Sulfur and iron diagenesis in temperate unsteady sediments of the East China Sea inner shelf and a comparison with tropical mobile mud belts (MMBs)

Redox cycling of iron (Fe) and sulfur (S) exerts profound influences on fates of numerous elements in coastal marine sediments. In this study, S and Fe cycling and its geochemical expressions in the East China Sea (ECS) inner shelf, a representative of temperate mobile mud belts (MMBs), were characterized and compared with tropical counterparts (the Amazon shelf and the Gulf of Papua). Fe and S speciation consistently points to the dominance of authigenic nonsulfidized Fe(II) phases (i.e., poorly crystalline clays (PCCs) and carbonates) and the prevalence of Fe redox cycling in the suboxic or weakly sulfidic regimes of the ECS-MMBs. High contents of authigenic magnetite may be a common diagenetic expression in all MMBs. Compared to the tropical MMBs, three main differences of diagenetic expressions in the ECS-MMBs are (i) light S-34(pyrite) in the ECS-MMB versus characteristically heavy S-34(pyrite) in the Amazon shelf MMBs; (ii) lower total reactive Fe (Fe-TR), total diagenetic Fe(II), and ratio of Fe-TR to total Fe in the ECS-MMBs; and (iii) Fe(II) carbonates and PCCs are equally important sinks for nonsulfidized Fe(II) in the ECS-MMBs, whereas PCCs are the predominant sink in the tropical counterparts. These differences are ascribable to factors including low degradability of organic matter, small diffusion scales, less intense chemical weathering in the drainage basin, and/or weaker reverse weathering in the ECS-MMBs. Despite the differences above, Fe and S diagenetic expressions that characterize the prevalence of Fe redox cycling in the unsteady suboxic regimes are shared by the ECS-MMBs and tropical MMBs.