One of Europe’s largest methane ebullition field lies at 400 m below sea level in the Baltic Sea.

This seep area, estimated to extend over 17 km2 at depths of 400 m, is located in the west-central Baltic Proper at the Landsort Deep, the deepest part of the Baltic Sea. The Landsort Deep is a deep and narrow trough fault (Fromm, 1943) filled with around 100 m of late glacial and post-glacial sediments at its axis. The ebullition field is associated with a local drift deposit extending along the fault axis with higher than average sedimentation rates (1 cm/year; Jofesson, 2022). High current-associated sedimentation rates with relatively slow terrigenous deposition result in notable organic matter accumulation (TOC average of 11.4 weight %; Ketzer et al., 2024). The inflow of salty water from the North Sea and the freshwater runoff from the catchment area gives rise to a permanent halocline in the Baltic Proper at a depth of around 80 m. The euxinic waters below the halocline, resulting from limited vertical water exchange and eutrophication, combined with sapropel deposition, promote anomalous high biogenic methane production within the sediments.
Methane oversaturation in the sediment porewater leads to bubble formation, which escapes the seafloor intermittently and sporadically within the ebullition field. Mid-water acoustic data acquired at the study site reveal that many bubbles rise more than 300 m from the seafloor, with some reaching all the way to the sea surface (>400 m). Data analysis identified two groups of bubbles based on rise velocities, indicating two separate bubble size ranges. When comparing the observations with a bubble dissolution model, the results suggest that only extraordinarily large bubbles can explain the large rise heights.
Further methane flux estimations derived from acoustic data in combination with dissolution modelling will provide insights into the efficiency of the vertical methane flux from the ebullition field and help determine whether methane discharge from Landsort Deep sediments, at 400 m below the sea surface, can actually end up in atmosphere.

Fromm, E., 1943. Havsbottnens Morfologi Utanför Stockholms Södra Skärgård. Geografiska Annaler 25:3-4, 137-169. https://doi.org/10.1080/20014422.1943.11880722
Josefsson, S., 2022. Contaminants in Swedish offshore sediments 2003–2021. 103 pages. Geological Survey of Sweden.
Ketzer, M., Stranne, C., Rahmati-Abkenar, M., Shahabi-Ghahfarokhi, S., Jaeger, L., Pivel, M.A.G., Josefsson, S., Zillén, L., 2024. Near seafloor methane flux in the world's largest human-induced dead zone is regulated by sediment accumulation rate. Marine Geology 468, 107220. https://doi.org/10.1016/j.margeo.2024.107220