Australia: The Land Where Time Began
Permian Mass Extinction - About 251 million years ago
Lost : about 95 % of all species - 53 % of marine families, 84 % of marine genera, and an estimated 70 % of life on land.
At this time all the continents came together as the supercontinent Pangaea. For the first time the area of the land exceeded that of the ocean. Evidence has been presented that it may not have been the first time this occurred. The formation of such a vast land mass led to the a mixing of the fauna of all the continents. There would have been a large scale upheaval in ecosystems as the component species struggled to survive as the predator-prey-competitor relationships were realigned. Many would have succumbed while others were forced to adapt or evolve to maintain their place. As this was happening on the land, the loss of large areas of continental shelf would have caused a comparatively rapid loss of biodiversity among the marine organisms, the most diverse communities of which tended to occur on the very continental shelves that were being lost.
The dominant terrestrial group was the therapsids or mammal-like reptiles, the reptiles having arisen in the Carboniferous. These and other terrestrial groups such as insects, amphibians and reptiles were badly affected by the extinction event. On the land the gymnosperms, such as conifers, were the predominant flora of the time. Marine faunas were similar to those that survived, or arose after the Devonian extinction event.
The Permian mass extinction was the greatest known crisis for life in the entire history of life on earth. It came close to eliminating multicellular life on Earth, 90-95 % of all known life was extinct after this catastrophic event. Some groups were greatly reduced - sharks, bony fish, ammonoids, crinoids, eurypterids, ostracods, echinoderms, bryozoans, brachiopods. Among those groups particularly hard hit were the placoderms, acanthodians, pelycosaurs, blastoids, corals, trilobites, fusulinid foraminifera.
Prior to this extinction event most animals, about 67 % of marine animals, were sessile filter-feeders, after it there were many more mobile animals.
Several theories have been put forward to explain this most catastrophic of extinction events.
One that has some suggestive evidence is volcanism on a very large scale that occurred at the close of the Permian. The amount of material that was erupted during this volcanic phase was huge, up to about 2.7 million square miles of lava, about the area of Australia, and recent research has shown that about 20 % of the material from the Siberian Traps eruptions were of a pyroclastic nature, that is released explosively, throwing large amounts of ash and aerosols high into the atmosphere, the remainder being in the form of flood lava, the lava flows out without putting ash or dust into the atmosphere. It had been believed that this massive eruption had continued for a very long time but further recent evidence has dated the event to about 251 Ma, just before the end of the Permian.
Lava flows at Emeishan in China have been dated to this time, and the silica-rich nature of the lava indicates that the eruptions were of the explosive kind that put large amounts of material high into the atmosphere. These materials would include sulphates and very large ash clouds that could have been spread around the world. This 'nuclear winter' scenario would have lowered the temperature globally, and could have triggered glaciation on a continental scale. It has been found recently that the extinction event may have been a drawn out affair, not at all sudden.
The ash and dust clouds produced by these eruptions could have cut sunlight reaching the surface, the resulting reduction of terrestrial and marine photosynthesis could cause the collapse of food webs in both. The basalt lava released in the Siberian and Chinese eruptions and the associated intrusion of lava into carbonate rocks and coal beds that were forming would have released large quantities of CO2 into the atmosphere. It has been proposed that the ash and dust clouds would have produced a cooling effect, but once the dust and ash had been washed out of the air the raised CO2 levels, about double what it had been previously, would have lead to global warming, based on climate models the temperature would have been raised by about 1.5-4.5o C (2.7-8.1o F). Some have doubts that even such large eruptions in the Arctic Circle could have a global effect, suggesting that it would need to be closer to the equator for a global effect.
Another possible consequence of large-scale volcanism is acid rain. Some modern lakes are devoid of life as a result of acid rain from burning fossil fuel. The acid rain leaches aluminium out the surrounding soil and rocks and eventually reaches a concentration where it kills all aquatic animals.
Large areas of methane hydrate (methane ice) (methane clathrate) have been known of beneath the ocean floor in many places around the world for many years. In these deposits vast quantities of methane are locked up in a form that prevents the methane from escaping into the seawater. The majority of these deposits are in deep oceans where the temperature of the water is just above freezing, only a few degrees Celsius, and remains very stable over long periods, and the pressure at such depths is high. The methane hydrate remains stable only as long as the temperature and pressure requirements are met, but it only requires a very small rise in water temperature to result in the methane being released to the water. In the Arctic Ocean the low temperature of the water led to the accumulation of methane hydrate at relatively shallow depths that elsewhere required much greater depth, where the water temperature was very low and constant.
At the increased temperature levels mentioned above it is believed by some that the oceans would have warmed enough to release methane from the methane hydrates (methane clathrate) on the see floor. As methane is more than 20 times more efficient than CO2 as a greenhouse gas, this would have greatly exacerbated the atmospheric warming. It has been found that much of the lava produced by the Siberian Traps eruptions would have flowed over what was then shallow seas where they could have released vast quantities of methane from the sediments.
A decrease of the C13/C12 isotope ratio of 10 PPT has been found from end-Permian carbonate rocks around the world. This was the first and largest and most rapid of a series of changes of the isotope ratio that continued to occur until the Middle Triassic, when the ratio stabilised abruptly. It was only after the stabilisation of the C13/C12 ratio that organisms that form calcium carbonate structures like shells began to recover from the extinction event.
A similar situation to that during the closing Permian might be happening at the present. Scientists have been recording rising temperatures in the Arctic for some time. In 2008 a Russian research expedition vessel was measuring dissolved methane levels in the Arctic Ocean when they observed large patches of ocean where methane was bubbling to the surface along the north coast of Siberia. A British expedition also observed this phenomena in the seas closer to Britain. This indicates that the submarine permafrost is melting as well as the terrestrial permafrost, both of which are adding large amounts of methane to the atmosphere. Because of the low temperature of Arctic Ocean water the deposits of methane hydrates exist much closer to the surface of the ocean floor than they are elsewhere, where they can be several thousand metres below the sea bed.
On land melting permafrost has led to Arctic lakes, as in Siberia, becoming much larger, and with methane bubbling out of them.
Another proposed mechanism, also involving large scale glaciation, postulates cooling, but doesn't include volcanism as the trigger. It included rapid warming, severe climatic fluctuations, and concurrent glaciation at both poles. Evidence has been found in sediments from temperate zones of cooling and drying, in the form of dune sands and evaporites, and evidence of glaciation at the poles, at the close of the Permian.
Some believe the eruptions could have led to global warming. Others believe it was caused by flood volcanism from the Siberian Traps, possibly with the addition of an impact that may have triggered the massive volcanism.
There is evidence in the form of sediments dated to the latter part of the Permian that indicate that these sediments were deposited under anoxic conditions. Such sediments have been found at places like East Greenland. It is known that under anoxic conditions large amounts of hydrogen sulphide can be released from sediments, so if organisms could survive in the very low oxygen levels they would be poisoned by the hydrogen sulphide.
According to the authors of the article in PNAS a large influence on the history of life on Earth has been the periods when the oceans became anoxic, such occurrences being coincident with mass extinction events. In their study reported in PNAS they used uranium system proxies to establish the occurrence of global changes in the oceans, as opposed to local changes as were used in previous studies. In their study they used 238U/235U (δ238U) and thorium/uranium ratios is a carbonate section in Dewan in southern China. Their results indicate a 6-fold increase in the flux of uranium to anoxia facies, indicating an increase in the extent of anoxia in the oceans, represented by the isotopic shift. They say the oceanic anoxia intensification coincides with the extinction horizon, or slightly precedes it, and that the intensification extended for 40,000-50,000 years, at least, past the extinction horizon.
In an article in Cosmos Online the lead author Gregory Brennecka explained that earlier studies suggesting the occurrence of greatly expanded anoxia in the oceans were based on local measurements but the latest study was based on a process that relates to the entire ocean system, confirming the existence of expanded anoxia, as suggested by the previous study, and showing that the anoxic conditions found occurred in the entire ocean system.
The findings of this study also indicate that the loss of oxygen from the oceans occurred over a very short period of time, just at the time of the extinction event
Recent findings indicate that the areas of anoxic deep sea water are expanding to such an extent that they have been found to be spilling over the edge of the continental shelf along the northwestern coast of the United States.
Under the severe anoxic conditions apparently prevailing, at least in places, if not globally, sulphate-reducing bacteria in the sediments would flourish, producing large quantities of hydrogen sulphide. The production of large quantities of this gas would poison not only marine life but terrestrial life, and unlike most other mass extinctions, the plants would also be killed off as well as the animals.
Formation of Pangaea
Another hypothesis postulates that it was the formation of Pangaea that caused the mass extinction, but Pangaea was formed about half way through the Permian, after which life apparently prospered for millions of years until the close of the period. Its formation would have certainly have caused problems for marine animals living on the continental shelves as the area of continental shelf was greatly reduced as the continents merged. The fauna and flora of the inland parts would have been badly affected as the central parts of the supercontinent dried out now that they were much further from the sea.
Wilkes Land Impact
A possible impact crater beneath several kilometres of ice in Wilkes Land, Antarctica, called the Wilkes Land Anomaly or the Wilkes Land Mascon (mass concentration), was proposed as the site of an impact that led to the near-extinction of life on Earth about 250 Ma. It was first proposed in 1962 by R.A.Schmidt, who based his claim on geophysical data. He suggested that it could be the source of the tektites from the Australasian strewnfield. The proposed impact crater was supported in 1976 by J.G.Weilhaupt, who cited evidence of a large negative gravity anomaly at the site of the proposed subglacial topographic depression that was about 243 km in diameter and 848 m deep. In 1979 C. R.Bentley refuted these proposals, based on air-borne radio echo-sounding data that failed to find the crater.
In 2006 the Wilkes Land mascon (mass concentration) (70 S, 120 E) was reported based on data from NASA's GRACE satellite that identified a large mass concentration, about 300 km (200 mi) across that was surrounded by a large ring-like structure that had been seen on radar images of the land surface beneath the glacier (Von Frese & Potts, 2006). They suggested the crater probably formed less than 500 Ma, and also suggested that it appears to have been disturbed by the rift valley that formed as Australia began to separate from Antarctica, They suggest that the impact may have weakened the crust at the site which led to the final separation from Antarctica, and the last stage in the fragmentation of Gondwana.
Another large impact site from this time, is at Bedout to the north of the Western Australian coast.
The lack of evidence of a well-defined impact ejecta layer where it outcrops in Victoria Land and the Transantarctic Mountains that could support the impact at the close of the Permian argues against this being an impact that resulted in the mass extinction. (8)
Von Frese et al. report a subglacial basin about 500 km wide based on data from GRACE, centred on (70°S, 120°E), in Wilkes Land, Antarctica. They suggest that the data is quantitatively consistent with thinned crust from a giant impact crater that is underlain by an isostatically disturbed mantle plug. The proposed crater is 3 times the size of the Chicxulub crater, and the impact is assumed to have occurred before the formation of the east Antarctic coast that cuts across the ring faults. The crust has been disrupted and thinned extensively where the Kerguelen Hot Spot and where Gondwana rifting developed, though the adjacent Australian crust was relatively undisturbed. They also suggest the micrometeorite and fossil evidence indicate the impact may have occurred at the beginning of the mass extinction at the close of the Permian, possibly triggering it, about 260 Ma. At this time the Siberian Traps were in effect antipodal to the proposed Wilkes Land impact. The authors point out that on Mars and on the Moon antipodal volcanism is common to large impact craters. The authors suggest this antipodal relationship between large impact craters and igneous provinces may also apply on Earth. They suggest the impact may have contributed to the development of the hot spot that produced the Siberian Traps and is now beneath Iceland (Von Frese et al., 2009).
This hypothesis has included all the above suggested mechanisms for the mass extinction at the end of the Permian.
There is accumulating evidence that acidification of the oceans was probably involved in, and even may have been the primary cause of, all mass extinction events that are known of in the history of life on Earth.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|