Research On Typical Seabed Fluid Flows In China Marginal Seas

Seabed fluid flow mainly refers to the process that fluids traverse the seabed and discharge into the water,vice versa.It is an important part of the global hydraulic system and links the spheres of the earth.Based on the comprehensive acoustic data,in-situ geochemical data and near seabed videos and photos,typical seabed fluid activities in the North Yellow Sea,the mid-Okinawa Trough and Southwestern Taiwan were described and analyzed,in order to shine some light on the study of seabed fluid flow in china marginal seas.Numerous isolated shallow seabed domes have been discovered in the North Yellow Sea(NYS)via a high-resolution multibeam echosounder survey and twelve such domes were found within an 11 km by 17 km survey area.These domes are between 0.1 m and 1.0 m high with diameters from 250 to 1700 m and display a characteristic circular to elliptical shape with surrounding depression.The acoustic anomalies presented on the high resolution sub-bottom profile images indicate that shallow gas accumulates just beneath these domes and suggest that the migration and accumulation of the shallow gas may be responsible for the formation of these features.However,analysis of sediment pore water samples from one of the domes confirmed the presence of strongly diluted seawater,indicating that sub-marine fresh groundwater discharge may also play a role in forming these structures.The presence of fresh groundwater flows is of interest for hydrological budgets and may be a significant part of the regional hydrologic balance.Our findings suggest that shallow gas accumulation and migration is strongly coupled to submarine fresh groundwater discharge at this domed site and may provide some new information in the study and management of the submarine fresh water resources in the NYS coastal areas.Based on the in situ chemical data and video data collected at the Iheya North Knoll hydrothermal field and the CLAM hydrothermal filed located at the mid-Okinawa Trough,hydrothermal communities and geochemical features of in situ seabed water were described and analyzed.Hydrothermal activities in the CLAM site was relatively low and dominated by diffusion patterns.However,at the Iheya North Knoll site,typical hydrothermal chimneys,up to 30 m and with vigorous fluid erupting through the vent site,were observed.Communities at both site show somehow zonation distribution patters with that of CLAM site at the seabed surface and that of Iheya North Knoll on the chimney.It was suggested that the composition and distribution patterns of the communities was mainly controlled by temperature near the venting site,while leaving the venting site,that of the communities would controlled by the concentrations of the nutrients which supported the chemosynthetic communities.The south summit of the Formosa Ridge is one of the most active cold seep site located at the passive South China Sea continental margin.Based on the acoustic data and video data collected using multibeam echo sounder system,side scan sonar system and high-definition cameras mounted on Remotely Operated Vehicle,the seabed topographic features and the surficial expressions of the cold seep system were described.The main surficial expressions of the cold seep system include authigenic carbonate structures,chemosynthetic communities,gas plumes and reductive sediments.Based on the acoustic data,which were ground-truthed with video data,the exposure of the carbonate structures with various morphological features caused the anomalous seabed topographic features of the cold seep site.The distribution of the chemosynthetic communities was controlled by the positions that cold seep fluids venting or emission through the seabed,which correlate well with the fissures or fractures developed within the authigenic carbonate mound.The distribution patterns of the surficial expressions of the cold seep system indicated that the authigenic carbonate mound might vertically channelize and horizontally redirect the cold seep fluids,which in turn effect the distribution patterns of the surficial expressions.