Sedimentary Geochemical Response to Gas Hydrate Episodic Release on the Northeastern Slope of the South China Sea

The resource of gas hydrate in South China Sea is plentiful. It redound to resource development by investigating how the gas hydrate decompose. A sedimentology and geochemistry study has been done in gas-hydrate-bearing borehole sediment collected at site GMGS08 within the gas hydrate drilling area in the northeastern South China Sea. Results show that 11 layers of authigenic carbonate and bivalve bioclast, and 2 layers of carbonate limestone are distributed from top to bottom in hole GMGS08. Authigenic carbonate and bivalve bioclast primarily occurred in soupy structure sediments with high water content. The δ¹³C values of authigenic carbonate ranged from -41.36×10^-3 to -56.74×10^-3 except one sample with a higher δ¹³C value of -38.85×10^-3, and the δ¹⁸O values were apparently heavy ranging from 2.94×10^-3 to 5.37×10^-3, suggesting they were attributed to gas hydrate dissociation via microbial anaerobic oxidation of methane, which was predominantly biogenic gas. The δ¹³C values of organic carbon varied from -21.33×10^-3 to -82.44×10^-3, in which the δ¹³C negative excursion intervals corresponded to authigenic carbonate layers. According to the distribution of authigenic carbonate, it is concluded that at least 6 episodes of gas hydrate release existed in geologic record, and the varying sizes of authigenic carbonate among the layers indicated the intensity of methane flux in each episode was different.