Summary of Scientific Results: Changes in Physical Properties | Table of Contents


Gas Hydrates
Although no solid gas hydrate was recovered during Leg 190, their presence was documented indirectly. Both temperature measurements of cores on the catwalk and pore fluid Cl concentrations indicate the existence of gas hydrates at two slope sites, Site 1176 and Site 1178. Gas hydrate dissociates upon recovery because it is unstable at ambient temperature and pressure. Recovery of solid hydrate is unlikely unless it is extremely abundant.

Sites 1176 and 1178 are within the stability field of seawater-methane hydrate from the seafloor to the BSR. Because methane is the dominant gas in the sediments at these sites, if gas hydrate is present it should be primarily methane hydrate as it is at nonthermogenic oceanic sites. Formation of methane hydrate is a highly exothermic reaction; its decomposition consumes much heat and cools the cores. At Site 1176, colder than background temperatures, by 4°—5°, between ~220 and 240 mbsf, were measured in two cores (190-1176A-25X and 26X). Because of poor core recovery no data exist between 240 and 320 mbsf. Pore fluid Cl concentrations suggest minor dilution of Cl by ~1% beyond dilution by other processes.

At Site 1178, gas hydrate appears to be considerably more abundant. Based on pore fluid Cl concentrations, methane hydrate (inferred from gas composition) is present between ~120 and 400 mbsf, with the highest concentrations between 150 and 200 mbsf. At ~200 mbsf, in Core 190 1178A-23X, the lowest catwalk core temperature of —0.5°C was measured. Temperatures colder than background by 4°—6°C were measured in several cores, mostly between 150 and 200 mbsf.

At Site 1178, the Cl concentration-depth profile has a steep, continuous trend of freshening between 90 and 200 mbsf with two intense Cl minima. The first is between 170 and 185 mbsf. The second minimum with the lowest Cl value of 524 mM compared with that of bottom-water value of 557 mM was measured in Core 190-1178A-23X (the one with the —0.5°C catwalk temperature). This corresponds to >6% dilution by methane hydrate decomposition. The background dilution throughout the 150—200 mbsf interval is 3%. Between 200 and 400 mbsf, Cl concentrations continue to gradually decrease with depth from 545 mM to a minimum of 517 mM at the BSR depth (~420 mbsf), which corresponds with >7% dilution. Superimposed on the background Cl dilution profile are numerous smaller Cl minima. This suggests that throughout the section, from 90 to 400 mbsf, disseminated gas hydrate is present and is responsible for the background 3%—4% Cl dilution and that specific sediment horizons, probably the coarsest grained ones, have higher hydrate concentration, equivalent to 6%—7% Cl dilution.

Cl concentrations sharply decrease below the BSR depth and reach a minimum of 470 mM, almost a 6% dilution, centered around 500 mbsf. The origin of this low-Cl zone is as yet unclear. It may represent a more hydrate rich young paleo-BSR, which has not had enough time to dissipate the dilution signal caused by dissociation of the hydrate. Consistent with this scenario, higher concentrations of methane are found at this depth. Sedimentation and/or tectonics could have caused such an upward migration of the BSR.

Summary of Scientific Results: Changes in Physical Properties | Table of Contents