In situ detection of the fine scale heterogeneity of active cold seep environment of the Formosa Ridge, the South China Sea

Abstract

The active cold seep at Formosa Ridge (Site F) sustains the most flourishing ecosystem on the continental slope of the northern South China Sea. However, the physicochemical environment around the cold seep has scarcely been investigated. The knowledge about the variability of the seafloor habitat around the cold seep was poor and the key processes within the associated chemosynthetic ecosystem remain unclear. Here, we used an in situ integrated sensors detection approach combined with geochemical measurements to investigate the environmental condition around the chemosynthetic ecosystems fueled by the hydrocarbon seepage. Methane, hydrogen sulfide, dissolved oxygen and other biogenic elements in the bottom water were detected on a fine scale. The mixed gas composition and carbon isotope value of methane from the vent fluids suggested its origin was predominantly biogenic with slight thermogenic incorporation. Both horizontal and vertical variations in methane and oxygen concentrations were observed, which showed contrasting trends in fields from the center of flourishing communities to the margin of sediments. The concentration of methane decreased from 31,200 ppm at the flourishing center to 381 ppm at the margin on a horizontal gradient. The low concentration of hydrogen sulfide indicated that sulfide could be an energy-limiting factor for chemoautotrophic symbionts. The in situ oxygen levels were variable near the flourishing community but homogeneous at the margins. The results suggested that the availability of oxygen and methane played a substantial role in structuring the seep communities. Overall, from the flourishing center to the periphery, the chemical gradients drive the spatial distribution of chemosynthetic community at site F. The observations of the fine scale survey highlight the heterogeneity of microenvironments and provide novel information on the environmental dynamics within site F for the first time. • In situ detection of the fine scale environment in the cold seep were conducted. • Variations of methane and oxygen were observed from the center to the periphery. • The low sulfide level could be a limiting factor for the chemosynthetic community.