Secondary radioactive contamination of the Black Sea: New evidence from plutonium vertical distribution in its deep bottom sediments

The vertical distribution of anthropogenic plutonium isotopes (^239+240Pu and ²³⁸Pu) in the bottom sediments from eastern deep-water area of the Black Sea was studied. Three peak concentrations were obtained for both ^239+240Pu and ²³⁸Pu, but their positions in sediment profiles coincided only partially. The combination of the distribution of plutonium isotope concentrations and ^239+240Pu/²³⁸Pu activity ratios allowed dating of sediments and determining the sedimentary characteristics of the studied area. Dynamics of plutonium sedimentation fluxes was studied using geochronological reconstruction technique. Obtained trends of changes in plutonium sedimentation fluxes during 1940s-2013 revealed its secondary input into the Black Sea marine environment both in early 2000s and 2010s. The isotopic signature of plutonium in these increased fluxes indicated radionuclides most likely of Chernobyl NPP origin. Thus the entry of Chernobyl derived secondary radioactive contamination into the Black Sea was confirmed by new data on plutonium vertical distribution in its bottom sediments.