This is the second of a three-part series. To view the previous post of the series, go here.

The Permian Triassic extinction is the largest known extinction to date, wiping out an estimated 96% of all species. The surface of the Earth 251 million years ago looked much different than it does today. All the continents were connected, forming one large super continent, known as Pangea. Ocean circulation was set up similarly today, where upwelling of cold water to the surface would have been in the Tethys Sea. 

The cause of the Permian Triassic extinction was mostly uncertain until recently. It was known that oxygen levels in the atmosphere were lower than they are today, but the reason for this was unclear. It was thought that the intense volcanism occurring at the time drastically increased atmospheric CO₂ levels, causing global warming and lower atmospheric O₂ levels. This alone would not have been a sufficient cause for the massive amount of species that went extinct during this time. In particular, it especially could not explain the number of plant species that went extinct considering the fact that plants thrive in conditions of elevated CO₂. The discovery of some very important marine fossils brought some light to the situation.

Marine fossils from the Permian Triassic boundary contain large amounts of photosynthetic green and purple sulfur bacteria. Today, these bacteria are found in anoxic (without oxygen) aquatic areas such as deep, stagnant lakes and the Black Sea. They need light in order to photosynthesize, meaning that they need to stay fairly close to the surface. One of their main functions is the production of H₂S gas, which is very toxic to most life forms.

 Typically, these bacteria reside fairly deep down in the ocean, because the surface ocean contains a fair amount of oxygen that these bacteria don’t like. The low light at these depths slows their photosynthesis and proliferation, preventing a bloom of the toxic bacteria. The abundance of these bacteria suggests that the world’s oceans were in an anoxic state right up to the surface. The high atmospheric CO₂ levels from intense volcanism was most likely the main contributor to anoxic oceans, because warmer waters have a lower ability to absorb oxygen.

The conditions of the ocean were perfect for rapid and widespread proliferation of these bacteria, and this would have led to huge bubbles of H₂S gas erupting into the atmosphere. This toxic gas would have been detrimental to nearly all life forms on Earth at the time. In addition to this, elevated atmospheric CO₂ and warming would have increased the lethality of H₂S.

This process did not become so intense overnight, it took hundreds of thousands of years for levels of H₂S in the atmosphere to reach toxic levels. Models of H2S levels line up with estimations of past ocean circulation, where highest levels were in primary areas of upwelling of deep ocean waters. It is estimated that amount of H₂S entering the atmosphere from the ocean was 2000 times greater than what is released by any volcano.

More than just fossil evidence of the H₂S producing bacteria is needed to confirm this explanation for the biggest known extinction. Elevated levels of toxic H2S gas in the atmosphere would have surely caused great damage/thinning to the Earth’s ozone layer, the layer of O3 gas protecting the surface from the UV radiation of the sun. Fossil spores found in Greenland show deformities that are known to be caused by high UV exposure. High UV levels would have also caused great distress to any life on land, further contributing the extinctions.

In summary, the Permian Triassic is a classic example of “priming the pump” meaning that all the conditions were lined up perfectly for everything to go wrong causing the extinction of almost all life on Earth. High volcanic activity led to high CO₂ and low O₂ levels in the atmosphere causing rapid global warming. This made it difficult for the oceans to absorb oxygen, leading to anoxic oceans and an ocean wide bloom of oxygen-hating-H₂S-producing bacteria. Massive bubbles of H₂S gas were being emitted into the atmosphere from the oceans. The amplified lethality of H₂S from the warm conditions of Earth at the time was slowly killing off life on Earth and damaging the ozone layer, exposing plants and animals to dangerously high levels of UV radiation. 

The likelihood of all these events lining up so perfectly is extremely small and rare, but not impossible. Considering that mass extinctions are also rare (but very important) occurrences, it would make sense that more significant and complex events than just volcanos caused them. Next week, we will explore how we could be currently entering the 6th mass extinction.