In situ measurements of methane cycling in cold seep sediments containing gas hydrates and brines

The spatial distributions of dissolved in situ methane concentrations and biogeochemical processes were investigated in cold seep sediments containing gas hydrates and brine fluids along the northern continental slope of the Gulf of Mexico and the Northern Cascadia Margin, offshore Vancouver Island. The measured distributions elucidate the role of biogeochemical processes in controlling hydrate stability and provide evidence for a potentially large source of methane directly from brine seeps. To obtain in situ methane concentrations, two novel pore-fluid samplers were developed and tested. While both samplers retained gas samples at in situ pressures, HYDRA used active suction to collect discreet samples while the Pore-Fluid Array used pumps driven by osmosis to slowly collect a temporal record of pore-fluid chemical composition. Within hydrate-bearing sediments, the successful retrieval of 14 in situ methane concentration profiles with HYDRA showed that from immediately adjacent to and as far as 350 cm away from outcropping hydrate, porefluids contained less than 15 mM dissolved methane, a factor of four lower than methane saturated fluids. While undersaturated conditions are expected near the sediment-water interface, these results suggest that hydrates in the shallow sediments are not thermodynamically stable and could be dissolving. However, the observed hydrates may be more stable than theory predicts if their dissolution is kinetically controlled by impurities concentrated on their surfaces. Hydrate stability was not strongly affected by biogeochemical processes occurring adjacent to hydrate. Spatial variability in rates of sulfate reduction and anaerobic methane oxidation were largely controlled by the availability of petroleum and/or chemosynthetically derived carbon, rather than by distance from hydrate deposits. In brine filled sediments, in situ dissolved methane concentrations reached 25 mM and steep chloride concentration gradients could be explained by upward advection rates that varied between 3 and 65 cm/year. As there was no indication of methane oxidation in these brine sediments, these sites may be an important direct source of methane to the oceans. The combined results of this study demonstrate the importance of in situ quantification of methane concentrations and biogeochemical processes within hydrate and brine sediments.