Abstract: Seafloor Gas-Hydrates: A Video Documenting Oceanographic Influences on Their Formation and Dissociation

I. R. MacDonald, N. L. Guinasso Jr., J. M. Brooks, R. Sassen, L. L. Lee III, K. T. Scott

Gas hydrates form in the upper few meters of the sediment column at hydrocarbon seeps in the northern Gulf of Mexico. At a site located in 540 m water depth, we found yellow hydrate material outcropping in a lobed mound that was about 1 m. high, 3 m wide, and mostly covered with a thin drape of sediment. We observed mytilid bivalves (mussels) with methanotrophic symbionts in the vicinity of the mount, but not on the mount itself. Attempts to sample the hydrate caused pieces of it to break off and float upward in the water column. Photographs and video taken from a submarine during 1992 and 1993 document the growth of one lobe of the hydrate mound and the disappearance of a second lobe. We postulate that accreting masses of gas hydrate rise in the uppermost sediment column due to their buoyancy, eventually breaking free from the seafloor to float upward as intact units. Samples of the gas stream that vented continuously around the hydrate mound consisted of 11.4% N₂, 8% CO₂, 0.2% O₂, 69.6% methane, 6.3% ethane, 1.7% propane, 0.2% i-butane, 0.9% n-butane, 0.3% i-pentane and <0.1% n-pentane. Gas hydrate formed from such a mixture of hydrocarbons at 540 m depth should remain stable in temperatures up to about 14 C. However, we constructed an in-situ device, the bubblometer that monitored gas flow and water temperature during a 44-day deployment near the mound. The bubblometer documented intense gas discharge events that occurred during a 10-d interval when bottom water temperature temporarily exceeded 8 C. Gas hydrates form d from pure methane would dissociate at temperatures above about 7.5 C. The gas discharge was either the sporadic result of events in the sediment column or the disassociation of pure-methane hydrates due to increased temperature. This 15-min video presents these findings with narrative and data displays, as well as footage of the hydrate, and deployment of the bubblometer as taken by the submarine Johnson Sea-Link.

AAPG Search and Discovery Article #90960©1995 AAPG Southwest Section Meeting, Dallas, Texas