Gas seeps and rock formation in the northern Adriatic Sea

Gas seeps in the Adriatic Seabed were first documented in 1940, but investigation have been conducted only much more recently. The distribution of shallow gas, seeps and related features is reviewed. The presence off the Italian northern Adriatic Sea coast of scattered rock outcrops has been studied in detail, but just recently some of them have been attributed to methane-induced cementation of local seabed sediments. Isotopic and geochemical data have been combined with the geological and sedimentological framework by means of very high resolution geophysical investigations, allowing the development of a general model and several different case histories of gas seeps. The general model is in fact given by the classic diagenetic evolution of organic matter. The decomposition processes generate CH 4 and CO 2 , as final products. Organic matter is recycled and a small fraction only is resistant to a complete biochemical transformation. Carbon and oxygen stable isotopes suggest that diffusive fluxes of gas and pore waters can take place only in a zone controlled by hydraulic conductivity (either by tectonics or by the abundance of clasts in the sediment) and in consequence point out different local case histories.