CHARACTERISTICS OF SEDIMENTARY GEOCHEMISTRY AND THEIR RESPONSES TO COLD-SEEP ACTIVITIES IN DONGSHA, THE NORTHERN SOUTH CHINA SEA

The northern South China Sea is a typical active cold methane seeping area and the seeping fluids certainly have critical impact on the geochemistry of the sediments. Two gravity cores, 1375-cm-long at Site 973-4 (with water depth of 1,666 m) and 935-cm-long at Site 973-5 (with water depth of 2,998 m) respectively, were recovered from cold methane seeps in the Dongsha area. Total carbon and total sulfur analysis, foraminiferal oxygen and carbon isotope tests of the sediment samples at both sites are studied. According to the total sulfur and their porewater data from adjacent sites, it is deduced that the two sites have shallow sulfur methane interface and high methane fluxes. The depth of sulfur methane interface is 900 cm under the sea bottom at Site 973-4 and 750 cm under the sea bottom at Site 973-5. TOC and TS data demonstrate that cold-seep fluids and their AOMs are the controlling factors of the precipitations of pyrites. Oxygen and carbon isotopes compositions of Uvigerina spp. indicate that the activities of cold seeps in Dongsha area has been attenuated since the last glacier maximum and this might be the results of rising global sea level.