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

Relationships between organic carbon, total nitrogen and organic nitrogen c oncentrations and variations in delta C-13(org) and delta N-15(org) are exa mined in surface sediments from the eastern central Arctic Ocean and the Ye rmak Plateau. Removing the organic matter from samples with KOBr/KOH and de termining residual as well as total N shows that there is a significant amo unt 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 signi ficantly higher (8-16). This correction of organic TOC/N ratios for the pre sence of soil-derived bound ammonium is especially important in samples wit h high illite concentrations, the clay mineral mainly responsible for ammon ium adsorption. The isotopic composition of the organic N fraction was esti mated by determining the isotopic composition of the total and inorganic ni trogen fractions and assuming mass-balance. A strong correlation between de lta N-15(org) values of the sediments and the nitrate concentration of surf ace waters indicates different relative nitrate utilization rates of the ph ytoplankton in various regions of the Arctic Ocean. On the Yermak Plateau, low delta N-15(org) values correspond to high nitrate concentrations, where as in the central Arctic Ocean high delta N-15(org) values are found beneat h low nitrate waters. Sediment delta C-13(org) values are close to - 23.0 p arts per thousand in the Yermak Plateau region and approximately - 21.4 par ts per thousand in the central Arctic Ocean. Particulate organic matter col lected 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 l ow CO2(aq) concentrations in these environments. A maximum terrestrial cont ribution of 30% of the organic matter to sediments in the central Arctic Oc ean is derived, based on the carbon isotope data and various assumptions ab out the isotopic composition of the potential endmembers. (C) 2000 Elsevier Science Ltd. All rights reserved.