Atypical diagenesis of sulfur and iron in sediments of the river-dominated Bohai Sea (China)

Abstract Diagenesis of iron (Fe) and sulfur (S) in river-dominated marine systems is complex and has not been well documented. In this study, sediments from the Yellow River-dominated Bohai Sea (BHS) were used to examine the diagenesis of S and Fe and interpret it in light of anthropogenic disturbances. Organic carbon (OC) source characterization indicates that the bulk OC in the sediments is dominated by marine source (62.8 ± 9.4%). Based on low total reduced inorganic sulfide (TRIS) contents and abundant unsulfidized Fe(II), it is inferred that dissimilatory iron reduction (DIR) dominates over sulfate reduction (SR) in the sediments. DIR-dominated regimes and active physical reworking of the sediments are limiting SR and TRIS accumulations, resulting in a significant deviation of total OC (TOC) to TRIS ratio from that for typical marine sediments. Speciation of Fe and S indicates that, besides carbonate-associated Fe(II), other phases are also an important sink of unsulfidized Fe(II), and that reactive Fe is capable of quick buffering towards episodically produced dissolved sulfide on one month timescale. Pulsed sedimentation, seasonal sediment erosion, translocation, and sorting have exerted significant influences on the cycling of S and Fe, resulting in atypical patterns of Fe and S diagenesis. Nevertheless, there is no a discernable impact of eutrophication and algal blooms on geochemistry of the two elements in the sediments, which is ascribable to active physical reworking of sediments as well as the dilution of marine OC by rapid accumulation of terrestrial material.