Evolution of gas hydrate stability zone in the deep water of Dongsha sea area since the Last Glaciation Maximum

The evolutionary history of the gas hydrate stability zone (GHSZ) in the Dongsha deep water area since the last glacial maximum (LGM) is simulated and predicted using the CSMHYD program, and the fluctuations of sea level and bottom water temperature and their effects on the thickness of gas hydrate stability zone as well as the effects of hydrate decomposition on environment are carefully investigated and discussed. The results show that: (1) Gas hydrate could form in the sea area at a water depth more than 595 m; the current theoretical thickness of GHSZ is 245 m on average, and the maximum could be over 380 m which is located in the eastern part of the study area. Another area with large thickness is found at the juncture of the Dongsha continental slope and the Taiwan shoal continental slope. (2) The thickness of GHSZ in the Dongsha sea area has changed in an asymmetrical pattern since the LGM. It can be divided chronologically into five complete cycles, named TC1, TC2, TC3, TC4 and TC5 respectively. The thinning half-cycles are longer in time than those of the thickening ones. The thickness of GHSZ in cycles of TC1-TC4 is controlled by sea level fluctuation, while the thickness of cycle TC5 mainly controlled by sea bottom temperature. (3) The seabed temperature and sea level changes bring stronger effects on GHSZ thickness in the intermediate water area rather than in the deep water area. Meanwhile, the pressure effect is relatively obvious in the intermediate water depth area. The influence of sea level variation on GHSZ in the deep water area is limited. The abnormal decrease in CaCO₃ content in the Dongsha sea area is possibly due to the acidification caused by the methane released from gas hydrates dissociation.