Главная > Реферат >Остальные работы
The End Permian Mass Extinction
Think of a world which existed 290 million years ago. As you look out over the terran in front of you, you think that you are on an alien planet. You see volcanoes spewing ash and lava. Beside them is the ocean which is swarming with many different species of echinoderms, bryozoans and brachiopods. As you look down onto the sea floor you are amazed at the countless number of starfish and urchins. Some animals leave you can?t even describe and you have no idea even what phylum they belong to. This is a world at its height in diversity of oceanic species. Millions of wondrous species existed at this time in the ocean and most of them will never appear again in earth?s history. In the geologic time scale, a million years means nothing but this time things are different. In the blink of an eye things now look vastly different. The world once again looks alien but it looks worse than before. The sky is dark. Oceans are no longer teaming with life. The stench of rotting flesh and plants hangs in the air. The ground trembles under your feet. You feel an intense heat burning you face. You look up and see one of the greatest show of force mother nature has ever shown. Whole mountains are being thrown in the air. Lava and debris are everywhere. You ask yourself, what has happened? Will life ever exist on earth again?
The above paragraph is a primitive example of what the end of the Permian period could have looked like. Marine life was devastated, with a 57% reduction in the number of families (Sepkoski, 1986) and an estimated 96% extinction at the species level (Raup, 1979). Oceanic life suffered the most but terrestrial life forms were also greatly affected. There was a 77% reduction in the number of tetrapod families (Maxwell and Benton, 1987). All major groups of oceanic organisms were affected with the crinozoans (98%), anthozoans (96%), brachiopods (80%) and bryozoans (79%) suffering the greatest extinction (McKinney, 1987). The end of the Permian and beginning of the Triassic periods marked the single greatest extinction event the world has ever faced.
II. Timing of the Extinction
There are many questions regarding the timing of the extinction at the end of the Permian. One of the main questions was the even a catastrophe or gradual. There is evidence for both scenarios. Some of the evidence supports an extraterrestrial event such as a meteor. Other evidence supports the theory of the ocean and terrestrial environments slowly changing.
A. Geochemical evidence
The research done by Xu Dao-Yi and Yan Zheng (1993)gives evidence for an extraterrestrial event. They made a table which showed the distribution of carbon 13, iridium, and microspherules across the P/T (Permian and Triassic) border. The section was over a thickness of 35 cm. They found a sudden depletion in C-13 falling from a value near zero to a minimum of less than ?6% in some samples. Similar patterns of C-13 have been observed in more than five P/T sections in China. Some other scientists like Baud et al (1989) argue that what could have caused this anomaly is the result of a depositional hiatus or erosional disconformity. Xu and Yan argue that there is no evidence for a significant hiatus and that Baud et al. Even made a mistake in the timing of their rock layers. ?If the PTB [Permian Triassic boundary] is considered a catastrophic event, a short-time hiatus should be expected and is in fact a reasonable consequence of a catastrophic event? (by Xu Dao-Yi and Yan Zheng, 1993). But what is the significance of C-13 being associated with catastrophic events? Hsu et al. (1982) said that they suggested that carbon isotope anomalies are related to microplankton productivity. We will touch again on this later in the paper. Therefore, the sudden C-13 change may indicate the exact stratigraphic position of the mass killing event at the PTB. Analysis of iridium (Xu Dao-Yi and Yan Zheng, 1993)in the layer reviled some interesting results. High Ir values only occurs in the uppermost part of the layers. This means that the layer is close to the PTB. The concentration of Ir was at least an order of magnitude higher than the background values and this is characteristic of most Upper Permian and Lower Triassic boundaries. The scientists go on to say that ?the existence of a rich Ir anomaly on a global scale within the K/T boundary layers of both marine and continental facies has been interpreted as highly impressive evidence for an impact origin. Another discovery that may serve as a marker of an event is microspherules. A variety of microspherules have been discovered in the PTB layers of the Meishan section (Xu et al., 1989). The origin of the microspherules could be multiple. They are small circular indentations in the rocks and the most abundant elements are Si or Si-Al. Mircospherules are similar to cosmic dust. Since a large amount of microspherules occurs in a thin layer of PTB layer it can serve as another event marker.
Maxwell (1989) who got his information from Clark et al. (1986) said that
The elemental in boundary clays across China suggest that there is a remote possibility that the predominantly illite boundary clay is a remote possibility that the predominantly illite boundary clay resulted from the alteration of ejecta dust from a comet impact, but the most likely source was ash from a massive volcanic eruption.
The trace elements suggested that the dust was highly acidic and the ratios of TiO2 and AL2O3 are low enough to support the volcanic dust scenario (Clark et al. 1986).
There is some research which gives evidence of a gradual extinction event. Magaritz et al. (1988) reported that carbon-isotope ratios are known to shift or change at some boundaries associated with a mass extinction event. A shift can occur due to a decrease in plant production following a meteor impact or from a large decrease in sea level that reduces shelf area, exposing the shelf and its accumulated organic carbon to erosion. There are sections examined in the Alps of Italy and Austria that actually show a gradual change in the C-13 content of marine organisms across the PTB. These sections show no dramatic shifts that can be associated with a mass extinction. Thus as you can see, the findings of Clark et al. (1985) and Magaritz et al. (1988) shows geochemical evidence that the mass extinction was a gradual event and not a catastrophic extinction event.
B. Faunal evidence
Faunal evidence is much harder to come by and explain that geochemical evidence due to major gaps in the PTB boundary layers. Also marine faunal evidence is much more linear than terrestrial. Yoram Eshet et al. (1995) said that fungal evidence can be used to mark the PTB layer. It can also be used for evidence to show how the extinction event occurred. There is a sharp fungal spike in the PTB layer which is made up of Lueckisporites virkkiae, Endosporites papillatus, and Klausipollenites schaubergeri spores. Yoram Eshet et al. (1995) defined four stages across the Permian-Triassic boundary. Stage one, consisted of low abundance of spores which became increasingly abundant. At the top, the disappearance of more than 95% of the Late Permian pollen and spore taxa became apparent. Stage two contained and abundance of fungal remains and here it is defined as the ?fungal spike?. Also there is quite a bit of organic detritus, composed of carbonized plant debris. Stage three and four will be described later in this paper. Since this fungal evidence can be seen throughout the world it makes it highly unlikely that the increase is everywhere an artifact resulting from sedimentary processes or local conditions. Also it should be noted that the fungal spike is very thin which suggests that remains could have been missed at many PTB layers. The reason there is a large fungal spike should be obvious. Fungi are known to adapt and respond quickly to environmental stress and disturbance (Harris and Birch, 1992). During a high stress period, like an extinction event, decimation of autotrophic life occurs which creates a large pool of decaying organic matter. This is evident by the abundant plant debris seen in the fungal spike.
Marine evidence for the PTB extinction event has the greatest impact. According to Douglas H. Erwin (1993), the world?s leading expert on the Permian crisis, marine organisms such as bivalves and gastropods suffered the greatest so that most are unfamiliar even to students of invertebrate zoology. But findings by Erik Flugel and Joachim Reinhardt (1990) contains contradictory evidence that marine life suffered in the end Permian and early Triassic. It is commonly assumed that reefs are affected more severely at major extinction events than other biotopes. Another assumption is that there is a decrease in diversity of shallow-marine organisms during the Late Permian. In analyzing the Permian-Triassic reefs using very sophisticated equipment the scientists found that there was no reduction in diversity of reef organisms during the last part of the Permian. That there was evidence of high and even increasing diversity of the uppermost Permian reef communities. The argument of Erik Flugel and Joachim Reinhardt (1990) was again countered by a number of scientists. Sweet (1992) showed that strata previously assigned to the topmost Permian stage was mistaken and that the strata should have been moved lower. If Sweet?s scheme is accepted, then the mass extinction becomes an intra-Triassic event. The differences in data could be due to inadequate sampling as proposed by Sepkoski (1986). The evidence for this statement is found in that there is virtually no complete late Permian sections and complete sections across the PTB layers.
As you can see, nothing is fool-proof in the study of the Permian-Triassic extinction event. Since there is conflicting evidence of when, what, and how the extinction event occurred, there will be will be many different theories and hypothesis on the causes of the end Permian extinction. This paper will explore a few of the possibilities.
One of most agreed with reasons of the cause of the extinction was made up by Newell (1963).
III. Causes of the end Permian extinction
There are many theoretical causes of the Permian mass extinction. The causes are divided up into two main groups: diversity-dependent and diversity independent. Diversity dependent hypotheses have just recently been formed and thus they are not very popular but they do make quite a bit of sense when looked at clearly. Diversity-dependent factors limit population growth as population size get larger. It involves a depletion of environmental factors such as oxygen, nitrogen, and carbon dioxide. Bramlette (1965) and Tappan (1968) evolved on a scenario of nutrient reduction. In the model, landscapes where flat and thus streams were not capable of transferring nutrients to the oceans. Also a reduction of upwelling activity helped the effect. They also proposed that oxygen levels may have declined as a result of a loss of primary productivity. Tappan went on to say that heavy extinction of suspension feeders at the end of the Devonian, Permian, and Cretaceous implicated changes in primary productivity as the main cause of the extinction through accumulation of organic material in the ocean and thus starving the ocean and land of nutrients. Once again let it be noted that the oceans would starve if there was no upwelling. Through this mechanism the end Permian is very gradual and it would selectively remove different species at different times. Many scientists criticize this mechanism because it would cause the oceans to be virtually sterile. Wingnall (1993) criticized this hypothesis by saying ?It appears unlikely that the oxygen-deficiency was induced by high productivity for, as we have shown, organic-rich facies are only patchily developed in the Griesbachian [early Triassic].?When thought through carefully, nutrient accumulation or sequestration would have reached a peak during the development of the extensive Carboniferous coal swamps and not during the Permian period.
One very interesting hypotheses is based on biogeography. Erwin (1993) said that,
Since most species occur only within a single marine province, one of the major controls on global diversity should be the number of marine provinces. Similar communities in different areas of a single province tend to have roughly similar community composition (at least for the more abundant species). Thus the species within a nearshore sandy-bottom community will tend to recur throughout a province but will differ between provinces.
Since continents usually define marine boundaries then when continents are dispersed there will be more marine provinces and thus more diversity. Erwin goes on to say that the formation of Pangea (the great super-continent) in the late Permian times forced a reduction in sea-floor spreading.
Since the depths of the ocean basins are a function of the age of oceanic crust, a reduction in the rate of sea-floor spreading will allow the mean age of the oceanic crust to increase, increasing the size of the ocean basins. The volume of the mid-ocean ridge spreading centers will also decline. The net effect should be a regression.
Richard Leakey (1995) adds an interesting parallel.
Imagine four one-inch squares, each of which has a total edge length of four inches, giving a grand total of sixteen inches. Now bring them together as a single square of side two inches. The total edge length is now a mere eight inches, just half of the previous figure. The same thing happens with individual continents and available shallow-water habitats. The formation of Pangea therefore must have devastated species in these habitats by this mechanism alone??
Regression causes in increase in the continent?s surface area and it also alters climate patterns. There will be an increase in seasonality in nearshore waters along with an increase in nutrients and competition as provinces merge together. Therefor global diversity should be at its lowest when the supecontinent exists. The more continental climates and higher seasonality will increase the instability of nutrients, primary productivity, and other trophic resources. Here species that are affected seasonally will be affected greatest while species with a broad trophic and environmental tolerances will be favored. Since the study of instability if very complex we should treat these kinds of hypotheses carefully. In concluding, the above factors may well have played a role with other factors in causing the greatest extinction on the earth (Erwin, 1993).
Now we move away from diversity-dependent factors to diversity-independent hypotheses which are more common and accepted. This involves models that affect all individuals of a species equally and is independent of the number of species present. As mentioned before, most extinction fall into this category.
Extraterrestrial phenomena is one of the favorite explanations for the Permian extinction. There is quite a bit of evidence to support it. In Science News (1993), Monastersky reported on the findings of a Canadian team working with well-preserved shales and cherts from northeastern British Columbia. These rocks formed when the region lay at the bottom of an inland basin. The researchers got information about an ancient ocean during the Permian time by isolating from the rock small amounts of kerogen. Kerogen is the decomposed residue of Permian plankton. At the PTB the kerogen records drop sharply in the ratio between heavy C-13 atoms and light C-12 atoms. Monastersky goes on to say
To interpret the shift in Carbon isotopes, the researchers exploited the fact that plants tend to avoid Carbon 13 as they grow during photosynthesis. Because of the vast number of phytoplankton competing for carbon-12 during normal times, however, the plants typically incorporate some carbon-13. But a sudden die-off of most phytoplankton would give survivors greater access to carbon-12. When they fall to the ocean floor and get incorporated into sedimentary rocks, they reduce the ratio of carbon-13 to carbon-12 within a rock?.
Geochemists who have studied inorganic carbon which came from shells of ancient plankton have also detected abrupt drops in carbon isotopic ratio at the end of the Permian. Due to the many factors that can alter this ratio they have not been able to isolate what caused the change. Fewer processes affect the carbon isotopic ratio in kerogen. This greatly strengthens the case that the surface ocean suffered from a biological crisis. ?It is consistent with some sort of catastrophic event like an asteroid?.?(Monastersky, 1993). Another paper written by Richard Monasterky (1997) gives more evidence. It seems that a scientists named Gregory J. Retallack went searching in the Southern Hemisphere and reported that he found ?shocked? quartz at two sites in the Antarctica and one site in Australia. This type of quartz is riddled with intersecting sets of fractures and is born only during impacts. Iridium also adds to the evidence. Scientists now know that an impact caused the extinction at the Cretaceous-Tertiary boundary and there is also an increase in Iridium at this boundary. Thus they can deduce that if they find and increase of iridium at the PTB above background levels then they will have evidence of an impact. The discovery of a significant increase in iridium and microspherules at the PTB boundary by Xu Dao-Yi et al. (1993, 1985, 1989) gives imperical evidence of an impact.
There are many problems with the impact theory. First there is evidence that shows that the Permian extinction started gradually and had a more rapid pulse at the end (Monastersky, 1993). Also some scientists have argued that the quartz crystals found by Retallack were not shocked because Retallack only studied them under a light microscope, where it is difficult to distinguish shock features from more prosaic deformations caused by normal tectonic stress in the Earth?s crust. Monastersky (1997) said that ?an impact capable of triggering unparalleled losses should have strewn telltale clues around the world.? Western geologists have attempted to verify the Chinese reports of iridium and they are fruitless. Anomalies can cause a build up of iridium in one place which would lead to what the Chinese have discovered (Erwin, 1993).
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.
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.
- The Spanish Civil War 3 Essay, Research Paper The Spanish Civil War In the ... the son of the old dictator Primo ... tools. In the end they triumphed ... that the eventual extinction of the Catholic ... much political debate and numerous assassinations, two of which began the ...
- Extinction Essay, Research Paper Introduction Earth probably began ... host to at least one mass extinction each. The Precambrian era was a period in ... a least three mass extinctions, in the late Ordovician, the late Devonian, and the end Permian. It many ...
- The Colloseum Essay, Research Paper of all time. The city ... , back in the days of its prime, it was ... the end of the Emperor Nero and the triumph of the ... animals to near extinction in the surrounding areas. However ... held was the mock sea battles. The Romans were ...
- The Buddha Essay, Research Paper In the sixth century before the ... (austerities) and Pranayama (practice of ... the music dies, The string overslack is dumb and the music ... gained coolness by the extinction of all ... THE END Buddha went to Sravasti, the capital of the ...
- ... those bring the most revenue when the pound pays profit. The prime target, ten ... the end of their line can generate money through the state. One of the ... see their extinction at hand. It is a public campaign to make sellers ...