Assess the different hypotheses put forward for the mass extinctions at the end of the Permian and Cretaceous (KT) Periods. A mass extinction is an event in which at least 25-75% of species in the global environment are eradicated in a short period of time. Where as a regional extinction event is when the extinction is confined to a specific zone. Five mass extinctions have occurred throughout time, two of the most well known of these are the Permian and Cretaceous extinction events. There are several hypotheses that are used to explain the causes of mass extinctions.
Climate change, the warming or cooling of global environments over a short period of time, can lead to other occurrences. Shifts in climate can cause extinction by moving or eliminating entire habitats. Global cooling most impacts the tropical areas as in the polar and deep-water flora and fauna have already adapted to cool environments. The major effect of global warning is on the circulation of ocean currents and the reduction of dissolved oxygen in water bodies. Anoxia, no or little dissolved oxygen in the sea, is the major cause of marine extinction.
Anoxia is linked to global warming and sea levels rising. Bolide, impact by asteroids or meteorites, can cause several environmental causatums. The dust from the impact can block out sunlight for years, creating permanent a winter, leading to the collapse of ecosystems. With no sunlight, plants cannot photosynthesis, so they will eventually die. Decaying plant and animal life leads to release of noxious gases, lessening the O2 levels in both atmosphere and lithosphere. The bolide impact can cause huge earthquakes and tsunamis.
Molten debris raining down after the impact can start wildfires across the globe, further adding to the noxious atmosphere. The release of carbon dioxide and sulfur dioxide from mega-volcanoes can impact global temperatures, leading to global warming, sea level rises and acid rain. Volcanic eruptions have are commonly located around the times of mass extinctions, and can add to the already stressed environment. The Permian extinction event occurred around 248 million years ago and marks the end of the Paleozoic Era. It is the largest extinction event in history, with approximalty 96% of all life becoming extinct.
There are several hypotheses about what caused the Permian extinction, but as much of the evidence about what occurred has been lost over time, it is hard to tell exactly what caused this mass extinction. At the time of the Permian extinction, the earth was joined as the large super-continent Pangea. One theory to account for this extinction is that the earth experienced an ice age for approx 40 000 years, which was followed by an extreme global greenhouse effect. The regression and transgression of fluctuating climate caused rainfall patterns to change.
Very few life forms would have been able to withstand the extreme climatic changes. There is evidence supporting this theory in Greenland and Russian rock strata. The fluctuating climate caused a fluctuation of sea levels, with first a fall in levels then with a rise. This rise, so soon after a fall in sea levels would have placed extreme stress on the marine community that was trying to adjust to the already change marine environment. The regression in sea levels exposed organic matter that was decomposed to form large amounts of CO2 causing a drop in oxygen levels and an anoxic marine environment.
The large carbon dioxide levels could have contributed to more global warming. Nearing the end of the Permian period there were mass eruptions in the Siberian traps approx 2 million km2. This volcano released massive amounts of carbon dioxide and sulfur dioxide, leading to acid rain and blackout of sunlight. It is thought that this caused the global cooling, and once the eruptions ended, intense global warming cause by the noxious gases released during the eruptions, which lead to the fluctuating sea levels.
Much flora and fauna suffered severely because of these environmental issues. There is new evidence indicating that a bolide impact could have occurred near the end of the Permian period. A crater, the Bedout crater, in Australia has been discovered. A bolide impact would have lead to mass death of marine and terrestrial organisms. The force of the impact could have released methane from stored organic matter and large earthquakes. These earthquakes would have triggered the volcanism in the Siberian traps, leading to the release of large amounts of CO2 and sulfur dioxide.
These gases combined with methane previously released would have created a noxious atmosphere, combined with climate change that killed a large scale of life. It is most likely that a simultaneous occurrence of the different global environmental changes was responsible for ending the Permian period, and the lives of 96% of species. Each environmental issue is a major geological event, and amplified the other events, leading to catastrophic environmental situation, in which barely any life could survive.
The Cretaceous – Tertiary mass extinction, commonly referred to as the KT extinction, occurred 65 million years ago. As the most recant extinction event it is more easily studied as more evidence has been preserved than for the earlier extinctions. This extinction is the most commonly known, as it is the extinction of the dinosaurs, but the smallest scale mass extinction with only 76% of species dying out. Evidence for catastrophism at the KT boundary can be found in a layer of greenish sediment, which is located at different sites all over the world.
The sediment makes the divide between the Cretaceous fossils from the newer, evolved Tertiary fossils. This dark sediment contains high concentrations of Iridium, an element only found in the earth’s mantle, or on meteorites. There are two main hypotheses that are used to explain the cause of the KT extinction and the end of the reign of the dinosaurs. The gradualist theory suggests that the KT extinction was caused by a regression of sea levels combined with global climate change caused by volcanism. There is evidence of a large deposit of Basalt located at the Deccan Plateau in India.
The release of volcanic gases and ash would lead to climate change and the darkening of the sky. The volcanic ash released into the atmosphere at the time of the eruption, which then settled slowly all over the globe, could have caused the iridium layer mentioned above. However, there is evidence that a bolide impact struck the earth in the Gulf of Mexico, the Chicxulub crater. The theory is known as the catastrophists theory. A bolide impact would explain the high levels of iridium and the shocked quartz, which has also been found throughout the globe in the same layer containing iridium.
Shocked quartz is only formed under extremely high pressure, naturally formed in bolide impacts. The quartz contains fractures that show the impact shock. The bolide impact would have cause mega tsunamis and wildfires throughout the world from the molten bits of meteorite raining down on the earth. Tektites have been found in sediments around the Chixulub crater, which provides substantial evidence that a bolide impact did occur there, as tektites are a special type of debris formed from a meteorite impact.
Evidence for impact craters is very hard to find as most craters are under the sea. While many support the catastrophists theory for causing the KT extinction, a combination of the gradualist theory, with the final blow coming from a meteorite impact is much more likely, as there has been evidence that the organisms living during the KT period were already dying out, before the mass extinction event occurred ending the reign of the dinosaurs and allowing mammals to reign the earth.
Even though some hypotheses are more likely to have caused the coup de grace than others, all evidence would have had an impact in the mass extinction process. Bibliography 1, Hubble, T. (2009). Earth and environmental science: the HSC course. Cambridge University press, Melbourne. 2. Prof Benton, M. (2001). Cassell’s Atlas of Evolution. Andromeda Oxford Limited, Oxfordshire. 3. Palmer, D. (2000). The Atlas of the Prehistoric World. Marshall Publishing Ltd, London. 4. (2010). Disaster down under – Australian impact Craters. [Internet].
Available from: (Accessed 8th May 2011). 5. Goodwin, A. (2001). Paleobiology and biodiversity Research Group: The Permo Triassic Mass Extinction. [Internet]. University of Bristol’s Department of Earth Sciences, UK. Available from: http://palaeo. gly. bris. ac. uk/Palaeofiles/Permian/SiberianTraps. html (Accessed 29th of April). 6. (2011). Discovery earth: Mass Extinctions. [Internet]. Discovery Communications, LLC. Available from: http://dsc. discovery. com/earth/wide-angle/mass-extinctions-timeline. html (Accessed 15th of April).