Relation between Pore Fluid Chemistry and Gas Hydrates Associated with Bottom-Simulating Reflectors at the Cascadia Margin, Sites 889 and 892

Abstract

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 Deg temp. measured in a core .apprx.8 m above the BSR depth at Site 889, and the obsd. coincidence of low pore fluid Cl and high headspace methane concns. 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 min. of 15% and .apprx.10% at Sites 889 and 892, resp., is inferred from geochem. and geophys. evidence. Assuming, however, bottom-water Cl concn., the max. Cl dilns. obsd., 36% at Site 889 and 15% at Site 892, correspond to pore-space hydrate occupancies of 39% and 16%, resp. Mixing with a diffusing, or upward-migrating, low-Cl fluid from a deeper source at Sites 889 and 892 accounts for the difference between the two ests. As gas hydrate was not recovered, 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 concn. of free gas below the BSR. The persistence of the zone of max. Cl diln. 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 temps. at the depth of the seismic BSRs are lower by approx. 2 Deg than the calcd. temps. for the base of a pure H2O-pure CH4 hydrate stability field at the corresponding pressures. Addn. of gases such as ethane, CO2, or H2S further increases the hydrate stability temp. at corresponding pressures. The measured temps., 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 assocd. 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 amts. of ethane and some CO2. [on SciFinder (R)]