Published online 21 April 2009 | Nature | doi:10.1038/news.2009.380
Why big eruptions don't always fuel mass extinctions
Rocks near the site of a volcano could determine whether an eruption causes catastrophic climate change.
Huge volcanic eruptions have sometimes been followed by mass extinctions — but not always. The reason seems to lie in the chemistry of the rocks near the volcano.
Mass extinctions have been blamed on carbon dioxide released during big eruptions, because the gas was thought to have warmed the planet, eradicating much of its animal and plant life.
It is not certain, however, that even the very largest volcanic eruptions could eject enough gas into the atmosphere to drive the sort of climate change that would cause a mass extinction. Indeed, geologists have found evidence of many huge ancient volcanic eruptions that seem to not be connected to mass extinctions at all.
Now a team of researchers has analysed just how much CO2 non-volcanic rocks around volcanoes might release if they are super-heated. They have found that in some cases the rocks might spew out much more CO2 than the volcano itself.
Clément Ganino and Nicholas Arndt at Joseph Fourier University in Grenoble, France, went to explore a volcanic site roughly 260 million years old in southwestern China that seems to have formed at the same time as a mass extinction wiped out 35% of all genera on the planet. Catastrophic as it was, this extinction, which took place in the middle of the Permian period, was minor compared with the one that followed 10 million years later. That was the largest extinction in history, wiping out more than 70% of all genera including 90% of life in the sea and thousands of species including trilobites, many ancient sharks, the armoured fish known as placoderms and the fin-backed reptiles called pelycosaurs.
What caused the end-Permian and mid-Permian extinctions is still hotly debated, however.
Ganino and Arndt looked closely at dolomite, one of the most common rocks in the region of the mid-Permian eruption. Dolomite is composed of calcium, magnesium, and carbonate; when heated, it breaks down into magnesium oxide, calcium carbonate and CO2. The researchers calculated that 1 kilogram of dolomite heated by a volcanic source would produce 240 grams of carbon dioxide. They also found that impure marbles in the area would have released between 220 and 290 grams of CO2 per kilogram of rock when heated.
Today, because of erosion, the Chinese volcanic site is not particularly large. Before it was weathered away, however, geologists estimate that volcanic rock covered 500,000 square kilometres. Based on the abundance of heated sedimentary rocks that would have released carbon dioxide, Ganino and Arndt estimate that between 61,600 and 145,600 gigatonnes of CO2 were released1.
This would have overwhelmed the mere 16,800 gigatonnes of CO2 typically emitted by magma alone during eruptions, explains Ganino. “The mass of CO2 released from sedimentary rock is 3.6 to 8.6 times larger than the mass of CO2 released from magma. We did not expect such a huge difference,” he says.
Fuelling the fire
Henrik Svensen at the University of Oslo, Norway, was one of the first researchers to suggest that the rocks below volcanic eruptions might play a role in the devastation caused by eruptions. “More than 99% of all carbon on the Earth's surface is stored in sedimentary rocks, and heating those rocks with the high-temperature material that comes out of volcanoes is a good way to put a lot of carbon into the atmosphere quickly,” he says.
“There is a growing consensus that volcanics played a big role in mass extinctions, particularly the huge extinction at the end of the Permian,” says palaeobiologist David Bottjer at the University of Southern California in Los Angeles. “This finding — that various magma-heated sedimentary rocks produced the gases needed to cause climatic stress — fits in well with the conclusions that we have started coming to.”
The finding also raises the possibility that the heating of sedimentary rocks by the impact of a huge asteroid or comet and by volcanic activity at the time of the dinosaurs may have played a role in their extinction.
However, both Svensen and Ganino are quick to add that these findings do not mean that volcanic eruptions that took place at the same time as mass extinctions were necessarily connected to them.
“Destructive potential seems to depend upon the sedimentary rocks in the region,” says Svensen. It is for this reason that researchers must take a case-by-case approach when studying the effect of eruptions on the environment, he explains.
References Ganino, C. & Arndt, N. Geology 37, 323–326, doi:10.1130/G25325A.1 (2009).