GAS HYDRATE THAT BREACHES THE SEA-FLOOR ON THE CONTINENTAL-SLOPE OF THE GULF-OF-MEXICO

We report observations that concern formation and dissociation of gas hydrate near the sea floor at depths of approximately 540 m in the nor thern Gulf of Mexico. In August 1992, three lobes of gas hydrate were partly exposed beneath a thin layer of sediment. By May 1993, the most prominent lobe had evidently broken free and floated away, leaving a patch of disturbed sediment and exposed hydrate. The underside of the gas hydrate was about 0.2-degrees-C warmer than ambient sea water and had trapped a large volume of oil and free gas. An in situ monitoring device, deployed on a nearby bed of mussels, recorded sustained releas es of gas during a 44 day monitoring period. Gas venting coincided wit h a temporary rise in water temperature of 1-degrees-C, which is consi stent with thermally induced dissociation of hydrate composed mainly o f methane and water. We conclude that the effects of accumulating buoy ant force and fluctuating water temperature cause shallow gas hydrate alternately to check and release gas venting.