End Permian Mass Extinction

Finally, the big one. Around 90% of all species went extinct: 70% of land vertebrates and 80-96% marine animals (Chen & Benton, 2012). After millions of years of living beings surviving the challenges of the changing Earth, what happened that almost wiped them out?

Fig. 1: A marine ecosystem before and after the mass extinction. Chen & Benton, 2012. © John Sibbick

The end Permian extinction event was 252 million years ago and it happened "quickly", in less than 100,000 years (Shen & Bowring, 2014). The exact causes are still debated, though there is one that stands out. This time the culprit seems to be volcanism (Payne & Clapham, 2012) - though not a lone volcano erupting, but massive eruptions, flooding extensive areas with over 5 million km³ of basalt. The eruption of the Siberian traps, located in central Russia, went on over a period of 1 to 2 million years (Shen & Bowring, 2014). The volcanic activity released huge amounts of CO₂ to the atmosphere; this could have even been increased due to interactions with organic deposits. Svensen et al. (as cited by Payne & Clapham, 2012) calculated that over 30,000 GT of carbon was released to the atmosphere! As you probably expect, there is evidence for a rise in global temperatures of around 8°C (Shen & Bowring, 2014). 

However, not only did the CO₂ influence the temperature; it likely caused ocean acidification and carbonate saturation. This would affect the marine animals, specially those with shells, leaving them without refugia (Payne & Clapham, 2012). 

Another mechanism involved in the extinction was marine hypoxia/anoxia, perhaps caused by the excessive chemical weathering of the Siberian traps, releasing phosphorous to the oceans (Payne & Clapham, 2012). The excess nutrients would have promoted eutrophication and hypoxia.

The emissions from the eruptions would have included not only CO_2, but sulfur, HCl and CH₃Cl, among others. Black et al. (2014) used a model to assess the effect of these emissions on the ozone layer and acid rain with different eruption scenarios. They found that the ozone layer could have been depleted up to 30% due to HCL and up to 67% due to CH₃Cl when these emissions reached the atmosphere. The effects would have been felt worldwide - increased UV B radiation with mutagenic effects. For the acid rain they considered a CO₂ concentration 10 times higher than present. This effect alone would have meant acid rain with a pH of around 4, and it would have been present worldwide. Combined with sulfate, for a 1 year eruption of 2400 km³, the pH may have dropped as low as 2, with the effects mainly in the Northern hemisphere (the location of the traps).

It took about 8-9 million years for life to make a full recovery (Chen & Benton, 2012), after almost being wiped out in 100,00 years. But this mass extinction paved the way for a new revolution of life, and the soon to begin age of the reptiles.

RIP
520 - 252 million years ago
Source: Wikipedia.com