WHEN WAS THE MASS EXTINCTION AND WHAT WERE CONDITIONS LIKE?
The Earth has undergone five previous mass extinctions, occurring in the Ordovician, Devonian, Permian, Triassic and Cretaceous periods. The worst of these extinctions was the one that marks the change from the Permian into the Triassic and is estimated to have occurred around 252.5 ± 0.3 million years ago (Mundil, 2004). It is responsible for killing off 96% of marine invertebrates and 70% of land species (Benton, 2018).
The Permian period began 299 million years ago and was marked by significant climate variations and fluctuations. This was a direct result of the continents colliding to form the supercontinent Pangaea. Due to its large size, there was spatial climate variation; the north was hot with fluctuations between wet/dry conditions whilst the south was cold and arid (National Geographic, 2017).
At the end of the Permian environmental conditions plummeted and became unsuitable for a majority of life thus causing the extinction.
Not until the end of the early Triassic, around 5 million years after the extinction, did signs of recovery start to show. The recovery period is the longest recorded out of all the past extinctions (Brayard, et al., 2011).
WHAT CAUSED THE EXTINCTION?
The cause of the Permian-Triassic extinction event is unknown and scientists are unclear whether it was caused by a single significant event or a gradual decline in environmental conditions.
One theory suggested the trigger was the eruption of volcanic rock in Russia. The series of eruptions released large amounts of CO2 into the atmosphere and marine environments, resulting in a condition known as hypercapnia, which impacted marine life in particular by preventing formation of carbonate shells. Most marine invertebrates have calcium carbonate shells and it prevented them from reproducing and lead to death. This was proven by scientists who subjected sea urchins and copepods to high levels of CO2 which were found to have reduced fertilisation rates and changes to their bodily functions (Knoll, et al., 2007).
Another theory suggested that the increased CO2 levels resulted in ocean anoxia (Joachimski, et al., 2012). The surface of the ocean is normally rich in oxygen as it is in contact with the atmosphere. This oxygen-rich water is mixed downwards to supply the deep ocean with oxygen. Increased CO2 levels and decreased O2 levels during the Permian meant less oxygen was diffused into the ocean and mixed downwards which deprived the deep ocean of oxygen. The ocean anoxia may have also arisen due to the formation of Pangaea and the uplift of the land resulting in changes to weathering and transport of nutrients into the ocean (Cao, et al., 2009). The ocean isotopes of conodont species were analysed from each sedimentary bed from the two periods and at the Permian-Triassic boundary the oxygen isotopes start to decrease (Joachimski, et al., 2012).
Another theory suggested the formation of Pangaea resulted in the continentality effect. This effect combined with the increased CO2 levels added to the already warming climate. Evidence of widespread deforestation was found in China through distinct charcoal layers. The widespread distribution of charcoal suggests global warming and increasing aridity reached a climax which increased forest fires. This was followed by soil erosion and fungal virulence due to rapid deforestation. This increased weathering and caused disastrous soil erosion on the continent (Shen, et al., 2011).
The most likely cause of the extinction was a combination of factors that made conditions hostile for life. However no one will know the true cause of the extinction until scientists understand the environmental conditions and timing of events that occurred at the Permian-Triassic boundary.
Bibliography and Further reading
Benton, M. (2018). Hyperthermal-driven mass extinctions: killing models during the Permian–Triassic mass extinction. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 376(2130), p.20170076.
Brayard, A., Vennin, E., Olivier, N., Bylund, K., Jenks, J., Stephen, D., Bucher, H., Hofmann, R., Goudemand, N. and Escarguel, G. (2011). Transient metazoan reefs in the aftermath of the end-Permian mass extinction. Nature Geoscience, 4(10), pp.693-697.
Cao, C., Love, G., Hays, L., Wang, W., Shen, S. and Summons, R. (2009). Biogeochemical evidence for euxinic oceans and ecological disturbance presaging the end-Permian mass extinction event. Earth and Planetary Science Letters, 281(3-4), pp.188-201.
Geography.name. (2019). continentality. [online] Available at: http://geography.name/continentality/ [Accessed 22 Apr. 2019].
Dem.ri.gov. (2019). Hypoxia and Anoxia- Rhode Island -Department of Environmental Management. [online] Available at: http://www.dem.ri.gov/programs/emergencyresponse/bart/hypoxia.php [Accessed 22 Apr. 2019].
Joachimski, M., Lai, X., Shen, S., Jiang, H., Luo, G., Chen, B., Chen, J. and Sun, Y. (2012). Climate warming in the latest Permian and the Permian-Triassic mass extinction. Geology, 40(3), pp.195-198.
Knoll, A., Bambach, R., Payne, J., Pruss, S. and Fischer, W. (2007). Paleophysiology and end-Permian mass extinction. Earth and Planetary Science Letters, 256(3-4), pp.295-313.
Mundil, R., Ludwig, K., Metcalfe, I. and Renne, P. (2004). Age and Timing of the Permian Mass Extinctions: U/Pb Dating of closed System Zircons. Science, 305(5691), pp.1760-1763.
Shen, S., Crowley, J., Wang, Y., Bowring, S., Erwin, D., Sadler, P., Cao, C., Rothman, D., Henderson, C., Ramezani, J., Zhang, H., Shen, Y., Wang, X., Wang, W., Mu, L., Li, W., Tang, Y., Liu, X., Liu, L., Zeng, Y., Jiang, Y. and Jin, Y. (2011). Calibrating the End-Permian Mass Extinction. Science, 334(6061), pp.1367-1372.
Suzanne Falck, F. (2019). Hypercapnia: Causes, treatments, and diagnosis. [online] Medical News Today. Available at: https://www.medicalnewstoday.com/articles/320501.php [Accessed 22 Apr. 2019].
Nationalgeographic.com. (2019). The Permian Period and Extinction. [online] Available at: https://www.nationalgeographic.com/science/prehistoric-world/permian/ [Accessed 22 Apr. 2019].
Twitchett, R. (1999). Palaeoenvironments and faunal recovery after the end-Permian mass extinction. Palaeogeography, Palaeoclimatology, Palaeoecology, 154(1-2), pp.27-37.
Images:
Anon, (n.d.). [image] Available at: https://www.ocean.washington.edu/courses/climate_extremes/Tutorial/Paleomaps/PERMIAN.jpg [Accessed 22 Apr. 2019].
British Geological survey (2019). BGS geological timechart. [image] Available at: https://www.bgs.ac.uk/discoveringGeology/time/timechart/home.html [Accessed 22 Apr. 2019].
Sam Noble Museum (n.d.). Permian terrestrial communities. [image] Available at: https://samnoblemuseum.ou.edu/common-fossils-of-oklahoma/paleocommunities/terrestrial-communities/permian/ [Accessed 22 Apr. 2019].