This report is a contribution to the Strategic Environmental Assessment (SEA6) conducted by the Department of Trade and Industry (now Department of Energy and Climate Change). Methane-derived authigenic carbonate (MDAC) is formed as a consequence of the anaerobic oxidation of methane by consortia of bacteria. MDAC develops as a rock-like concretion when a carbonate precipitate cements normal seabed sediment; the carbon of this carbonate is derived from methane. It occurs as crusts or slabs at the seabed, or as exposed lumps. The occurrence of MDAC is evidence of methane seepage, although the seepage is not necessarily going on at the present time. Large quantities of MDAC indicate that methane seepage has occurred over a prolonged period of time. 'Submarine structures formed by leaking gas' have been identified by the European Commission's Habitats Directive as habitats worthy of protection.

The distribution of MDAC in UK waters is poorly known. This report summarises the available information relevant to MDAC in the SEA6 area, including data acquired by surveys specifically conducted in support of this SEA in 2004. Carbon isotope data for samples recovered in 2004 have confirmed that the cemented hard grounds from two of the areas studied, Texel 11 and Holden's Reef, are composed of MDAC.