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

Abstract

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 239,24OPu 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 137Cs and 239,24OPu and the concentration of fine (< 63 μm) 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 137Cs (138 Bq/kg), 60Co (92 Bq/kg), 241Am (433 Bq/kg), and especially 239,24OPu (8.47 × 103 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 238Pu 239,240Vu activity ratio of 0.0245 in Chernaya Bay is equivalent to values measured in global fallout. The 240Pu 239Pu 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 241Am 241Pu ratio, is consistent with the timing of the 1955 and 1957 underwater nuclear tests. Relatively low initial yields of 241Pu ( 241Pu 239Pu atom ratio = 0.00 123) in these tests have resulted in relatively low 241Am 239,240Pu 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 137Cs and 239,240Pu 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.