Carbon mineralization in Arctic sediments northeast of Svalbard: Mn(IV) and Fe(III) reduction as principal anaerobic respiratory pathways

Carbon oxidation rates and pathways were determined in 3 sediments at latitude 79 degrees to 81 degrees N in the Barents Sea, where the ice cover restricts primary production to a few months of the year. Oxygen uptake (1.5 to 3.5 imnol m(-2) d(-1)) and sulfate reduction (< 0.1 to 0.22 mmol m(-2) d(-1) over 0 to 10 cm depth) rates were measured by whole core incubation. Pathways of anaerobic carbon oxidation were determined by combining results of anoxic sediment bag incubations with pore water and solid phase analyses of the sediments. In accordance with the high contents of solid Mn (>= 60 mu mol cm(-3)) and Fe(III) (>= 108 mu mol cm(-3)), dissimilatory Mn(IV) and Fe(III) reduction contributed between 69 and >= 90% to anaerobic carbon mineralization in the upper 10 cm of the sediments. At 2 of the 3 stations, sulfate reduction rates were below our detection limit of 1 nmol cm(-3) d(-1). Solid Mn and Fe(III) were abundant from the surface to 10 cm sediment depth and were apparently the only important anaerobic electron acceptors. At the third station, vertical zonation of anaerobic mineralization was observed, with Mn(IV) reduction at 0 to 3 cm followed by concurrent Fe(III) and sulfate reduction at 3 to 5 cm and sulfate reduction at 5 to 10 cm. Rates of microbial carbon oxidation were low compared to those in fjords on the west and south coasts of Svalbard. This is in accordance with the limited organic carbon supply by primary and secondary productivity caused by long periods of ice coverage.