Researchers from Linnaeus University and Stockholm University on a Baltic Sea expedition unearthed a significant methane gas release in the Landsort Deep, 30 kilometers from Nynäshamn. This deep-sea methane eruption surprised Christian Stranne, an associate professor at Stockholm University, due to its unprecedented scale and depth.
The Phenomenon
- The research project, led by Marcelo Ketzer, a professor of environmental science at Linnaeus University, aims to enhance understanding of methane sources and sinks in the Baltic Sea’s oxygen-deprived depths.
- The team discovered a 20-square-kilometer area emitting methane at a depth of approximately 400 meters, prompting further analysis to unravel the reasons behind this unusual release, likely linked to sediment characteristics and deep ocean currents.
Interesting Discovery
Another interesting discovery made during the expedition concerns how high up through the water column the methane bubbles 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. The methane in the bubbles dissolves in the ocean and therefore they usually gradually decrease in size as they rise towards the sea surface,” says Christian Stranne.
“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.”
Great surprise
To the researchers’ great surprise, they could see some bubbles rising to 370 meters from the ocean floor.
“Bubbles from deep-sea sediments (around a thousand meters and deeper) can rise significantly higher due to a coating of ‘frozen methane’ that forms around the bubble,” says Stranne.
“This summer I participated in a French expedition to the Amazon outlet where we observed bubbles rising up to 700 meters above the seabed. But I don’t know of any study where such persistent bubbles have been observed at these depths—it could be a new world record, and it could force us to re-evaluate the role of deep basins in the Baltic Sea, in terms of contribution to the surface water methane content.”
Oxygen-free bottoms
- The researchers have observed methane bubbles rising at least 40 meters from the sea surface, potentially even higher.
- They attribute this phenomenon to oxygen-depleted conditions in the Baltic Sea, where high dissolved methane levels help preserve bubble integrity, making methane transport more efficient.
- This hypothesis, if confirmed, suggests that worsening oxygen conditions could lead to increased methane transport from the Baltic’s depths, though the exact impact on atmospheric methane release remains uncertain. Ketzer and Stranne speculate that similar methane emissions may occur elsewhere 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. There are potentially another half dozen places to explore,” Marcelo Ketzer adds.
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Source : Phys Org
