Bottom-simulating reflectors were observed beneath the southeastern slope of the Dongsha Islands in the South China Sea, raising the potential for the presence of gas hydrate in the area. We have analyzed the chemical and isotopic compositions of interstitial water, headspace gas, and authigenic siderite concretions from Site 1146. Geochemical anomalies, including a slight decrease of chlorine concentration in interstitial water, substantial increase of methane concentration in headspace gas, and 18O enrichment in the authigenic siderite concretion below 400 meters below seafloor are probably caused by the decomposition of gas hydrate. The low-chlorine pore fluids contain higher molecular-weight hydrocarbons and probably migrate to Site 1146 along faults or bedded planes.

Supplement to: Zhu, Youhai; Huang, Yongyang; Matsumoto, Ryo; Wu, Bihao (2003): Geochemical and stable isotopic compositions of pore fluids and authigenic siderite concretions from Site 1146, ODP Leg 184: implications for gas hydrate. In: Prell, WL; Wang, P; Blum, P; Rea, DK; Clemens, SC (eds.) Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 184, 1-15