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

Seep carbonate nodules were firstlycollected from the southwestern Dongsha area onnorthern continental slope of South China Sea formineralogical and geochemical studies. The resultsof X-ray diffraction show that carbonate nodules arecomposed of Fe-rich domolite,siderite and a few cal-cite and aragonite, as well as a small quantity ofnon-carbonate minerals such as goethite, quartz andclay minerals. Fe-rich dolomite and siderite charac-terized by euhedral microcrystalline structure weredirectly precipitated at cold seeps. The δ 13C valuesfor Fe-rich dolomite and siderite in the bulk nodulesvary from ?18.24‰ to ?36.07‰, and the δ 18O valuesrange from 0.42 to 2.76‰. Their moderate depleted13C reflects that carbon origin is possibly thermoge- netic gas or mixed gas, which is evidence of gasventings in the seafloor. Moreover, massive wormtube fossils related to cold seeps were found on thesurfaces of carbonate nodules, and the conduitsand/or channels in semi-solidified nodules whichwere not filled by carbonate or sediment were alsoobserved. A preliminary conclusion is that activemicro gas venting with a conduit/channel diameterrange from 200 μm to 600 μm possibly exists inmodern seafloor of the carbonate nodule area.