Input and biogeochemical transformation of dissolved carbon in the Siberian shelf seas

The input of dissolved carbon to the Arctic Ocean by eight Eurasian rivers is estimated both from historical measurements (1980–1988) in river water and from analysis of seawater samples collected in the Laptev and East Siberian seas (1993 and 1994). The weighted mean concentration of total dissolved inorganic carbon (CT) of the historical Eurasian runoff measurements amounts to 0.77 mmol kg−1. This can be compared with a mean runoff concentration of 0.57 ± 0.02 mmol kg−1 estimated for the Laptev and East Siberian seas from a CT vs salinity plot for the discrete seawater samples. When including the contribution from North American rivers, an annual riverine input of CT to the Arctic Ocean of 33 or 39 × 1012 gC is obtained, depending on which of the concentrations that is used for the Eurasian runoff. The total organic carbon concentrations (TOC) found in the Laptev and East Siberian seas are quite variable, with concentrations of up to 0.35 mmol kg−1. Telang et al. (1991) (in Biogeochemistry of Major World Rivers, pp. 75–104, Wiley, New York) report historic TOC river data of 0.6–0.8 mmol kg−1. This implies that riverine input of TOC could be in the same order as that of CT. From the historical data it becomes evident that the Ob, Yenisey and Lena rivers provide close to 90% of the runoff to the Eurasian shelf. The CT signature of the runoff can be traced along the Eurasian shelf seas out into the deep Arctic Ocean. The distribution indicates that the majority of the Ob and Yenisey discharge flows into the Laptev Sea before entering the central Arctic Ocean. Silicate concentrations of more than 60 μmol kg−1 and less than 70% oxygen saturation found in bottom waters of the eastern East Siberian Sea indicate a high decay rate of organic matter at the sediment surface. The distribution of oxygen, silicate and CT in the water column clearly shows the necessity of considering biochemical transformation in determining the water mass characteristics in the Arctic Ocean.