Abstract:
Cold fluids and authigenic carbonate precipitates on the seafloor are closely associated with gas hydrate dissociation, thus the identification and study of authigenic carbonates and cold fluids can be key in the investigation for gas hydrates and seep communities. In this paper a comprehensive comparison has been performed on the lithology, chemical composition, isotope geochemistry and evaluation of seep carbonate precipitates collected from the Dongsha Islands area, Shenhu area, and the deep-sea core from ODP Site 1146, Leg 184, South China Sea. Those seep precipitates are different in lithology and in their stable isotopes of carbon and oxygen, possibly reflecting different diagenetic environments. Chemical compositions of Fe, Al (sub 2) O (sub 3) , TiO (sub 2) , K (sub 2) O, Na (sub 2) O, MnO and P (sub 2) O (sub 5) are different in seep precipitates from the Dongsha Islands area and Shenhu area, implying that they were possibly influenced by clay minerals and micro Fe-, Mn-nodules or crusts. Cold fluid activity occurred once in the Shenhu area, forming during early and later stages of seep precipitates. At least three periods of cold fluid activity and several stages of carbonate precipitation were identified in the Dongsha Islands area, reflecting differences in the carbon sources and tectonics systems of two areas. It is concluded that gas hydrates could exist in the area of southern Dongsha Island, which was associated with thermogenic methane from deep sediments, and seep communities could also occur on that seafloor. This is a new avenue for the investigation of gas hydrates and seep communities in the southwestern Dongsha area.