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 CO_2, 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

Cao. C, Love. G.D, Hays. L.E, Wang. W, Shen. S, Summons. R.E, 2009. Biogeochemical evidence for euxinic oceans and ecological disturbance presaging the end-Permian mass extinction event. Earth and Planetary Science Letters. 281 (188-201).

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.

Knoll. A.H, Bambach. R. K, Payne. J.L, Press. S, Fischer. W.W, 2007. Palaeophysiology and end-Permian mass extinction. Earth and Planetary Science Letters 256 (295-313).

Kump. L.R, Pavlov.A, Arthur. M.A, 2005. Massive release of hydrogen sulphide to the surface ocean and atmosphere during intervals of oceanic anoxia. Geology. 33 (397-400).

Mundil. R, Ludwig. K.R, Metcalfe. I, Renne. P.R, 2004. Age and Timing of the Permian Mass Extinctions: U/Pb Dating of Closed-System Zircons. Health Research Premium Collection. 305 (1760-1763).

Payne. J.L, Lehrmann. D.J, Follett. D, Seibel. M, Kump. L.R, Riccardi. A, Altiner. D, Sano. H, Wei. J, 2007. Erosional trunctation of the uppermost Permian shallow-marine carbonates and implications for Permian-Triassic boundary events. GSA Bulletin. 199 (771-784).

Retallack. G.J, Smith. R.M.H, Ward. P.D, 2003. Vertebrate extinction across Permian-Triassic boundary in Karoo Basin, South Africa. GSA Bulletin. 115 (1133-1152).

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Svensen. H, Planke. S, Polozov. A.G, Schmidbauer. N, Corfu. F, Podladchikov. Y.Y, Jamtveit. B, 2009. Siberian gas venting and the end-Permian environmental crisis. Earth and Planetary Science Letters. 277 (490-500).

Twitchett. R.J. 1999. Palaeoenvironments and faunal recovery after the end-Permian mass extinction. Palaeogeogr. Palaeoclimatol. Palaeoecol. 154 (27-37).

Wignall. P.B, Hallam. A, 1992. Anoxia as a cause of the Permian-Triassic mass extinction: facies evidence from norther Italy and the western United States, Palaeogeogr. Palaeoclimatol. Palaeoecol. 93 (21-46).