58 Chapter 2 Table 3. Diffusive fluxes of sulfate and methane (mmol m-2 day-1) as calculated from porewater profiles (intervals in parentheses), and, for the benthic flux, from porewater and bottom water concentrations (see text). SMTZ; sulfate-methane transition zone. Site Downward sulfate flux into SMTZ Upward methane flux into SMTZ* Benthic methane efflux Site 3 (Sandöfjarden) 0.88 (9.5-19 cm) 0.08 (29-34 cm) 0 Site 5 (Lilla Värtan) 1.49 (1.75-7.5 cm) 0.54 (8-13 cm) 0.15 Site 7 (Skurusundet) 1.27 (2.5-4.5 cm) 0.50 (0-15 cm) 1.02 *Methane fluxes into the SMTZ at Sites 5 and 7 are likely underestimated because of degassing during sampling. In sediments of eutrophic, low-oxygen coastal systems, sulfate-dependent anaerobic oxidation of methane (S-AOM) is expected to dominate methane removal1. Sulfate is presumably also the major terminal electron acceptor for AOM by ANME-2 archaea in the investigated sediments of the Stockholm Archipelago at all three sites. Estimated diffusive fluxes of sulfate and methane into the SMTZ (Table 3) suggest that S-AOM accounts for at least 40% of the observed sulfate reduction. Given the potential degassing of methane, especially for sediment intervals with high methane concentrations (Egger et al., 2016), in situ S-AOM rates are likely higher. The differences in calculated benthic effluxes of methane between our three sites suggests that the removal of methane becomes less effective with more reducing bottom water redox conditions (Table 3). To investigate if alternative terminal electron acceptors for AOM such as Fe and Mn oxides were available, we determined the concentration and reactivity of the Fe and Mn oxides present in the sediment. The sequential extractions dissolved between 70% and 80% of the total sediment Fe at the three sites (Figure 3, Supplementary Table 3). Except for the surface sediment at Site 3, the poorly ordered Fe(III)- oxides (e.g. ferrihydrite and lepidocrocite) content was generally low . Crystalline Fe(III)-oxides (e.g. goethite and hematite) accounted for a substantial fraction of the extractable Fe at all sites (15-30%). The fraction consisting of a mixture of
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