The Great Dying: What caused Earth's largest mass extinction event?
If you think of an extinction event, you probably think of the asteroid that killed the dinosaurs, but Earth has gone through five mass extinction events (so far) and is perhaps going through its sixth.
Among the several extinctions our planet has faced, the Cretaceous mass extinction that ended non-avian dinosaurs isn't even the largest. That honor goes to the Permian mass extinction, also known as The Great Dying, which wiped out approximately 70% of all terrestrial species and 96% of all marine species on Earth.
In addition to this, a strange 10-million-year gap in coal created around the time of the extinction – known as the “coal gap” – suggests that large numbers of coal-forming trees went extinct during the event, taking millions of years to recover.
Scientists have proposed a number of explanations for the extinction and the causes behind it: from a catastrophic release of methane from the ocean floor to our old friend, an asteroid impact. From studying rocks formed at the time of the extinction, we know that in the Late Permian the oceans and shallow waters were devoid of oxygen. It appears that lack of oxygen (also known as anoxia) definitely played a role in the extinction event, as well as having a domino effect.
Sulfate-reducing microorganisms, which can perform anaerobic respiration using sulfate rather than the old reliable O2 likely thrived in these low-oxygen environments. The hydrogen sulfide byproduct they produce, in addition to making the oceans sulfide as a secondary consequence of their lack of oxygen, may have been released into the atmosphere. Here it may have poisoned plants and damaged the ozone layer, exposing life to killer levels of UV rays.
Ocean warming, in turn, could have caused ocean methane to be released into the atmosphere, exacerbating the problem.
An alternative explanation for the extinction, proposed by an MIT team in 2014, is perhaps the most concerning. Could the greatest extinction the world has ever seen have been caused by microbes? Researchers noticed the rise of a certain microbe around the time of extinction. Methanosarcin, a single-celled organism, became capable of digesting organic matter, producing methane as a byproduct, thanks to the transfer of a single gene from the bacterium Clostridia.
The hypothesis is that Methanosarcina thrived during this period, emitting methane into the atmosphere and interrupting the carbon cycle, causing (or adding to) the disruption of the carbon cycle and ultimately fueling the extinction event. The chemical process involved in methane-creating microbes involves metallic nickel, meaning that if the scolars fails to find a correspondingly higher amount of nickel during the extinction event, the hypothesis could be discarded.
However, scientists examined the best-studied sediments in southern China and found high levels of nickel, perhaps supporting the theory.
“ A single horizontal gene transfer triggered a biogeochemical change, massive volcanism acted as a catalyst, and the resulting expansion of Methanosarcina acetoclastica acted to perturb CO2 and O2 levels. The resulting biogeochemical disruption would likely have been widespread. For example, anaerobic oxidation of methane may have increased sulfide levels, possibly resulting in a toxic release of hydrogen sulfide into the atmosphere, causing extinctions on land .”
The team stressed that while more evidence is needed to support the theory, the study could show how sensitive the Earth is to the evolution of microbial life.
