The highest bubble streams on record raise questions on methane emissions from the deep Baltic Sea.

During a scientific cruise in August 2023, we made an important discovery for the understanding of the methane dynamics in the Baltic Sea that clearly requires further exploration into its implications.

We came across potentially one of the largest active seafloor ebullition sites in Europe, where bubbles rise >400 m in the water column (highest on record) and reach the sea-air interface. This is the first time that direct methane emissions to the atmosphere are identified in the deep Baltic Sea.

The objective of this project is to quantify seafloor and atmospheric methane emissions from the above site, to establish a geological model to explain the anomalous methane concentration there, and to explore another seep site in the Baltic Sea with similar geological conditions.

This four-years project will include two offshore missions for the acquisition of new data: (1) acoustic imagery of sediment and water column, (2) seafloor inspections with remotely operated vehicle, (3) profiles (temperature, salinity, oxygen) and samples of the water column, and (4) sedimentological logs and geochemical samples (gas, pore water) from gravity and twin cores.

Major possible implications of this project include the identification of a new mechanism to transfer methane from sediments to the water, the existence of several other potential seafloor ebulition sites in the deep Baltic Sea, and a new, unnacounted, and potentially important methane source to the atmosphere in the region.