Background
Organic material that sinks out of the water column and becomes buried in the seabed undergoes anaerobic degradation, first with sulphate reduction and deeper down with methanogenesis as the predominant terminal pathway. Two thirds of the global marine methane production takes place in continental shelf sediments, although these comprise only 8% of the ocean area (1, 2).
Coastal seas are hotspots of methane formation, often enhanced by eutrophication from coastal populations and from river discharge. In the Baltic Sea, methane supersaturation has led to the accumulation of free gas in the form of dense bubbles beneath the depth of sulphate penetration, several meters sub-surface. This “shallow gas” has been detected in hundreds of square kilometres of the Baltic seafloor (e.g. 3). Although the gas is continuously rising up towards the sediment surface, it is effectively broken down sub-surface when reaching into the sulphate zone.
Under the influence of global warming and prolonged conditions of eutrophication, however, gassy sediments may loose their capacity to retain methane (4). Recently, gas ebullition has been discovered in several areas of the Baltic Sea, including the eastern Gulf of Finland, the Gdansk Basin, and the central Baltic. The associated escape structures, such as pock-marks on the sea floor, have been recognized by high-resolution multibeam bathymetry. Ebullition drives out methane, which is a 25-fold stronger green-house gas than carbon dioxide on a molecular basis. A part of the methane may escape into the atmosphere while the rest is broken down aerobically in the water column and thereby adds to the oxygen demand. Importantly, methane carries hydrogen sulphide from the anoxic sediment, where it is a main product of bacterial sulphate reduction, and into the water column. Hydrogen sulphide is a highly toxic gas which, even at low concentration, may have adverse, sub-lethal effects on fish larvae and other sensitive organisms.
1Canfield, D.E., E. Kristensen, and B. Thamdrup (2005) Aquatic Geomicrobiology. Elsevier, San Diego, 640 pp
2Jørgensen, B. B., and S. Kasten (2006) Sulfur cycling and methane oxidation, pp. 271-309. In H. D. Schulz and M. Zabel (eds.), Marine Geochemistry, 2nd ed. Springer, Berlin.
3Laier, T., and J. B. Jensen (2007) Shallow gas depth-contour map of the Skagerrak - western Baltic Sea region, Geo-Marine Letters 27: 127-141.
4Best, A.I, M.D. Richardson, B.P. Boudreau, et al. (2006) Shallow seabed methane gas could pose coastal hazard. EOS, 87: 213-220.