Microbes caused Earth's biggest extinction 250 million years ago

Written by PTI | Washington | Updated: Apr 2 2014, 00:15am hrs
ExtinctionVolcanoes are not entirely off the hook, they have simply been demoted to accessories to the crime, researchers said. (Reuters)
Methane-producing microbes may be responsible for the largest mass extinction in Earth's history - an event that wiped out 90 per cent of the planet's species 252 million years ago, a new MIT study has found.

Researchers have found enough evidence to show that the perpetrators of the largest of the planet's five known mass extinctions, were not asteroids, volcanoes, or raging coal fires, all of which have been implicated previously.

Rather, they were a form of microbes - specifically, methane-producing archaea called Methanosarcina - that suddenly bloomed explosively in the oceans, spewing prodigious amounts of methane into the atmosphere and dramatically changing the climate and the chemistry of the oceans.

Volcanoes are not entirely off the hook, they have simply been demoted to accessories to the crime, researchers said.

The reason for the sudden, explosive growth of the microbes may have been their novel ability to use a rich source of organic carbon, aided by a sudden influx of a nutrient required for their growth: the element nickel, emitted by massive volcanism, they said.

Massachusetts Institute of Technology (MIT) professor of geophysics Daniel Rothman, Gregory Fournier, and five other researchers at MIT and in China build their case based on three independent sets of evidence.

First, geochemical evidence shows an exponential (or even faster) increase of carbon dioxide in the oceans at the time of the so-called end-Permian extinction.

Second, genetic evidence shows a change in Methanosarcina at that time, allowing it to become a major producer of methane from an accumulation of organic carbon in the water.

Finally, sediments show a sudden increase in the amount of nickel deposited at exactly this time, said researchers.

The carbon deposits show that something caused a significant uptick in the amount of carbon-containing gases carbon dioxide or methane produced at the time of the mass extinction.

Some researchers have suggested that these gases might have been spewed out by the volcanic eruptions that produced the Siberian traps, a vast formation of volcanic rock produced by the most extensive eruptions in Earth's geological record.

But calculations by the team showed that these eruptions were not nearly sufficient to account for the carbon seen in the sediments.

Even more significantly, the observed changes in the amount of carbon over time don't fit the volcanic model.

"A rapid initial injection of carbon dioxide from a volcano would be followed by a gradual decrease. Instead, we see the opposite: a rapid, continuing increase," Fournier said.

"That suggests a microbial expansion. The growth of microbial populations is among the few phenomena capable of increasing carbon production exponentially, or even faster," he added.

The study was published in the Journal Proceedings of the National Academy of Science.