Characterization of Iron, Sulfur, and Phosphorus Diagenesis in Muddy Sediments of the South Yellow Sea Using the Diffusive Gradients in Thin Films (DGT) Technique

The muddy sediments of the central South Yellow Sea (SYS) are not only an important depocenter, but also an important area for early diagenesis. A systematic study of the interlinked sulfur (S), iron (Fe), and phosphorus (P) in sediments of the sea could provide important information on the diagenetic cycle of these elements; however, their interactions have not been well documented. In this study, an in situ, high-resolution diffusive gradient in thin films (DGT) technique, together with solid-phase speciation was used to reveal the diagenesis of S, Fe, and P in muddy sediments, and to quantify the benthic flux of dissolved inorganic phosphate (DIP). Solid-phase and porewater chemistry indicate that ferruginous conditions prevail in the sediments, dissimilatory iron reduction (DIR) dominated Fe reduction, and the sulfate reduction rate is generally low due to the low lability of sedimentary organic carbon. Two-dimensional porewater distributions provide direct evidence of high spatial heterogeneity of sulfide and DIP on a microscale, and spatial coexistence of DIR and sulfate reduction. A positive linear correlation (R2 > 0.75) between porewater Fe and DIP measured by the DGT over the depth interval of rapid increase in DGT-labile Fe suggests that DIP release is primarily driven by DIR. Overall, the muddy sediments are a weak source of DIP, with an average benthic flux of 0.422 ± 1.004 × 10−3 mmol/m2/day, which accounts for only 0.02−0.04% of DIP required for the primary production of the SYS, and thus has only minor impact on P budget in the sea.