Spatial–Temporal Distribution and Interrelationship of Sulfur and Iron Compounds in Seabed Sediments: A Case Study in the Closed Section of Mikawa Bay, Japan

Herein, the distribution of sulfur and iron compounds (dissolved sulfide: H2S and HS−, iron sulfide: FeS, and ionized iron: Fe2+ and Fe3+) in sediments (0–15 cm depth) at four stations in Mikawa Bay, Japan, was evaluated from April 2015 to March 2016. The maximum dissolved sulfide concentrations in the upper part of the sediment porewater (0–4 cm depth) (within 1.4–8.1 mmol·L−1) varied among stations located in a waterway of a large-scale port with a significant dead zone. Moreover, the iron sulfide and ionized iron concentrations in the upper part were highest at a station where the dissolved sulfide concentration was relatively low compared with that of the other sites. Analysis of the theoretical and hypothetical accumulation of particulate oxidized iron (FOOH) at the stations located in the dead zone revealed that the estimated particulate oxidized iron accumulation was higher (2303 mmol·m−2) at a station in which the dissolved sulfide concentration was low compared with the other stations (142–384 mmol·m−2). Altogether, these findings suggest that the sulfur–iron cycling can determine the amount of dissolved sulfides that accumulate in sediments. Hence, artificially adding iron compounds to the seabed may help mitigate free sulfides accumulation and prevent extreme hypoxia.