Abundance and sinking of particulate black carbon in the western Arctic and Subarctic Oceans

The abundance and sinking of particulate black carbon (PBC) were examined for the first time in the western Arctic and Subarctic Oceans. In the central Arctic Ocean, high PBC concentrations with a mean of 0.021 ± 0.016 μmol L⁻¹ were observed in the marginal ice zone (MIZ). A number of parameters, including temperature, salinity and ²³⁴Th/²³⁸U ratios, indicated that both the rapid release of atmospherically deposited PBC on sea ice and a slow sinking rate were responsible for the comparable PBC concentrations between the MIZ and mid-latitudinal Pacific Ocean (ML). On the Chukchi and Bering Shelves (CBS), PBC concentrations were also comparable to those obtained in the ML. Further, significant deficits of ²³⁴Th revealed the rapid sinking of PBC on the CBS. These results implied additional source terms for PBC in addition to atmospheric deposition and fluvial discharge on the western Arctic shelves. Based on ²³⁴Th/²³⁸U disequilibria, the net sinking rate of PBC out of the surface water was −0.8 ± 2.5 μmol m⁻³ d⁻¹ (mean ± s.d.) in the MIZ. In contrast, on the shelves, the average sinking rate of PBC was 6.1 ± 4.6 μmol m⁻³ d⁻¹. Thus, the western Arctic Shelf was probably an effective location for burying PBC.

New insights on the role of sea ice in intercepting atmospheric pollutants using (129)I

Measurements of (129)I carried out on sea ice samples collected in the central Arctic Ocean in 2007 revealed relatively high levels in the range of 100-1400×10(7) at L(-1) that are comparable to levels measured in the surface mixed layer of the ocean at the same time. The (129)I/(127)I ratio in sea ice is much greater than that in the underlying water, indicating that the (129)I inventory in sea ice cannot be supported by direct uptake from seawater or by iodine volatilization from proximal (nearby) oceanic regimes. Instead, it is proposed that most of the (129)I inventory in the sea ice is derived from direct atmospheric transport from European nuclear fuel reprocessing plants at Sellafield and Cap La Hague. This hypothesis is supported by back trajectory simulations indicating that volume elements of air originating in the Sellafield/La Hague regions would have been present at arctic sampling stations coincident with sampling collection.