A Rapid Gas Hydrate Dissociation in the Northern South China Sea since the Late Younger Dryas
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摘要: 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，11.3~8.0 ka 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δ13C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.Abstract: 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 δ13C 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.
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Key words:
- global warming /
- the South China Sea /
- Younger Dryas /
- gas hydrate /
- seawater
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图 1 南海北部地形及站位
三角形代表本研究实测站位，十字符代表参考站位，条带状阴影指示珠江口海底峡谷
Fig. 1. Schematic geographical map of the northern SCS and the location of the cores
图 2 南海周边“低钙事件”对比
箭头代表AMS14C测年点，ZHS-176孔AMS14C测年数据葛倩等(2008), 17940引自Jian et al.(1999), MD2905杨文光等(2008), MD2142引自Chen et al.(2003), SA12-19李学杰和江茂生(2003), MD2151引自Zhao et al.(2006)；灰色框指示“低钙事件”
Fig. 2. Comparison of the CMs around the SCS
图 3 神狐海域“低钙事件”剖面
箭头及数据指示B64孔AMS14C测年层位及年代; 南海17940孔CaCO3含量引自Wang et al.(1999)，其他各孔数据为本次分析结果；灰色框指示“低钙事件”
Fig. 3. The section of the CM in the Shenhu area
图 4 神狐海域“低钙事件”期间各种环境指标的变化特征
1266C孔CaCO3含量和δ13C引自Zachos et al.(2005); 南海17940孔CaCO3含量、底栖有孔虫C.wuellerstorfi和C.kullerbergi壳体δ13C引自Wang et al.(1999); 17940孔Corg MAR引自Jian et al.(1999); 海平面变化及上升速率数据引自Hanebuth et al.(2000); 湖光岩玛珥湖Ti元素含量和太阳辐射量引自Yancheva et al.(2007)
Fig. 4. Comparison of the different environmental proxies against the CM
表 1 主要沉积物柱状样站位信息
Table 1. Location of the studied cores
钻孔 经度E 纬度N 水深(m) 17940 117°23.0′ 20°07.0′ 1 727 MD2905 117°21.6′ 20°08.2′ 1 647 MD2142 119°27.9′ 12°41.1′ 1 557 MD2151 109°52.2′ 8°43.7′ 1 598 SA12-19 110°25.4′ 17°09.2′ 1 300 ZHS-176 115°33.3′ 20°00.0′ 1 383 ZHS-174 115°30.2′ 20°10.2′ 640 B64 112°28.8′ 21°33.1′ 19 注：17940引自 Jian et al.(1999) 和Wang et al.(1999) ，MD2905杨文光等(2008)，MD2142引自Chen et al.(2003) ，MD2151引自Huang et al.(1999) 和Zhao et al.(2006) ，SA12-19李学杰和江茂生(2003)，ZHS-176、ZHS-174和B64为本次研究站位.
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