Methane Hydrate Estimates from the Chloride and Oxygen Isotopic Anomalies: Examples from the Blake Ridge and Nankai Trough Sediments

Abstract

Abstract: Oxygen isotopic fractionation between gas hydrate and ambient water is determined as αGH‐IW= 1.0037 at 12–16°C and 31 Mpa, on the basis of direct measurements of gas hydrate‐derived waters and ambient pore waters recovered from the Blake Ridge during ODP Leg 164. Oxygen isotopic anomalies give us the amount of gas hydrate of 7 to 9% (pore filling), which is almost twice as much as estimates from chloride anomalies. The difference is probably due to uncertainties in determining base‐line profiles of the in situ pristine pore waters, and partially due to the effects of selective filtration/adsorption during pore water extraction. Two 250 meter‐deep holes were drilled in the eastern Nankai trough off central Japan at a water depth of 950 m, where strong BSRs occur at about 300 mbsf. Massive hydrates were not recovered during this drilling but a number of soupy horizons suggest the existence of subsurface gas hydrate. Chloride concentration and δ18O of interstitial waters are observed to vary in a remarkable zigzag pattern with spiky anomalies, reflecting hydrate dissociation during core‐recovery and water extraction. The concentration of gas hydrate in sediments is estimated to be about 3–7% with a spiky maximum value of 30% from chloride anomalies and between 5 and 30% from δ18O anomalies. Significant difference in vertical distribution between nearby two holes in Nankai Trough probably reflect heterogeneous fluid migration through particular conduits in an accretionary wedge.