SUMMARY OF SCIENTIFIC RESULTS
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
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 150200 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