Abundance and speciation of iron across a subtropical tidal marsh of the Min River Estuary in the East China Sea

Abstract

Iron has profound influence on anaerobic organic matter oxidation and Fe–S–C biogeochemical cycling in tidal marshes. To examine the abundance and speciation of iron, six iron species (poor crystalline Fe(III), crystalline Fe(III), non-sulfidic Fe(II), porewater Fe2+, FeS, and FeS2) were investigated in a cross transect (span=250m, depth=∼1.2m) in a tidal marsh of the Min River Estuary in the East China Sea. The results suggested that sediment characteristics, pH, redox condition, conductivity, chloride, sulfate, and organic matter create a highly heterogeneous geochemical framework in the cross transect. Correspondingly, the zonation of iron species differed significantly across the tidal marsh. Poor crystalline Fe(III) (48±24μmolg−1) and crystalline Fe(III) (75±32μmolg−1) accumulated on the surface of the high-tide zone and decreased with elevation. The non-sulfidic Fe(II) minerals (140±61μmolg−1) accounted for the largest proportion of the six iron species. The porewater Fe2+ was enriched within the deep layer of the upland region (pH=6.2–6.5) and exhibited a maximum of 11.49mM. The aqueous sulfide concentration was below detection. FeS (39±6μmolg−1) and FeS2 (57±25μmolg−1) accumulated closer to the lower extremity. Tidal water brought O2 and removed certain amounts of dissolved organic carbon, dissolved inorganic carbon, and Fe(III)-bearing particles in an element budget during tidal inundation. Variations of topography, tidal hydrology, seawater intrusion, and organic matter altered the dominant organic matter oxidation pathway and further affected iron mineralization.