南海东沙东北部甲烷缺氧氧化作用的生物标志化合物及其碳同位素组成

Biomarkers and carbon isotope composition of anaerobic oxidation of methane in sediments and carbonates of northeastern part of Dongsha,South China Sea

南海东沙东北部碳酸盐岩和泥质沉积物中的生物标志化合物组合及其碳同位素组成分析表明,研究区内甲烷缺氧氧化作用(anaerobic oxidation of methane-AOM)发育。研究区内碳酸盐岩中含丰富的AOM标志化合物,2,6,11,15-四甲基十六烷(Crocetane-Cr.)、2,6,10,15,19-五甲基番茄烷(Pentamethylicosane-PMI)和2,6,10,15,19,23-六甲基二十四烷(Squalane-Sq角鲨烷)的13C亏损强烈(δ13C值介于-74.2‰^-119.0‰PDB之间),表明碳酸盐岩形成于AOM,同时反映该研究区曾发生过强烈、持续的富CH4流体释放活动。柱状泥质沉积物中,AOM生物标志化合物在硫酸岩-甲烷过渡带(SMI—Sulfate-Methane Interface)边界附近相对丰度高,SMI之上样品中含量低,或未检出,表明现代环境在SMI附近有大量嗜甲烷微生物生长,使得深部上升的甲烷被大量消耗,很少有甲烷逸出海底。AOM生物标志化合物可用来指示SMI边界。不同站位、不同岩性AOM生物标志化合物组成(包括碳同位素组成)的差异反映了嗜甲烷古细菌组成的不同。

Sediments and authigenic carbonates from the Dongsha region of the South China Sea show abun- dant AOM--related biomarkers （C20 Crocetane, C25 Pentamethylicosane, C30 Squalane） with highly depleted δ13C （δ13C= -74.2‰ to -119.0‰ PDB） signatures. These data identify the authigenic carbonates as microbially-mediated methane--derived precipitates. Furthermore, the distribution of AOM--biomarkers in sediments indicates that the modern AOM--process occurs around 600 cm below seafloor, coincident with the sulfate--methane--interface （SMI）. Above the SMI, the content of AOM--related biomarkers is either very low or undetectable. This implies that abundant methane consuming microbes thrieve just at the SMI and that methane becomes largely exhausted by methanotrophy before it reaches the seafloor. It is suggested that AOM--biomarkers serve as indicator of SMI. We conclude that vigorous methane--rich fluid expulsion has occurred at the seafloor when the carbonates formed, while the fluid activity within the sediment is rather weak.