南海神狐海域自生黄铁矿分布、形貌特征及其对甲烷渗漏的指示

自生黄铁矿是渗漏甲烷发生甲烷厌氧氧化和硫酸盐还原作用的产物之一，是海底甲烷渗漏活动的有效示踪剂。南海神狐海域是我国天然气水合物研究的重点区域，对神狐海域柱状沉积物中自生黄铁矿含量、分布、形貌等特征进行研究，结果发现自生黄铁矿的含量随深度增加而递增，存在两个异常富集峰段：在第一个黄铁矿富集峰段，黄铁矿以长条状为主，外形较粗，微晶形态以草莓状黄铁矿为主，且粒径均一；在第二个峰段主要以细长的条状黄铁矿为主，由带外壳结构的草莓球颗粒组成，晶粒大小不一，存在二次生长现象。另外还发现胶黄铁矿与自生黄铁矿共生。这些特征反映神狐海域沉积物中存在多期次的甲烷渗漏事件，高通量的甲烷渗漏发生在较浅的层位，可能发生甲烷的有氧氧化而变成缺氧环境，有利于黄铁矿富集在第一个峰段；较低的甲烷渗漏发生在较深的层位，甲烷厌氧氧化和硫酸盐还原作用而形成大量的黄铁矿保存在第二个峰段。因此，神狐海域沉积物中自生黄铁矿的异常富集可以作为地质历史时期甲烷渗漏通量和期次的指示矿物之一。

CROSS-SECTION DISTRIBUTION AND MORPHOLOGY OF AUTHIGENIC PYRITE AND THEIR INDICATION TO METHANE SEEPS IN SHENHU AREAS,SOUTH CHINA SEA

Sulfate reduction associated to anaerobic oxidation of methane (AOM-SR) is considered an important process of methane consumption in anoxic marine environments. This process results in the enrichment of authigenic pyrite, which may provide important information to active methane seepages. The Shenhu is one of the favorable areas for gas hydrate accumulation. We studied the content, distribution and morphology of the authigenic pyrite from two cores of sediments collected from the Shenhu area. Results show that the content of pyrite increases with depth and has two peaks. The pyrite in the shallow sediment is irregular tubelike and consists of pyrite framboids in similar size. In deep sediments, however, the pyrite occurs as straight tubes and consisting of framboidal cores and outer crusts. Microcrystals vary in different size. In addition, it is found that greigite always coexists with pyrite. Our results indicate a kind of temporal variation of methane flux occurs in the Shenhu sediment: relatively high methane flux occurs at the depth of shallow pyrite-rich peak, and the aerobic oxidation of methane influences bottom water redox conditions in the sea and promotes the precipitation of pyrite. Lower methane flux occurs in the deeper layers of sediments, AOM-SR is the main process to the precipitation of pyrite in the deep sediment. Overall, our results suggest that the anomalous enrichment of pyrite in sediment may be used as an indicator to methane seepage events in marine sediments.