Under some Arctic glaciers, meltwater brings methane to the surface that has remained trapped in rock for millions of years.
The phenomenon was studied in Svalbard, a Norwegian archipelago located far north of the Arctic Circle. Researchers collected 148 samples from rivers flowing from 19 glaciers. All these rivers contained more methane than the usual concentration in water. In the most pronounced cases, the concentration reached up to 425 times that level.
Glacial meltwater flows over rocks in Svalbard.
Credit: Billy Lindblom — CC BY 2.0 / Wikimedia Commons.
To understand where this gas comes from, the team analyzed its composition. They also looked for ethane and propane. These clues indicate that most of the methane comes from rocks beneath the ice, rather than from microbes living under the glaciers. This is known as thermogenic methane, formed underground under the effects of heat and pressure.
Not all glaciers release the same amount of gas, however. The highest concentrations appear when two conditions are met: the glacier rests on rocks rich in organic matter, particularly shale, and its bed is not completely frozen.
This second point makes a big difference. When part of the glacier bed is thawed, water can flow beneath the ice. It then passes through cracks and sediments, picks up methane, and carries it outward. Radar measurements conducted on several glaciers enabled the researchers to determine which areas of their beds were in this state.
Once it leaves the glacier, the water reaches fast-flowing rivers. Some of the dissolved methane then escapes into the atmosphere. The team estimates that Svalbard’s land glaciers could transport approximately 182 to 368 tonnes of methane per year.
The connection with warming is not linear, either. Greater melting brings more water beneath some glaciers and may, for a period, increase methane transport. But when a glacier becomes thinner, its bed may eventually freeze. Water then flows less easily beneath the ice, reducing this transport mechanism.
The researchers also found that the peak methane concentration was located well downstream of the glacier front in 68% of the rivers studied. Groundwater rich in gas can therefore join the waterways farther downstream. Previous work in Svalbard indicates that these sources located in front of glaciers could emit more methane than the glacial rivers themselves.
The next step is to look for this mechanism at other sites. Glaciers also cover hydrocarbon-rich terrain in other Arctic regions, in Canada and in parts of Antarctica. The nature of the rocks and the condition of the glacier bed will determine where this ancient methane can actually reach the surface.