Permian-Triassic extinction event
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The Permian-Triassic extinction event, labeled "End P" here, is the most significant extinction event in this plot for marine fossiliferous genera. |
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
extinct.
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. Organisms throughout the world, regardless of habitat, suffered similar rates of extinction, suggesting that the cause of the event was a global, not local, occurrence, and that it was a sudden event, not a gradual change. 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.
For some time after the event,
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.
[Eshet, Y. et al. (1995) Fungal event and palynological record of ecological crisis and recovery across the Permian-Triassic boundary. Geology, 23, 967-970.] However, some researchers argue that fungal species did not dominate terrestrial life, even though their remains have only been found in shallow marine deposits.
[Wignall, P.B. et al. (1996) The timing of palaeoenvironmental changes at the Permo-Triassic (P/Tr) boundary using conodont biostratigraphy. Hist. Biol. 12, 39-62.] Alternatively, others argue that fungal
hypha are simply better suited for preservation and survival in the environment, creating an inaccurate representation of certain species in the fossil record.
[Erwin, D.H. (1993) The Great Paleozoic Crisis: Life and Death in the Permian, Columbia University Press.]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 300-mile-wide crater was discovered in the
Wilkes Land region of
East Antarctica, which is believed to be linked with the extinction.
Plate tectonics
At the time of the Permian extinction, all the continents had recently joined to form the super-continent
Pangaea and the super-ocean
Panthalassa. This configuration radically decreased the extent and range of shallow aquatic environments and exposed formerly isolated organisms of the rich continental shelves to competition from invaders. As the planet's epicontinental systems coalesced, many marine ecosystems, especially ones that evolved in isolation, would not have survived those changes. Pangaea's formation would have altered both oceanic circulation and atmospheric weather patterns, creating seasonal
monsoons. Pangaea seems to have formed millions of years before the great extinction, however, and very gradual changes like continental drift alone probably could not cause the sudden, simultaneous destruction of both terrestrial and oceanic life.
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.
[Britt R. R.: "Giant Crater Found: Tied to Worst Mass Extinction Ever", http://space.com/scienceastronomy/060601_big_crater.html, June 1 2006] 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 plug of mantle materialâ€"a
mass concentration, or "mascon" in geological parlanceâ€"that occurs within the Earth's crust and appears to have been emplaced somewhere between 100 and 500 million years agoâ€"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 miles) 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 wide, while the Wilkes Land impactor (either an asteroid or perhaps a large
comet nucleus) could have been up to 30 miles wideâ€"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.
[Jones, A. et al. (2002) Impact induced melting and the development of large igneous provinces. Earth and planetary science letters, 202, 551.] 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 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.
It has also been proposed that such a collision might heat up ocean waters enough to produce "
hypercanes," gigantic storms with winds possibly exceeding the speed of sound. Although not impossible, this theory has little supporting evidence.
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 (
hypernova) or two objects as dense as
neutron stars colliding) that occurred within ~6000
light years would produce the same effect.
Volcanism
The P-T boundary was marked with many volcanic eruptions. In the
Siberian Traps, now a sub-Arctic wilderness, over 200,000 square kilometers were covered in torrents of lava. The Siberian
flood basalt eruption, the biggest volcanic effect on Earth, lasted for millions of years.
The acid rain, brief initial global cooling with each of the bursts of volcanism, followed by longer-term global warming from released volcanic gases, and other weather effects associated with enormous eruptions could have globally threatened life. The theory is debated if volcanic activity, over such a long time, could alter the climate enough to kill off 95% of life on Earth. Volcanic activity affects the concentration of atmospheric gases directly and indirectly affects the oceanic dissolved gases. Increases in carbon dioxide enhance the
greenhouse effect and cause global warming, which would reduce the temperature gradient between the equator and the poles. As a result,
thermohaline circulation would slow and eventually stop. The oceans would
stagnate, and nutrients would fail to disperse themselves. Many marine ecosystems rely on upwelling and circulation of nutrients, oxygen included; without the regular circulation, organisms would starve or suffocate. In addition, sulfur and particulates contribute to cooling, or volcanic winter, which usually lasts three to six months. Combinations of the two effects could produce a cooling cycle in which the climate alternately warms then cools. Such temperature fluctuations could cause convective overturn of the oceans, bringing
anoxic bottom waters to the surface; in an already oxygen-deprived environment, this would be fatal to many forms of life.
Significant evidence supports this theory. Fluctuations in air and water temperature are evident in the fossil record, and the
uranium/
thorium ratios of late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction. Numerous indicators of volcanic activity at the P-T boundary are present, though they are similar to bolide impact indicators, including iridium deposits. The volcanism theory has the advantage over the bolide theory, though, in that it is certain that an eruption of the
Siberian Trapsâ€"the largest known eruption in the
history of Earthâ€"occurred at this time, while no direct evidence of bolide impact has yet been confirmed to match the correct date.
Atmospheric hydrogen sulfide buildup
In 2005 Dr. Lee R. Kump, a geoscientist from
Pennsylvania State University, published a theory explaining a cascade of events leading to the Great Extinction. Several massive volcanic eruptions in
Siberian Traps, described above, started warming the atmosphere. The warming itself did not seem to be large enough to cause so massive an extinction event. However, it could have interfered with the ocean flow.
Cold water at the poles dissolves atmospheric oxygen, cools even more, and sinks to the bottom, slowly moving to the equator, carrying the dissolved oxygen. The warmer the water is, the less oxygen it can dissolve and the slower it circulates.
The resulting lack of supply of dissolved oxygen would lead to depletion of aerobic marine life. The oceans would then become a realm of bacteria metabolizing sulfates, and producing
hydrogen sulfide, which would then get released into the water and the atmosphere, killing oceanic plants and terrestrial life. Once such process gets underway, the atmosphere turns into a mix of methane and hydrogen sulfide.
Terrestrial plants thrive on carbon dioxide, while hydrogen sulfide kills them. Increase of concentration of carbon dioxide would not cause extinction of plants, but according to the fossils, plants were massively affected as well. Hydrogen sulfide also damages the
ozone layer, and fossil
spores from the end-Permian era shown deformities that could have been caused by
ultraviolet radiation.
Dr. Kump and his colleagues are now looking for biomarkers, indicating presence of
green sulfur bacteria in the ocean sediments. Such bacteria indicate lack of oxygen in combination with available sunlight. Such biomarkers were recently found in appropriately dated shallow water sediments by Kliti Grace and her colleagues from Curtin University of Technology, Australia.
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 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.
Sudden release of
methane hydrate has also been hypothesized as a cause of the
Paleocene-Eocene Thermal Maximum extinction event.
A combination
The Permian extinction is unequalled; it is obviously not easy to destroy almost all life on Earth. The difficulty in imagining a single cause of such an event has led to the following explanation: they all did it. 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.
There is no way to calculate the odds of some such combination occurring, but for it to have occurred once in the four-billion-year
history of Earth is not unbelievable.
*
"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"*
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*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.
*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)
