By Jonathan Amos
Science reporter, BBC News, San Francisco
Scientists drilling ocean sediments off Canada have discovered methane ices at much shallower depths than expected.
Hydrate nodules brought to the surface fizz as the methane escapes
The finding has important implications for climate studies, they believe.
The melting of hydrates, as they are known, is a suspected contributor to past and present increases in atmospheric methane, a greenhouse gas.
If shallow ices are destabilised in a warming world, it could have a positive feedback effect and drive temperatures even higher, the researchers warned.
"The rate of increase in the Earth's atmosphere for methane is much faster than that for carbon dioxide," said Timothy Collett, the co-chief scientist of the Integrated Ocean Drilling Program (IODP).
"Methane is 20 times more effective as a greenhouse gas than CO2. The source of this methane is uncertain, but there are a number of scientists who have looked at gas hydrates as contributing to this recent change."
Higher in the zone
Hydrates are a frozen mixture of water and gas, primarily methane.
They form under the frigid temperatures and high pressures found in ocean sediments and under the permafrost on land.
In the ocean, hydrates exist in a "zone of stability" under the seafloor in locations where water depths exceed 500m.
But the results of an expedition carried out by the IODP off Vancouver Island are putting a significant new perspective on this profile.
The IODP is a 21-nation science programme
The international marine research organisation used the drilling facility and laboratories of the US research vessel Joides Resolution to retrieve core samples from a geological area known as the (northern) Cascadia Margin.
The pressurised cores pulled back on to the ship had copious hydrate deposits - and at a level in the stability zone that was much higher than expected.
"Gas hydrates have been studied at Cascadia for 20 years, and there has been an established model for how hydrates form on such a margin," said IODP expedition co-chief Dr Michael Riedel of McGill University, Montreal.
"But we found from our expedition that this model is way too simple and has to be modified. We found anomalous occurrences of high concentrations of gas hydrate at relatively shallow depths, 60-100m below the seafloor."
As well as suggesting hydrates would be more concentrated at deeper levels below the seafloor, the old model also predicted the ices would be evenly distributed among the various grain sizes that comprise the sediments.
This has now been found wanting, too.
"After repeatedly recovering high concentrations of gas hydrate in sand-rich layers of sediment, we're reporting strong support for sediment grain size as a controlling factor in gas hydrate formation," said Dr Collett, who is affiliated to the US Geological Survey.
Vast reserves of the ices are thought to exist. One calculation suggests some 10,000 billion tonnes of carbon is stored in the form of gas hydrate around the world. That is twice the volume stored in all known reserves of fossil fuels - oil, coal and natural gas.
Fuel source: Companies would like to mine hydrates
"If you start looking at this as a carbon sink - the amount of carbon that could be available to climate change and to altering the atmosphere and its chemistry - this could be a very significant contribution," explained Dr Collett.
Hydrates have naturally excited the attention of mineral companies, and a number of them are now investing considerable sums of money in trying to exploit the resource.
BP will begin an exploratory programme to drill hydrates under the Alaskan permafrost in the New Year.
The IODP results were reported here at the American Geophysical Union Fall Meeting.