Diagenetic cycling and diffusive/benthic fluxes of iron and manganese in sediments of a large semi-enclosed sea remote to major rivers

Abstract

Dissolved iron (DFe) and manganese (DMn) diffusive/benthic fluxes and DFe species in sediments of marginal seas not dominated by major upwelling or riverine inputs have not been well documented. Here, solid-phase and porewater chemistry of sediments was used to quantify diffusive/benthic fluxes of DFe and DMn, and to elucidate the factors governing the fluxes from muddy sediments of the South Yellow Sea (SYS), a representative semi-enclosed shelf sea remote to major rivers. Results show that porewater DFe is produced mainly by dissimilatory iron reduction coupled to organic carbon (OC) mineralization, while DMn is produced dominantly by Mn-oxide reduction coupled to Fe2+ oxidation. Most upwardly diffusing Fe2+ is oxidized in the oxic sediment layer, while organic-Fe(III) complexes is mainly responsible for benthic DFe release, with its relative contributions from 63 % to nearly 100 %. Unlike DFe, DMn is expected to effectively diffuse upward into the overlying water, indicated by nearly 1:1 ratios of its diffusive fluxes to depth-integrated rates of production. Benthic fluxes of DFe (0.98–5.8 μmol/m2/d) and DMn (1.3–249 μmol/m2/d) from the SYS muddy sediments exhibit a high spatial heterogeneity, with a “sweet spot” at the western area, which is controlled largely by local/regional depositional settings and, to a less extent, by local diagenetic regimes. Overall, the semi-enclosed SYS sediment is not a hotspot of benthic DFe release, compared to other marginal seas under various depositional settings, which is attributable to low availability of labile OC and reactive Fe due to its remoteness to major rivers. Unlike DFe, benthic DMn fluxes in the SYS are comparable to those in other marginal seas including river-dominated areas, which is ascribable to refluxing-induced Mn enrichment in surface sediments as a common mechanism to sustain Mn redox cycling and benthic release.