Rates and Pathways of Carbon Oxidation in Permanently Cold Arctic Sediments

Abstract

We report here a comprehensive study of the rates and pathways of carbon mineralization in Arctic sediments. Four sites were studied at 115 to 329 m water depth in fjords on Svalbard and in coastal Norway. The Svalbard coastal region is characterized by permanently cold bottom water temperatures of -1.7 to 2.6 °C. Carbon oxidation (avg = 20 to 400 nmol d-') and sulfate reduction rates (avg = 10 to 350 nmol cm-3 d-l) were measured at high resolution to 10 cm depth in sediment incubation~ T. he distribution of oxidants available for microbial respiration was determined through porewater and solid phase geochemistry. By comparing the distribution of potential oxidants to the depth-integrated mineralization rates, the importance of various respiratory pathways to the oxidation of organic C could be quantified. Integrated C oxidation rates measured in sediment incubations (11 to 24 mm01 m-2 d-') were comparable to within a factor of 2 to dissolved inorganic carbon (DIC) fluxes measured in situ using a benthic lander. Sulfate reduction was the dominant microbial respiration pathway (58 to 92% of total C oxidation) followed by Fe(II1) reduction (10 to 26%), oxygen (5 to 14 %), and nitrate respiration (2 to 3%). At sediment depths where sulfate reduction was dominant, C oxidation equivalents, calculated from independently measured sulfate reduction rates, matched DIC production rates in incubations. Sediment geochemistry revealed that the same vertical sequence of oxidants is reduced/respired in these Arctic sediments as in temperate continental shelf sediments of equivalent water depths. Microbial communities in permanently cold Arctic sediments exhibited mineralization rates and pathways comparable to temperate nearshore environments. This study completely partitioned C oxidation pathways, showing a predominance of sulfate respiration and a substantial contribution of Fe(II1) reduction to organic matter mineralization in Arctic sediments for the first time. Microbial communities in cold sediments exposed to relatively high C deposition appear to respond to the input or availability of organic matter rather than to temperature.