Let’s Talk About Mass Extinctions: Permian Triassic

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 sulfideatmospheric 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.

Let’s Talk About Mass Extinctions: Uncovering Evidence of Past Extinctions

This is the first of a 3 part series that will teach you all about mass extinctions, travelling through time to share the details of the mass extinctions that have occurred in the past, all the way up to the one that is currently happening.

Extinctions are a normal part of life in fact, species of organisms are always going extinct. The background rate of extinction defines the ‘normal’ rate that extinctions occur, without human interference. This rate is has been recently estimated to be about 0.1 extinctions per million species per year (E/MSY). In addition to this, new species are always emerging in a type of punctuated equilibrium model, leading to a constantly changing amount of biodiversity on Earth through time. 

A mass extinction is defined as an event in which a least 75% of all species on Earth disappear. This must occur on a global scale, with corresponding evidence in both the geologic and biological records. For past extinctions, determining what 75% of species at that time was and what the coinciding geological markers are, are difficult and nearly impossible tasks.

There is no real way of determining with certainty how many species existed in the past and when they went extinct. The main source of evidence of past biodiversity is in the fossil record which has some major issues. A type of ‘bias’ exists in the fossil record, meaning that very specific geographic and geologic conditions need to be present for an organism to be fossilized.

Essentially, the organism needs to be in the right place at the wrong time, in other words, all the conditions for fossilization are there, but so is the thing that kills them. Ideally, that place is an anoxic environment that will slow or stall decomposition. It must also be saturated with carbonates, otherwise the soils will use the organism’s bones as a source of carbonate, dissolving them rather than preserving them. Chance is also a key element for fossilization: for fossils to be preserved for many years, it must occur in a place where sediments are deposited, lithified (compressed and built into the rock), then uplifted and preserved rather than eroded.

These very specific conditions were obviously not present everywhere, thus, only certain species over the past millions of years were fossilized. Marine animals, and some land animals and plants are the main components of the fossil record, leaving out a huge number of species of insects and bacteria that often don’t have opportunities to be fossilized. In addition to this, for these fossils to be discovered in modern times, they must be fairly abundant meaning that we are most likely only uncovering the most common species of the past. 

How different oxygen isotope ratios form depending on climatic conditions

In terms of the geologic record, it is where scientists can infer the conditions of the past depending on the thickness of layers of certain elements in the sedimentary rocks. For example, a thick iridium layer could infer a meteorite crash on Earth (however further evidence is required to confirm this) because iridium is uncommon on Earth, but very common in extra-terrestrial materials. Another example is the use of oxygen isotope ratios in sediments and ice cores as indicators of temperature (ie. markers for glaciations). Colder waters contain higher O18/O16 ratio because H2O16 is favored over H2O18 for evaporation since it is lighter. Therefore if conditions on Earth were cold, the H2O16 would be evaporated and trapped in ice and snow on land, leaving greater O18 in marine sediment records.

The main issue with the geologic record is that rocks don’t last forever. Earth’s crust is constantly being constantly renewed (sea floors spreading) and destroyed (subduction), meaning that most of the really old rocks are all gone. The oldest rocks on Earth are found in places where subduction does not occur, therefore they are fairly sporadic, making it difficult to discern past conditions.

Even today, the definitive number of species on Earth cannot be determined, but calculating extinction rates is a lot simpler, since we do not have to rely on old and sporadic fossils and sediments.

Massive spikes in extinction rates indicate a mass extinction

The periods when extinctions occurred are determined when, over a geologically “short” period of time, a significantly lower variety of fossils are discovered indicating a sharp drop in biodiversity during that time. If this drop coincides with addition markers such as an iridium spike (and many others) then this could indicate a mass extinction event.


The geologic markers and other astounding evidence found all over the world for what is believed to be the cause of the Permian-Triassic mass extinction will be discussed in next week’s post.