海洋沉积物孔隙水硫酸盐浓度和碳同位素对天然气水合物的指示

Sulfate Concentrations and Carbon Isotopic Compositions as Indicators of Marine Gas Hydrates

硫酸根离子（SO4^2-）是海洋沉积物孔隙水中的重要组分之一。硫酸盐还原菌利用孔隙水中SO4^2-作为氧化剂氧化沉积物中有机质或甲烷，造成孔隙水中SO4^2-离子浓度降低，同时使溶解在孔隙水中CO2的碳同位素组成降低。研究表明，在有天然气水合物出现的地区，强烈的甲烷缺氧氧化作用使孔隙水SO4^2-浓度急剧下降，表现为海底沉积物中硫酸盐-甲烷界面（SMI）较浅。如布莱克海台区，SMI界面为5．1～23．9m，界面附近溶解于孔隙水中CO2的δ^13C值低达-39％。笔者发现南海北部海区几个站位具有类似于布莱克海台区的较浅的SMI界面（7．5～17．2m）和极低的δ^13C值（-29％），结合其他地质、地球物理和地球化学证据，推测这些站位处可能赋存有天然气水合物，值得开展进一步详查工作。

Being an important constituent in pore water of marine sediments, sulfate （SO4^2-） acts as an oxidation agent involved in organic matter or methane oxidation reactions induced by microbial activity. These chemical reactions deplete sulfate within pore waters of the sulfate reduction zone, and also decrease carbon isotopic compositions of dissolved CO2. It is demonstrated that in gas hydrate areas such as the Blake Ridge, the strong anaerobic methane oxidation reaction produces linear and steeper sulfate gradients with shallow SMI depths. In the Blake Ridge, the SMI depths are between 5.1 m and 23.9 m, with δ^13C values as negative as - 39‰. It is found that several sites in the South China Sea also show shallow SM1 depths （7.5 m- 17.2 m） and low δ^13C values （ - 29‰）, similar to characteristics of the Blake Ridge. Combined with other geological, geophysical and geochemical evidence, it is considered that these sites may have great potential for gas hydrate accumulation at depths and therefore deserve further detailed exploration.