NATURAL AND ANTHROPOGENIC RADIONUCLIDE DISTRIBUTIONS IN THE NANSEN BASIN, ARCTIC-OCEAN - SCAVENGING RATES AND CIRCULATION TIMESCALES

Determination of the naturally occurring radionuclides Th-232, Th-230, Th-228 and Pb-210, and the anthropogenic radionuclides Am-241, Pu-239 ,Pu-240, (CS)-C-134 and Cs-137 in water samples collected across the N ansen Basin from the Barents Sea slope to the Gakkel Ridge provides tr acers with which to characterize both scavenging rates and circulation timescales in this portion of the Arctic Ocean. Large volume water sa mples (similar to 1500 1) were filtered in situ to separate particulat e (> 0.5 mu m) and dissolved Th isotopes and Am-241. Thorium-230 displ ays increases in both particulate and dissolved activities with depth, with dissolved Th-230 greater and particulate Th-230 lower in the dee p central Nansen Basin than at the Barents Sea slope. Dissolved Th-228 activities also are greater relative to Ra-228, in the central basin. Residence times for Th relative to removal from solution onto particl es are similar to 1 year in surface water, similar to 10 years in deep water adjacent to the Barents Sea slope, and similar to 20 years in t he Eurasian Basin Deep Water. Lead-210 in the central basin deep water also has a residence time of similar to 20 years with respect to its removal from the water column. This texture of scavenging is reflected in distributions of the particle-reactive anthropogenic radionuclide Am-241, which shows higher activities relative to Pu in the central Na nsen Basin than at the Barents Sea slope. Distributions of Cs-137 show more rapid mixing at the basin margins (Barents Sea slope in the sout h, Gakkel Ridge in the north) than in the basin interior. Cesium-137 i s mixed throughout the water column adjacent to the Barents Sea slope and is present in low but detectable activities in the Eurasian Basin Deep Water in the central basin. At the time of sampling (1987) the su rface water at all stations had been labeled with Cs-134 released in t he 1986 accident at the Chernobyl nuclear power station. In the simila r to 1 year since the introduction of Chernobyl Cs-134 to th, Nansen B asin, it had been mixed to depths of similar to 800 m at the Barents S ea Slope and to similar to 300 m in the central basin. ''Pre-Chernobyl '' inventories of Cs-137 (as well as Pu-239,Pu-240) are 10 times those expected from global atmospheric fallout from nuclear weapons testing and are derived principally from releases from the Sellafield, U.K., nuclear fuel reprocessing facility on the Irish Sea. Based on the sour ces of Cs-137 to the Nansen Basin, mixing time scales are 9-18 years f or the upper water column (to 1500 m) and similar to 40 years for the deep water. These mixing time scales, combined with more rapid scaveng ing at the basin margin relative to the central basin, produce residen ce times of particle-reactive radionuclides in the Nansen Basin compar able to other open ocean areas (e.g. north-west Atlantic) despite the presence of permanent ice cover and long periods of low-light levels t hat limit productivity in the Arctic.