The unknown methane leak at the deepest point of the Baltic Sea

The Landsort Deep northwest of the island of Gotland, at 458 metres, is the deepest point in the Baltic Sea. It was here that researchers discovered an intense release of methane gas from the seafloor during an expedition in the summer of 2023. Marcelo Ketzer, professor of environmental science, is one of the researchers behind the discovery.

– We already knew that methane can bubble up from shallow coastal areas in the Baltic Sea. But this is the first time we’ve seen it in deep water and on this scale.

Using sonar and sampling equipment, the researchers could see that gas was leaking from the seafloor almost everywhere across a 20-square-kilometre area. This makes the site one of the largest known methane emission areas in Europe. Some of the bubbles rise all the way to the surface and escape into the atmosphere, despite the depth being over 400 metres.

All samples are analysed in Linnaeus University's laboratory, one of the few places in Sweden with the equipment required for this type of research.

The microbial filter

Methane, a highly potent greenhouse gas, is second only to carbon dioxide in its contribution to global warming. Even small amounts of methane have a significant impact on the greenhouse effect.

Methane forms naturally deep in ocean sediments when algae and other organic materials break down in low-oxygen environments. In most cases, we are not aware of this process at all. Most of the methane is consumed by bacteria and other microorganisms in the sediments and therefore never reaches the water or the atmosphere. This process is called microbial filtering.

– Without the microbial filter, our oceans would resemble swamps with bubbling methane. It’s incredibly important that this filter functions. Thanks to it, methane emissions from the oceans are small compared to coastal and onshore areas, says Ketzer, and continues:

– The discovery at the Landsort Deep shows a location where the microbial filter doesn’t have the capacity to break down methane in the form of gas bubbles.

There may be more sites

It is no coincidence that the researchers chose to investigate the deepest part of the Baltic Sea.

– The Landsort Deep is like a large pit on the seafloor of the Baltic. It’s an interesting place to study because bottom currents transport algae debris there from wide areas, where it accumulates. You could say the pressure on the microbial filter is greatest there.

The findings in the Landsort Deep also strengthen suspicions that there may be many more undiscovered emissions on the seafloor of the deep parts of the Baltic Sea. With funding from, among others, the Swedish Research Council, Marcelo will be spending a considerable amount of time at sea in the near future to map the situation.

An expedition is planned to the Southern Quark, an area off the western coast of the island of Åland with similar geological seafloor conditions, where large amounts of organic material may have accumulated over time. Follow-up studies at the Landsort Deep are also planned, as well as investigations in the Bothnian Gulf.

– The expeditions are important for identifying more emission fields in the Baltic and for understanding the extent of this issue. With each research expedition, we build a richer knowledge of the mechanisms behind methane emissions.

Mapping methane on the seafloor has taken Marcelo Ketzer around the world. On expeditions to, among other places, the Sea of Japan, the Mediterranean, and the Atlantic Ocean, Ketzer studies the highly potent greenhouse gas that forms naturally deep within ocean floor sediments.

The Baltic Sea shows the future

Although the methane emissions were only recently discovered, they have likely been ongoing for thousands of years without our knowledge. It is a natural process, but researchers fear it is being intensified by human activity. For example, a warmer climate and eutrophication lead to more intense algal blooms, which results in more organic material sinking to the seabed and being converted to methane. This increases the pressure on the microbial filter that keeps the Baltic Sea from bubbling like a swamp.

The heavy environmental pressure on the Baltic Sea makes it especially important to study in order to understand how climate change and human activity may affect the world’s water resources in the future. The sea is shallow, with limited water exchange with the North Sea and a high level of human activity along its coasts.

– The Baltic is often described as a model for how other seas will be affected by climate change. The insights from our research are, therefore, of interest well beyond our immediate geographical area.

Understanding the scope is crucial

A central theme in Ketzer’s work is to use research to help diagnose many of the environmental problems caused by human activity. To do that, we first need to understand their scale.

– You feel conflicted with discoveries like this. When we were on the ship and discovered the emissions, we were thrilled by the significance of the find. But from a human perspective, it’s obviously bad news. At the same time, it’s good that we now know about the emissions. It’s like diagnosing a disease — it’s important to be aware of the problem in order to potentially be able to solve it in the future.

– It’s like diagnosing a disease – it’s important to be aware of the problem in order to potentially be able to solve it in the future.