Characteristics of active cold seepages in Qiongdongnan Sea Area of the northern South China Sea

Active cold seepage is a major concern, since the methane released from cold seepage may have an impact on the climate change and is greatly associated with the seafloor ecosystem. Using multi-beam data, sediment cores and multi-channel seismic data, we investigated active cold seep systems in the Qiongdongnan sea area of the northern South China Sea including gas bubble plumes, seafloor features, near-seafloor gas hydrate and the structure of the fluid flow. Multi-beam water column data identify several over 750 m high gas bubble plumes, suggesting that seabed fluid is highly active; Multi-channel seismic data show pockmarks, gas chimneys and fluid migration channel. Unlike the features of fluid migration pathway observed in other oceans, the fluid migration channel manifests itself as a string of "beads" on multi-channel seismic profile; Two massive gas hydrate samples were recovered through piston core, one of which occurs near active cold seepage and lies 8 m below seafloor. Based on these data and analysis, we proposed a model to describe the formation of the cold seep system. Free gases rise to the shallow sediments through gas chimney. Some of them are trapped in gas hydrate stability zone (GHSZ) as gas hydrate, while others enter the ocean water through GHSZ and trigger gas bubble plumes. Our results are significant to understand the migration of seabed fluid and the deposit of gas hydrate near seafloor.