The evidence for the existence of methane seepages in the northern South China Sea: abnormal high methane concentrations in bottom waters

The methane concentration of water samples at five stations collected by the CTD rosette water sampler in the areas of southwest Dongsha Islands and the Xisha Trough was analyzed by the gas-stripping method on aboard ship.It shows abnormal high methane concentrations in near bottom water samples at three stations.In the southwest Dongsha Islands area,the methane concentration of 4.25 and 10.64 nmol/dm3 occurs in near bottom water samples at Stas EIOSA and El06,respectively.In the Xisha Trough area,the high methane concentrations of 5.17,8.48 and 8.70 nmol/dm3 in water depths of 1 750,1 900 and 2 050 m,respectively,have been observed at Sta.E413.It is believed that the abnormal high methane concentrations are generated from the leakage of methane from sediments.Combining with previous geophysical and geochemical data from these two areas,this was probably relat-ed to the submarine gas hydrates decomposition and cold seep system.In May 2007,gas hydrate samples were successfully ob-tained by the drilling in the Shenhu Sea area located in the southwest Dongsha Islands area.It is called for further drilling surveys to confirm the existence of gas hydrate and cold seep system in the Xisha Trough as early as possible.