A massive reservoir of the greenhouse gas methane could be lying hidden beneath the Antarctic Ice Sheet, in a ticking time bomb for climate change.
An international team of scientists has discovered that old organic matter in sedimentary basins beneath the ice sheet may have been converted to methane by micro-organisms living under oxygen-deprived conditions.
The methane could be released to the atmosphere if the ice sheet shrinks and exposes these old sedimentary basins.
"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," says Professor Slawek Tulaczyk of UC Santa Cruz.
"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."
The team reckons that as much as half of the West Antarctic Ice Sheet and a quarter of the East Antarctic Ice Sheet is hiding pre-glacial sedimentary basins containing about 21,000 billion metric tons of organic carbon.
"This is an immense amount of organic carbon, more than ten times the size of carbon stocks in northern permafrost regions," says Jemma Wadham of the University of Bristol.
"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 modeled the accumulation of methane in Antarctic sedimentary basins using a tried-and-tested model. They found that the conditions beneath the ice favor the accumulation of methane hydrate - methane trapped within a structure of water molecules, forming a solid similar to regular ice.
They also calculated that the potential amount of methane hydrate and free methane gas beneath the Antarctic Ice Sheet could be up to four billion metric tons - comparable to some estimates made for Arctic permafrost.
And because they're only shallowly covered, they could be more susceptible to climate change.
"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," says Tulaczyk.