Sedimentation and mixing rates of radionuclides in Barents Sea sediments off Novaya Zemlya

Radionuclide measurements have been conducted on sediment cores collected in 1992 in the south-eastern region of the Barents Sea, known as the Pechora Sea. Cesium-137 and Pu-239,Pu-240 activities in surface sediments 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-240 and the concentration of tine (< 63 mu m) particles in surface sediments indicate that much of the variance in radionuclide concentrations throughout the Pechora Sea can be explained by particle size fractionation. However, elevated activities of Cs-137 (138 Bq/kg), Co-60 (92 Bq/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 radioactive contamination is two underwater nuclear tests conducted in Chernaya Bay in 1955 and 1957. The Pu-238/Pu-239,Pu-240 activity ratio of 0.0245 in Chernaya Bay is equivalent to values measured in global fallout. The Pu-240/Pu-239 atom ratio (0.0304), measured by mass spectrometry, is much lower than values (0.18) typical of global fallout, but is consistent with ratios measured for fallout from the early (1951-1955) series of weapons tests at the Nevada Test Site. The timing of the Chernaya Bay source term, estimated from the Am-241/Pu-241 ratio, is consistent with the timing of the 1955 and 1957 underwater nuclear tests. 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-239,Pu-240 activity ratios (0.05) in recent sediments in Chernaya Bay. Radionuclide tracer profiles in cores from the Pechora Sea can be simulated using a two-layer biodiffusion model with rapid, near-homogeneous mixing in the surface mixed layer and reduced mixing in the deep layer. Lead-210 profiles are consistent with a wide range of sedimentation and mixing rates in the deep sediment layer. However, the Cs-137 and Pu-239,Pu-240 results further constrain the model parameters and indicate that the downward transport of radionuclides in the sediments is governed primarily by sediment mixing, with sediment burial playing a secondary role.