Lake Siljan lies within the 370-million-year-old Siljan Ring, an impact crater created by a meteorite strike in central Sweden. While local methane seepage has been known for years, with recurring winter ice holes suggesting active gas release, the new study offers the first detailed and quantifiable evidence of emissions on this scale.
Researchers led by Professors Johan Mellqvist and Vladimir Conde conducted two measurement campaigns in 2023 and 2024, identifying several persistent hotspots of intense methane leakage. The emissions formed visible gas plumes and remained consistent over time. At peak sites, methane was released at rates up to 300 times higher than the norm for lakes, translating to around 3.5 tons of methane per year, or approximately 85 tons of CO2-equivalent.
"The natural methane emissions from the Siljan Ring lakes appear to be far more extensive than previously known. At certain sites, we recorded emission rates up to 300 times higher than what is typically seen in lakes," said Johan Mellqvist, Professor of Optical Remote Sensing at Chalmers.
Methane, a greenhouse gas far more potent than carbon dioxide in the short term, is typically released from lakes through bubble emissions caused by decaying organic matter. But in this case, the scientists were surprised to find emissions highly concentrated at specific locations.
"It's unusual - and quite strange - that the emissions were so extremely localised. To our knowledge, this kind of tightly concentrated methane leak has never been measured in a lake before. And we've only examined a small part of the Siljan Ring lakes so far. It's possible we've only seen the tip of the iceberg," Mellqvist added.
To detect the leaks, the team employed a tracer gas technique developed at Chalmers. This method allowed for more precise measurement of localized emissions compared to traditional tools like floating chambers, which are suited for broad, even surface emissions.
"The method allows us to measure concentrated emissions much more effectively than traditional approaches, such as floating chambers, which are designed for homogeneous surface emissions across wide areas. One of our key questions now is whether similar localised emissions exist in other lakes, but have gone undetected because our previous methods weren't suited to finding them," Mellqvist noted.
The origin of the methane remains uncertain. One theory links it to "deep gas" formed by ancient organic matter buried during the meteorite impact. Alternatively, the gas may stem from trapped methane in shallow sediments beneath the lakebed.
"If it turns out to be deep gas, then this may be unique to the Siljan Ring and the impact crater. But if it comes from more shallow sediment pockets, then this type of emission might be present in many more places," Mellqvist explained.
The research team now plans to extend its study to other parts of the Siljan system, including greater depths and nearby lakes, to map the full extent of the emissions and their potential drivers.
"We need to understand how many hotspots the area contains and identify the origin of the emissions. We also want to explore possible strategies to limit them. Our findings could have implications far beyond the Siljan region and help us reassess how we view natural gas seepage in geological formations worldwide," he concluded.
During their campaigns, researchers found five distinct hotspots in shallow waters (2-5 meters deep) across Siljan, Orsa Lake, Fudalsviken, and Vikarbyn. These formed gas plumes known as ebullition. Annual emissions from these sites total around 3.5 tons of methane, with an additional 50 kg coming from more diffuse sources. The origin of these emissions remains unresolved.
Research Report:Measurement of methane leakages from lakes in the Siljan Ring
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Chalmers University of Technology
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