Figure 3
From: H2S events in the Peruvian oxygen minimum zone facilitate enhanced dissolved Fe concentrations
The left part of the sketch illustrates Scenario 1: H2S and the majority of Fe(II) are removed at the sediment-bottom water interface by oxidizing both compounds with NO3− and NO2− to sulfate (SO4−) and iron oxyhyroxides (Fe(III)OOH). In the water column, Fe(II) is oxidized by NO3− and O2 with subsequent formation of particulate Fe(III)OOH and organically complexed Fe (Fe(III)L). A small fraction of Fe(II) is kept in solution by photochemical reduction processes in the surface. The middle part of the sketch illustrates Scenario 2: After H2S reduced most of the NO3−/NO2− present in the sediments, H2S and Fe(II) can diffuse without restriction across the sediment-water interface and upwards in the water column until they are oxidized at the oxycline, with subsequent formation of particulate Fe(III)OOH, organically complexed Fe (Fe(III)L) and sulfate (SO4−). Similar to Scenario 1, a small fraction of Fe(II) is kept in solution by photochemical reduction processes in surface waters. The right part of the sketch illustrates Advanced scenario 2: Similar to scenario 2, but due to long lasting sedimentary release, H2S and Fe(II) concentrations increase in the water column. We hypothesize that elevated concentrations of H2S and Fe(II) in the millimolar range facilitate the precipitation of mackinawite (FeSm) at the sediment–bottom water interface.
