SEDIMENTATION AND MIXING RATES OF RADIONUCLIDES IN BARENTS SEA SEDIMENTS OFF NOVAYA-ZEMLYA

Radionuclide measurements have been conducted on sediment cores collec ted in 1992 in the south-eastern region of the Barents Sea, known as t he Pechora Sea. Cesium-137 and Pu-239,Pu-240 activities in surface sed iments are generally less than 30 Bq/kg, with the highest levels being measured in sediments off the southwestern coastline of the island of Novaya Zemlya. High correlations between both Cs-137 and Pu-239,Pu-24 0 and the concentration of tine (< 63 mu m) particles in surface sedim ents indicate that much of the variance in radionuclide concentrations throughout the Pechora Sea can be explained by particle size fraction ation. However, elevated activities of Cs-137 (138 Bq/kg), Co-60 (92 B q/kg), Am-241 (433 Bq/kg), and especially Pu-239,Pu-240 (8.47 x 10(3) Bq/kg) were measured in one surface sediment sample from the fjord of Chernaya Bay on the southern coast of Novaya Zemlya. The source of rad ioactive contamination is two underwater nuclear tests conducted in Ch ernaya Bay in 1955 and 1957. The Pu-238/Pu-239,Pu-240 activity ratio o f 0.0245 in Chernaya Bay is equivalent to values measured in global fa llout. The Pu-240/Pu-239 atom ratio (0.0304), measured by mass spectro metry, is much lower than values (0.18) typical of global fallout, but is consistent with ratios measured for fallout from the early (1951-1 955) series of weapons tests at the Nevada Test Site. The timing of th e Chernaya Bay source term, estimated from the Am-241/Pu-241 ratio, is consistent with the timing of the 1955 and 1957 underwater nuclear te sts. Relatively low initial yields of Pu-241 (Pu-241/Pu-239 atom ratio = 0.00123) in these tests have resulted in relatively low Am-241/Pu-2 39,Pu-240 activity ratios (0.05) in recent sediments in Chernaya Bay. Radionuclide tracer profiles in cores from the Pechora Sea can be simu lated using a two-layer biodiffusion model with rapid, near-homogeneou s mixing in the surface mixed layer and reduced mixing in the deep lay er. Lead-210 profiles are consistent with a wide range of sedimentatio n and mixing rates in the deep sediment layer. However, the Cs-137 and Pu-239,Pu-240 results further constrain the model parameters and indi cate that the downward transport of radionuclides in the sediments is governed primarily by sediment mixing, with sediment burial playing a secondary role.