Meltwater rivers flowing from Svalbard glaciers are carrying ancient methane from rocks beneath the ice into the open air, new research shows.
The findings reveal a natural feedback loop that may worsen as the Arctic continues to get warmer. Melting glaciers can open hidden pathways for methane, a powerful greenhouse gas. Put simply, the more glaciers melt, the more methane they are likely to release.
Methane in the rocks
The study was led by Gabrielle Kleber , a researcher with the iC3 Polar Research Hub in Tromsø, Norway. Her team sampled rivers draining valley glaciers across central Svalbard. They found methane in every river they tested.
The study team took 148 water samples from 19 glacier-fed rivers in central Svalbard, providing by far the most extensive assessment yet of methane in glacier meltwater in the region. Every river in their survey contained more methane than expected from contact with the atmosphere, with the highest values reaching up to 425 times that level.
Co-author Silje Waaler says the team deliberately designed the survey to capture the diversity of glaciers across the region.
"We wanted to study many different glaciers, across a range of rock types and ice conditions. That gave us a clearer picture of why some glacier rivers carry more methane than others," she says.
A key finding is that this methane is mostly not being made by microbes under the ice, as has been observed beneath glaciers in Greenland.
Instead, it appears to come from Svalbard's geology. Many parts of the archipelago contain old shale layers rich in organic carbon. Over millions of years, heat and pressure can turn this material into methane and other gases.
They also analysed the carbon in the methane to identify its source. In some samples, they measured related gases, including ethane and propane, which helped confirm that much of the methane came from geological sources.
"These glaciers are mostly melting on their surfaces," Gabrielle says. "But this meltwater finds its way to the bottom of the glaciers through crevasses and holes. This means that it interacts with the rocks underneath, and where those rocks contain ancient gas, the water can flush methane out into rivers."
The most methane-rich waters came from glaciers resting on shale-bearing rock formations. But geology alone did not explain everything.
The team also found that the physical state of the glacier bed matters. Glaciers with thawed, wet and active beds were far better at picking up methane. Glaciers frozen to their beds were less connected to the rocks below, even when methane-rich geology was present.
A map of where methane can escape
To understand these processes, the researchers combined river chemistry with ice surveys. They used ground-penetrating radar to map ice conditions within selected glaciers. This allowed them to estimate how much of each glacier bed was thawed and able to carry water.
Co-author Leonard Magerl says that this combination was crucial.
"The temperature at the base of glaciers is an important piece of the puzzle," Leonard says. "We found that the biggest methane releases happened where the right rocks and the right glacier conditions came together. This insight can help to estimate emissions from other ice-covered regions."
The researchers estimate that land-terminating glaciers across Svalbard may transport roughly 182 to 368 tonnes of methane per year in meltwater, depending on how the estimate is scaled. This is in addition to previous, much higher estimates for methane released by groundwater springs in front of glaciers. But it still points to a widespread and undercounted pathway for ancient carbon to reach the atmosphere.
Why this matters
Methane is a powerful greenhouse gas. While this study focuses on Svalbard, similar methane-release pathways likely occur in other glaciated regions where ice overlies organic-rich rocks or sediments. These include large parts of the Arctic, the Himalayas and Antarctica.
Gabrielle explains that: "The amounts reported here are small compared with human-caused emissions from fossil fuels, farming and waste. But they matter because they reveal a natural feedback loop that is not confined to Svalbard, and that may grow as the Arctic warms."
As glaciers thin and retreat, more meltwater may reach their beds. This can increase contact with fractured rock, sediment and groundwater. In some places, that may flush out more methane.
However, the story is not simple. Some Svalbard glaciers are also becoming colder at their beds as they shrink. If a glacier becomes frozen to its bed, its ability to flush methane through subglacial rivers may fall.
"Our results show that future methane release will depend on both geology and glacier change," Gabrielle says. "That makes it important to know what lies beneath the ice, not only how fast the ice is melting."
Links to previous research
The new study builds directly on previous iC3 work on methane around retreating Svalbard glaciers. Gabrielle Kleber and Leonard Magerl have previously found that meltwater from one Svalbard glacier could carry geologic methane from beneath the ice , making it release more methane per area than Greenland glaciers. Meanwhile, newly uncovered groundwater springs are also releasing the potent greenhouse gas in Svalbard forefields, demonstrating the many understudied sources of methane in these environments.
The new study, published in Nature Communications today , takes the next step. It shows that methane-rich meltwater is not unique to one glacier. It is widespread across central Svalbard, but strongest where shale-rich geology and thawed glacier beds overlap.
