Seems that global warming was the culprit but the exact cause of same is still up for grabs...
Explanatory theories
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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.
[edit] 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.
[edit] 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.[7] 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.[citation needed]
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.[citation needed]
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.[8] 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.[9] 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.[9]
[edit] 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.
[edit] 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 (77,000 mi2) were covered in torrents of lava. The Siberian flood basalt eruption, the largest known volcanic event on Earth, lasted for millions of years.
Weather effects associated with enormous volcanic eruptions, such as acid rain and the 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 threatened life; however, it is debated whether long-term volcanic activity, alone, could alter the climate enough to kill off 95% of life on Earth. Volcanic activity directly affects the concentration of various atmospheric gases, which in turn affects oceanic concentrations of dissolved gases. Increases in atmospheric carbon dioxide may enhance the greenhouse effect enough to lead to increased global temperatures and reduce the temperature gradient between the equator and the poles. As a result, thermohaline circulation would slow and potentially stop. Without these currents, the oceans could stagnate, and nutrients would fail to be dispersed. 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 cycle in which the climate alternately warms and 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 Traps—the largest known eruption in the history of Earth—occurred at this time, while no direct evidence of bolide impact has yet been confirmed to match the correct date.
[edit] 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. An increase in the concentration of carbon dioxide would not cause plants to become extinct, 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.
[edit] 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.[10]
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.
[edit] 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.
en.wikipedia.org
Of course it's only Wikpedia... but at least it's a start.
This is also Wickpedia... but copyrighted...
en.wikipedia.org
LOoks like it used to be a lot hotter... than now..
This figure shows climate change over the last 65 million years. The data is based on a compilation of oxygen isotope measurments (d18O) on benthic foraminifera by Zachos et al. (2001) which reflect a combination of local temperature changes in their environment and changes in the isotopic compositon of sea water associated with the growth and retreat of continental ice sheets.
Because it is related to both factors, it is not possible to uniquely tie these measurements to temperature without additional constraints. For the most recent data, an approximate relationship to temperature can be made by observing that the oxygen isotope measurments of Lisiecki and Raymo (2005) are tightly correlated to temperature changes at Vostok, Antarctica as established by Petit et al. (1999). Present day is indicated as 0. For the oldest part of the record, when temperatures were much warmer than today, it is possible to estimate temperature changes in the polar oceans (where these measurements were made) based on the observation that no significant ice sheets existed and hence all fluctuation in (d18O) must result from local temperature changes (as reported by Zachos et al.).
The intermediate portion of the record is dominated by large fluctuations in the mass of the Antarctic ice sheet, which first nucleates approximately 34 million years ago, then partially dissipates around 25 million years ago, before reexpanding towards its present state 13 million years ago. These fluctuations make it impossible to constrain temperature changes without additional controls.
Significant growth of ice sheets did not begin in Greenland and North America until approximately 3 million years ago, following the formation of the Isthmus of Panama by continental drift. This ushered in an era of rapidly cycling glacials and interglacials (see figure at upper right).
Also appearing on this graph are the Eocene Climatic Optimum, an extended period of very warm temperatures, and the Paleocene-Eocene Thermal Maximum (labeled PETM). The PETM is very short lived high temperature excursion possibly associated with the destablization of methane clathrates and the rapid buildup of greenhouse gases in the atmosphere. Due to the coarse sampling and averaging involved in this record, it is likely that the full magnitude of the PETM is underestimated by a factor of 2-4 times its apparent height.
Seems to me the ice core records are far too current in the geological clock ? I know my ski season is shot for this year though :o(
Al |
