Kristina Olsson

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

Abstract 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 ( C T ) 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 C T vs salinity plot for the discrete seawater samples. When including the contribution from North American rivers, an annual riverine input of C T to the Arctic Ocean of 33 or 39 × 10 12 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 C T . 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 C T 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 C T in the water column clearly shows the necessity of considering biochemical transformation in determining the water mass characteristics in the Arctic Ocean.

Anthropogenic carbon dioxide in the Arctic Ocean - inventory and sinks

An inventory and sequestering rate of anthropogenic carbon dioxide CTanthro in the Arctic Ocean, calculated by a plume-entrainment model, are presented. The plume is initiated by a fraction rj leaving the shelf break at 200 m, followed by an entrainment of rj for every 150 m depth the plume descends. The model is constrained by the CFC-12 and carbon tetrachloride (CCl4) distributions, with the concentrations of CFC-12, CCl4, and CTanthro in the source water calculated assuming a water in 100% equilibrium with the atmosphere. The model is run from 1750 to 1991, the latter being the year in which measurements of the transient tracers in the water column of the central Arctic Ocean were made. The output from the model gives sinks of anthropogenic carbon dioxide in 1991 of 0.026±0.009 Gt C yr−1, of which 0.0194 Gt C yr−1 is in the Eurasian Basin and 0.0070 Gt C yr−1 in the Canadian Basin. This amounts to about 1% of the total oceanic uptake of anthropogenic CO2. The Arctic Ocean inventory of anthropogenic carbon dioxide in 1991 was 1.35(+0.12/−0.06) Gt C, which is about 1% of the total oceanic inventory. The sensitivity of the computed sinks and inventories to various model assumptions was estimated.