Microbial Mn(IV) and Fe(III) reduction in northern Barents Sea sediments under different conditions of ice cover and organic carbon deposition Maren Nickel, Verona Vandieken à , Volker Bru ¨ chert, Bo B. Jørgensen Max Planck Institute for Marine Microbiology, Celsiusstr. 1, 28359 Bremen, Germany article info Available online 7 July 2008 Keywords: Marine sediments Sediment incubations Carbon mineralization Arctic Svalbard abstract Carbon oxidation rates and pathways were determined in two sediments at latitude 751 and 771N 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 (480% 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 CO 2 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. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction The organic material in Arctic environments is derived from primary production by phytoplankton in the upper water column or from sea ice and may additionally be released from melting sea ice (Schubert and Calvert, 2001) or glaciers. In the northern Barents Sea, plankton productivity is interseasonally highly variable and depends on the availability of light by ice coverage and complete darkness in winter. The partial or complete retreat of sea ice in summer allows light to become available for photosynthesis. When the ice melts in spring or summer, a stratified water column with a nutrient-rich euphotic zone develops due to the input of low-salinity meltwater and suppressed wind mixing (Wassmann and Slagstad, 1993). This supports the growth of intensive phytoplankton blooms that follow the receding ice edge. Greatest primary production is found along the marginal ice zone (Sakshaug and Slagstad, 1991). Areas that are influenced by relatively warm, nutrient-rich Atlantic water (south and west of Svalbard) either are year-round ice-free or the ice cover melts in spring (Hebbeln and Wefer, 1991; Falk- Petersen et al., 2000). Whereas in areas along the north and east coast of Svalbard, influenced by waters of the Arctic Ocean, the open-water season is relatively short with a maximum ice extent during late winter/spring and a minimum in late autumn (Falk- Petersen et al., 2000). Thus, primary production is restricted to a short summer to fall period. The sea ice conditions influence the amount of organic carbon produced by primary production, which partly settles to the sea floor and becomes available to the benthic community (Hulth et al., 1996). The oxidation of deposited organic material by benthic organisms is mediated through a sequence of respiratory processes with different inorganic electron acceptors (O 2 , NO 3  , Mn(IV), Fe(III) and SO 4 2 ). The general depth sequence of these oxidants corresponds to a gradual decrease in their redox potential and, as a result, a vertical zonation of the pathways is observed in sediments (Froelich et al., 1979). The different electron-accepting pathways can thereby spatially overlap with each other, depending on substrate availability or substrate reactivity (Canfield et al., 1993b; Kostka et al., 1999). The quantitative importance of the various carbon mineraliza- tion pathways has been studied intensely, and it is generally found that oxygen and sulfate play a major role in shelf sediments (Canfield et al., 2005a). An important factor controlling the relative significance of each microbial process is the availability of organic carbon. Oxic mineralization is the dominant degrada- tion pathway at low carbon fluxes (Wijsman et al., 2002). With increasing carbon deposition rates, oxygen is rapidly consumed in the surface sediments and anaerobic degradation processes ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsr2 Deep-Sea Research II 0967-0645/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2008.05.003 à Corresponding author. Present address: Institute of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark. E-mail address: vvandieken@daad-alumni.de (V. Vandieken). Deep-Sea Research II 55 (2008) 2390– 2398