10. RELATION BETWEEN PORE FLUID CHEMISTRY AND GAS HYDRATES ASSOCIATED WITH BOTTOM-SIMULATING REFLECTORS AT THE CASCADIA MARGIN, SITES 889 AND 8921

Prominent seismic bottom-simulating reflectors (BSRs) were penetrated at two sites on the Cascadia Margin, off Vancouver Island (Site 889, at 224 mbsf) and off Oregon (Site 892, at 74 mbsf) Although solid gas hydrate was not recovered at the depth of either of these prominent BSRs, the -1.4°C temperature measured in a core ~8 m above the BSR depth at Site 889, and the observed coincidence of very low pore fluid Cl and very high headspace methane concentrations at the depth of both BSRs, together with an increase in seismic velocities, strongly imply the presence of gas hydrate in situ with methane as the dominant gas within the hydrate cages. Pore-space occupancy by hydrate of a minimum of 15% and -10% at Sites 889 and 892, respectively, is inferred from geochemical and geophysical evidence. Assuming, however, bottom-water Cl concentration, the maximum Cl dilutions observed, 36% at Site 889 and 15% at Site 892, correspond to pore-space hydrate occupancies of 39% and 16%, respectively. Mixing with a diffusing, or upward-migrat ing, low-Cl fluid from a deeper source at Sites 889 and 892 accounts for the difference between the two estimates. As gas hydrate was not recovered, it is suggested that finely disseminated hydrate prevails at these sites. Thus, the high amplitudes of these BSRs are not solely related to gas hydrate content, but also to the presence and concentration of free gas below the BSR. The persistence of the zone of maximum Cl dilution below the BSR depth (Site 889) probably reflects a rather recent, interglacial, upward migration of the base of the hydrate stability field. At both sites the measured in situ borehole temperatures at the depth of the seismic BSRs are lower by approximately 2°C than the calculated temperatures for the base of a pure H2O-pure CH4 hydrate stability field at the corresponding pressures. Addition of gases such as ethane, CO2, or H2S further increases the hydrate stability temperature at corresponding pressures. The measured temperatures, however, are within the uncertainties of the base of the stability field of a seawater-CH4 hydrate. This observation has important implications for using seismic BSRs for mapping heat flow. Solid gas hydrate was recovered only at Site 892 between 2 and 19 mbsf, but this gas hydrate was not associated with the BSR, which occurs at a depth of 74 mbsf. This is a mixed hydrate that contains both CH4 and up to 10% H2S, with minor amounts of ethane and some CO2.