Nitrogen and carbon isotopic composition of marine and terrestrial organic matter in Arctic Ocean sediments: implications for nutrient utilization and organic matter composition

Relationships between organic carbon, total nitrogen and organic nitrogen concentrations and variations in delta C-13(org) and delta N-15(org) are examined in surface sediments from the eastern central Arctic Ocean and the Yermak Plateau. Removing the organic matter from samples with KOBr/KOH and determining residual as well as total N shows that there is a significant amount of bound inorganic N in the samples, which causes TOC/N-total ratios to be low (4-10 depending on the organic content). TOC/N-org ratios are significantly higher (8-16). This correction of organic TOC/N ratios for the presence of soil-derived bound ammonium is especially important in samples with high illite concentrations, the clay mineral mainly responsible for ammonium adsorption. The isotopic composition of the organic N fraction was estimated by determining the isotopic composition of the total and inorganic nitrogen fractions and assuming mass-balance. A strong correlation between delta N-15(org) values of the sediments and the nitrate concentration of surface waters indicates different relative nitrate utilization rates of the phytoplankton in various regions of the Arctic Ocean. On the Yermak Plateau, low delta N-15(org) values correspond to high nitrate concentrations, whereas in the central Arctic Ocean high delta N-15(org) values are found beneath low nitrate waters. Sediment delta C-13(org) values are close to - 23.0 parts per thousand in the Yermak Plateau region and approximately - 21.4 parts per thousand in the central Arctic Ocean. Particulate organic matter collected from meltwater ponds and ice-cores are relatively enriched in C-13 (delta C-13(org) = - 15.3 to - 20.6 parts per thousand) most likely due to low CO2(aq) concentrations in these environments. A maximum terrestrial contribution of 30% of the organic matter to sediments in the central Arctic Ocean is derived, based on the carbon isotope data and various assumptions about the isotopic composition of the potential endmembers. (C) 2000 Elsevier Science Ltd. All rights reserved.