新仙女木末期南海北部天然气水合物分解事件

A Rapid Gas Hydrate Dissociation in the Northern South China Sea since the Late Younger Dryas

深海天然气水合物分解与全球变暖密切相关．南海北部是重要的天然气水合物蕴藏区，ZHS-176、ZHS-174、17940和MD2905孔CaCO3含量记录均表明，11．3～8．0ka B．P．神狐海域存在一次典型的“低钙事件”（CM），该事件具有“快速降低、缓慢升高”不对称的变化结构，CaCO3含量降幅高达9％．“低钙事件”期间，底栖有孔虫Cibicidoides wuellerstorfi和Cibicidoides kullenbergi壳体占”C分别负偏了1．4‰和0．7‰，海底有机碳的堆积速率（MAR）也突然升高了1倍．综合分析表明，新仙女木末期南海北部天然气水合物很可能发生了一次较大规模的快速分解，大量甲烷气体从天然气水合物中逸散，氧化后使底层海水快速酸化，从而导致了神狐海域碳酸盐的溶解．底层水团温度上升很可能是神狐海域天然气水合物分解的主要触发因素．

Gas hydrate in the deep sea is closely related to the global warming. One of the most important gas hydrate stability zones （GHSZ） is located in the Shenhu of the northern South China Sea （SCS）. All records of carbonate content in cores ZHS- 176, ZHS-174, 17940 and MD2905 reveal a carbonate minimum （CM） from 11.3-8. 0 ka B. P. , which is characterized with an asymmetric pattern of a rapid decrease of 9 % value followed by a gradual recovery. The benthic foraminifer δ^13C levels in the shells of Cibicidoides wuellerstorfi and Cibicidoides kullenbergi are depleted by 1.4‰ and 0. 7‰, respectively, during the CM period. Meanwhile, the mass accumulation rate （MAR） of the organisms suddenly increased nearly twofold on the seabed. These findings indicate a likely release of a large amount of methane from the gas hydrates since the late Younger Dryas （YD）. Oxidation and absorption of the methane should have lowered pH of the bottom seawater, thereby triggering a shoaling of the carbonate lysocline. Temperature increasing of the bottom seawater in the northern SCS provides a possibility to induce gas hydrates dissociation.