Gas Hydrate Occurrence Inferred from Dissolved Cl- Concentrations and δ18O Values of Pore Water and Dissolved Sulfate in the Shallow Sediments of the Pockmark Field in Southwestern Xisha Uplift, Northern South China Sea

Deep-water pockmarks are frequently accompanied by the occurrence of massive gas hydrates in shallow sediments. A decline in pore-water Cl-concentration and rise in delta O-18 value provide compelling evidence for the gas hydrate dissociation. Mega-pockmarks are widely scattered in the southwestern Xisha Uplift, northern South China Sea (SCS). Pore water collected from a gravity-core inside of a mega-pockmark exhibits a downward Cl-concentration decrease concomitant with an increase in delta O-18 value at the interval of 5.7-6.7 mbsf. Concentrations of Cl-, Na+, and K+ mainly cluster along the seawater freshening line without distinct Na+ enrichment and K+ depletion. Thus, we infer that the pore water anomalies of Cl-concentrations and delta(18)ZO values are attributed to gas hydrate dissociation instead of clay mineral dehydration. Moreover, the lower delta O-18 values of sulfate in the target core (C14) than those in the reference core (C9) may be associated with the equilibrium oxygen fractionation during sulfate reduction between sulfate and the relatively O-18-depleted ambient water resulting from gas hydrate formation. The gas hydrate contents are estimated to be 6%-10% and 7%-15%, respectively, according to the offset of Cl-concentrations and delta O-18 values from the baselines. This pockmark field in southwestern Xisha Uplift is likely to be a good prospective area for the occurrence of gas hydrate in shallow sediments.