PORE WATER GEOCHEMISTRY IN SHALLOW SEDIMENTS FROM SOUTHEASTERN SHENHU AREA OF NORTHERN SOUTH CHINA SEA AND THEIR IMPLICATIONS FOR GAS HYDRATE OCCURRENCE

Proper conditions occur in the southeastern Shenhu area of northern South China Sea for gas hydrate accumulation.The geophysical signature of gas hydrate, i.e.the bottom simulating reflector (BSR), has been discovered.Site SH-CL13, SH-CL16 and SH-CL17 are within the area of BSR.Cl^-concentrations and δD values of pore water samples from gravity-piston cores of SH-CL16 and SH-CL17 significantly decrease and increase with depth, respectively, which suggests gas hydrate accumulation underneath.The contents of methane in headspace gas samples from the three sites vary between 10.0 and 37.7 μL/kg, indicating very low methane fluxes in shallow sediments.The depths of sulfate-methane interface (SMI) are between 28.5 and 39.5 m, and sulfate (or methane) fluxes change between 3.6 and 5.9 mmol·m^-2·a^-1.As we know, the depth of SMI and sulfate fluxes are constrained by methane fluxes.The trend of SO₄^2- variations and δ¹³C_DIC values of pore water indicates that consumption of sulfate is dominated by organoclastic sulfate reduction (OSR) in shallow sediments and anaerobic oxidation of methane (AOM) occurs in relatively deep layers.In summary, the three sites are within the potential areas of gas hydrate occurrence.