Distributions of nuclear fuel-reprocessing tracers in the Arctic Ocean: Indications of Russian river influence

Radionuclide sampling in 1986 and 1993 in the Canada Basin, and in 1993 in the Amundsen Basin and on the adjacent Laptev shelf, provides new insights into the origin, timing, pathways, and mechanisms for dispersal of non-fallout radioactive tracers in the Arctic Ocean. First, samples from the Beaufort Sea shelf, slope, and adjacent basin show a four-fold increase in I-129 concentrations from 1986 to 1993. Second, anthropogenic non-fallout radionuclide concentrations in the Beaufort Sea increase with proximity to slope boundary currents. Third, there is evidence for riverine contributions of anthropogenic radionuclides to surface waters of the Amundsen Basin and the Laptev continental shelf. This evidence includes high surface water burdens of Np-237 and I-129, with the maximum in anthropogenic I-129 found in the least saline and most O-18-depleted waters, consistent with an origin in high-latitude runoff. Additionally, the Np-237/I-129 atom ratios in the Laptev Sea and Amundsen Basin in 1993 were significantly lower than observed elsewhere in the Arctic Ocean and can be reasonably explained by I-129 added during transit of the Russian shelves. The Pu-240/Pu-239 ratios in the water column were mostly near 0.18, consistent both with stratospheric bomb fallout and with the discharged-weighted mean Sellafield ratio during 1966-1985. In the least saline water samples collected at the most shallow Laptev shelf station, however, the Pu ratios were lower, consistent with a non-European nuclear fuel reprocessing source. There are clear secondary maxima in Np-237 and I-129 near 1000 m in the Amundsen Basin, likely associated with the Barents Sea branch of Atlantic water. Finally, the I-129/salinity and I-129/delta(18)O relationships in the Amundsen and Canada Basins at mid-depths are indistinguishable, suggesting effective horizontal dispersion.