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

Abstract

A comprehensive investigation of carbon and nutrient cycling in Arctic marine sediments is presented. The high-Arctic fjord Young Sound In Northeast Greenland was chosen as study site. The fjord was covered by sea ice for approximately 10 mo during 1996. Despite hlghly fluctuating seasonal air temperatures, the bottom water temperature remained almost constant at -1.2 to -1.8C throughout the year. When sea ice broke in mid-July, benthic mineralization was immediately stimulated by a significant peak in sedimentation of organic material. Due to rapid mineralization of the easily degradable fraction of the settling organic material, respirat~on rates returned to their basic lower level within 1 mo and remained low for the rest of the season. Benthic mineral~zation rates in the Young Sound sediment are comparable with rates from much warmer locations, suggesting that benthic minerahzation in this high-Arctic coastal sediment was regulated by the availability of organic matter and not by temperature. Rate measurements covered oxygen respiration, denitrification, manganese, iron, and sulfate reduction as well as DIC and nutrient flux from the sediment. In response to enhanced mineralization following sea ice break-up, sediment water fluxes of 02, DIC, NO3+ NO,-, NH,+, urea, and Si increased and rapidly recycled nutrients to the water column, indicating an efficient benthic-pelagic coupling in Young Sound. Sediment porewater concentrations of O2 were affected by the input of organ~c matter, leading to higher O2 consumption rates near the sediment surface during summer. In contrast, no seasonal alterations In concentration profiles of DIC, NH,', NO; + NO2-, Mn2+, Fe and SO4'were observed Furthermore, depth distributions of e--acceptors (02, NO3-, Fe(II1) and SO4'-) and reduction rate measurements supported the classical orderly progression from 0, respiration to NO,reduction followed by bacterial iron reduction and finally sulfate reduction. On an annual scale, O2 respiration accounted for 38% of total oxidation of organic carbon, denitrification, 4%; iron reduction, 25 /I,; and sulfate reduction, 33 %. Rates of carbon oxidation by manganese reduction were insignificant ( < l 96) and the fraction of refractory carbon buried was approximately equal to the amount of carbon being mineralized in Young Sound.