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2013年中国地质调查局广州海洋地质调查局对南海北部实施第二次天然气水合物钻探并成功钻获可视天然气水合物实物样品,共有五个站位获取沉积物岩芯.本文对其中的GMGS2-08站位沉积物开展有孔虫同位素分析研究,以了解天然气水合物地质系统有孔虫的碳同位素特征及其对甲烷释放的响应.GMGS2-08岩芯沉积物中顶空气甲烷浓度最高达到39300μmol L~(-1),气态烃δ~(13)C值在-69.4~-72.3‰PDB,明确指示其为生物成因,根据甲烷δD测试结果(-183~-185‰SMOW)可进一步判别为CO2还原型微生物气.同位素分析结果显示GMGS2-08站位所获得的岩芯中共出现5期有孔虫碳同位素轻值事件,有孔虫δ~(13)C值明显低于南海冰期-间冰期的正常变化区间,底栖Uvigerina peregrina出现-15.85‰PDB的δ~(13)C极轻值,浮游Globigerinoides ruber的δ~(13)C值同样低至-5.68‰PDB.电镜扫描发现δ~(13)C负偏层位有孔虫壳体经历后期成岩改造被自生碳酸盐所充填,有孔虫壳体的成岩矿化程度与埋深并不相关.与有机碳的相关性计算表明有机质的厌氧氧化对底栖有孔虫的δ~(13)C组成影响微弱,意味着有孔虫的δ~(13)C异常负偏可能主要来源于甲烷厌氧氧化成因的次生碳酸盐的叠加影响.GMGS2-08岩芯记录中有孔虫δ~(13)C负异常和δ~(18)O正异常的耦合性是地质历史时期研究区天然气水合物分解释放的重要证据.
In 2013, the China Geological Survey Guangzhou Marine Geological Survey carried out the second natural gas hydrate drilling on the northern part of the South China Sea and successfully obtained physical samples of visible natural gas hydrate, with a total of five stations for sediment cores.In this paper, GMGS2 -08 station sediments to carry out foraminiferal isotope analysis to understand the carbon isotope characteristics of foraminifera in the natural gas hydrate geological system and its response to methane release.GMGS2-08 core sediments in the top of the methane concentration reached the highest 39300 μmol L -1, and δ 13 C values of gaseous hydrocarbons ranged from -69.4 to -72.3 ‰ PDB, indicating that it is a biogenic cause. According to δD test results of methane (-183 ~ -185 ‰ SMOW), it can be further identified Is carbon dioxide-reduced microbial gas.The results of isotope analysis showed that there were five light-burned carbon isotope values in the cores obtained from GMGS2-08 stations, and the δ 13 C values of foraminifera were significantly lower than those from the South China Sea glacial- The δ13C value of -15.85 ‰ PDB was very low in the normal interval of interglacial period, while the δ13C value of Globigerinoides ruber was also as low as -5.68 ‰ PDB. The δ ~ (13) C negative partial layer foraminifera shell experienced late The alteration of rocks was filled with carbonatite, and the diagenetic mineralization of the foraminifera shell was not related to the depth of burial.Correlation with organic carbon indicated that the anaerobic oxidation of organic matter affected δ ~ () 13) C is weak, which implies that the δ 13 C anomaly of foraminifera may be mainly due to the superimposition of secondary carbonates caused by methane anaerobic oxidation. The coupling between δ 13 C negative anomaly and δ 18 O positive anomalies is an important evidence for the decomposition and release of natural gas hydrate in the study area during geologic history.