Microbial Mn(IV) and Fe(III) reduction in northern Barents Sea sediments under different conditions of ice cover and organic carbon deposition

Carbon oxidation rates and pathways were determined in two sediments at latitude 75° and 77°N southeast of Svalbard in the northern Barents Sea. Seasonal ice cover restricts primary production to few months a year, which determines the sedimentation rate of organic material to the seafloor. At one station, with seasonally extended ice cover, low organic carbon content and sedimentation rate combined with relatively high concentrations of Mn and Fe(III) oxides favored dissimilatory Fe and Mn reduction (98% of anaerobic carbon oxidation) over sulfate reduction in the top 12 cm of the sediment. In contrast, in a sediment that had not been ice covered for at least 12 months and with more organic carbon and a higher sedimentation rate, sulfate reduction was the most important anaerobic electron-accepting process (>80% of anaerobic carbon oxidation). In the upper 3 cm, microbial Fe and sulfate reduction occurred simultaneously, and sulfate reduction dominated at 3–12 cm. Oxygen consumption rates (1.9 and 3.7 mmol m−2 d−1) and anaerobic CO2 production rates (1.3 and 6.4 mmol m−2 d−1) of both stations were similar to rates from open-ocean sediments farther north in the Barents Sea but lower compared to those in fjords of Svalbard.