Methane Hydrate Exploration, Atwater Valley, Texas-Louisiana Shelf: Geophysical and Geochemical Profiles

From May 14-20, 2004, piston core and heat flow measurements were collected across two mound structures, mound D and mound F, in an area designated as Atwater Valley, a shallow trough on the continental slope south of the Mississippi Delta. Atwater Valley lies at 1,200 to 1,500 m water depth. Several small mound structures occur in the valley, rising less than 50 m above the surrounding seafloor. The UNOLS vessel RV Gyre was contracted to support this work. The work took place in about 1,300-m water depth through a 4-km transect at lat. 27.9356 degrees N, Ion. 89.2794 degrees W (lease block Atwater Valley 14). Previous USGS seismic lines AV65, AV97 and AV82 and a 3.5 kHz echosounder profile collected during the cruise were used to guide operations. Total sampling included mounds D and F and a transect between the mounds. During 4 days on site, we acquired 15 piston cores and 23 thermal profiles on a transect from mound F to mound D. Extensive piston coring and heatflow was conducted at mound F. Dense heat flow measurements and one piston core were obtained at mound D. All coring and heat flow measurements were conducted at the sites planned for deep drilling by the Chevron-Texaco JIP. All cores had good penetration (shortest was 255 cm, longest 842 cm), and no carbonate pavement or hydrate mounds were encountered. The cores were immediately sectioned and sampled for chemical analysis. All attempts at thermal probing resulted in sufficient penetration, except for one instance where the instrument laid horizontal on the seafloor. The sulfate-methane interface (SMI) estimated from pore water sulfate profiles indicated a range in the vertical flux of methane. Sulfate and methane pore water profiles from piston cores on mound F indicated the greatest vertical methane flux in this study region.