Multi-azimuth exploration of the physicochemical environment around a cold seep in the South China Sea- from sparse to luxuriant

The leakage of cold, methane-rich fluids from subsurface reservoirs to the seafloor at specifc sites on continental slopes, termed cold seeps, sustains some of the richest ecosystems on the sea bed. Here, we used an integrated sensors detection approach to investigate the seafloor environmental condition of the chemosynthetic communities fueled by a SE-NW oriented cold seep in the South China Sea (SCS) in 2017 with Remotely Operated Vehicle (ROV) named Faxian. Methane from the vent fluids has a carbon isotope value about -70‰, suggesting its biogenic dominant origin. However, the small amount of ethane and propane indicated slight thermogenic incorporation. Methane levels varied both within and among assemblages, but were generally the highest in mussel beds. Vertical gradients in methane measured in all assemblages, show a decrease in methane from the bottom to the top of individuals. Horizontal gradients were also observed, with methane decreasing from the centre to the margin of mussel clusters (ranging from 10⁵ ppm to 3000ppm). Methane concentrations were higher above living mussel clusters (vent fluid 10⁵ ppm) in the centre of the cold seep than in external areas. In all, the methane concentration was more variable in the vicinity of the chemosynthetic communities. Hydrogen sulphide concentrations in seawater were very low or undetectable. No significant anomalies or variations in carbon dioxide and pH were detected. Oxygen level ranges homogeneous around 3.30 mg/L at the short distance about 140 cm above from the mussel clusters on the seafloor. When the distance dived to 70 cm from the mussel bed, the oxygen level was depleted at about 3.01mg/L. Although the anaerobic oxidation may dominate methane consumption in the sea bed, most methane reaching the hydrosphere will be consumed by aerobic oxidation in the water column, or transported into the upper mixed layer of the ocean, eventually reaching the atmosphere.