Sedimentary organic matter in two Spitsbergen fjords: Terrestrial and marine contributions based on carbon and nitrogen contents and stable isotopes composition

The aim of this study was to estimate the spatial variability of organic carbon (C-org) and total nitrogen (N-tot) concentrations, C-org/N-tot ratios, stable isotopes of carbon and nitrogen (delta C-13(org), delta N-15(tot)) and the proportions of autochthonous and allochtonous organic matter within recently deposited sediments of two Spitsbergen fjords: the Hornsund and the Adventfjord, which are affected to a different degree by the West Spitsbergen Current. C-org concentrations ranged from 1.38% to 1.98% in the Hornsund and from 1.73% to 3.85% in the Adventfjord. In both fjords the highest C-org concentrations were measured at the innermost stations and they decreased towards the mouths of the fjords. This suggests fresh water runoff to be an important source of organic matter (OM) for surface sediments. The results showed that both fjords differ significantly in terms of sedimentary organic matter characteristics. The samples from the Hornsund, except those from the innermost station in the Brepollen, had relatively low C-org/N-tot ratios, all within a narrow range (from 9.7 to 11.3). On the other hand significantly higher C-org/N-tot ratios, varying within a broad range (from 14.6 to 33.0), were measured in the Adventfjord. The samples from the Hornsund were characterized by higher delta C-13(org) (from -24.90 parts per thousand to -23.87 parts per thousand) and delta N-15(tot) (from 3.02 parts per thousand to 4.93 parts per thousand) than those from the Adventfjord (-25.94 parts per thousand to -24.69 parts per thousand and from 0.71 parts per thousand to 4.00 parts per thousand, respectively). This is attributed to a larger proportion of marine organic matter. Using the two end -member approach proportions of terrestrial organic matter were evaluated. Terrestrial OM contribution for the Adventfjord was in the range of 82-83%, while in case of the Hornsund the results were in the range of 69-75%, with the exception of the innermost part of the fjord, where terrestrial organic matter contribution ranged from 80 to 82%. The strong positive correlation between delta C-13(org) and delta N-15(tot) was revealed. This was taken as an indicator that fresh, labile organic matter is the base of the trophic pyramid. The conclusion regarding a more complex food web in the Horsund, based on larger delta N-15(tot) values, is further substantiated by the larger proportion of autochthonous, labile organic matter found in the Hornsund's sediments. (C) 2015 Elsevier Ltd. All rights reserved.