Permian–Triassic extinction event

K–T

Tr–J

P–Tr

Late D

O–S

Millions of years ago

The Permian–Triassic extinction event is the most significant extinction event in this plot for marine genera. (source and image info)

The Permian–Triassic (P–Tr) extinction event, informally known as the Great Dying,^[1] was an extinction event that occurred 251.4 million years ago,^[2][3] forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with up to 96% of all marine species^[4] and 70% of terrestrial vertebrate species becoming extinct; it is the only known mass extinction of insects.^[5][6] Fifty-seven percent of all families and 83% of all genera were killed. Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than after other extinction events.^[4] This event has been described as the "mother of all mass extinctions".^[7]

Researchers have variously suggested that there were from one to three distinct pulses, or phases, of extinction.^[8][2][9] There are several proposed mechanisms for the extinctions; the earlier phase was likely due to gradual environmental change, while the latter phase has been argued to be due to a catastrophic event. Suggested mechanisms for the latter include large or multiple bolide impact events, increased volcanism, and sudden release of methane clathrate from the sea floor; gradual changes include sea-level change, anoxia, increasing aridity,^[10] and a shift in ocean circulation driven by climate change.

Dating the extinction

Until about the year 2000 it was 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.^[11] However, a study of uranium/lead ratios of zircons from rock sequences near Meishan, Changxing, Zhejiang Province, China^[3] date the extinction to 251.4  ±0.03 Ma, with an ongoing elevated extinction rate occurring for some time thereafter.^[2] A large (-9‰^[12]), abrupt global change in the ratio of ¹³C to ¹²C, denoted δ¹³C, coincides with this extinction,^{[13][14][15][16]} and is sometimes used to identify the Permian-Triassic boundary in rocks that are unsuitable for radiometric dating.^[17]

It has been suggested that the Permian-Triassic boundary is associated with a sharp increase in the abundance of marine and terrestrial fungi, and that this was caused by the sharp increase in the amount of dead plants and animals fed upon by the fungi.^[18] For a while this "fungal spike" was used by some paleontologists to identify the Permian-Triassic boundary in rocks that are unsuitable for radiometric dating or lack suitable index fossils, but even the proposers of the fungal spike hypothesis pointed out that "fungal spikes" may have been a repeating phenomenon created by the post-extinction ecosystem in the earliest Triassic.^[18] More recently the very idea of a fungal spike has been criticized on several grounds, including that: Reduviasporonites, the most common supposed "fungal spore", was actually a fossilized alga;^[12][19] the spike did not appear worldwide;^[20][21] and in many places it did not fall on the Permian-Triassic boundary.^[22] The algae which were mis-identified as fungal spores may even represent a transition to a lake-dominated Triassic world rather than an earliest Triassic zone of death and decay in some terrestrial fossil beds.^[23] However, newer chemical evidence agrees better with a fungal origin for Reduviasporonites, diluting these critiques.^[24]

Uncertainty exists regarding the duration of the overall extinction and about the timing and duration of various groups' extinctions within the greater process. Some evidence suggests 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.^[22][25] Statistical analyses of some highly fossiliferous strata in Meishan, South China suggest that the main extinction was clustered around one peak.^[2] Recent research shows that different groups went extinct at different times; for example, while difficult to date absolutely, ostracod and brachiopod extinctions were separated by between 0.72 and 1.22 million years.^[26] In a well preserved sequence in east Greenland, the decline of animals is concentrated in a period 10 to 60 thousand years long, with plants taking several hundred thousand further years to show the full impact of the event.^[27] An older theory, still supported in some recent papers,^[28] is that there were two major extinction pulses 5 million 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.^[29] For example, all but one of the surviving dinocephalian genera died out at the end of the Guadalupian,^[30] as did the Verbeekinidae, a family of large-size fusuline foraminifera.^[31] The impact of the end-Guadalupian extinction on marine organisms appears to have varied between locations and between taxonomic groups - brachiopods and corals had severe losses.^[32][33]

Extinction patterns

The event had a profound effect on the terrestrial ecosystem, which is still being felt today, a quarter of a billion years later.^[6] In the late Permian, there were many sorts of reptiles and amphibians on land, together with many plants, especially ferns but also conifers and ginkgos. There were also complicated coral reef ecologies undersea.^[34] By this time, Pangea was in existence, and animals could roam freely. There were lush jungles, deserts, and oceanic environments. After the extinction, one genus of land vertebrate dominated: a medium-sized herbivore called Lystrosaurus. Only one genus of sea life is common after the extinction as well: a brachiopod called Lingula. Eventually other genera and species seem to reappear - the so-called "Lazarus taxa", named after the Biblical character who returned from the dead. Clearly they must have survived the extinction event, but in very low numbers.^[34] Like the end-Ordovician event, it seems to have been composed of two bursts, separated by an interval of about 10 million years, the second being the larger of the two. Notable extinction happened again amongst brachiopods, ammonoids, and corals, as well as gastropods and, unusually, insects. It took about 50 million years for life on land to fully recover its biodiversity. Nothing resembling a coral reef shows up until 10 million years after the Permian extinction, and full recovery of marine life took about 100 million years.^[34]

Marine organisms

Marine invertebrates suffered the greatest losses 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.^[2]

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.^[36] The extinction primarily affected organisms with calcium carbonate skeletons, especially those reliant on ambient CO₂ levels to produce their skeletons.^[37]

Among benthic organisms, the extinction event multiplied background extinction rates, and therefore caused most damage to taxa that had a high background extinction rate (by implication, taxa with a high turnover).^[38][39] The extinction rate of marine organisms was catastrophic.^{[2][7][40][41]}

Marine invertebrate groups which survived include: 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.

The groups with the highest survival rates generally had active control of circulation, elaborate gas exchange mechanisms, and light calcification; more heavily calcified organisms with simpler breathing apparatus were the worst hit.^[42][43] In the case of the brachiopods at least, surviving taxa were generally small, rare members of a diverse community.^[44]

The ammonoids, which had been in a long-term decline for the 30 million years since the Roadian (middle Permian), suffered a selective end-Guadalupian extinction pulse. This extinction greatly reduced disparity, and suggests that environmental factors were responsible for this extinction. Diversity and disparity fell further until the P-Tr boundary; the extinction here was non-selective, consistent with a catastrophic initiator. During the Triassic, diversity rose rapidly, but disparity remained low.^[45]

The range of morphospace occupied by the ammonoids became more restricted as the Permian progressed. Just a few million years into the Triassic, the original morphospace range was once again occupied, but shared differently between clades.^[46]

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,^[5] 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.^[7]

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.^[5]

Terrestrial plants

Plant ecosystem response

The geological record of terrestrial plants is sparse, and based mostly on pollen and spore studies. Interestingly, plants are relatively immune to mass extinction, with the impact of all the major mass extinctions "negligible" at a family level.^[12] Even the reduction observed in species diversity (of 50%) may be mostly due to taphonomic processes.^[12] However, a massive rearrangement of ecosystems does occur, with plant abundances and distributions changing profoundly.^[12]

At the P–Tr boundary, the dominant floral groups changed, with many groups of land plants entering abrupt decline, such as Cordaites (gymnosperms) and Glossopteris (seed ferns).^[47] Dominant gymnosperm genera were replaced post-boundary by lycophytes - extant lycophytes are recolonizers of disturbed areas.^[48]

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 flora 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 δ¹³C values, but occurs many years after. ^[49] The recovery of gymnosperm forests would take 4-5 million years.^[12]

Coal gap

No coal deposits are known from the Early Triassic, and those in the Middle Triassic are thin and low-grade.^[50] This "coal gap" has been explained in many ways. It has been suggested that new, more aggressive fungi, insects and vertebrates evolved, and killed vast amounts of trees. However these decomposers themselves suffered heavy losses of species during the extinction, and are not considered a likely cause of the coal gap.^[50] It could simply be that all coal forming plants were rendered extinct by the P/Tr extinction, and that it took 10 million years for a new suite of plants to adapt to the moist, acid conditions of peat bogs.^[50] On the other hand, abiotic factors (not caused by organisms), such as decreased rainfall or increased input of clastic sediments, may also be to blame.^[12] Finally, it is also true that there are very few sediments of any type known from the Early Triassic, and the lack of coal may simply reflect this scarcity. This opens the possibility that coal-producing ecosystems may have responded to the changed conditions by relocating, perhaps to areas where we have no sedimentary record for the Early Triassic.^[12] For example in eastern Australia a cold climate had been the norm for a long period of time, with a peat mire ecosystem specialising to these conditions. Approximately 95% of these peat-producing plants went locally extinct at the P-Tr boundary;^[51] Interestingly, coal deposits in Australia and Antarctica disappear significantly before the P-Tr boundary.^[12]

Terrestrial vertebrates

Even the groups that survived suffered extremely heavy losses of species, and some terrestrial vertebrate groups very nearly became extinct at the end-Permian. Some of the surviving groups did not persist for long past this period, while others that barely survived went on to produce 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).^[52][53]

Possible explanations of these patterns

The most vulnerable marine organisms were those which produced calcareous hard parts (i.e. from calcium carbonate) and had low metabolic rates and weak respiratory systems - notably calcareous sponges, rugose and tabulate corals, calciate brachiopods, bryozoans, and echinoderms; about 81% of such genera became extinct. Close relatives which did not produce calcareous hard parts suffered only minor losses, for example sea anemones, from which modern corals later evolved. Animals which had high metabolic rates, well-developed respiratory systems and non-calcareous hard parts had negligible losses - except for conodonts, in which 33% of genera died out.^[54]

This pattern is consistent with what is known about the effects of hypoxia, a shortage but not a total absence of oxygen. However, hypoxia cannot have been the only killing mechanism for marine organisms. Nearly all of the continental shelf waters would have had to become severely hypoxic to account for the magnitude of the extinction, but such a catastrophe would make it difficult to explain the very selective pattern of the extinction. Models of the Late Permian and Early Triassic atmospheres show a significant but protracted decline in atmospheric oxygen levels, with no acceleration near the P-Tr boundary. Minimum atmospheric oxygen levels in the Early Triassic are never less than present day levels - the decline in oxygen levels does not match the temporal pattern of the extinction.^[54]

The observed pattern of marine extinctions is also consistent with hypercapnia (excessive levels of carbon dioxide). Carbon dioxide (CO₂) is actively toxic at above-normal concentrations, as it reduces the ability of respiratory pigments to oxygenate tissues, and makes body fluids more acidic, thereby hampering the production of carbonate hard parts like shells. At high concentrations, carbon dioxide causes narcosis (intoxication).^[55] In addition to these direct effects, CO₂ reduces the concentration of carbonates in water by "crowding them out", which further increases the difficulty of producing carbonate hard parts.

Marine organisms are more sensitive to changes in CO₂ levels than are terrestrial organisms for a variety of reasons. CO₂ is 28 times more soluble in water than is oxygen. Marine animals normally function with lower concentrations of CO₂ in their bodies than land animals, as the removal of CO₂ in air-breathing animals is impeded by the need for the gas to pass through the respiratory systems membranes (lungs, tracheae, and the like). In marine organisms, relatively modest but sustained increases in CO₂ concentrations hamper the synthesis of proteins, reduce fertilization rates, and produce deformities in calcareous hard parts.^[54] In addition, an increase in CO₂ concentration leads to ocean acidification, consistent with the preferential extinction of heavily calcified taxa and signals in the rock record that suggest a more acidic ocean.^[56]

It is difficult to analyze extinction and survival rates of land organisms in detail, because there are few terrestrial fossil beds that span across the Permian-Triassic boundary. Triassic insects are very different from those of the Permian, but there is a gap in the insect fossil record spanning approximately 15M years from the late Permian to early Triassic. The best known record of vertebrate changes across the Permian-Triassic boundary occurs in the Karoo Supergroup of South Africa, but statistical analyses have so far not produced clear conclusions.^[54]

Biotic recovery

Earlier analyses indicated that life on Earth recovered quickly after the Permian extinctions, but this was mostly in the form of disaster taxa, such as the hardy Lystrosaurus. The most recent research indicates that the specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to recover. It is thought that this long recovery was due to the successive waves of extinction which inhibited recovery, as well as to prolonged environmental stress to organisms which continued into the Early Triassic. Recent research indicates that recovery did not begin until the start of the mid-Triassic, 4 million to 6 million years after the extinction;^[57] and some writers estimate that the recovery was not complete until 30M years after the P-Tr extinction, i.e. in the late Triassic.^[58]

A braided river - the Waimakariri River in the South Island of New Zealand.

During the early Triassic (4-6M years after the P-Tr extinction), the plant biomass was insufficient to form coal deposits, which implies a limited food mass for herbivores.^[50] River patterns in the Karoo changed from meandering to braided, indicating that vegetation there was very sparse for a long time.^[59]

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 worldwide, for example: the herbivorous therapsid Lystrosaurus (which accounted for about 90% of early Triassic land vertebrates) 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.^[47][60]

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); Dicroidium (a seed fern).^{[10][60][61][62]}

Sessile filter feeders like this Crinoid were significantly less abundant after the P-Tr extinction.

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.^[63]

Before the Permian mass extinction event, both complex and simple marine ecosystems were equally common; after the recovery from the mass extinction, the complex communities outnumbered the simple communities by nearly three to one,^[63] and the increase in predation pressure led to the Mesozoic Marine Revolution.

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.^[64]

Crinoids ("sea lilies") suffered a selective extinction, resulting in a decrease in the variety of forms in which they grew.^[65] Their ensuing adaptive radiation was brisk, and resulted in forms possessing flexible arms becoming widespread; motility, predominantly a response to predation pressure, also became far more prevalent.^[66]

Land vertebrates

Lystrosaurus was by far the most abundant early Triassic land vertebrate.

Lystrosaurus, a pig-sized herbivorous dicynodont therapsid, constituted as much as 90% of some earliest Triassic land vertebrate faunas.^[10] Smaller carnivorous cynodont therapsids also survived, including the ancestors of mammals. In the Karoo region of southern Africa the therocephalians Tetracynodon, Moschorhinus and Ictidosuchoides survived but do not appear to have been abundant in the Triassic.^[67]

Archosaurs (which included the ancestors of dinosaurs and crocodilians) were initially rarer than therapsids, but they began to displace therapsids in the mid-Triassic.^[10] In the mid to late Triassic the dinosaurs evolved from one group of archosaurs, and went on to dominate terrestrial ecosystems for the rest of the Mesozoic.^[68] This "Triassic Takeover" may have contributed to the evolution of mammals by forcing the surviving therapsids and their mammaliform successors to live as small, mainly nocturnal insectivores; nocturnal life probably forced at least the mammaliforms to develop fur and higher metabolic rates.^[69]

Some temnospondyl amphibians made a relatively quick recovery, in spite of nearly becoming extinct. Mastodonsaurus and trematosaurians were the main aquatic and semi-aquatic predators during most of the Triassic, some preying on tetrapods and others on fish.^[70]

Land vertebrates took an unusually long time to recover from the P-Tr extinction; one writer estimates that the recovery was not complete until 30 million years after the extinction, i.e. not until the Late Triassic, in which dinosaurs, pterosaurs, crocodiles, archosaurs, amphibians and mammaliforms were abundant and diverse.^[4]

Causes of extinction event

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 volcanism, or sudden release of methane hydrates from the sea floor. The latter include sea-level change, anoxia, and increasing aridity.^[10] Any hypothesis about the cause must explain the selectivity of the event, which primarily affected organisms with calcium carbonate skeletons; the long (4-6 million year) period before recovery started; and the minimal extent of biological mineralization (despite inorganic carbonates being deposited) once the recovery began.^[37]

Impact event

Artist's impression of a major impact event. A collision between Earth and an asteroid a few kilometers in diameter would release as much energy as several million nuclear weapons detonating.

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;^[71][72] fullerenes trapping extraterrestrial noble gases;^[73] meteorite fragments in Antarctica;^[74] and grains rich in iron, nickel and silicon, which may have been created by an impact.^[75] However, the accuracy of most of these claims has been challenged.^{[76][77][78][79]} Quartz from Graphite Peak in Antarctica, for example, once considered "shocked," 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.^[80]

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,^[72] the so-called Wilkes Land crater of East Antarctica,^[81][82] and, even more speculatively, the Gulf of Mexico.^[83] In each of these cases the idea that an impact was responsible has not been proven, and some have 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 likely that the crater would no longer exist. As 70% of the Earth's surface is sea, an asteroid or comet fragment is more than twice as likely to hit ocean as it is to hit land. However, Earth has no ocean-floor crust more than 200 million 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.^[84]

One attraction of large impact theories is that theoretically they could trigger other cause-considered extinction-paralleling phenomena^[85], such as the Siberian Traps eruptions (see below) as being either an impact site^[86] or the antipode of an impact site.^[85][87] Subduction should not be taken as an excuse that no firm evidence can be found; much like the K-T event, an ejecta blanket stratum rich in siderophilic elements (e.g. iridium) would be found in a great many formations from the time. The abruptness of an impact would also explain why species did not rapidly evolve in adaptation to more slowly-manifesting and/or less than global-in-scope phenomena.

Volcanism

The world around the time of the P-Tr extinction. The Siberian Traps eruptions occurred on the eastern shore of the shallow sea (paler blue) at the north of the map. The earlier Emeishan eruptions occurred on the north edge of the almost enclosed shallow sea just north of the equator - at this time the blocks that currently form China and South-East Asia were just emerging.

The final stages of the Permian saw two flood basalt events. A small one, Emeishan Traps in China, occurred at the same time as the end-Guadalupian extinction pulse, in an area which was close to the equator at the time.^[88][89] The flood basalt eruptions which produced the Siberian Traps constituted one of the largest known volcanic events on Earth and covered over 2,000,000 square kilometers (772,204.3 sq mi) with lava.^[90][91][92] 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.^[2][93]

The Emeishan and Siberian Traps eruptions may have caused dust clouds and acid aerosols which would have blocked out sunlight and thus disrupted photosynthesis both on land and in the photic zone of the ocean, 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 molluscs 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 and the warming would have proceeded without any mitigating effects.^[85]

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.^[94] The basalt lava erupted or intruded into carbonate rocks and into sediments which were in the process of forming large coal beds, both of which would have emitted large amounts of carbon dioxide, leading to stronger global warming after the dust and aerosols settled.^[85]

There is doubt, however, about whether these eruptions were enough on their own to cause a mass extinction as severe as the end-Permian. Equatorial eruptions are necessary to produce sufficient dust and aerosols to affect life worldwide, whereas the much larger Siberian Traps eruptions were inside or near the Arctic Circle. Furthermore, if the Siberian Traps eruptions occurred within a period of 200,000 years, the atmosphere's carbon dioxide content would have doubled. Recent climate models suggest that such a rise in CO₂ would have raised global temperatures by 1.5 °C (2.7 °F) to 4.5 °C (8.1 °F), which is unlikely to cause a catastrophe as great as the P-Tr extinction.^[85]

Methane hydrate gasification

Scientists have found worldwide evidence of a swift decrease of about 10 ‰ (parts per thousand) in the ¹³C/¹²C isotope ratio in carbonate rocks from the end-Permian (δ¹³C_carbonate of -10 ‰).^[41][95] This is the first, largest and most rapid of a series of negative and positive excursions (decreases and increases in ¹³C/¹²C ratio) that continues until the isotope ratio abruptly stabilises in the middle Triassic, followed soon afterwards by the recovery of calcifying life forms (organisms that use calcium carbonate to build hard parts such as shells).^[42]

A variety of factors may have contributed to this drop in the ¹³C/¹²C ratio, but most turn out to be insufficient to account fully for it:^[96]

• Gases from volcanic eruptions have a ¹³C/¹²C ratio about 5 to 8 ‰ below standard (δ¹³C about -5 to -8 ‰). 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.^[97]
• 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  ‰ in ¹³C/¹²C (δ¹³C -3 to -4 ‰) at the Paleocene-Eocene Thermal Maximum (PETM) concluded that even transferring all the organic carbon (in organisms, soils, and dissolved in the ocean) into sediments would be insufficient: even such a large burial of material rich in ¹²C would not have produced the smaller drop in the ¹³C/¹²C ratio of the rocks around the PETM.^[97]
• Buried sedimentary organic matter has a ¹³C/¹²C ratio 20 to 25 ‰ below normal (δ¹³C -20 to -25 ‰). 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 10 ‰. This is not thought to be a realistic possibility.^[7]
• Rather than a sudden decline in sea level, intermittent periods of ocean-bottom hyperoxia and anoxia (high-oxygen and low- / zero-oxygen conditions) may have caused the ¹³C/¹²C ratio fluctuations in the Early Triassic;^[42] and global anoxia may have been responsible for the end-Permian blip. The continents of the end-Permian and early Triassic were more clustered in the tropics than they are now (see map above), and large tropical rivers would have dumped sediment into smaller, partially enclosed ocean basins in low latitudes. Such conditions favor oxic and anoxic episodes; oxic / anoxic conditions would result in a rapid release / burial respectively of large amounts of organic carbon, which has a low ¹³C/¹²C ratio because biochemical processes use the lighter isotopes.^[98] This, or another organic-based reason, may have been responsible for both this and a late Proterozoic/Cambrian pattern of fluctuating ¹³C/¹²C ratios.^[42]

Other hypotheses include mass oceanic poisoning releasing vast amounts of CO₂^[99] and a long-term reorganisation of the global carbon cycle.^[96]

However, only one sufficiently powerful cause has been proposed for the global 10 ‰ reduction in the ¹³C/¹²C ratio: the release of methane from methane clathrates;^[7] and carbon-cycle models confirm that it would have been sufficient to produce the observed reduction.^[96][99] Methane clathrates, also known as methane hydrates, consist of methane molecules trapped in cages of water molecules. The methane is produced by methanogens (microscopic single-celled organisms) and has a ¹³C/¹²C ratio about 60 ‰ below normal (δ¹³C -60  ‰). 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 sea floor, but usually only about 1,100 meters (3,609 ft) below the sea floor.^[100]

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.^[101]

One would expect a vast release of methane to cause significant global warming, since methane is a very powerful greenhouse gas. A "methane burp" could have released 10,000 billion tons of carbon dioxide equivalent - twice as much as in all the fossil fuels on Earth.^[34] There is strong evidence that global temperatures increased by about 6 °C (10.8 °F) near the equator and therefore by more at higher latitudes: a sharp decrease in oxygen isotope ratios (¹⁸O/¹⁶O);^[102] 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.^[10][103]

However, the pattern of isotope shifts expected to result from a massive release of methane do not match the patterns seen throughout the early Triassic. Not only would a methane cause require the release of five times as much methane as postulated for the PETM,^[42] but it would also have to be re-buried at an unrealistically high rate to account for the rapid increases in the ¹³C/¹²C ratio (episodes of high positive δ¹³C) throughout the early Triassic, before being released again several times.^[42]

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, their loss increasing competition for food sources.^[104] There is some correlation between incidents of pronounced sea level regression and mass extinctions, but other evidence indicates there is no relationship and that regression may itself create new habitats.^[10] 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.^[105]

Anoxia

There is evidence that the oceans became anoxic (severely deficient in oxygen) 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.^[106] The uranium/thorium ratios of several late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction.^[107]

This would have been devastating for marine life, producing widespread die-offs except for anaerobic bacteria inhabiting the sea-bottom mud. There is also evidence that anoxic events can cause catastrophic hydrogen sulfide emissions from the sea floor (see below).

The possible sequence of events leading to anoxic oceans might have involved a period of global warming that reduced the temperature gradient between the equator and the poles which slowed or perhaps even stopped the thermohaline circulation. The slow-down or stoppage of the thermohaline circulation could have reduced the mixing of oxygen in the ocean.^[107]

However, one research article suggests 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.^[108]

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 vast 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.^[109] 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.^[110]

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

Map of Pangaea showing where today's continents were at the Permian–Triassic boundary

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.^[111] 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.^[54] 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.^[54]

However, there may be some weak links in this chain of events: the changes in the ¹³C/¹²C ratio expected to result from a massive release of methane do not match the patterns seen throughout the early Triassic;^[42] and 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.^[108]

In a paper published in the December 2002 issue of the American Association of Petroleum Geologists' publication "Explorer", Canadian geologist Michael S. Stanton proposed that the Gulf of Mexico is a giant astrobleme created at the time of the Permian extinction event. Stanton stated the various reasons for his theory by explaining that if the gulf is of impact origin, it would be similar in size to the Moon's smallest maria.^[112]

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)

External links

• "The Permo–Triassic extinction" Introduction.
• "The Permo–Triassic extinction" A more detailed introduction. Bibliography.
• "Siberian Traps Website"
• BBC2 'The Day the Earth Nearly Died' website.
• 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 [http://www.sciencemag.org/cgi/content/abstract/289/5485/1740 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?"
• 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
• Ward, P.D. (2006) "Impact from the Deep". Scientific American October 2006.