Published 2024-09-12.

Methane is a potent greenhouse gas, the second most important contributor to global warming after carbon dioxide. Hence, it is critical to understand its origin and behaviour to mitigate climate change. Although methane is largely formed through natural processes, emissions can be amplified by human activities, such as excessive nutrient inputs to the sea causing eutrophication.

In a new study, researchers from Stockholm University and the University of Helsinki have examined the dynamics of methane in brackish coastal environments to improve understanding of its emissions to the atmosphere. They discovered that dissolved methane in the surface water originates from dissolution of bubbles that are being formed in the seafloor and traverse up through the water column – so-called ebullition – rather than diffusion of dissolved methane from the seafloor.
“This is important, because it suggests that the total methane emissions from brackish coastal areas are likely being underestimated as some bubbles make it to the atmosphere”, comments Martijn Hermans, the lead researcher of the study.

Methane transport in the sea – two pathways

Methane is formed when organic material, such as dead plants, algae and animals decompose. In saline coastal waters this process usually takes place deeper down in the seafloor, and almost no methane makes it to the water column, let alone the

Researcher Martijn Hermans. Photo: Lisa Bergqvist

atmosphere. However, in a brackish system like the Baltic Sea, and especially when there is a high input of organic matter, the production of methane occurs closer to the sediment surface. From there it can be transported to the water, and potentially even to the atmosphere, via two different ways: Dissolved methane can diffuse from the seafloor into the bottom water and then upwards through the water column. When the methane production is high, however, the seafloor becomes oversaturated and bubbles are being formed. These bubbles can ascend rapidly through the water, in contrast to diffusion, which is a slow process.

In the new study, the researchers examined the origin of dissolved methane in surface waters by analysing the presence of different flavours of methane, known as isotopes, within the seafloor as well as in the water column. The isotopic composition of methane varies depending on the processes involved in its formation and oxidation, serving as a strong indicator of its origin.

"The results show that the methane in the surface water of the stratified zone had the same isotopic composition as methane found further down the seafloor where methane bubbles are formed", Martijn Hermans says. "The portion from diffusion out of the seafloor was negligible. The stronger the stratification in the water column, the greater the proportion of methane in the surface water originates from ebullition."

The likely explanation for this is that the majority of the dissolved methane that is diffused from the seafloor is oxidised within the deeper waters before reaching the surface waters, especially in stratified waters where the thermocline serves as an effective barrier. The bubbles, on the other hand, can ascend quicker through the water column, where they undergo partial dissolution.

The fact that the methane in the surface water origins from the partial dissolution of bubbles, however, suggests that the remaining bubbles are likely to reach the atmosphere, thereby contributing to global warming.

Most studies typically only measure the dissolved methane in water column, rather than methane in the form of gas bubbles, so there might be more methane released from brackish coastal areas than previously estimated.

See the illustration as high resolution pdf:  Methane dynamics illustration (1513 Kb)

Measurements in Tvärminne archipelago

The study is based on measurements conducted in August 2022 by a team of researchers from different disciplines at Stockholm University. They utilized the R/V Electra for an expedition to the Tvärminne Archipelago, located near Hanko in Finland. This expedition was part of the ongoing cooperation between the universities – CoastClim – exploring the connections between the coastal environments and climate feedbacks.

The study area in Tvärminne Archipelago is not only representative for coastal areas in the Baltic Sea, but also for many other estuarine systems characterized by low salinity and high nutrient inputs. 

“The results suggest that ebullition could play a key role in methane transport to surface waters worldwide, which can have significant implications for the global estimations of methane emissions”, says Martijn Hermans.

Text: Lisa Bergqvist

Read the full study

Hermans et al 2024: Ebullition dominates methane emissions in stratified coastal waters