The discovery of a new ebullition field in the deep Baltic Sea

A previously unknown gas ebullition field has been identified at ~400 m water depth in the Landsort Deep, the deepest part of the Baltic Sea. Hydroacoustic mapping reveals persistent seabed gas release over an area of approximately 17 km². Although no direct gas samples were collected, elevated methane concentrations in sediment pore waters and bottom waters suggest that the bubbles are methane-dominated. Stable carbon isotope signatures of dissolved methane indicate a predominantly microbial origin, consistent with in situ production from the degradation of organic matter rather than migration from deeper thermogenic reservoirs.

The seep field is spatially associated with a drift deposit characterized by enhanced sedimentation rates, pointing to a tight coupling between organic matter accumulation and methane production. Acoustic flux estimates indicate an average seabed methane release on the order of ~10 mol m^-2 yr^-1, comparable to fluxes reported from the well-studied Tommeliten seep area in the North Sea. However, the Landsort Deep seep field is roughly two orders of magnitude larger in areal extent, implying substantially higher integrated methane emissions.

These findings highlight the potential for deep, hypoxic basins in eutrophied marginal seas to host large, previously unrecognized methane sources. The Landsort Deep provides a natural laboratory for investigating how sedimentation, redox conditions, and water-column stratification regulate methane production, oxidation, and escape from the seabed in coastal and semi-enclosed marine systems.