南海北部东沙海域天然气水合物分解事件及其与海底滑塌的关系

东沙海域是南海北部一个重要的天然气水合物成藏区, 其陆坡广泛发育滑塌构造。文章对采自东沙陆坡中部973-4柱样和下部平坦区973-5柱样开展了沉积学粒度、底栖有孔虫种属特征和稳定同位素等的综合分析。研究结果表明: 两个柱样中底栖有孔虫的δ¹³C在末次冰期均出现明显负偏现象, 同时δ¹⁸O增高, 指示该时期东沙海域存在持续的天然气水合物分解事件; 末次冰消期以来, δ¹³C负偏现象逐渐消失, δ¹⁸O值降低, 可能是由于海平面上升阻止了天然气水合物分解。973-4柱样仅在末次盛冰期对应层位440~600cm段存在明显的滑塌沉积, 且该层段对应的特征底栖有孔虫Uvigerina spp.和Bulimina spp.的数量突增, 推测该区的海底滑塌可能是由于末次盛冰期海平面大幅度下降引起天然气水合物大量分解诱发所致; 973-5柱样同样记录到了海底滑塌现象, 但其滑塌沉积晚于973-4柱样的滑塌时间, 且其规模较小。

Gas hydrate dissociation event and its relationship with submarine slide in Dongsha Area of northern South China Sea

Dongsha area is a critical gas hydrate area of the northern South China Sea, and a submarine slide is extensively developed on its slope. In this paper, we analyzed the sedimentological particle size, species characteristic and stable isotope of benthic foraminifera in the cores 973-4 and 973-5, respectively, recovered from the middle of the slope and the flat area at the base. The results of the distinctly negative δ¹³C values and the heavier δ¹⁸O values in both cores in the Last Glacial period suggested that there were persistent gas hydrate dissociation events in the Dongsha area during this period. The δ¹³C negative gradually disappeared and the δ¹⁸O values decreased since the Last Deglacial period, indicating that gas hydrate dissociation was prevented because of global sea level rise. In the core 974, obvious submarine slide deposits only occurred in 440~600 cm according to the Last Glacial Maximum and the number of Uvigerina spp. and Bulimina spp. sharply increased in this location, which implied the submarine slide was probably caused by an intense methane release event induced by the descending sea level in the Last Glacial Maximum period. A smaller scale submarine slide was also recorded in core 973-5, but the deposition time was later than that of core 973-4.