit land. Furthermore, the Earth has no ocean surface older than 200 million years, given that the process of convective motion and subduction has now destroyed the crust oceanica dating back to that period. It has also been speculated that craters formed by very large impacts may have been masked by an extensive lava flow caused by the breaking or weakening of the crust following the impact.
The collision idea also has other support, as it could theoretically have triggered other phenomena considered to cause the extinction, such as the eruptions of the Siberian Traps (see below) if they were the site of the impact or if they were found at the antipodes (regions of chaotic terrain). However, subduction cannot be taken as an excuse for the fact that no evidence of an impact is found; such as the extinction event at the end of the Cretaceous, there should be a fallout layer rich in siderophilic elements (e.g. iridium) in the geological formations of the period. The abrupt change caused by an impact would also explain why species have not evolved rapidly by adapting to slower phenomena and on a non-global scale.
Volcanism
The final stages of the Permian period witnessed two large-scale volcanic events. The smaller one, which formed the Emeiscian trap in China, occurred around the same time as the extinction peak at the end of the Guadeloupe, in an area near the equator. The effusive eruptions of basalt that formed the Siberian Traps constituted one of the largest volcanic events that have ever occurred on Earth and covered over 2,000,000 km2 of surface with lava. The eruptions of the Siberian Trappo were previously thought to have lasted for millions of years, but some research has dated them to 251.2 ± 0.3 million years - immediately before the end of the Permian.
The eruptions of the Emeiscian and Siberian Traps would have produced clouds of dust and acid aerosols that would have blocked sunlight and therefore interrupted photosynthesis both at sea and on land, collapsing the food chain. These eruptions may have also caused acid rain as the aerosols were washed out of the atmosphere. This would have killed land plants, molluscs and plankton with a calcium carbonate shell. The eruptions would also have emitted carbon dioxide, causing global warming. When the dust clouds and aerosols were cleared from the atmosphere, the excess carbon dioxide would remain and warming would continue without mitigating effects.
The Siberian Trap possesses particular characteristics that made it even more harmful. Pure basalt outpourings produced high-velocity lava flows and did not eject debris into the atmosphere. It appears, however, that 20% of the emissions from the Siberian Traps were pyroclastic, i.e. they consisted of ash and other debris ejected up to high altitudes, increasing the short-term cooling effect. There basaltic lava erupted or intruded carbonate rocks and sediments that would have given rise to large deposits of coal, and from here would have produced large quantities of carbon dioxide, leading to more global warming after the dust and aerosols settled.
It remains questionable, however, whether these eruptions were sufficient to cause a mass extinction as severe as the one at the end of the Permian. Equatorial eruptions are needed to produce enough dust and aerosols to affect life worldwide, whereas the Siberian Trap eruptions were in or near the Arctic Circle. Furthermore, if the Siberian eruptions took place over a period of 200,000 years, the carbon dioxide content in the atmosphere it would double. Some climate models suggest that such an increase in CO2 would have raised global temperatures by 1.5 °C up to 4.5 °C, an increase thought unlikely to cause a catastrophe like the PT extinction.
However one theory, popularized by the 2005 documentary Miracle Planet, is that minimal volcanic heating caused a methane hydrate release, and this would have created a vicious warming cycle, given that methane is 25 times more efficient than CO2in exacerbate global warming.
Methane hydrates
Scientists have found global evidence of a rapid decrease of about 10‰ (parts per thousand) in the 1C13/C12 ratio in carbonate rocks dating back to the Late Permian (C13 delta of -10‰). This is the first, largest, and fastest of a series of negative and positive excursions (increases and decreases in the C13/C12 ratio) that continued until the rate of isotopes abruptly stabilized in the Middle Triassic, followed by the recovery of life forms that used calcium carbonate to build solid parts such as shells).
Various factors may have contributed to this decline in the C13/C12 ratio, but most are not sufficient to fully explain it.
Gases erupted from volcanoes have a C13/C12 about 5 to 8‰ below the standard (C13 delta about -5 to -8 ‰). But the amount needed to cause a reduction of about 10‰ around the globe would require eruptions more powerful than any recorded on the face of the earth ever before.
A reduction in organic activity would have removed C12 from the environment more slowly, so that more would be left to incorporate into the sediment, thus reducing the C13/C12 ratio. Biochemical processes use the lighter isotopes as chemical reactions are basically driven by the electromagnetic forces between atoms and the lighter isotopes react more rapidly to these forces. However, an evaluation of a small decrease of 3 to 4‰ in the C13/C12 ratio (C13 delta from -3 to -4 ‰) at the Paleocene-Eocene Thermal Maximum (PETM) established that even by transferring all the organic carbon (of organisms, soils and dissolved in the oceans) in sediments would be insufficient: even such a burial of C12-rich material would not have produced even the smallest decrease in the C13/C12 ratio observed over the PETM period.
Buried sedimentary organic matter has a C13/C12 ratio 20 to 25‰ below normal (C13 delta -20 to -25‰). Theoretically, if sea level fell sharply, surface marine sediments would have remained exposed to oxidation. But it would have taken 6,500 to 8,400 billion tons of organic carbon oxidized and returned to the ocean and atmosphere in less than a few hundred thousand years to reduce the C13/C12 ratio by 10‰. It is not believed that this could have been a realistic possibility.
Rather than a sudden drop in sea level, intermittent periods of ocean floor hyperoxia and anoxia (high oxygen / low oxygen conditions) may have caused the C13/C12 ratio fluctuations in the Early Triassic, and global anoxia could be responsible for the extinction at the end of the Permian. Continents in the late Permian and early Triassic were more clustered in the tropical zone than now, and large tropical rivers would have accumulated sediments in small, partially enclosed oceanic basins at low latitudes. These conditions would have favored episodes of anoxia interspersed with episodes of hyperoxia, which would have released large amount of organic carbon that has a low C13/C12 ratio since biochemical processes use the lightest isotopes. This or another organically based reason may have been responsible for this fluctuation in the C13/C12 ratio and for that which occurred earlier in the Proterozoic-Cambrian boundary.
Other hypotheses include large-scale ocean poisoning following a large amount of CO2 release and a long-term reorganization of the global carbon cycle.
However, only one cause has been proposed that is powerful enough to justify the global 10‰ reduction in the C13:C12 ratio: the release of methane from methane hydrates, and carbon cycle models confirm that this would have been sufficient to produce the observed reduction. Methane hydrates, also known as methane clathrates, are methane molecules trapped in cages of water molecules. Methane is produced by methanogens (microscopic single-celled organisms) and has a C13/C12 ratio of 60‰ below normal (delta C13 -60 ‰). With the right combination of pressure and temperature it is trapped in clathrates very close to the permafrost surface and in even greater quantities at the margins of the continental shelf and in the nearby ocean floors. Oceanic methane hydrates have been found in sediments dating to depths of at least 300 m underwater. They can be found down to 2,000 m deep, but generally do not form below 1,100 m.
The area covered by lava from Siberian Trap eruptions is about twice as large as originally thought, and most of this area was shallow sea during the Late Permian. It is probable that the seabed contained deposits of methane hydrates and that the lava caused these deposits to dissociate, releasing large amounts of methane.
Since methane is a very potent greenhouse gas, a huge release of methane is expected to cause significant global warming. A "methane eruption" may have released the equivalent of 10,000 billion tons of carbon dioxide - double what can be found in fossil fuels on Earth. There is strong evidence that global temperature increased by about 6 °C near the equator and perhaps even more at higher latitudes: a sudden decrease in the oxygen isotope ratio (O18/O16), the extinction of plants in the type Glossopteris (the Glossopteris and the other plants that grew in the same areas), which needed a cold climate and the replacement with a flora typical of lower latitudes.
However, the expected pattern of isotope changes as a result of a massive methane release does not match the situation observed across the Early Triassic. Not only would the methane hypothesis require the release of five times the amount of methane postulated for the PETM, but this amount would have to have been reabsorbed at an unrealistic rate to explain the rapid increase in the C13/C12 ratio (episodes of very positive delta C13) during the Early Triassic, before being further released several times.
Sea Level Fluctuations
A marine regression occurs when submerged areas near the coastline remain exposed above sea level. This lowering of sea level causes a reduction of surface marine habitats, leading to biotic replacement. Shallow marine habitats are productive areas for bottom-of-the-food-chain organisms, and their disappearance increases competition for food sources. There is some correlation between the effects of a sea level regression and mass extinctions, but other evidence indicates that there is no relationship and that the regression creates other habitats anyway. It has been hypothesized that changes in sea level resulted in changes in sediment deposition and affected water temperature and salinity, and thus leading to a decline in diversity in the sea.
Anossia (the anosic Event)
There is evidence that the oceans became anoxic (severely oxygen deficient) towards the end of the Permian. There was a noticeable and rapid onset of anoxic deposition in marine sediments around East Greenland. The uranium/ thorium ratio of various Permian sediments indicates that the oceans were highly anoxic during the period of the mass extinction.
This would have been devastating to life in the sea, causing high mortality, except for anaerobic bacteria that inhabited the ocean floor muds. There is also evidence that anoxic events can cause catastrophic emissions of hydrogen sulphide from the seabed.
The possible sequence of events leading to anoxic oceans may have involved a period of global warming that reduced the temperature gradient between the equator and the poles, which would have slowed or even stopped the thermohaline circulation. Slowing or stopping that circulation would have reduced the mixing of oxygen in the ocean.
However, research has suggested that the types of thermohaline oceanic circulation that existed at the end of the Permian could not have sustained deep-sea anoxia.
Hydrogen sulfide emissions
A severe anoxic event at the end of the Permian may have made sulfur-reducing bacteria the dominant species of oceanic ecosystems, causing vast emissions of hydrogen sulfide (H2S) that would have poisoned plant and animal life both on land and in the seas, as well how it would have severely depleted the ozone layer, exposing surviving organisms to lethal levels of ultraviolet radiation. The anaerobic photosynthesis of chlorobes (green sulphurous bacteria), and the consequent emissions of hydrogen sulphide, occurred from the end of the Permian up to the lower Triassic. The fact that this anaerobic photosynthesis continued into the Early Triassic is consistent with the fossil record of the remarkably slow recovery from the mass extinction.
This theory has the advantage of explaining the mass extinction of plants, which otherwise would have had to thrive in an atmosphere with high levels of carbon dioxide. The spores fossils point to another factor in support of the theory: many show deformations that could have been caused by ultraviolet radiation, which would have been much more intense after hydrogen sulfide emissions had weakened the ozone layer.
The supercontinent Pangea
About the middle of the Permian period (in the Kungurian age of the Cisuralian epoch) all the continents merged to form the supercontinent Pangea, surrounded by the superocean Panthalassa, although some blocks of present- day Asia did not join the supercontinent up to the Upper Permian. This configuration greatly reduced the extension of the surface aquatic environments, the most productive section of the sea, and exposed the isolated organisms of the rich continental shelves to competition with other invading organisms. The formation of Pangea would also have altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons near coasts and an arid climate within the supercontinent.
Marine life suffered very high, but not catastrophic, rates of extinction after the formation of Pangaea, almost as high as other mass extinctions. It appears that the formation of Pangea did not cause a significant increase in extinction levels on land, and indeed the expansion and increase in diversity of therapsids appears to have occurred in the Late Permian, after Pangea was almost fully formed. Thus it seems probable that Pangea initiated a long period of extinctions in the sea, but was not directly responsible for the "great die-off" and the end of the Permian.
Combination of Causes
The possible causes that are supported by solid evidence appear to describe a sequence of catastrophes, each worse than the preceding: the eruptions of the Siberian Trap were severe enough on their own, but because they occurred near coal beds and the continental shelf, they also caused the release of large quantities of carbon dioxide and methane. The resulting global warming may have caused the most severe anoxic event in ocean history: According to this theory, the oceans became so anoxic that anaerobic sulfur-reducing organisms came to dominate ocean biochemistry and caused large emissions of hydrogen sulfide.
However, there may be some relatively weak links within this chain of events: predicted changes in the C13/C12 ratio as a result of a massive methane release do not fit patterns observed during the Early Triassic, and it does not appear that the types of oceanic thermohaline circulation that existed at the end of the Permian were capable of sustaining anoxic even in the high seas.
