The volcanic eruptions that created Iceland might also have triggered one of the most catastrophic episodes of global warming ever seen on Earth, a new study suggests.

Michael Storey at Roskilde University in Denmark and colleagues have found evidence that a huge volcanic eruption, 55 million years ago, unleashed so much greenhouse gas into the atmosphere that world temperatures rose by as much as 8°C – with the Arctic ocean reaching a toasty 25°C.

"It was already a warm Earth, and it got a lot warmer," says Storey. The climatic turmoil that ensued was disastrous for most life, he says, killing off many deep-sea species.

Ancient ocean sediments that record this episode, called the Palaeocene-Eocene Temperature Maximum (PETM), also contain an unusually small amount of the heavier isotope of carbon, carbon-13. The sediments point to a sudden influx of available carbon dioxide or methane – which would explain the sudden warming – from some source with reduced carbon-13 levels.

Eocene meltdown

Trillions of tons of carbon would have been needed to cause the so-called Eocene meltdown. "Where did all this organic carbon come from?" says Storey. The leading theory has been that methane-bearing sediments called clathrates, which accumulate on the sea floor, suddenly released their gas bubbles into the atmosphere (see Earth's ancient heat wave gives a taste of things to come).

Then, in 2004, Henrik Svensen, at the University of Oslo in Norway, suggested an alternative theory. The north Atlantic Ocean opened up around 55 million years ago, an event accompanied by violent volcanic eruptions that built layers of basalt rock 7 kilometres thick. Some of these lava flows could have heated up organic sediments such as coal, producing large amounts of greenhouse gas.

The precise dates of the basalt build-up and the greenhouse gas accumulation were unknown, however, so the connection was uncertain. Storey's team looked at small crystals of feldspar, taken from layers of volcanic ash both in the seafloor sediments and also within the basalts in eastern Greenland.

"Feldspar acts as time capsule," says Storey. The isotope argon-40 steadily accumulates inside the crystal, as radioactive potassium decays, so its concentration tells you how old the crystal is.

Ocean's birth

The team used lasers to extract traces of argon gas – "a millionth of a millionth of a gram" – from each crystal, and date them. They found that the release of greenhouse gases did correspond with the massive surge in flood-basalt activity. Storey now thinks this is more likely than the great clathrate belch, because of the team's new evidence of a possible trigger mechanism: "It's the birth of an ocean."

"The hypothesis sounds interesting and plausible," says climatologist Drew Shindell of the Goddard Institute for Space Studies in New York, US. "But the question that comes to mind is why the many other comparable volcanic eruptions did not trigger a similar event."

More often, volcanic eruptions cool the Earth. The last known flood-basalt eruption, for example, was in 1783 to 1784 at Laki in Iceland, when 15 cubic kilometres of lava spewed in fountains from a crack in the ground. Sulphur aerosols that emerged with them caused one of the most severe winters ever recorded across the northern hemisphere.

Man-made emissions

The Eocene eruptions were vast by comparison, with a total volume of 10,000,000 cubic kilometres, enough to build a proto-Iceland in the newly-born north Atlantic. It remains to be seen whether they generated more heating from greenhouse gases than cooling from aerosols.

So, could volcanoes be responsible for today's global warming? Not according to Terry Gerlach of the Cascades Volcano Observatory in Washington State, US. "I presently put the global volcanic CO₂ output at 0.2 billion tons a year. This is less than 1% of man-made CO₂ pollution."

Whatever the ultimate cause of the Eocene meltdown, a link to greenhouse gases seems highly likely. "There are obvious lessons from this," says Storey. "The rate at which greenhouse gases are now being added to the atmosphere exceeds by far the rate of 55 million years ago," he warns.

Journal reference: Science (vol 316, p 587)