Geochemical constraints for the formation and dissociation of gas hydrate in an area of high methane flux, eastern margin of the Japan Sea

Abstract

The Umitaka Spur in the Joetsu basin on the eastern margin of the Japan Sea is characterized by a number of features indicative of high methane flux such as a BSR and gas plumes rising through the water column. These are likely linked to the presence of hydrocarbon source rocks underneath the spur. The interstitial water chemistry at the spur was measured in order to delineate the behavior and evolution of the gas hydrate system across the region. The sulfate depth profiles indicate that the SMI depths become shallower toward the crest of the spur and are shallowest (less than 200 cmbsf) at two plume sites, suggesting that the methane fluxes are highest around the plumes. Depth profiles of dissolved Cl − all fall into one of four distinguishable trends: Type-I where concentrations linearly increase with sediment depth, Type-II where concentrations linearly decrease with depth, Type-III where concentrations remain constant and Type-IV where concentrations exhibit negative spikes caused by dissociation of gas hydrate during core recovery and handling. Type-I cores commonly recovered at central plume site are characterized not only by steep Cl − gradients (+ 13.2 to + 51.0 mM/m), but also extreme depletions in D and 18O (− 0.97 to − 0.54‰VSMOW/m for δD H2O and − 0.16 to − 0.10‰VSMOW/m for δ 18O H2O) suggesting gas hydrate formation at shallow depth. The Cl − gradients of Type-II cores recovered from the southern part of the spur have little variation (− 14.8 to − 8.3 mM/m). Gas hydrate dissociation along base of the gas hydrate stability zone best explains both the similar pattern of freshening with depth and the widespread distributions of Type-II cores. Although gas hydrate dissociation is known to release isotopically heavy water, Type-II cores show progressive depletions of both D and 18O with downward depth. The apparent contradiction between Cl − concentrations and the isotopic compositions of Type-II cores is likely due to re-equilibration by strong burial diagenesis observed all over the Japan Sea sediments. Regional dissociation of gas hydrate is likely to have been triggered by sea level drop which facilitated dissociation of subsurface gas hydrate.