Natural methane seepage in the Danish offshore area – spatial distribution and morphology of methane-derived authigenic carbonates (MDACs)

Natural, cold seeps are important processes affecting seafloor geochemistry, ecosystems and greenhouse gas cycling. However, spatial and temporal constraints on offshore seepage remains limited. Methane seeps can form a variety of structures at the seabed, including pockmarks (depressions), or carbonate crusts in the form of methane-derived authigenic carbonates (MDACs), which can therefore be indicators of methane pathways through the seabed sediments.MDACs form through microbial mediated anaerobic oxidation of methane and provide a unique record of both relict and active seep-related carbonated formation. Here, we investigate the spatial distribution of MDACs, their morphology, relation to the subsurface geology, and whether they are still forming in the Danish offshore areas of Kattegat and Skagerrak. Their exposure at the seafloor is linked to cementation of unconsolidated sediment, combined with glacio-isostatic uplift and erosion, resulting in various morphologies such as pillars, mushroom-like structures or slabs. As the hard substrates occur at a predominantly sandy sea floor, MDACs can act as local ‘oases’ and provide a foundation for benthic ecosystems.Occurrences and morphologies of MDACs are identified by a combination of side-scan sonar, multibeam echosounder and sub-bottom profiler data, and confirmed with videos by remotely operated vehicles (ROVs) or divers. The majority of the MDACs align with the Sorgenfrei–Tornquist Zone, suggesting a tectonic control on fluid migration pathways. Previously published data indicated a stable carbon isotope signature (δ¹³C) of predominantly microbial methane source, likely derived from Late Quaternary organic-rich marine sediments (Jørgensen et al., 1990).