August 30, 2012
Antarctic Ice Sheet Could Be Hiding Immense Methane Reservoir
April Flowers for redOrbit.com - Your Universe Online
A recent study of the Antarctic Ice Sheet suggests that it could be a largely overlooked source of the potent greenhouse gas methane.
An international team of scientists from the University of Bristol, the University of California, Santa Cruz, the University of Alberta, Edmonton, and the University of Utrecht, demonstrate that old organic matter in sedimentary basins located beneath the Antarctic Ice Sheet may have been converted to methane by microbes living under oxygen-deprived conditions. The methane could be released into the atmosphere if the ice sheet shrinks and exposes these old sedimentary basins, the study, published in Nature, warns.
Slawek Tulaczyk, a professor of Earth and planetary sciences at UC Santa Cruz, said the project got its start five years ago in discussions with first author Jemma Wadham at the University of Bristol School of Geographical Sciences, where Tulaczyk was on sabbatical.
"It is easy to forget that before 35 million years ago, when the current period of Antarctic glaciations started, this continent was teeming with life," Tulaczyk said. "Some of the organic material produced by this life became trapped in sediments, which then were cut off from the rest of the world when the ice sheet grew. Our modeling shows that over millions of years, microbes may have turned this old organic carbon into methane."
Fifty percent of the West Antarctic Ice Sheet (1 million square kilometers) and 25 percent of the East Antarctic Ice Sheet (2.5 million square kilometers) overlies pre-glacial sedimentary basins containing about 21,000 billion metric tons of organic carbon, the team estimates.
"This is an immense amount of organic carbon, more than ten times the size of carbon stocks in northern permafrost regions," Professor Wadham said.
"Our laboratory experiments tell us that these sub-ice environments are also biologically active, meaning that this organic carbon is probably being metabolized to carbon dioxide and methane gas by microbes."
The researchers numerically simulated the accumulation of methane in the sedimentary basins using an established one-dimensional hydrate model. What they found is that sub-ice conditions favor the accumulation of methane hydrate (methane trapped within a structure of water molecules forming a solid similar to regular ice).
Additional calculations show the potential amount of methane hydrate and free methane gas beneath the Antarctic Ice Sheet could be up to 4 billion metric tons, a similar order of magnitude to some estimates for the Arctic permafrost. The team predicts the shallow depth of these reserves makes them more susceptible to climate forcing than other methane hydrate reserves on Earth.
The relationship between the amount of energy we receive from the Sun, and the amount of energy we radiate back into space is climate forcing. Climate forcings are the initial drivers of climate shift.
Dr. Sandra Arndt, a NERC fellow at the University of Bristol, who conducted the numerical modeling, told BBC News, "It's not surprising that you might expect to find significant amounts of methane hydrate trapped beneath the ice sheet. Just like in sub-seafloor sediments, it is cold and pressures are high, which are important conditions for methane hydrate formation."
If these calculations are correct, the methane hydrate and gas beneath the Antarctic Ice Sheet released during episodes of ice-sheet collapse could act as a positive feedback on global climate change during past and future ice-sheet retreat.
"The Antarctic Ice Sheet could constitute a previously neglected component of the global methane hydrate inventory although significant uncertainty exists," the scientists said.
Levels of methane, which can stay in the atmosphere for up to 15 years, have been rising in recent years following a period of stability that has lasted since 1998.
According to Reuters, scientists have already identified thousands of sites in the Arctic where methane is bubbling into the atmosphere but the potential for methane formation under the Antarctic Ice Sheet has been less well studied.
In 2007, the United Nations Intergovernmental Panel on Climate Change (IPCC) estimated that the smaller West Antarctica ice sheet is shrinking and the East sheet growing. And while a complete melting of the ice sheets isn't likely for thousands of years, smaller changes may release some of the methane trapped in hydrates.
“That hydrate is stable as long as you don´t change the temperature or pressure,” Wadham told Bloomberg News. “In Antarctica, though you might not have a big temperature change at the bed of the ice sheet, if the ice thins, the pressure drops and some of that hydrate could be converted into gas bubbles and then lost.”
"Our study highlights the need for continued scientific exploration of remote sub-ice environments in Antarctica, because they may have far greater impact on Earth's climate system than we have appreciated in the past," Tulaczyk said.