西沙海槽晚更新世冰期以来原地微生物成因水合物储库的变化

利用基于热力学理论的CSMHYD程序,计算现今及晚更新世冰期琼东南盆地西沙海槽天然气水合物的稳定带厚度及资源量,讨论晚更新世冰期以来海平面、底水温度和沉积速率变化对西沙海槽天然气水合物储库变化的影响。结果表明：①研究区水深超过600 m的海域具备天然气水合物赋存的温压条件。天然气水合物稳定带最大厚度约300 m,位于研究区的中东部和东南部。②晚更新世冰期以来,底水温度的升高抵消了海平面上升对天然气水合物稳定性的影响。研究区冰期和间冰期旋回中沉积速率发生显著变化,沉积速率大的区域,原地微生物成因天然气水合物的浓度也相应较大;导致冰期-间冰期旋回过程中原地微生物成因水合物储库变化的关键因素是甲烷供给及沉积速率,而不是温压条件的变化。③西沙海槽天然气水合物现今的储库比晚更新世冰期的减少了0.78 × 10¹² m³的甲烷气。

Change of In Situ Microbial Gas Hydrate Reservoir in Xisha Trough since the Last Pleistocene Glaciation

Sloan’s CSMHYD program was used to calculate the thickness of the gas hydrate stable zone (GHSZ) and the amount of gas hydrate in the Xisha Trough at present and at the last Pleistocene Glaciation, respectively. The effects of the changes in the bottom water temperature and the sea level on GHSZ also discussed. Results showed that the temperature and pressure conditions for gas hydrate formation were available in the Xisha Trough of Qiongdongnan Basin, where water depth is more than 600 m. The thickest GHSZ was located in the east of the study area, with a maximum thickness of more than 300 m. The thickness of GHSZ decreased with the bottom water temperature increase and increased with the sea level increase, wherein the effect of the former was stronger than that of the latter. In the study area, the deposition rate changed significantly during the glacial and interglacial cycles, which changed the concentration of in-situ microbial hydrates. The concentration of methane hydrate formed by in-situ microbial hydrates was also higher in the area with high deposition rate. 0.78 × 10¹² m³ of methane is released from gas hydrate since the Last Pleistocene Glaciation. The released methane should have greatly affected the environment, such as inducing submarine landslides.