It is well known that some event took place over 250 million years in the past that caused a mass extinction on Earth, taking out 90% of all the species. However, nobody knows for sure what caused this event, known as the Great Dying.
The previous leading candidate was extreme volcanic activity releasing massive amounts of carbon dioxide into the atmosphere. However, a new study by researchers at MIT suggests that volcanic activity was not the entire cause, but was likely an enabler in the Great Dying. The group's research was published in a recent edition of the Proceedings of the National Academy of Sciences.
While the volcano hypothesis makes sense on some level, elements of it did not add up-primarily with the amount of carbon released.
Geochemical analysis showed an unusually high level of carbon in sediments that correlate to the time of the Great Dying, indicating a large increase in carbon compounds in gaseous form. This had been potentially attributed to a series of erupting volcanoes that formed the volcanic stone formation known as the Siberian traps.
While these eruptions make up the largest known volcanic emissions in the history of the Earth, the team at MIT determined that the eruptions by themselves did not account for all of the carbon in the sediments.
As an added refutation, the carbon deposits showed variations over time that did not fit with volcanic eruption. Volcanic activity should produce a large initial increase with a gradual decline over time as the carbon dissipated, but the deposits indicated a continued increase beyond the eruption period.
What could survive and add still more carbon to the atmosphere under this environment? The answer is microbes. In particular, the team focused on Methanosarcina, an archaea that is a prolific producer of methane. The volcanic eruptions caused a significant release of nickel into the atmosphere, according to a new analysis of sedimentary material found in China, and nickel is a necessary ingredient for the growth of Methanosarcina.
While compiling a map of the genetic history of Methanosarcina, it was noted that in the approximate time of the mass extinction, the microbe managed to obtain a method of rapid methane production from another microbe.
Combined with an initial feedstock of atmospheric carbon and nickel, this created the perfect set of circumstances for explosive growth of Methanosarcina. Once the carbon dioxide and nickel-laden byproducts of the volcanic eruptions entered the oceanic environment of Methanosarcina, rapid expansion followed. The resulting massive increase in methane accounts for the larger volume of carbon, and matches up better with the growth rates in the sediments. The Great Dying subsequently took place, as many species could not adapt to the new, harsh environment.
It's fair to say that we still don't know for sure what caused the Great Dying-although thanks to the research team at MIT, we now have a more plausible theory than before. As with any theory, it's always subject to change based on newer findings.