By Julianna Kettlewell
BBC News Online science staff
Scientists have found a series of vents in the Nordic Seas that may have burped enough methane to cause massive global warming 55 million years ago.
Early horses: The warming benefited mammals, such as Propalaeotherium
The early Eocene Period witnessed a dramatic increase in temperature, which was triggered by a sudden surge of greenhouse gases in the atmosphere.
But just where these gases came from has been something of a mystery.
Nature magazine reports the discovery of gas vents dating from the right time and which could represent the source.
At the beginning of the Eocene, the Earth's temperature suddenly shot up by about five Celsius. It was one the most dramatic global warming events that has ever been recorded in geologic history.
For human evolution, it was a vital period. The warm climate allowed primitive mammals to disperse across the Earth, diverging as they did so into many different groups.
Among these newly emerging groups were mankind's primate ancestors.
Scientists usually agree about the cause of this 100,000-year heatwave: a great pulse of carbon dioxide (CO2) hit the atmosphere to launch an extreme greenhouse event.
However, what does cause a large amount of discussion within the scientific community is just how this gas was produced.
One popular theory relates the existence of hydrates - a frozen mixture of water and methane produced by microbes - which are locked in ocean sediments. If the sea warmed up enough, the argument goes, then these hydrates would have melted, disgorging their load of methane.
The methane (a potent greenhouse gas itself) would have then broken down to produce a CO2 surge.
This process needs to be kick-started by minor global warming, and some scientists suggest that the collision of a comet or asteroid might have done the trick. But these accounts have suffered from a lack of direct evidence.
Now a team of Norwegian scientists have hit upon something that could explain the creation of these greenhouse gases.
While studying charts of sedimentary rock produced by companies prospecting for petroleum, they noticed a series of hydrothermal vents.
They believe that trapped organic matter was heated so intensely by angry seams of lava that it broke down into methane and forced its way to the surface forming these structures.
Don't try this: Water drips away from a lump of burning methane ice taken from the ocean floor
"We believe these hydrothermal vents are a likely explanation for the gases that caused the climatic change," Bjorn Jamtveit, of the University of Oslo, told BBC News Online.
However, the theory has been met with some scepticism. This is largely because the carbon these vents belched would have been rich in a form - or isotope - that does not tally with geological records.
Rock deposits indicate the massive CO2 surge of 55 million years ago contained a lot of the isotope carbon-12. But the carbon produced when organic matter is heated intensely by lava contains a greater proportion of carbon-13.
"The problem is that the carbon released is the wrong sort of carbon," explained Mark Maslin, of University College London, UK. "I just don't find it a plausible theory."
The only way this theory could explain the high levels of carbon-12 at the beginning of the Eocene is if truly enormous amounts of gas were produced. That way, even though it contains proportionally less carbon-12, the carbon cycle is still flooded with it.
"It is a bit like having a swimming pool filled with white paint," explained Gerald Dickens, of Rice University, Houston, US. "If you want to turn that swimming pool pink, you can either add a little red paint or a lot of pink paint."
In other words, the gas produced by hydrothermals represents pink paint. To get the effect recorded, you need an awful lot of it.
According to Professor Jamtveit, there was indeed an awful lot of it. "When we calculate the amount of gas that can be released by this process it is enormous," he said. "It corresponds to the amount of gas in the petroleum resources in the whole Earth today."
But Dr Maslin remains unconvinced: "I can't imagine that amount of carbon could be released just through the Nordic Seas," he said.
If the hydrothermal vents did not cause the Eocene thermal maximum that could be worrying news for us.
The vents only spewed methane because of a rare spate of laval activity. It could happen again, but probably not in the near future.
On the other hand, if the hydrate-release theory is true; if minor global warming can cause gas hydrates in ocean sediments to melt and shed their methane load, we could be heading for trouble.
Dr Maslin put it bluntly: "We've got global warming occurring - and if we keep it up will all the gas hydrate be released to cause super global warming? Unfortunately I think the answer to that is 'yes'."