59 Sulfide toxicity as key control on anaerobic oxidation of methane in eutrophic coastal sediments Fe(II)-carbonates and monosulfides, merely consisting of Fe(II)-monosulfides in this setting with abundant porewater sulfide (van Helmond et al., 2020), was the dominant Fe fraction at depth in the sediment at Site 3. At Sites 5 and 7, this fraction, as well as Fe(II)-pyrite, accounted for the majority of the extractable Fe at depth. Figure 3. Solid-phase iron and manganese speciation (μmol g-1 dry sediment) depth profiles for the three study sites. The arrow at the bottom indicates decreasing bottom water (BW) oxygen concentrations from Site 3 to 7. cmbsf; centimeters below the seafloor. About 50 to 60% of the total sedimentary Mn was extracted in the analyzed steps, i.e. steps 1, 2 and 5 (see Supplementary section on solid-phase analysis), at all three sites. At Site 3, poorly ordered Mn(III/IV)-oxides (e.g. birnessite and pyrolusite) dominated the extracted Mn pool with only a minor (< 10%) contribution of pyriteassociated Mn. At Site 5, poorly ordered Mn(III/IV)-oxides were never dominant, and concentrations were low at depth. Sedimentary Mn could generally be divided into two relatively equal fractions of Mn(II)-carbonates (rhodochrosite) and pyriteassociated Mn. At Site 7, pyrite-associated Mn dominated the extracted Mn pool with only a minor (< 10%) contribution of poorly ordered Mn(III/IV)-oxides (Figure 3 and Supplementary Table 3). 2
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