Harry H. Roberts¹, Wm. J. Wiseman¹, James Hooper¹, Gary Humphrey¹
(1) Coastal Studies Institute, Baton Rouge, LA

Abstract: Gas hydrate at or near the modern seafloor: effects of water column temperature variations of hydrate stability

Since 1991 when gas hydrates were first found exposed on the seafloor (GC 232), it has been clear that the special geologic framework of the northern Gulf of Mexico continental slope provides an optimum setting for the study of these interesting solids composed of hydrocarbon gas and water. Although gas hydrates have been identified as enormous potential sources of energy and as possible drivers of global climate change (CH4 release and sequestering), in the deepwater hydrocarbon province of the Gulf they are at present considered potential geohazards and sites where protected communities of chemosynthetic organisms are likely to be found. Data derived from an experiment on the stability of surficial gas hydrates (GC 185) in the context of changing ocean temperatures provides insight into both geohazards and environmental impacts of these unusual components of the slope's surface and near-surface sediments. Data collected in 1997-98 (1-year instrument deployment-water depth ~ 542 in) indicate that gas hydrate decomposition and out-gassing is coupled to slight changes in water temperature related to the shifting of water masses at frequencies coherent with tides (diurnal and semi diurnal) and mesoscale eddies (4-20 days) that are known to exist on the upper slope. Maximum water temperature excursions over the 1-year deployment were in the realm of ~ 3°C and no warm Loop Current eddies were measured. Outgassing at tidal frequencies was induced by temperature changes of < 1°C (resulting in a 3-4 cm3/min change in outgassing rate). Longer decomposition cycles (mesoscale eddy-induced were induced by temperature changes of up to ~ 3°C with peak outgassing at 125 cm3/min from a 1-m diameter collector cone positioned over a surficial exposure of gas hydrate. It is unclear as to the response of these gas hydrates during Loop Current intrusions when bottom temperatures may quickly change by more than 10°C.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana