Research expedition discovers extensive emissions of methane from bottom of Baltic Sea

The research project aimed "to expand knowledge about methane and its sources and sinks in the oxygen-free environments in the deeper parts of the Baltic Sea."

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Image Credit: Linnaeus University and University of Stockholm

In a recent research expedition to the depths of the Baltic Sea led by Linnaeus University and Stockholm University, researchers discovered extensive emissions of the greenhouse gas, methane, in the Landsort Deep. Landsort Deep or Landsortsdjupet is located about 30 kilometers southeast of the coastal town Nynäshamn and is the deepest part of the Baltic Sea.

The research team saw “an intense bubble release” from the deep area, which they determined to be about 20 square kilometers by 400 meters deep.

“Knowledge about the factors that control how much methane is produced in these deeper areas and where the methane goes is limited,” Marcelo Ketzer, the project lead and professor of environmental science at Linnaeus University, said. “How does the system react to, for example, eutrophication or a warmer climate? I knew from one of my previous projects that the methane levels in the sediments in this area are higher than elsewhere in the Baltic Sea, but I never expected methane to bubble out into the sea in this way.”

The research project was funded by the Carl Trygger Foundation and aimed “to expand knowledge about methane and its sources and sinks in the oxygen-free environments in the deeper parts of the Baltic Sea,” according to a press release. The researchers collected a large number of sediment cores and water samples during the expedition and they will now further analyze the samples collected to provide answers to why so much methane gas is released from this specific area in the Baltic Sea.

“We already have a pretty good idea of why it looks the way it does,” Ketzer said. “The size of the sediment grains in the area and the form of the seafloor gives us an indication. It seems like deep ocean currents are causing sediments to accumulate in this particular area, but we need to do more detailed analyses before we can say anything definitive.”

During the expedition, researchers said the methane bubble rise reached 370 meters from the ocean floor, which was higher than the expected rise.

“At the depths we are working with here, you can expect the methane bubbles to reach at most perhaps 150-200 meters from the seabed,” Christian Stranne, associate professor of marine geophysics at Stockholm University, said. “The methane in the bubbles dissolves in the ocean and therefore they usually gradually decrease in size as they rise towards the sea surface. It is actually quite a complicated balance between pressure effects and diffusion of gases that together determine how size and gas composition develop in a bubble, but the net effect for smaller bubbles is that they lose both size and rise velocity with increased distance from the bottom.”

While the researchers said the bubbles can rise significantly higher due to a coating of “frozen methane” that forms around the bubble, so far they have not been able to find out exactly how high the bubbles reach, but they know the bubbles rose at least 40 meters from the sea surface from the sonar. The researchers believe the oxygen-free conditions in Landsort Deep leads to “the bubbles not losing methane as quickly,” according to a press release.

“The bubbles are thus kept more intact in this environment, which means that methane transport towards the sea surface becomes more efficient,” Stranne said. “It is a hypothesis that we are currently investigating and if it proves to be correct, it could have consequences—if the oxygen conditions in the Baltic Sea deteriorate further, it would probably lead to a greater transport of methane from the deeper parts of the Baltic Sea, but it remains to be investigated how much may leak into the atmosphere.”

The researchers believe their discovery in the Landsort Deep may also occur in other places in the Baltic Sea.

“Now we know what to look for and we look forward to testing this model in other areas of the Baltic Sea with similar geological conditions,” Ketzer said. “There are potentially another half dozen places to explore.”

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