Discovery of seep carbonate nodules as new evidence for gas venting on the northern continental slope of South China Sea

Seep carbonate nodules were firstly collected from the southwestern Dongsha area on northern continental slope of South China Sea for mineralogical and geochemical studies. The results of X-ray diffraction show that carbonate nodules are composed of Fe-rich domolite, siderite and a few calcite and aragonite, as well as a small quantity of non-carbonate minerals such as goethite, quartz and clay minerals. Fe-rich dolomite and siderite characterized by euhedral microcrystalline structure were directly precipitated at cold seeps. The δ13C values for Fe-rich dolomite and siderite in the bulk nodules vary from 18.24‰ to 36.07‰, and the δ18O values range from 0.42 to 2.76‰. Their moderate depleted 13C reflects that carbon origin is possibly thermogenetic gas or mixed gas, which is evidence of gas ventings in the seafloor. Moreover, massive worm tube fossils related to cold seeps were found on the surfaces of carbonate nodules, and the conduits and/or channels in semi-solidified nodules which were not filled by carbonate or sediment were also observed. A preliminary conclusion is that active micro gas venting with a conduit/channel diameter range from 200 μm to 600 μm possibly exists in modern seafloor of the carbonate nodule area.