Seasonal carbon and nutrient mineralization in a high-Arctic coastal marine sediment, Young Sound, Northeast Greenland

A comprehensive investigation of carbon and nutrient cycling in Arctic mari ne sediments is presented. The high-Arctic fjord Young Sound in Northeast G reenland was chosen as study site. The fjord was covered by sea ice for app roximately 10 mo during 1996. Despite highly fluctuating seasonal air tempe ratures, the bottom water temperature remained almost constant at -1.2 to - 1.8 degrees C throughout the year. When sea ice broke in mid-July, benthic mineralization was immediately stimulated by a significant peak in sediment ation of organic material. Due to rapid mineralization of the easily degrad able fraction of the settling organic material, respiration rates returned to their basic lower level within 1 mo and remained low for the rest of the season. Benthic mineralization rates in the Young Sound sediment are compa rable with rates from much warmer locations, suggesting that benthic minera lization in this high-Arctic coastal sediment was regulated by the availabi lity of organic matter and not by temperature. Rate measurements covered ox ygen respiration, denitrification, manganese, iron, and sulfate reduction a s well as DIC and nutrient flux from the sediment. In response to enhanced mineralization following sea ice break-up, sediment water fluxes of O-2, DI C, NO3- + NO2-, NH4+, urea, PO43-, and Si increased and rapidly recycled nu trients to the water column, indicating an efficient benthic-pelagic coupli ng in Young Sound. Sediment porewater concentrations of O-2 were affected b y the input of organic matter, leading to higher O-2 consumption rates near the sediment surface during summer. In contrast, no seasonal alterations i n concentration profiles of DIC, NH4+, + NO3- + NO2-, Mn2+, Fe2+ and SO42- were observed. Furthermore, depth distributions of e(-)-acceptors (O-2, NO3 -, Fe(III) and SO42-) and reduction rate measurements supported the classic al orderly progression from O-2 respiration to NO3- reduction followed by b acterial iron reduction and finally sulfate reduction. On an annual scale, O-2 respiration accounted for 38% of total oxidation of organic carbon, den itrification, 4%; iron reduction, 25%; and sulfate reduction, 33%. Rates of carbon oxidation by manganese reduction were insignificant (<1%) and the f raction of refractory carbon buried was approximately equal to the amount o f carbon being mineralized in Young Sound.