海底天然气渗漏系统微生物作用及冷泉碳酸盐岩的特征

Characteristics of Cold Seep Carbonates and Microbial Processes in Gas Seep System

海底天然气渗漏系统是全球海洋环境中广泛分布的自然现象。部分渗漏天然气通过细菌作用转变为二氧化碳,同时海水硫酸盐被还原为硫化氢, 与孔隙水中的钙和铁结合而沉淀冷泉碳酸盐岩。冷泉碳酸盐岩的常见矿物有微晶方解石、文石、白云石和黄铁矿。冷泉碳酸盐岩常发育一些特殊的组构, 如黄铁矿环带结核、溶蚀面、平底晶洞、凝块和向下生长的叠层石组构等。碳酸盐岩特别负的δ13C值指示碳来源于生物成因的甲烷, 而18O富集可能与天然气水合物的分解有关。冷泉碳酸盐岩中黄铁矿的δ34S值低于海水的, 这指示硫来源于微生物还原的海水硫。冷泉碳酸盐岩中的生物标志化合物及其极负的δ13C值指示微生物的生命代谢活动。

Gas seep system is a common phenomenon on the seafloor in the continental shelf and slope. Venting gas is converted partially into CO_2 and H_2S by methane oxidizing archaea and sulfate reducing bacteria in gas seep sites,inducing seep carbonates precipitation. The most common minerals in seep carbonates are microcrystalline calcite, microcrystalline aragonite and dolomite, and pyrite as well. Cold seep carbonates usually (occur) as chermoherm, nodule, chimney, hardground, cement and fine vein. Pyrite rim carbonate nodules, (corrosion) surfaces, inverted stromatactoid cavities, clotted fabrics,and stromatolitic are the most common sedimentary fabrics in seep carbonates. Gas seep carbonates generally are typified by low δ^(13)C values, as they inherit the stable isotope signature from their carbon source, ^(13)C-depleted methane. Due to the enrichment of ^(18)O in gas (hydrates), anomalously positive δ^(18)O values in seep carbonates could argue in favour of gas hydrate destabilization at gas seep sites. The pyrite enclosed in the seep carbonates generally shows low ^(34)S values. The ^(13)C (depleted-lipid) biomarkers in cold seep carbonates are believed to reflect in-situ anabolic activity of methane oxidizing archaea and sulfate reducing bacteria.