Pockmarks in the southern Baltic Sea

This study is part of the KOMSO project that focusses on standardised measurement methods for the long term carbon storage potentials in the Baltic Sea while the contemporaneous methane release of the seafloor works as an antagonistic player. We want to understand how pockmarks are formed and how stable these structures are. Another open question is whether vessel traffic affects the shape and degassing amount of the structures.

The Baltic Sea was formed during the last Weichselian glaciation and underwent a multi-phase development. It was finally flooded during the Littorina transgression, while the basins and bays of the southern Baltic Sea contain thick glacial and post-glacial sediments. In particular, the Littorina and late Holocene deposits contain sediments rich in organic matter. Depending on the varying depth of the sulphate-methane transition zone in the different basins, which is partly modified through submarine groundwater discharge, methane is produced below this zone. This results in the accumulation of free gas within the sediments. The migration of the shallow biogenic gas forms gas fronts and leakage zones in the form of pockmarks at the seafloor.

In a first step, we mapped the pockmarks in the various basins using a bathymetric grid with a general resolution of up to 10 m, in smaller areas also up to 1 m. We catalogued the mapped pockmarks according to the following criteria: (i) their location (near the coast or in the central basin, and water depth), (ii) their lateral and vertical dimensions (diameter and depth), and (iii) their form (elongate or round, single structure or clustered, depression/negative or upbending/positive). Additional parametric sediment echo sounder (SES) profiles (vertical 2D sections) allow further conclusions to be drawn, such as the stratigraphic affiliation of the gas escaping from the pockmarks and the depth of the underlying gas front.

Adjacent to some existing pockmarks, the seafloor forms upward bulges above gas chimneys that may indicate the build-up of methane overpressure in the shallow subsurface. Whether these doming structures develop into new pockmarks needs to be evaluated with future differential bathymetric surveys. During the course of the projects, we will repeat SES profiles in order to determine any possible temporal or seasonal variation in the size of the pockmarks.