Seabed features associated with cold seep activity at the Formosa Ridge, South China Sea: Integrated application of high-resolution acoustic data and photomosaic images

High resolution acoustic data, together with high-definition photomosaic images and video observations of the Formosa Ridge Cold Seep Site were obtained using a remotely operated vehicle (ROV). Seabed features of the cold seep site were described in detail and the scales and distribution patterns of these features displayed on the seabed were quantitatively analyzed. The cold seep site is characterized by irregular and hummocky topography on the seabed relief map and patches with high to medium backscatter intensities on side-scan sonar (SSS) images. Based on visual data, six kinds of seabed features related to cold seep activity were identified: gas flares, dense benthic chemosynthetic communities, mussel beds with low population densities, shells and/or shell debris areas, authigenic carbonate concretions, and reduced sediments. Spatially, these seabed features are distributed in an approximately zoned pattern with gas plumes and benthic communities in the innermost part of Site F and the reduced sediments in the outermost part. Based on the integrated acoustic and visual images and in-situ geochemical data, a conceptual model describing the cold seep fluid conduits within the shallow subsurface at Site F at the present is established. Briefly, favorable conduits have developed within the authigenic carbonate mound. The conduits channelize the cold seep fluids and result in relatively focused discharge of the seep fluids at the intersections of the conduits and the seabed, to sustain the dense communities there. Additionally, the authigenic carbonate concretions redirect some of the cold seep fluids to seeps at the periphery of the site, resulting in the reduced sediments. Integrated acoustic and visual investigations combined with in situ measurements are an effective way to quantitatively study the spatial extent of a cold seep site and can provide detailed information for further study of the cold seep system.