Ash from burning coalfields, sparked by massive volcanic eruptions, may have added to the volley of crises that led to the biggest extinction event in Earth’s history.

Many factors combined to cause the mass extinction at the end of the Permian period 251 million years ago, during which about 96 per cent of marine species and 70 per cent of those on land went extinct. Several researchers have speculated that volcanism might have set off extensive coal fires that contributed to the extinction, but no clear evidence had been found – until now.

In Permian sediments from the Sverdrup basin in the Canadian Arctic, a team led by Stephen Grasby at the Geological Survey of Canada in Calgary found tiny spherical particles that are evidence of open coal combustion – very different from the ash that comes from burning vegetation.

“It’s the first literal smoking gun to show that coal combustion was occurring,” says Grasby.


Fire starters

He speculates that major volcanic eruptions some 1000 kilometres east of the Sverdrup basin – in what is now Siberia – may have ignited overlying coal deposits and released huge quantities of ash into the atmosphere.

Modern-day coal-fired power plants scrub this so-called “fly ash” from their emissions because it contains a lot of toxic metals such as chromium. When Grasby looked more closely at his sediments dating to the very end of the Permian, he found that they, too, contained high levels of chromium. This suggests that fly ash may have poisoned ancient oceans and lakes and contributed to the extinction.

Further studies of other end-Permian sediments around the world should show how widely the fly ash spread as the coalfields burned, and therefore how important it is likely to have been in causing the global extinction, says Andrew Knoll, a palaeobiologist at Harvard University.

If it turns out to be one of the main drivers of the extinction, he notes, it would probably have killed off species relatively indiscriminately. Another factor must explain why some groups – such as corals – suffered massive extinctions, while others – such as sea anemones – did not, he says.

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo1069