Ancient microbial methane release detected in the coastal Arctic

 Methane is the second-largest contributor to global warming after carbon dioxide (CO2), and atmospheric methane concentrations are increasing rapidly due to a combination of human and natural sources. Understanding how much methane comes from each source, both now and in the future, is crucial for predicting atmospheric methane concentrations and anticipating climate change. However, natural methane sources are difficult to quantify, particularly when emissions increase as a result of global warming through positive feedback mechanisms.

 In this study, the researchers describe a natural methane source from below the Arctic Ocean that is likely to become increasingly important as the climate continues to warm.

“Our findings suggest that we are overlooking a methane source that contributes to the rising greenhouse gas emissions, thereby amplifying climate change”, says Marenka Brussee, PhD student at the Department of Environmental Science. “By analysing real samples collected at remote sites, we provide evidence on the dominant methane sources releasing methane. These sources can now be incorporated into climate models to improve estimates of large-scale greenhouse gas emissions.”

Diagnosing release of ancient methane

The team developed new analytical methods that enabled, for the first time, the direct identification of ancient microbial methane release from methane pools trapped in deep frozen sediments beneath the seafloor of the East Siberian Arctic Shelf.

The methods, which were developed at Stockholm University, facilitated measuring the triple-isotopic composition of methane in seawater.

“Methane consists of one carbon atom and four hydrogen atoms, each of which can occur in slightly heavier or lighter forms known as isotopes,” explains Brussee. “By measuring the ratios of these isotopes, we can determine how the methane was formed, how old its precursor material is, and whether it has been altered over time. With our methods, we can trace methane back tens of thousands of years. The procedures require extremely high precision since gases leak easily, as anyone with a bike tire knows,” says Brussee.

Using this approach, the research team detected methane more than 48,000 years old in the inner Laptev Sea that had been microbially produced. Combined with the region’s geological characteristics and observations of large bubble flares, the results pointed to methane escaping from preformed pools trapped within the frozen seabed sediments.

A vast and vulnerable region

The East Siberian Arctic Shelf is the world’s largest continental shelf sea and has a geological structure unlike any other shallow marine system. Beneath the shallow sea lies a several-hundred-meter-thick layer of long-frozen ground known as subsea permafrost. This formed around 15,000 years ago, when rising sea levels flooded the Arctic tundra following the last ice age.

The shelf seabed is thought to contain multiple types of methane deposits and methane precursors at different depths, which may release methane at different rates and respond differently to global warming.

The East Siberian Arctic Shelf is extremely difficult to access due to its remote location and harsh climate conditions. As a result, it remains underrepresented in scientific studies and global datasets. Methane emissions from this Arctic region have been observed for more than two decades, but uncertainty about their origin has limited their inclusion in climate models and greenhouse gas emission estimates.

To address this knowledge gap, a long-term collaborative project was initiated 20 years ago by Professor Örjan Gustafsson at the Department of Environmental Science, to identify methane sources across the East Siberian Arctic Shelf. Marenka Brussee joined the team in late 2020 and has primarily contributed to the development of analytical methods to directly diagnose methane sources.

The researchers found consistent release of ancient microbial methane in the inner Laptev Sea, which was surprising, as fossil thermogenic methane had previously been documented roughly 350 kilometres further offshore.

“This means multiple methane sources are active in this region at the same time,” says Brussee. “To accurately estimate greenhouse gas emissions, we need to account for all of them.”

Thawing subsea permafrost

Subsea permafrost is thawing rapidly at a rate estimated to be around 35 times faster than nearby terrestrial permafrost. As these frozen sediments destabilise, methane trapped within them may be released more readily.

The researchers conclude that emissions of ancient microbial methane from subsea permafrost are likely to increase as warming continues, underscoring the vulnerability of the East Siberian Arctic Shelf and the importance of incorporating its methane emissions into future climate projections.

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