Australasian Science: Australia's authority on science since 1938

Small Crater Responsible for the Great Dying

By Stephen Luntz

An Australian scientist believes he has identified the crater responsible for the greatest extinction in our planet’s history.

The crater was once considered far too small to have caused the event known as The Great Dying, but a new angle suggests that its formation might indeed have been the cause.

The Chicxulub crater in Mexico is thought to have been formed by the asteroid that wiped out the dinosaurs. However, one-quarter of the animal and plant species alive during the Cretaceous survived that event.

Since Chicxulub is 180 km across it was thought that the object responsible for the Permian–Triassic (P-Tr) extinction event 187 million years earlier must have been even larger. The P-Tr event wiped 96% of marine species from the fossil record, and marks the only known mass extinction of insects.

The Araguainha crater in Brazil is 40 km across, making it an unlikely cause for such a large extinction. Moreover, previous estimates of its age put it 8 million years after the P-Tr extinction event.

However, Dr Eric Tohver of the University of Western Australia’s School of Earth and Environment has been working with fellow WA-based researchers Martin Schmieder and Fred Jourdan to improve the dating of impact structures around the world (AS, October 2013, pp.16–20). “We were particularly interested in the Araguainha crater, since the original age determined in the 1990s was relatively close to the Permo-Triassic boundary,” says Tohver. “The refinements in geochronological techniques that we are applying are helping to reveal the true age of these structures.”

In work published in Geochimica et Cosmochimica Acta the team re-dated the crater at 254.7 Ma with an error margin of 2.5 Ma, encompassing The Great Dying 252.3 million years ago.

However, this still leaves the problem of how a medium-sized crater could cause such destruction.

Tohver thinks the answer lies in the fact that much of the rock in the area is oil shale laid down around 30 million years earlier. He believes the impact would have set off a chain of earthquakes and tsunamis in the region, releasing vast quantities of methane. This in turn would have caused a drastic warming of the planet for a period of a few decades – long enough to wipe out many species.

The theory remains controversial. Around the same period the Siberian volcanic traps released even larger amounts of gas. However, Tohver says that volcanic province emissions are spread over a few million years, and the microfossil record suggests the Permian extinction occurred over a particularly short amount of time – consistent with a single event.

Biological methane has a high ratio of carbon-12 to heavier isotopes. Tohver says the hydrocarbon deposits of the area are depleted in carbon-12 to an extent that suggests 1500–1600 Gt of methane have been released since the rocks were formed.

Tohver acknowledges the findings only show that a massive release at just the right time is “possible rather than probable”. However, his estimate is four times the amount of carbon that humans have put into the atmosphere since the start of the Industrial Revolution. Released as methane, rather than primarily as carbon dioxide, it would have had a warming effect several times larger than the one we have so far generated.

However, even this quantity of gas is only half what was thought to have escaped to the atmosphere 55 million years ago, forming the boundary between the Palaeocene and Eocene epochs. Temperatures rose 6°C as a result, but the event does not rank as a mass extinction. It is possible that this could have been because it took place over 20,000 years, giving species a chance to adapt.

However, Tohver also suggests that the arrangement of the continents 200 million years earlier made life forms more vulnerable to sudden changes in temperature. “At the end of the Permian the continents were all together in Pangea,” he says. “We know the interior of continents is subject to much more extreme temperature variations, even in the course of a day, so this may have made life more vulnerable.”