Impact event
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; fullerenes trapping extraterrestrial noble gases; meteorite fragments in Antarctica; and grains rich in iron, nickel and silicon, which may have been created by an impact. However, the veracity of most of these claims has been challenged. The shocked quartz from Graphite Peak in Antarctica 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.
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, the so-called Wilkes Land crater of East Antarctica, and, even more speculatively, the Gulf of Mexico. 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.
One attraction of large impact theories is that theoretically they could trigger other cause-considered extinction-paralleling phenomena, such as the Siberian Traps eruptions (see below) as being either an impact site or the antipode of an impact site. 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 final stages of the Permian saw two flood basalt events. A small one,
 |
|
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. (Picture Source) |
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 upper layers of the seas, 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.
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. 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.
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.
However, one theory, popularized by the 2005 documentary Miracle Planet, is that the slight volcanic warming caused a melting of methane hydrate, and this created a positive-feedback warming loop, as methane is 45 times more efficient than CO2 at exacerbating global warming.
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 (δ13Ccarbonate of -10 ‰). 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).
There are a variety of factors that may have contributed to this drop in the 13C/12C ratio, but most turn out to be insufficient to account fully for it. Some hypotheses include mass oceanic poisoning releasing vast amounts of CO2 and a long-term reorganisation of the global carbon cycle.
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; and carbon-cycle models confirm that it would have been sufficient to produce the observed reduction. 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.
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.
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. 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); 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.
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, 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.
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. 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. 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.
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. The uranium/thorium ratios of several late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction.
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.
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.
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. 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.
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
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. 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. 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.
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; 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.
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. (AAPG Article.
Return to the Old Earth Ministries Online Earth History Curriculum homepage.
Source Pages: Wikipedia - Permian, Permian Extinction
Copyright issues. Wikipedia pages may be re-used on other websites. A link back to the original page is provided. Changes to the text in this curriculum were made to clarify the interpretation from a Christian perspective. Thus, all statements in the text that are religious in nature have been added. Some duplicate material has been removed, and some material not relevant for a high school class was removed.