During a recent research expedition led by Linnaeus University and Stockholm University to the deepest parts of the Baltic Sea, specifically in the Landsort Deep, scientists made a remarkable discovery. They stumbled upon an extensive release of methane, a potent greenhouse gas, emanating from the seafloor sediments.
This methane leak was found in the Landsort Deep, situated approximately 30 kilometers southeast of the coastal town of Nynäshamn. Christian Stranne, an associate professor specializing in marine geophysics at Stockholm University, expressed his astonishment at this revelation.
“While it’s known that methane gas can escape from shallow coastal seabeds in the Baltic Sea, I’ve never witnessed such a vigorous and concentrated release, especially not from such profound depths,” remarked Christian Stranne.
A poorly understood phenomenon
The research initiative has a primary objective of advancing our understanding of methane and its origins and destinations within the oxygen-depleted regions of the deeper Baltic Sea. Leading this endeavor is Marcelo Ketzer, a distinguished professor specializing in environmental science at Linnaeus University.
Marcelo Ketzer explains, “Our current knowledge regarding the factors governing methane production in these deep-sea regions and the pathways of methane dispersion remains quite limited. We’re in the dark about how this system responds to factors like eutrophication or a warming climate. While I was aware, based on a prior project, that methane levels in the sediments here are notably elevated compared to other Baltic Sea areas, the sudden emergence of methane bubbling into the sea took me by surprise.”
The researchers identified the affected area, encompassing about 20 square kilometers (equivalent to nearly 4,000 football fields), lying at a depth of approximately 400 meters. During the expedition, they meticulously gathered numerous sediment cores and water samples. These samples now hold the potential to unveil the reasons behind the significant release of methane in this specific location.
Marcelo Ketzer elaborates, “We already have some insights into why this phenomenon occurs. The size of the sediment grains in the area and the seafloor’s topography provide us with initial clues. It appears that deep-sea currents are responsible for depositing sediments in this particular region, but before we can provide definitive answers, we need to conduct more comprehensive analyses.”
The bubbles rise to the surface of the sea
The expedition also yielded an intriguing revelation regarding the ascent of methane bubbles through the water column, defying conventional expectations.
Christian Stranne explains, “Typically, at the depths we were investigating, methane bubbles would ascend at most about 150 to 200 meters from the seabed. These bubbles tend to dissolve in the ocean as they rise, resulting in a gradual decrease in size.”
He continues, “The dynamics governing the development of bubble size and gas composition are rather intricate, influenced by both pressure effects and gas diffusion. However, for smaller bubbles, the overall trend is that they diminish in size and ascent speed as they move farther from the seabed.”
To their astonishment, the researchers observed some methane bubbles rising as high as 370 meters from the ocean floor, significantly surpassing the anticipated range.
Stranne elaborates, “Bubbles emanating from deep-sea sediments, typically found around depths of a thousand meters or more, can achieve remarkable altitudes due to a coating of ‘frozen methane’ enveloping the bubble.”
He adds an intriguing note, “During a recent expedition to the Amazon outlet in the summer, we observed bubbles ascending as high as 700 meters above the seabed. To my knowledge, there hasn’t been a study documenting such persistent bubbles at these depths. This could potentially set a new world record and prompt a reevaluation of the role deep basins play in contributing to surface water methane levels in the Baltic Sea.”
Oxygen-free bottoms mighty be the explanation
Determining the exact altitude to which these methane bubbles ascend has proven elusive for the researchers. While sonar observations have confirmed their presence up to 40 meters from the sea surface, it’s plausible that some bubbles reach even greater heights. The size of these bubbles might contribute, but the team leans towards a different explanation they find more plausible.
Christian Stranne explains, “Our current belief ties back to the oxygen-depleted conditions prevalent in the deep Baltic Sea waters. In the absence of oxygen, the levels of dissolved methane in the ocean can be notably elevated. Consequently, these bubbles retain their methane content for longer periods. This preservation effect makes the transport of methane towards the sea surface more efficient.”
He adds, “This is a hypothesis we’re actively investigating, and if substantiated, it could have significant implications. Should oxygen conditions in the Baltic Sea deteriorate further, it might lead to increased methane transport from its deeper regions. However, the extent of potential atmospheric leakage remains to be studied.”
Marcelo Ketzer and Christian Stranne harbor the belief that methane emissions akin to those discovered in the Landsort Deep could manifest in other areas of the Baltic Sea.
Ketzer remarks optimistically, “Now equipped with a clearer understanding, we’re eager to apply this model in other Baltic Sea regions featuring similar geological conditions. There are potentially half a dozen places awaiting exploration.”
Source: Linnaeus University