project		background
intro goals challenges background methane fluxes the methane barrier AOM hydro-acoustics microbiology methods modeling database work packages working areas publications			Methane is produced in ocean margin sediments as a consequence of the microbiological degradation of organic matter buried below the zone of sulfate penetration. As methane builds up, it migrates upwards and may reach the sediment surface, where it leads to the formation of complex sediment structures. Pockmarks or carbonate chimneys and platforms, or large scale topographies on the deep sea floor such as mud volcanoes and carbonate mounds, are manifestations of methane ebullition and oxidation1 2. Although these consequences of gas formation through deep biosphere processes are important, the methane involved comprises only a small fraction, perhaps less than 10%, of the overall methane flux within the sea floor. Vast amounts of methane occur in deep sea sediments in the form of gas hydrates which in the past may have triggered catastrophic slope instabilities and mud slides or even caused rapid global warming. Several EU funded projects integrated in the Ocean Margin Cluster (OMARC) currently study the occurrence of gas hydrates or the physics of methane driven sea floor structures and their biological implications. In ocean margin sediments, on the shelf or upper slope, the hydrostatic pressure is below the stability field for gas hydrates and high methane concentrations lead instead to the formation of gas bubbles3. The bubbles are trapped in soft clayey sediments or may rise to the sediment surface in a diffuse or eruptive manner. The quantity and dynamics of sediment-bound gas bubbles are still poorly understood, although some recent studies have provided useful examples. Literature: 1) Hovland, M. And A.G. Judd (1988) Seabed pockmarks and seepages. Graham and Trotman, London, 293 pp. 2) Dando, P.R., S.C.M. OHara, U. Schuster, et al. (1994) Gas seepage from a carbonate-cemented sandstone reef on the Kattegat coast of Denmark. Mar. Petrol. Geol. 11: 182-189. back to top