OS4A-4:AUTHIGENIC MINERALS IN SHALLOW CORE SEDIMENTS AND THEIR IMPLICATIONS FOR GAS HYDRATE EVOLUTIONS, NORTHERN SOUTH CHINA SEA

Jiasheng WANG1, *, Lushan WU2, Qi LIN3, Lei XIE4, Yuansheng WANG3
1. State Key Laboratory of Biogeology and Environment Geology, China University of Geosciences, CHINA; 2. Guangzhou Marine Geological Survey, CHINA; 3. Faculty of Earth Sciences, China University of Geosciences CHINA; 4. Jianghan Oil Production Plant of Jianghan Oilfied Company, SINOPEC, CHINA

    A total of 11 shallow cored sediments from northern South China Sea including 1 cores in XiSha, 6 cores in Shenhu and 4 cores in Dongsha area are researched to focus on the authigemic mineral types, distributions and the stable C, O, S isotopes with the depth. Pyritization is observed widely in all researched shallow cored sediments, most of pyrites occur in the forms of framboidal, rod, dumbbell, plate, foraminifera-filled and un-regular aggregations. The crystals of pyrites in aggregations are mainly octahedral. The stable sulfur isotopes of pyrites show a strong negative value low to -50‰ CDT, indicating their precipitation closely relative to the sulfate bacterial reduction and/or the sulfur bacterial disproportionation. The authigenic carbonates could be identified by the binocular microscope and SEM, they occur mainly in the forms of rod, plate, ball and un-regular concretions. The crystals of carbonates seem mainly in rhombus features. The stable carbon isotopes of carbonates show no extreme negative values, the minimal down to about -11‰ PDB, however a good negative relationship between the stable carbon and oxygen isotopes of carbonate could be figured out. Some radio-form gypsums are distributed in sediments as the authigenic minerals mainly accompanied with the pyrites on the surface of pyrite aggregations and/or within the cut-profiles of rod pyrite aggregations, which indicates that the gypsum precipitated most likely with the pyrites in situ. Moreover, the observed above authigenic minerals are distributed in varied abundance and isotopic compositions with the depth, especially in Shenhu and Dongsha area. Compared to the reported authigenic minerals in cored sediments from IODP 311 Exploration in Cascadia margin, the types and morphology of authigenic minerals and sulfur isotopes of pyrites are much similar. Taking to the lithology, texture and structure of our researched sediments in northern South China Sea, as well as the reported hydrate-bearing cored sediments from Shenhu area in 2007, we propose that the authigenic minerals in shallow cored sediments could also be used as one of the best proxies to indicate the complex geobiological processes of microbe-mediated sulfate reduction and anaerobic oxidation of organic matters or methane during the evolution of marine gas hydrate geo-system.