The Qiongdongnan Basin is one of the target areas for marine gas hydrate exploration in the northern margin of the South China Sea. In this study, major anion (e.g., SO42 , Cl, Br, I), cation (e.g., Ca, Mg, K, Na), and trace element (e.g., Sr, Ba) con- centrations of pore water samples collected from site HQ-1PC in the Qiongdongnan Basin were analyzed. These geochemical data suggest that the process of AOM (Anaerobic Oxidation of Methane) is dominant in sulfate-reduction zone in this site due to high upwelling iodine flux and strong microbial activities. The iodine-rich fluids, which may carry methane for the gas hydrate formation, pass through the gas hydrate stability zone, mix with brine released during gas hydrate formation, and cause the geochemical anomalies observed at site HQ-1PC. The pore water geochemical characteristics and anomalies in the Qiongdongnan Basin are quite similar to those found in other gas hydrate locations in the Shenhu area in the northern South China Sea, and a genetic link is suggested with the possibility of gas hydrate occurrence in the study area. The Qiongdongnan Basin is one of the target areas for marine gas hydrate exploration in the northern margin of the South China Sea. In this study, major anion (eg, SO42, Cl, Br, I), cation (eg, Ca, Mg, K, Na), and trace element (eg, Sr, Ba) con- centrations of pore water samples collected from site HQ-1PC in the Qiongdongnan Basin were analyzed. These geochemical data suggest that the process of AOM (Anaerobic Oxidation of Methane) is dominant in sulfate-reduction zone in this site due to high upwelling iodine flux and strong microbial activities. The iodine-rich fluids, which may carry methane for the gas hydrate formation, pass through the gas hydrate stability zone, mix with brine released during gas hydrate formation, and cause the geochemical anomalies observed at site HQ-1PC. The pore water geochemical characteristics and anomalies in the Qiongdongnan Basin are quite similar to those found in other gas hydrate locations in the Shenhu area in the northern South China Sea, a nd a genetic link is suggested with the possibility of gas hydrate occurrence in the study area.