南海北部琼东南海域活动冷泉特征及形成模式

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

近年来,活动冷泉的研究越来越受到关注.本文利用多波束数据、多道地震数据以及底质取样结果研究琼东南海域活动冷泉系统,分析活动冷泉的羽状流特征、海底地貌与底质特征以及流体活动构造特征.多波束水体数据上,观测到多个延伸高度超过750m的气泡羽状流,海底流体活动非常强烈;多道地震上识别出麻坑、流体运移通道、气烟囱等流体渗漏相关的构造,与其他海域观测到的反射特征不同,羽状流的下方流体运移通道呈强振幅"串珠"反射;重力活塞取样在两个站位上获得浅表层块状天然气水合物.其中一个站位位于活动冷泉附近,天然气水合物赋存于海底以下8m左右.基于以上三方面的数据,笔者提出了一个用于描述活动冷泉系统的形成模式,游离气通过气烟囱向上运移到达浅层,一部分在天然气水合物稳定带内形成天然气水合物,另一部分穿透天然气水合物稳定带到达海底,形成活动冷泉的羽状流.

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 shallowsediments 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.