2. Cosmic Radiation
Hatfield and Camp (1970) found a crude correlation between the galactic position of the solar system and major faunal extinction?s. They said that if the earth moved through a galactic plane (one which extreme radiation passes through) it would subject the to huge amounts of radiation and magnetic fields. This statement itself can be focused on because it could cause breeding patterns in some animals to stop or be altered. Hatfield goes on to show how this is possible. Our galaxy has one revolution around the galactic center every 200 million years. At the same time the sun completes three vibrations which are perpendicular to the galactic plane. Thus there is one vibration about every 80-90 million years. Therefore Hatfield speculated that when the earth moves through the plane, it could produce a faunal extinction like the Permian extinction. The increased radiation would produce an increase of mutations and deaths in some species. Species that live deep in the ocean and lakes would not be affected directly. Erwin (1993) using information from many studies said that ?an increase an increase in cosmic radiation would have eliminated many groups and increased the rate of mutation among the survivors, thus explaining both extinction and the subsequent radiation.? This statement makes some sense when one thinks about how all of the new species were created so fast during the Triassic era. Dickens (1992) supported the theory of cosmic radiation. He said that the cause for extinction could be changes in the planetary or galactic system; change in the angle of the earth?s axis; changes in the atmosphere, probably as a result of magmatic and volcanic activity; or a combination of these factors. There is good evidence to reject this proposal. Cosmic radiation will affect terrestrial and very shallow organisms more than benthic organisms. Evidence suggests that both benthic and shallow organisms were greatly reduced and terrestrial organisms were not as affected as oceanic species. The cosmic radiation model can not explain these differences.
3. Global Cooling ? Global Cooling.
Before much study was done on the mechanics of mass extinction?s some people believed that the Permian extinction was due to global cooling or ice age conditions. This is not the case. There is no evidence to support the fact that there was global cooling at the end of the Permian. On the contrary, there is major evidence which support the theory that warm climates existed. As you will see, most of the theory?s are based on global warming. Dickens (1992) gives evidence of glaciation in the upper Carboniferous and it is widespread in the lowest stage of the Permian. Above the lower Permian there is no evidence for glaciation. ?After the mid-Permian, world climate became steadily warmer until in the latest Permian and earliest Triassic a universally hot climate, substantially warmer than the present prevailed? (Dickens, 1992). Warm waters are indicated by the development in the sedimentary sequence of reefs, desert deposits, fine-grained red beds, and evaporites.
The hot climate of the latest Permian and earliest Triassic, together with marine regression, widespread volcanism, and tectonic instability, would have subjected the fauna and flora to extremely rigorous conditions and would no doubt have been sufficient to effect a great change in the biota (Dickens, 1992).
Detailed studies about the causes are lacking according to Dickens but more likely causes for climatic change may be fluctuations in solar energy as stated before in the cosmic radiation section.
4. Salinity
The hypothesis that salinity decrease caused the mass extinction of oceanic life was first formed by Beurlen in 1956 (Maxwell, 1989). Evidence for this phenomena was based mainly on stenohaline groups such as the bryozoans, ostracodes and corals which were greatly reduced at the PTB. The least affected groups were gastropods and fresh water fishes. Organisms with some tolerance of salinity variations survived and proliferated in the early Triassic. Therefore it was found that a selective extinction of marine families occurred in the BTB. Beurlen proposed that salinity was progressively reduced during the second half of the Permian and also that salinity reached critically low values at the PTB, before persisting into the early Triassic. Early marine faunas are sparse and many groups that were diverse before and after the PTB are not present at the PTB. Beurlen said that this was due to a few places in the world where normal salinities were maintained. A return to normal salinities world-wide would allow the surviving species to repopulate the seas and as a result, crop up again in the fossil record after their temporary absence. This leaves us with one main question, what would cause such a large reduction in ocean salinity? Maxwell (1989) gives some answers based on the work of many scientists. In the 1950?s and 60?s it was thought that the drop in salinity was due to large-scale evaporite sedimentation accompanied by the formation of large quantities of dense brine which was stored deep down on the sea floor. Salinity could have been reduced to a value around 30 parts per thousand (which is safe to drink). If this occurred than the result would be huge volumes anhydrite, gypsum, salt, and halite deposited on the sea floor. Beurlen (1956) estimated that 5*10^14 tones would need to be deposited. Other scientists strongly criticized Beurlen stating that this would only be 15% of the amount of evaporites that would need to be stored. A figure of 200,000 cubic kilometers was postulated but some scientists say that this is only 10% of the real amount. Therefor it would seem that Permian evaporite deposits can not explain the lowering of salinity levels. The best reason that I could find to explain salinity decreases was put forward by Fisher (1963) called the brine-reflux hypothesis. The evaporation of sea water and the deposition of salts produced dense brines which sank deep onto the floor of the ocean. This leaves the top circulating water free if salt. In looking at this proposal carefully, I think Fisher would come into opposition with scientists say that the extinction was due to a temperature decrease. A temperature decrease would cause less evaporation and should cause the oceans to be saltier due to fresh water being accumulated in glaciers.
Erwin (1993) said that a scientists named Bowen in 1968 actually argued that Permian climates triggered an increase in Permian salinity of approximately 20% above today?s levels. His study was based on the volume if massive Louann salt deposits from the Gulf Coast and other Paleozoic evaporites. So as you can see there is a great deal of uncertainty if even salinity had anything to do with the PTB extinction. Erwin goes on so slam all of the hypotheses.
These salinity hypotheses are instructive examples of how often ?explanations? are nothing of the kind. Stenohaline taxa are also largely stebotopic, and often independent evidence must be advanced that salinity changes were the selective factor. Contrary to several of these papers, nautiloids did not particularly suffer during the extinction, blastoids and crinoids disappeared long before the ammonoiads or the brachiopods, and ?strophomenid? brachiopods suffered far greater extinction did spiriferid brachiopods? In summery, non of these patterns is consistent with the salinity gypothesis.?
5. Species Area effects
If you look back through the geological column, you will find a correlation between marine regressions and major mass extinction?s. But what really is the connection. Erwin gives use a good base from which we can conclude many new things. His statements are based on MacArthur and Wilson?s theory of island biogeography.
They suggested that species diversity on an island is a function of immigration to the island from a continental source, and extinction on the island due largely to competition. Thus the immigration rate should be a declining function of the number of species on the island and should approach zero when all the species from the source pool have reached the island. Similarity, as species diversity increases, the extinction rate should climb as competition for resources increases. The equilibrium species diversity will be the point where the immigration rate and extinction rate are the same. Among the implication of the theory are that smaller islands and more distant islands should have fewer species than larger islands or those closer to the source area? (Erwin, 1993).
Evidence for a regression is quite good according to Maxwell (1989).
1. The greatest level of regression of shallow seas from continents of any Phanerozoic interval occurred at this time.
2. Reef environments are unknown during the latest Permian and early Triassic.
3. There are few taxa up to class level of early Triassic benthic and pelagic organisms, contrasting with large numbers before and after this time.
4. Early Triassic taxa were organized into small number of shelly invertebrate communities with very low species diversity.
5. Biogeographic diversification was a Phanerozoic low in the early Triassic.
6. There are abundant late Permian evaporite deposits.
Using the species-area hypothesis we can deduce several facts. We already know that during the PTB the sea level declined and that one single continent was formed. This reduces shelf area and thus reduces the area a species can live in causing greater competition for resources. You will then get species dying off and lower species diversity. Some scientists claim that a reduction of shelf area alone would have cause the extinction. Since most organisms on land are connected to the sea, we can postulate that there would also be a reduction in the number of species on land.
Many scientists, as reported by Erwin, have rejected the species-area hypothesis. Their rejection is based on many facts. Some point to an example during the Middle Eocene where there was a 50% reduction is shelf area along the Gulf Coast. According to the species area hypothesis there should have been a reduction in diversity of species but evidence supports that there wasn?t. Some argue that it is only the change in number of marine provinces that affects diversity. To me it would seem that there if there is a reduction in species area there should be a reduction in marine provinces are at least the area of space to live in each province. Erwin (1993) makes some very challenging suggestions.
If the species-area relationship is valid, regressions should have a far greater effect on continents than on islands since, in general, the area of an island will increase during a regression. Modern tropical reef biotas are among the richest environments in the world, rivaling if not surpassing the tremendous diversity found in tropical rain forests. If most marine families have representatives on oceanic islands they will be relatively immune to regression-induced extinction.
6. Anoxia-Stagnant Ocean
Anoxia-stagnant ocean hypothesis was first presented by Berry and Wilde (1978). To me this is one of the most complicated and intriguing hypothesis which tries to explain the mass extinction. Berry and Wilde based their theory and conclusions on their research of the extensive black shales of the Paleozoic which indicate that the oceans were depleted of oxygen. Oceans now have a minimum zone of oxygen at middle depths. As you descend down further oxygen levels increases. The Berry-Wilde hypothesis replaces this with an anoxic deep ocean. While this fact alone does nothing to enhance our knowledge of the extinction, combining it with the isotopic record of carbon, oxygen, and sulfur and the various explanations for shifts in isotopes and we can generate a good number of hypotheses based on global anoxia and global warming (Erwin, 1993). We will examine a couple of these hypotheses individually.
The first explanation proposed that there was a regression which caused exposed organic materials to be oxidated and this resulted in a drop of C-13 levels and an increase in carbon dioxide. The erosion and oxidation of organic compounds caused an increase in surface temperature of about six degrees centigrade. Since warm water can hold less ?air? or oxygen than cold water, the increase in temperature caused a drop in the solubility of oxygen in seawater. This compounded that fact that the oceans were already depleted of oxygen due to the long-term oxidation of carbon. Thus an anoxic ocean caused the mass extinction of ocean species and this also affected terrestrial species.
A second model based on Erwin (1993) is a little different. The core of the hypotheses is that there was a regression than the formation of the anoxic layer (by a mechanism already described). There was a rapid transgression which resulted in the spread of anoxic waters which resulted in extinction. There is quite a large body of evidence to support this theory. The early Triassic communities were low in diversity but species abundance was very high. This is characteristic of opportunistic expansion in an environmentally stressed setting. Laminated black shales are lacking which is a diagnosis of anoxic conditions. This means that most of the earth did not have anoxic conditions which supports a regression. This theory also supports evidence that a fast extinction occurred and not a gradual one. As with all theories there doubters. The most damaging piece of evidence is that biota was decimated prior to the onset of the transgression. There is also evidence in some sediments that there was extensive bioturbation. This is a characteristic of oxygen rich waters (Erwin, 1993).
The third and last model which is based on the evidence of C, O, and S isotopes sets up the ocean into two boxes. The upper box which represents shallow oxygen rich waters while the lower box and far larger portion represents the lower ocean which is anoxic. The two are separated by a redoxcline which causes minimal interchange between them. Supporters of this hypothesis argue that the transition between these two states occurred at the PTB. It involved a rapid destratification of the oceans and establishment of vigorous circulation. It resulted in the oxidation of deep carbon that had been stored previously. This resulted in the increase in atmospheric carbon dioxide and sulfur but a decrease in oxygen and C-13. This is supported by evidence in layers in the Eastern Tethys. Most importantly, the oxidation of nitrogen and phosphorous induced marine extinction by nutrient deficiency. Terrestrial extinction?s were the result of a drop between 10 and 90 % in oxygen and probable climate cooling (Erwin, 1993). A paper written by Tom Waters (1996) tried to explain how anoxia might have caused the particular patterns that the extinction?s took. His paper was based on a stagnant ocean so that there was little flow from the deep to shallow regions. As dead organisms rained down from the surface into this nearly stagnant water, the decay of all that material gradually sucked the oxygen out of it. With few currents flowing into the deep, there was no way to bring fresh oxygen from the surface. While oxygen disappeared, carbon dioxide was building up in the Permian deep. The deep ocean they argue was a disaster waiting to happen. What finally unleashed it was a cooling climate. The cooling was the result of a decrease in atmospheric carbon dioxide which weakens the greenhouse effect. This chilled surface waters sending them down and pushed up the anoxic waters into the shallow areas. It killed marine life and the carbon dioxide which escaped from the water warmed the atmosphere melting the glaciers causing a transgression. There is evidence that during the Neoproterozoic Era, from 800 to 543 million years ago, that this same thing happened four times.
The survivors were the active breathers who could flush out the excess carbon dioxide. For instance, passively respiring corals suffered heavy losses in the Permian extinction, while active breathers like snails and clams fared much better (Waters, 1996).
Thus species with higher extinction rates were the ones less able to handle carbon dioxide poisoning.
7. Volcanism
There is very good evidence that volcanism caused or triggered the PTB extinction. A paper by Paul R. Renne et al (1995) puts together numerous papers into one sound thesis. The evidence of a bolide impact and for volcanism were synchronous within sever hundred years. Thus more scientists believe in the volcanism theory rather than an asteroid simply because there is more evidence for volcanism and also that volcanism can produce some of the effects of an asteroid collision with earth which can eliminate the asteroid hypothesis all together. Erwin (1993) stated four ways in which volcanism might be able to cause mass extinction?s.
1. Creation of a dust cloud that reduces photosynthesis and initiates global cooling; injection of massive amounts of carbon dioxide and sulfates into the atmosphere into the atmosphere causing global warming.