Biogeochemistry of the Russian Arctic. Kara Sea: Research results under the SIRRO project, 1995–2003

The Kara Sea is an area uniquely suitable for studying processes in the river-sea system. This is a shallow sea into which two great Siberian rivers, Yenisei and Ob, flow. From 1995 to 2003, the sea was studied by six international expeditions aboard the R/V Akademik Boris Petrov. This publication summarizes the results obtained, within the framework of this project, at the Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Various hydrogeochemical parameters, concentrations and isotopic composition of organic and carbonate carbon of the sediments, plankton, particulate organic matter, hydrocarbons, and dissolved CO^sub 2^ were examined throughout the whole sea area at more than 200 sites. The δ^sup 13^C varies from -22 and -24[per thousand] where Atlantic waters enter the Kara Sea and in the North-eastern part of the water area to -27[per thousand] in the Yenisei and Ob estuaries. The value of δ^sup 13^C of the plankton is only weakly correlated with the δ^sup 13^C of the organic matter from the sediments and is lower by as much as 3-4[per thousand]. The paper presents the results obtained from a number of meridional river-sea profiles. It was determined from the relations between the isotopic compositions of plankton and particulate matter that the riverwaters carry material consisting of 70% detrital-humus matter and 30% planktonogenic material in the river part, and the material contained in the off-shore waters consists of 30% terrigenous components, with the contribution of bioproducers amounting to 70%. The carbon isotopic composition of the plankton ranges from -29 to -35[per thousand] in the riverine part, from -28 to -27[per thousand] in the estuaries, and from -27.0 to -25% in the marine part. The relative lightness of the carbon isotopic composition of plankton in Arctic waters is explained by the temperature effect, elevated CO^sub 2^ concentrations, and long-distance CO^sub 2^ supply to the sea with riverwaters. The data obtained on the isotopic composition of CO^sub 2^ in the surface waters of the Kara Sea were used to map the distribution of δ^sup 13^C^sub CO^ ^sub 2^. The complex of hydrocarbon gases extracted from the waters included methane, C^sub 2^-C^sub 5^, and unsaturated C^sub 2=^-C^sub 4=^ hydrocarbons, for which variations in the concentrations in the waters were studied along river-estuary-sea profiles. The geochemistry of hydrocarbon gases in surface fresh waters is characterized by compar