Geochemical Evaluation of Piston Core Porewater on Atwater Valley, Gulf of Mexico: Variation in Vertical Methane Gradients

From May 14 to May 20, 2004 piston core and heat flow measurements were collected across two mound structures in an area designated as the Atwater Valley, a shallow trough on the continental slope south of the Mississippi Delta . Atwater Valley lies at 1,200 to 1,500 meters water depth. Several small mound structures occur in the valley, rising less than 50 meters above the surrounding seafloor. The USGS collected several multichannel seismic lines in this area in 2003. During 4 days on site we acquired 15 piston cores and 23 thermal profiles on a transect from mound F to mound D. A previous USGS seismic line (AV65) and a 3.5 kHz echosounder profile collected during the cruise were used to guide operations. 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. Geochemical data were coupled with USGS seismic data and NRL heatflow data. The sulfate-methane interface estimated from pore water sulfate profiles provides a range in the variation of vertical methane fluxes. Sulfate and methane pore water profiles from piston cores on mound F indicated the largest vertical methane flow. Sulfate was depleted in surface core samples and methane concentrations were raised suggesting a flow of methane into the water column. Overall the SMI on the transect ranged from 45 to 410 cm. Stable carbon isotope ratios and speciation of gases sampled from the piston cores indicate biogenic methane. Chloride data from piston cores did not indicate hydrates were sampled and dissociated during transport from the sediment and deck processing. However, high chloride concentrations were measured on mound F. It is expected that the chloride originates from deep salt mounds. DIC concentrations and stable carbon isotope analysis confirm anaerobic methane oxidation in the pore water profiles. Mound F sites show more shallow increases in DIC concentration and 13C depleted DIC pools, indicating an increased vertical methane flow in this region of the survey. The heatflow probing was conducted at the same location of the piston coring. The data show clear anomalies in sediment temperature and heat flow associated with the mounds. Measurements collected on the top of mound F show elevated sediment temperatures, and heat flow values of around 160 mW/m2. Sediment temperatures decrease as one moves away from the summit of the mound, and heat flow values drop to a background level of 40 to 50 mW/m2. Sediment temperatures at the summit of Mound D are similar to what was observed at Mound F, and heat flow values are slightly lower at around 132 mW/m2, partly as a result of the slightly