The release of large amounts of the greenhouse gas methane from shallow marine gas reservoirs and dissociating gas hydrates at continental margins is discussed to have advanced warming episodes in Earth’s history. However, methane emitted at the seafloor is consumed partly in the ocean by microbial oxidation reducing the portion reaching the atmosphere. It is the main purpose of this project to quantify the fraction of CH4 oxidised in the water column in relation to the amount discharging, dispersed in the water column and escaping into the atmosphere at different seep settings. In the outgoing phase, Dr. Susan Mau will learn the 14C/3H-tracer technique including the synthesis of the tracers at the University of California, Santa Barbara, to measure oxidation rates of CH4 in the water column. Thereby acquired microbiological techniques will ideally complement her skills in geochemistry, oceanography and geology as well as her expertise in the marine CH4 cycle. This will allow integrated investigation of sever all vent sites situated in 10-100m water depth in the Santa Barbara Basin. The area is ideally suited as it is one of the most active areas of natural marine hydrocarbon seepage in the world. The new techniques and gathered knowledge during the outgoing phase will then be integrated in the department of Marine Biogeochemistry at IFM-GEOMAR and in the SFB 574 at the Christian-Albrechts-Universität zu Kiel, Germany, during the return phase. Planed investigations at vent sites at the continental slope offshore Chile, where recently gas hydrates have been discovered, and in the North Sea will benefit considerably from the additional expertise of Dr. Mau. The comparison of the results from the distinct settings will provide new insight to the buffer system of the ocean with regard to hydrocarbon seepage. This is of great value for the assessment of the climatological response of a sudden release of CH4 from shallow marine gas reservoirs and gas hydrates.