南海北部天然气水合物远景区末次冰期以来底栖有孔虫稳定同位素特征及其影响因素

STABLE ISOTOPIC CHARACTERISTICS AND THEIR INFLUENCING FACTORS OF BENTHIC FORAMINIFERA IN THE PROSPECTIVE GAS HYDRATE AREA FROM THE NORTHERN SOUTH CHINA SEA SINCE THE LAST GLACIAL

底栖有孔虫碳同位素负偏移是地质记录中天然气水合物释放的重要证据之一。对南海北部西沙海槽和东沙陆坡等天然气水合物远景区XH-27PC和DS-4PC柱状样分别进行顶空气甲烷含量分析、有机碳含量分析、粒度分析和有孔虫氧碳同位素分析。结合碳酸盐含量及AMS 14C测年，揭示研究区末次冰期以来底栖有孔虫的稳定同位素特征。结果显示：西沙海槽BSR区沉积物中甲烷含量较低；底栖有孔虫碳同位素负偏不明显，与顶空气甲烷含量呈弱正相关（R=0．32），与有机碳含量有强负相关（R=-0．82），说明低通量甲烷不足以引起底栖有孔虫碳同位素显著偏移，在无甲烷或甲烷轻微渗漏的环境中有机碳的厌氧氧化是影响底栖有孔虫碳同位素组成的主要因素。东沙陆坡BSR区沉积物中含有大量的甲烷气体；底栖有孔虫氧同位素记录在末次冰期异常偏重，可能与天然气水合物的分解释放有关；同时可识别出多期碳同位素快速负偏事件，其成因很可能是末次冰期海平面下降导致海底沉积物的温度、压力条件发生变化，从而引发水合物甲烷失稳分解，底栖有孔虫吸收富。℃的甲烷源碳致使壳体碳同位素负偏移。

The South China Sea （SCS） is the largest marginal sea in the West Pacific, located at the junction of three plates, the Eurasian, the Pacific and the Indo-Australian plate. Since the Mesozoic, the northern continental margin of the SCS had gone through a succession of geotectonic regimes and formed hydrocarbon-bearing basins such as the Pearl River Mouth, the Yinggehai, the Qiongdongnan and the Taixinan basin, where gas and oil resources are abundant. According to the recent GMGS gas-hydrate drilling programs in the anomalous bottom simulating reflector （BSR） areas, the northern SCS marginal sediments bear large abundances of methane hydrate. Carbon isotope negative excursions of benthic foraminifera have been used as indicators for periodic release of gas hydrate in the geologic records. Here we present methane and organic carbon content, sediment grain, oxygen and carbon isotopic signatures of planktonic and benthic foraminifera from core XH-27PC （water depth 782m; length 8.05m） in the Xisha Trough and core DS-4PC （water depth 166; length 13.75m） on the Dongsha slope, within the prospective gas hydrate areas of the northern SCS. The analytical results, combined with carbonate content and AMS 14C dating data, are used in this paper to reveal stable isotopic characteristics of the study areas since the Last Glacial. In the Xisha BSR area, the methane content in the shallow sediment is quite scarce, and the δ13C depletion of infaunal Uvigerina peregrina is not apparent. It shows a weak positive correlation between δ13Cand CH4 content （R = 0.32） and a strong negative correlation between δ13Cand organic carbon content （R =-0. 82）, suggesting low CH4 current is not capable of inducing δ13Cdepletion of benthic foraminiferal tests, the anaerobic oxidation of organic matter is the predominant influence on the carbon isotopic composition of benthic foraminifera in the slight- or non-seep environment. However, in the Dongsha BSR area, the CI-I4 content in the shallow sediment is extremely high （up to over 8000μL/kg）. In comparison with core XH-27PC, the δ18Ovalues of infaunal Uvigerina hispida in core DS-4PC were 0.55%o higher on average during MIS 1, whereas the mean variations rose to 0.6%o during MIS 2. Thus the anomalous δ18Oenriched records of core Ds-gPC during the Last Glacial were likely related to ancient gas hydrate release. Moreover, several remarkable δ13Cdepleted events have been recognized in core DS-dPC during MIS 2 and 3 stages. The decreases in δ13Cwere -0.25‰ to -0.42%o, and lasted for less than 1000 years in general. The minimum value was -2.05 PDB at the depth 13.23m, lower than the normal variability range of glacial- interglacial cycles in the SCS. Consequently, it was probably attributed to sea-level fall or lowstands, which altered the temperature and pressure condition at or below the sediment-water interface during the Last Glacial, and that led to the destabilization and dissociation of methane hydrates. Benthic foraminifera incorporated the isotopically light carbon derived from anaerobic oxidation of methane into their calcitic tests via dissolved inorganic carbon pool, resulting in δ13C negative excursion.