A pore-fluid array (PFA) was constructed and deployed in surficial sediments to assess hydrate stability over time at an outcropping hydrate mound, Mississippi Canyon 118, Gulf of Mexico. The PFA contains an instrument package, connector, cement weight and sea-floor probe. It is deployed from a surface ship similar to a gravity core under its own cement weight. A seafloormounted instrument package houses four OsmoSamplers (Jannasch et al., 2004) that continuously collect and store sedimentary porefluids in coils of gas-tight, small-diameter tubing (?70m long) using osmotic pumps. The samplers collect fluids at a flow rate of ?0.4mL/day at 4°C and need no electrical power. Each sampler is connected to individually filtered ports along the 10-meter sub-seafloor probe via small diameter tubing and a custom-made, low dead-volume connector. The connector allows a remotely operated vehicle to replace the seafloor instrument package without disturbing with the sample probe. Once the instrument package is collected, the porefluids will be measured for chloride, sulfate, and in situ methane concentrations to determine temporal changes in hydrate and address processes controlling hydrate stability in the shallow subsurface. The PFA was deployed in May 2005 and will be retrieved in the summer of 2006. Data gained from this study will be coupled to geophysical measurements to correlate seismic events relating to gas hydrate formation or decomposition.
Gas hydrates are crystallized mixtures of hydrocarbon gas (mainly methane) and water that occur in areas of high pressure and/or low temperature, as found in continental shelves and permafrost. Based upon interpretations of seismic data, gas hydrate deposits are thought to represent one of the largest carbon reservoirs on Earth, containing up to 1016 kg of methane carbon, about twice the amount stored in fossil fuels (Kvenvolden, 1988). Current global methane budgets include hydrate reservoirs as a small source of methane to the atmosphere but do not consider their dynamic nature (Lelieveld et al., 1998). Changes in overlying water pressure due to sea level fluctuations and/or ocean water warming could dissociate hydrate and release methane to the overlying water and possibly the atmosphere. Hydrate stability needs to be assessed over the long term and factors controlling their stability understood.
The hydrate stability zone (HSZ) is defined by overlying water pressure and temperature as well as the sedimentary geothermal gradient. Studies have shown that changes in bottom water temperature of <2°C increased gas flux from outcropping hydrate at Green Canyon 185 (GC 185), Gulf of Mexico, USA (MacDonald et al., 1994, Roberts et al., 1999). Using a program to estimate equilibration conditions for hydrate formation (CSMHYD; Sloan, 1998), we calculated the upper temperature limit at GC 185 (540m water depth) to be ?17°C (using hydrate-gas concentrations from Sassen et al., 1999). However, measured temperature ranges did not exceed 10°C within a year (Roberts et al., 1999) therefore a change in temperature should not account for the observed gas emission.