Permian-Triassic extinction event

The Permian-Triassic (P-T or PT) 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 and 70 percent of terrestrial vertebrate species becoming extinctAdditional resource:

Ceratopsian crest as acoustic amplifier can be found in published work:

Anton, J.A. Dinosaurs Incognito. 2009. VDM Verlag. Germany. pp. 192..

The Permian-Triassic extinction event, labeled "End P" here, is the most significant extinction event in this plot for marine fossiliferous genera.

Duration of event

At one time, this die-off was assumed to have been a gradual reduction over several million years. Now, however, it is commonly accepted that the event lasted less than a million years, from 252.3 to 251.4 Ma (both numbers ±300,000 years), a very brief period of time in geological terms. A detailed study of plutonium-to-lead decay in zircons in ash beds in China dates the extinction 252.6 ± 0.2 million years ago, synchronous with the Siberian flood volcanism (Mundil 2004).

Organisms throughout the world, regardless of habitat, suffered similar rates of extinction over the same relatively short period, showing that the extinction was global and sudden, not gradual or localized.

New evidence from strata in Greenland shows evidence of a double extinction, with a separate, less dramatic extinction occurring 9 Ma before the Permian-Triassic (P-T) boundary, at the end of the Guadalupian epoch. Confusion of these two events is likely to have influenced the early view that the extinction was extended.

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. (Palaeos)

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.

Complex ecosystems in which species interacted with one another became much more common after the event.

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. 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. 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.

Land vertebrates

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

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

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

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. Recently, a group of scientists claimed they have discovered a 300-mile-wide (480 km) crater in the Wilkes Land region of East Antarctica, which they believe may be linked with the extinction.

The supercontinent Pangaea

In the Kungurian age of the Permian's Cisuralian epoch (about half way through the Permian) all the continents joined to form the super-continent Pangaea and the super-ocean Panthalassa.

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

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

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

Antarctic impact event

In June of 2006, Dr. Ralph von Frese announced the discovery of the Wilkes Land crater in the Wilkes Land region of East Antarctica, which may mark the site of the impact that caused the Permian-Triassic extinction. A 300-mile-wide crater more than a mile beneath the East Antarctic Ice Sheet was found using gravity fluctuations measured by NASA's GRACE satellites to peer beneath Antarctica's icy surface, imaging a 200-mile-wide (320 km) plug of mantle material—a mass concentration, or "mascon" in geological parlance—that occurs within the Earth's crust and appears to have been emplaced somewhere between 100 and 500 million years ago—a broad time span that brackets the specific age of the Permian-Triassic extinction.

When the scientists overlaid their gravity image with airborne radar images of the ground beneath the ice, they found the mascon perfectly centered inside a circular ridge some 500 km (300 mi) wide. The Wilkes Land crater is more than twice the size of the Chicxulub crater in the Yucatan peninsula, which marks the impact that may have ultimately killed the dinosaurs 65 million years ago. The Chicxulub impactor (most likely an asteroid) is thought to have been 6 miles (10 km) wide, while the Wilkes Land impactor (either an asteroid or perhaps a large comet nucleus) could have been up to 30 miles (50 km) wide—four or five times as wide.

The gravity measurements also suggest that it could have set the stage for the breakup of the ancient Gondwana supercontinent by creating the tectonic rift that later pushed Australia northward. Approximately 100 million years ago, Australia split from Gondwana and began drifting north, pushed away by the expansion of a rift valley into the eastern Indian Ocean.

When large bolides (asteroids or comets) impact Earth, the aftermath weakens or kills much of the life that thrived previously. Release of debris and carbon dioxide into the atmosphere reduces the productivity of life and causes both global warming and ozone depletion. Analysis of the ratios of carbon and boron isotopes in the fossil record provides evidence of increased levels of atmospheric carbon dioxide. Material from the Earth's mantle released during volcanic eruption has also been shown to contain iridium, an element associated with meteorites. At present, there is only limited and disputed evidence of iridium and shocked quartz occurring with the Permian event, though such evidence has been very abundantly associated with an impact origin for the Cretaceous-Tertiary extinction event.

If the estimated date of the Wilkes Land, Antarctica, event is not correct, and a different extraterrestrial impact triggered the Permian extinction, the crater record of such an event would most likely be erased because there is no Permian-age oceanic crust remaining; all of it has been subducted, so plate tectonics during the last 252 million years have erased any possible P-T seafloor crater.

Adrian Jones, at University College London, has modeled the effects of impacts on the Earth's geological crust and suggests that after an impact, the crust rebounds to form a large shallow crater. In a truly massive impact, the combined heat of the impact and rebound is enough to melt the crust. Lava floods through and the crater disappears beneath new crust. If Jones is right, the Permian meteorite crater can't be found because it doesn't exist.

But in the past geologist John Gorter of Agip found evidence of a circular structure 200 kilometers (125 mi) in diameter called the Bedout, in currently submerged continental crust off the northwestern coast of Australia, and geologist Luann Becker, of the University of California, confirmed it, finding shocked quartz and brecciated mudstones. The geology of the area of continental shelf dates to the end of the Permian. The Bedout impact crater is also associated in time with extreme volcanism and the break-up of Pangaea. "We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time," Dr. Becker explains. "This is what happened 65 million years ago at Chicxulub but was largely dismissed by scientists as merely a coincidence. With the discovery of Bedout, I don't think we can call such catastrophes occurring together a coincidence anymore," Dr. Becker added in a news release.

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. The resulting direct ultra-violet radiation from the sun would weaken or kill nearly all existing species. Only those deep in the oceans would be unaffected. Statistical frequency of supernovae suggests that one at the P-T boundary would not be unlikely. A gamma ray burst (the most energetic explosions in the universe; believed to be caused by a very massive supernova or two objects as dense as neutron stars colliding) that occurred within ~6000 light years would produce the same effect.

Unfortunately there appears to be no independent evidence that a supernova occurred near the earth at the right time.

Volcanism

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

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

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

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

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

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.

This would have been devastating for marine life, except for anerobic 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:

• 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 is more saline 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:

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

Methane hydrate gasification

In 2002 a BBC2 'Horizon' documentary, 'The Day the Earth Nearly Died,' summarized some recent findings and speculation concerning the Permian extinction event. Paul Wignall examined Permian strata in Greenland, where the rock layers devoid of marine life are tens of meters thick. With such an expanded scale, he could judge the timing of deposition more accurately and ascertained that the entire extinction lasted merely 80,000 years and showed three distinctive phases in the plant and animal fossils they contained. The extinction appeared to kill land and marine life selectively at different times. Two periods of extinctions of terrestrial life were separated by a brief, sharp, almost total extinction of marine life. Such a process seemed too long, however, to be accounted for by a meteorite strike. His best clue was the carbon isotope balance in the rock, which showed an increase in carbon-12 over time. The standard explanation for such a spike – rotting vegetation – seemed insufficient.

Geologist Gerry Dickens suggested that the increased carbon-12 could have been rapidly released by upwellings of frozen methane hydrate from the seabeds. Experiments to assess how large a rise in deep sea temperature would be required to sublimate solid methane hydrate suggested that a rise of 5°C (10 F) would be sufficient. Released from the pressures of the ocean depths, methane hydrate expands to create huge volumes of methane gas, one of the most powerful of the greenhouse gases. The resulting additional 5°C rise in average temperatures would have been sufficient to kill off most of the life on earth.

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

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.

External links

• "The Permo-Triassic extinction" Introduction.
• "The Permo-Triassic extinction" A more detailed introduction. Bibliography.
• BBC2 'The Day the Earth Nearly Died' website.
• BBC: "The Extinction Files" (ed: this link is now dead and a search reveals nothing matching the title...)
• PBS series Evolution: "Extinction!" video segment
• Luann Becker, "Exploring Antarctica: Understanding Life on Earth and Beyond": includes links to scientific papers
• SpaceRef: "Big Bang in Antarctica: Killer Crater Found Under Ice" Radar images courtesy of Ohio State University.
• Science Daily: Global warming led to atmospheric hydrogen sulfide and Permian extinction
• Science Daily: Big Bang In Antarctica: Killer Crater Found Under Ice
• Lee Siegel, "Rocks Reveal Details of Mass Extinction" Based on Peter D. Ward, David R. Montgomery, Roger Smith, "Altered River Morphology in South Africa Related to the Permian-Triassic Extinction", in Science 8 September 2000
• David Morrison, "Did an Impact Trigger the Permian-Triassic Extinction?"
• Gregory J. Retallack, John J. Veevers, and Ric Morante, "Global coal gap between Permian-Triassic extinction and Middle Triassic recovery of peat-forming plants" GSA Bulletin, 108/2 (February 1996) pp 195-207
• Giant Crater Found: Tied to Worst Mass Extinction Ever Robert Roy Britt (SPACE.com) 1 June 2006 06:07 p.m. ET
• Rocks Reveal Details of Mass Extinction Lee Siegel (SPACE.com) 02:44 p.m. ET 7 September 2000
• "Permian Extinction Event" BBC News extract on the History Files
• Grice et al (2005) "Photic Zone Euxinia During the Permian-Triassic Superanoxic Event" (Science Vol. 307. no. 5710) - abstract
• Kump, L.R., Pavlov, A., and Arthur, M.A. (2005). "Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia". Geology v. 33, p.397–400
• Wignall, P.B., and Twitchett, R.J. (2002) Permiam-Triassic sedimentology of Jameson Land, East Greenland: Incised submarine channels in an anoxic basin (Journal of the Geological Society, Nov 2002)

Other resources

• Becker L, Poreda R J, Hunt A G, Bunch T E, Rampino M, "Impact Event at the Permian-Triassic Boundary: Evidence from Extraterrestrial Noble Gases in Fullerenes" Science (2001) 291 pp 1530-33.
• Benton M J (2003) When Life Nearly Died: The Greatest Mass Extinction of All Time, Thames & Hudson. Overview written for the layman.
• Erwin D, 2006. Extinction - How life on earth nearly ended 250 million years ago. Princeton University Press, Princeton, New Jersey.
  Summarised by National Geographic
• Mundil, Roland, Kenneth R. Ludwig, Ian Metcalfe, Paul R. Renne, 2004. "Age and Timing of the Permian Mass Extinctions: U/Pb Dating of Closed-System Zircons", Science Magazine, (17 September 2004) pp 1760-63. (On-line abstract)
• Over, Jess (editor), Understanding Late Devonian and Permian-Triassic Biotic and Climatic Events, (Volume 20 in series Developments in Palaeontology and Stratigraphy (2006). The state of the inquiry into the extinction events.
• Sweet, Walter C. (editor), Permo-Triassic Events in the Eastern Tethys : Stratigraphy Classification and Relations with the Western Tethys (in series World and Regional Geology) (2003)