Uncovering Hidden Methane Emissions in the Baltic Sea

What is the main goal of this cruise, and what kinds of data are you collecting?

The main goal of the cruise was to investigate methane ebullition sites in the deep Baltic Sea, which are sites where methane bubbles rise from the seafloor into the water column and potentially the atmosphere. We collected acoustic data to detect the presence of gas in the sediment and the water column. We also collected geochemical data via sediment coring, the WEGAS system, and CTD deployments to measure methane concentrations in the sediment and water. In addition, we analysed dissolved inorganic and organic carbon in water and sediment.

To collect this data, what instruments are you using? 

We used a variety of instruments, including seismics, mid-water sonar, a gravity corer, CTD, ROV, and the WEGAS system.

What’s a typical day like onboard for the research team? 

It was very intense days onboard with a multidisciplinary team. We normally started the days with acoustic acquisitions to establish the best sites for coring. After looking at the data and defining the sites for investigation, we continued with CTD profiles and water column sampling, followed by multi-coring and gravity coring. The WEGAS system was running most of the time, collecting continuous data during the expedition. This “ideal” sequence of work had to be adjusted on many occasions due to weather conditions and the ship’s workflow. 

Did you encounter any surprising findings or challenges during the cruise?

We confirmed our previous hypothesis that ebullition sites in the deep Baltic Sea were related to accumulation of organic-rich material by bottom currents. We mapped the ebullition sites with hull-mounted sonar to identify bubbles. However, we now know, based on ROV visual observations and acoustics, that the bubbles streams are ephemeral and very difficult to be visually observed. In addition, we know now that the gas is accumulated deep in sediments (some meters below the seafloor), and it is not possible to sample gas bubbles with our current gas sampler

Why is this research important, and how might it impact people and/or the planet?

We discovered that there are several ebullition fields continuously releasing methane in the deep Baltic Sea environment. The ebullition is likely the source for the high methane concentrations found in the bottom water (this is a new information). Some of the bubbles may even reach the atmosphere, representing a previously unknown source of atmospheric methane, as such emissions were previously only known in the coastal Baltic Sea.

What’s one thing you wish more people understood about marine research?

That investigations in the deep sea are quite difficult to do and expensive to carry out, but the data is fundamental to understand how our planet works!

What’s the most challenging part of doing science at sea?

Dealing with bad weather, as well as limited and uncomfortable facilities onboard. A lab on a vessel is very different from a lab onshore – it is smaller, and everything needs to tighten up.

What has receiving the SWERVE funding and ship time meant for you? 

SWERVE ship time was a fundamental part of the project. 
Without it, our existing resources would have been very limited, constraining what we could achieve. It provides access to resources that otherwise will be only accessible to a few researchers. In a sense, it democratises the resources we have in Sweden. In addition, it positions Sweden among the few countries in the world with a research fleet available for all.