Diagenetic relationships of methanogenesis, nutrients, acoustic turbidity, pockmarks and freshwater seepages in Eckernfِrde Bay

High organic loading (4–5 wt%) and sedimentation rates (1.4 mm yr−1) in Eckernförde Bay sediments lead to anaerobic conditions within the uppermost 15 cm. Intense bacterial sulphate reduction (0.011–0.15 mM SO4 red2− yr−1) exhausts dissolved sulphate around 150 cm sediment depth, resulting in methanogenesis at greater sediment depth by carbonate reduction (1.8–8.5 μM CH4 yr−1). Extensive regions of Eckernförde Bay are characterized by acoustically turbid sediments (>300 cm sediment depth), resulting from deeper sediments supersaturated in methane, forming free gas accumulation zones. Methane migrating upward into the sulphate reduction zone is effectively consumed by sulphate reducing bacteria at rates 3–10 times greater than methanogenesis, i.e. 46–95 μM CH4 ox yr−1. The pathways of methane formation and anaerobic oxidation are confirmed by stable carbon and hydrogen isotope evidence. Pockmarks, i.e. shallow surface depressions in some Eckernförde Bay sediments, are not associated with gas ebullition; rather, these physical features result from the expulsion of freshwater. These episodic springs or seepages of groundwater from the underlying, Holocene glacial lags and sands into the basin at pockmark sites have characteristic low chloride and methane concentrations. The geochemical evidence indicates that the commercial accumulation of petroleum in the lower Cretaceous, Dogger–Beta–Hauptsandstein Schwedeneck field in Eckernförde Bay (1500 mbsf) has no surface manifestation and does not influence either the occurrence of acoustic turbidity or pockmarks.