珠江口盆地东部陆坡末次冰期天然气水合物-冷泉活动的记录与时间

Records and timing of gas hydrate and cold seep activity during the Last Glacial Period from the east of the Pearl River Mouth Basin,South China Sea

对珠江口盆地东部陆坡GMGS 08C孔的3层自生碳酸盐岩(结核状)和有孔虫(底栖和浮游)开展高分辨率氧碳同位素分析和AMS^(14) C测年,第1层自生碳酸盐岩δ^(13) C为-38.85‰,第2、3层的δ^(13) C介于-41.36‰^-56.74‰,平均值-51.64‰,均低于-40‰。δ^(18) O值介于2.94‰~4.18‰,平均值3.68‰,明显偏重。这表明其为天然气水合物分解的产物,形成于微生物对甲烷的缺氧氧化作用,甲烷主要源自生物成因;自生碳酸盐岩层位的底栖有孔虫Uvigerina peregrina和浮游有孔虫Globigerinoides ruber的δ^(13) C值几乎同时出现极负偏,最低值分别达到-12.17‰和-9.06‰,有孔虫δ^(18) O总体上正偏,最大正偏幅度达1.50‰,说明其负偏与后期源于甲烷厌氧氧化作用(Anaerobic Oxidation of Methane,简称AOM)的成岩作用有关。自生碳酸盐岩及其与有孔虫异常碳(偏负)氧(偏正)、粥状沉积是GMGS 08C孔水合物分解甲烷渗漏的重要指标,据此推断该孔在末次冰期(MIS2—MIS3)发生过至少2次天然气水合物分解释放-冷泉甲烷渗漏活动,分别距今36ka和31~26ka,主要发生在低海平面期,说明冷泉活动明显受海平面变化影响。

Thirteen sites（including site GMGS2-08）were drilled during China＇s second gas hydrate drilling expedition（GMGS-2）in the east of the Pearl River Mouth Basin,South China Sea.Disseminated and massive gas hydrate deposits were found at site GMGS2-08.Hole GMGS 08C（water depth 798.7m,length 22.11 m and hole take rate 78.9%）is one of the 5holes at site GMGS2-08.Three authigenic carbonate layers and one soupy structure sediment layer were discovered in hole GMGS 08 C.Carbonate rocks occurred as nodules.Carbon and oxygen isotopic compositions of authigenic carbonates and foraminifera（planktonic and benthic）were analyzed.The δ^13C value of the first carbonate layer is-38.85‰,and the δ^13C values of the second and third ones vary between-41.36‰ and-56.74‰ with an average of-51.64‰（n=20）.The δ^18O values of these samples range from 2.94‰to 4.18‰ with an average of 3.68‰（n=20）.The extremely negative δ^13C values and δ^18O-enriched oxygen sources are related to the dissociation of gas hydrate,indicating biogenic methane is the main carbon source in hole GMGS 08 C.Meanwhile,the carbonate layer sediments document negative carbon isotopic excursions in planktonic and benthic foraminifera,the extremely negative values are-12.17‰ and-9.06‰,respectively.It suggests that these excursions are associated with anaerobic oxidation of methane（AOM）as the postdepositional authigenic carbonates from AOM filled in the foraminiferal shells.Therefore,the authigenic carbonates and their extremely negative δ^13C values and δ^18O-enriched oxygen,along with carbon isotope negative excursions of planktonic and benthic foraminifera and soupy sediment in hole GMGS 08 C,are indicators of release of gas hydrate during MIS2-MIS3.We infer that there were at least 2methane release events during the last glacial base on AMS^14）C dating of carbonates from hole GMGS 08 C.Our results show that the first methane release event occurred at 36 ka,and the second one occurred between 31 and 26ka while sea level was low.It suggests that cold seep activity and methane release events are obviously influenced by sea level change.