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This paper presents new geochemical data on gas-hydratebearing mud volcanoes discovered for the first time in the Gulf of Cadiz during cruises TTR9 and TTR10 of the R ⁄ V Professor Logachev in 1999-2000. The estimated gas hydrate content is 3-16% of sediment volume and 5-31% of pore space volume. Estimated values of the water isotopic composition for the Ginsburg mud volcano are very heavy for d 18 O (up to +53‰) and light for dD (up to ) 210‰). Gas released from the hydrates contains 81% of C 1 and 19% of C 2+ . The inferred source of the gas in the hydrates is enriched in C 2 -C 6 (£ 5%), indicating that the gas has a thermogenic origin. Gas hydrate of cubic structure II should be formed from a gas of such composition. It is interpreted that the composition of the mud volcano fluid corresponds to deep oil basins below the Gulf of Cadiz.

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PAGES 13,18Gas hydrates are natural gas reservoirs in ice-like crystalline solids, and are stable in pore spaces of submarine sediments in water depths greater than about 300-500 m.They have been recovered in many of the world's oceans, both at larger sub-bottom depths (up to 450 m) by drilling and near the seafloor in shallow cores by gravity-coring. In the latter case, the gas hydrates are related to the sites of enhanced seepage such as cold seeps and mud volcanoes [Ginsburg and Soloviev, 1998].Multidisciplinary field investigations during the two cruises have revealed new, large hydrate-bearing seepage structures in the Sea of Okhotsk, a northwestern marginal sea of the Pacific Ocean (Figure l The aim of the project was to clarify the distribution, the amount, and the nature of near-sea-bottom hydrate deposits and explain the role of the gas and fluid discharge for the hydrate formation.In an area of 200 km 2 , a side-scan-sonar survey identified more than 40 seepage structures. Intensive coring in three of these structures suggested that these seepage structures reflect the zones of hydrate accumulation due to fluid discharge.The successful mapping of near-seabottom hydrates allows better understanding of the formation mechanisms of these hydrates, and a more precise estimation of the total amount of shallow hydrates over a relatively large study area.