海洋沉积物中早期成岩作用地球化学研究进展

GEOCHEMISTRY OF EARLY DIAGENESIS IN MARINE SEDIMENTS:RESEARCH PROGRESS

海洋沉积物中早期成岩作用过程以有机质降解为基础。按照反应自由能大小,有机质依次被O2、NO-3、Mn4+、Fe3+、SO2-4氧化,在产甲烷菌作用下发生产甲烷作用,形成大量生物成因甲烷,这是海洋沉积物中甲烷主要来源之一。甲烷气体在向上运移过程中既可在水合物稳定带中与水分子结合形成水合物,游离态和溶解甲烷也可以在硫酸盐还原菌和甲烷菌联合作用下发生甲烷厌氧氧化作用,并与沉积物中的Ca2+、Fe2+结合,形成碳酸盐岩和黄铁矿等自生矿物。这一系列过程埋藏了大量甲烷(碳),成为重要的碳汇。早期成岩作用的各种反应机制,特别是沉积环境因素的影响及其产生的阳离子同位素分馏仍是未来的重点研究方向。通过大量的文献调研,概括了早期成岩作用过程中有机质降解及甲烷厌氧氧化作用的反应机理,黄铁矿、碳酸盐岩等自生矿物的形成机理以及同位素分馏特征等地球化学研究进展,并提出了未来重点研究的方向的建议。

The research progress of the reaction mechanisms of organic matter degradation, the anaero- bic oxidation of methane （AOM）, the formation mechanisms of authigenic minerals and the isotope fractionation were summarized in this paper. Organic matter drives the early diagenesis. Marine or- ganic matter could be oxidized by the oxidants and yield higher or lower free energy change per mol of organic carbon oxidized. According to the change in free energy the utilization of oxidants is in the or- der of 02, NOa , Mn4＋ , Fea＋ , SO42- and followed by methanogenesis. Microorganisms are the prima- ry catalysts for the methanogenesis and methane is produced in methanogenic zone. Upwardfluid transports methane from deeper layers into the near-surface sediments. Methane encountering down- ward diffusing sulfate is oxidized by a microbial consortium. Authigenic minerals （e. g. authigenic carbonates and pyrite） precipitate due to release of bicarbonate and hydrogen sulfide during the anae- robic oxidation of methane（AOM）. Plenty of carbon deposits in the deep sediments during above geo- chemical processes. These processes make the marine sediments to be a crucial carbon pool. Various reaction mechanisms during the early diagenesis, especially the effect of sedimentary environment on the degradation of organic matter and cation isotope fractionation, are the focus of future research.