Fate of iron sulfide compounds following simulated wetland sediment deposition

Sediment deposition in coastal wetlands is key to maintaining marsh elevation during periods of sea level rise. Deposition occurs naturally during storms to increase marsh elevation or when dredged sediments are intentionally introduced during restoration to build elevation capital. The deposition of sediments containing iron sulfide compounds (FeS and FeS₂) under natural or managed scenarios alters biogeochemical cycles, including the potential to induce dramatic declines in soil pH under prolonged oxidizing conditions. This 20-week mesocosm study investigated FeS and FeS₂ dynamics under continuously flooded, tidal, and drought treatments following a simulated natural or restoration deposition event by placing hyposulfidic sediment onto the marsh soil surface. The introduced FeS and FeS₂ rapidly oxidized (< 21 days) after sediment deposition across all treatments, followed by declining oxidation reduction potentials, increasing dissolved Fe concentrations, and subsequent FeS and FeS₂ re-precipitation in flooded and tidal treatments. Nominal pH declines occurred in flooded and tidal treatments due to re-precipitation of FeS and FeS₂ minerals. Conversely, pH decreased 1–2 units under simulated drought conditions. Observational and modeling results indicate that S⁻² generation limited the rate of FeS and FeS₂ formation following sediment deposition in alignment with previous field studies. Results suggest that the deposition of hyposulfidic sediments during storms and restoration events pose minimal risk of acidification when prolonged saturated conditions are present. However, short term FeS and FeS₂ oxidation and re-precipitation likely occur following both natural (over wash) and managed (restoration) scenarios as the deposited sediments achieve new dynamic biogeochemical equilibria. Coastal resource managers can use these results to ensure restoration projects maximize positive restoration outcomes while minimizing the risk of soil acidification through the informed management of FeS and FeS₂ minerals.