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.