Rates and pathways of carbon oxidation in permanently cold Arctic sediments

We report her-e a comprehensive study of the rates and pathways of carbon m ineralization in Arctic sediments. Four sites were studied at 115 to 329 m water depth in fjords on Svalbard and in coastal Norway. The Svalbard coast al region is characterized by permanently cold bottom water temperatures of -1.7 to 2.6 degrees C. Carbon oxidation (avg = 20 to 400 nmol cm(-3) d(-1) ) and sulfate reduction rates (avg = 10 to 350 nmol cm(-3) d(-1)) were meas ured at high resolution to 10 cm depth in sediment incubations. The distrib ution of oxidants available for microbial respiration was determined throug h porewater and solid phase geochemistry. By comparing the distribution of potential oxidants to the depth-integrated mineralization rates, the import ance of various respiratory pathways to the oxidation of organic C could be quantified. Integrated C oxidation rates measured in sediment incubations (11 to 24 mmol m(-2) d(-1)) were comparable to within a factor of 2 to diss olved inorganic carbon (DIC) fluxes measured in situ using a benthic lander . Sulfate reduction was the dominant microbial respiration pathway (58 to 9 2% of total C oxidation) followed by Fe(III) reduction (10 to 26%), oxygen (5 to 14%), and nitrate respiration (2 to 3%). At sediment depths where sul fate reduction was dominant, C oxidation equivalents, calculated from indep endently 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 communiti es in permanently cold Arctic sediments exhibited mineralization rates and pathways comparable to temperate nearshore environments. This study complet ely partitioned C oxidation pathways, showing a predominance of sulfate res piration and a substantial contribution of Fe(III) reduction to organic mat ter mineralization in Arctic sediments for the first time. Microbial commun ities in cold sediments exposed to relatively high C deposition appear to r espond to the input or availability of organic matter rather than to temper ature.