<p>Marine sediments are not merely passive repositories of deposited material, but dynamic geochemical systems in which organic-matter decomposition, sequential electron-acceptor use, mineral dissolution and precipitation, and porewater-solid phase interactions operate together. Within this system, early diagenesis refers to the set of reactions that begins at the sediment–water interface and continues through shallow burial, progressively reshaping the primary geochemical signal of the sediment. Recent studies show that early diagenesis cannot be reduced to single processes such as sulfate reduction or methane formation. It should instead be interpreted through an integrated framework that links alkalinity generation, Fe–Mn cycling, retention and release of redox-sensitive trace elements, silica and silicate alteration, organic sulfur formation, and benthic fluxes. This review synthesizes the geochemical basis of early diagenesis and redox zonation in marine sediments across different marine settings, including continental shelves, semi-enclosed basins, deltaic muds, gas seep environments, and abyssal plains. Organic matter mineralization, sulfate reduction, anaerobic oxidation of methane, Fe–Mn transformations, mobility of redox-sensitive elements, organic sulfur formation, and silica/silicate reactions are discussed together. The synthesis argues that early diagenesis is not a secondary noise that simply disturbs sedimentary records; rather, it is a primary interpretive layer that determines how marine sediment records acquire, preserve, and transform environmental meaning.</p>