GEOCHEMISTRY OF EARLY DIAGENESIS IN MARINE SEDIMENTS: RESEARCH PROGRESS

The research progress of the reaction mechanisms of organic matter degradation, the anaerobic 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 organic 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 order of O₂, NO₃^-, Mn⁴⁺, Fe³⁺, SO₄^2- and followed by methanogenesis. Microorganisms are the primary catalysts for the methanogenesis and methane is produced in methanogenic zone. Upwardfluid transports methane from deeper layers into the near-surface sediments. Methane encountering downward 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 anaerobic oxidation of methane(AOM). Plenty of carbon deposits in the deep sediments during above geochemical 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.