Introduction
Figure 1: Artists interpretation of what the Permian-Triassic Mass Extinction would look like
The Permian Triassic Mass Extinction (PTME) is better known as the era in which life was nearly completely eliminated. There have been many concepts to try and explain the reasoning for this. These have been tested in a variety of ways. For instance fossils or strata can be analysed and hypotheses can be formed based on the way in which fossils have been preserved, as seen in Knoll, A.H. et al 2007 and Twitchett, R.J. 1999. Another method is by looking at carbon and oxygen isotopes which can be interpreted to show the magnitude and the length of the mass extinction, as seen in Joachimski. M.M, et al 2012. Similar to this a method to date sediments is by looking at the uranium and lead isotopes in zircon or other sediments as seen in Mundil, R. et al. 2004 and Shen, S. 2011. In Cao, C. et al 2009 stratotype sections were analysed using isotopes for the oceanic environment at the time of the PTME.
Cause 1: Bolide Effect
Most commonly meteors are thought of as the cause of the mass extinction. This is also known as the bolide effect. Meteors will have ejected extra-terrestrial gases into the atmosphere and traces of shocked quartz, metallic particles and meteoritic elements have been located in a potential impact site in Australia (Wignall. P.B, et al. 1992). The bolide effect is linked to the similar effects of the Cretaceous-Paleogene mass extinction (Knoll. A.H, et al. 2007), where released methane oxidised to CO2, which eventually caused a knock on effect of dissolved CO2 in oceans, acid rain and ocean acidification as seen in Figure 5.
Figure 2: Artists interpretation of the levels of extinction due to worsening biotic conditions
Cause 2: Anoxic and Euxinic Oceans
Anoxia and euxinia have also been suggested as another reason as to why the PTME occurred (Shen, S. et al. 2011). In simple terms anoxia is where oceans have decreased in oxygen causing for solubility of oxygen to decrease and acidification to occur, making for living conditions to become harsh for species (Shen, S. et al. 2011). Euxinic oceans is where sulphur increases in the seas while simultaneously being anoxic (Shen, S. et al. 2011). Together CO2 and H2S (Shen, S. et al. 2011) are released which contributes to greenhouse gases in the atmosphere. As well as this anoxia can cause water temperatures and the respiration rates of oxygen to increase (Kump. L.R. et al. 2005) which is difficult for species to survive and thrive in.
Cause 3: Siberian Trap Volcanism
Figure 3: Knock-on effects of Siberian Trap Volcanism
Increased volcanism has frequently been suggested to contributed to the mass extinction. Evidence from carbon and oxygen isotopes in the rock record has suggested Siberian trap volcanism and flood basalts (Payne. J.L. et al. 2007) occurred around the time the mass extinction was estimated. Some of the effects of the volcanism includes the release of gases such as sulphates, CO2 and thermogenic methane (Knoll, A.H. et al. 2007) which ultimately will cause an increase in greenhouse gases and their effects. When methane oxidises it creates CO2 which, not only causes global warming, but also results in hypercapnic stress and marine anoxia (Retallack. G.J. et al. 2003).
Figure 4: Image showing the Permian-Triassic Boundary with proof of Volcanic Eruption in the rock record
What was the real cause?
On balance it could be said that a combination of all of the causes would have contributed to the near extinction of life identified between the Permian and Triassic boundary. Because all the events had a knock-on effect and contributed to the effects of other events, the explanation for all the causes combined together is viable. It is difficult to tell which single cause started first, as it currently not possible to calculate the exact dates for the different events. However each single event resulted in global warming, building on and exacerbating the effects of prior events, creating an increasingly hostile environment for many species to thrive and survive in (Figure 5).
Figure 5: Knock on effects of Global Warming and Anoxic Oceans
References
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Joachimski. M.M, Lai. X, Shuzhong. S, Jiang. H, Luo. G, Chen. B, Sun. Y, 2012. Climate Warming in the latest Permian and the Permian-Triassic mass extinction.
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