Radiation-Geochemical Stability of Bottom Sediments in the Ob and Yenisei Estuaries and Adjacent Shoal Area of the Kara Sea

This paper reports the results of investigations on the radioecological state of bottom sediments of the Ob–Yenisei shoal area and Ob and Yenisei estuaries. Materials were obtained in two periods: 1995–2001 and 2014–2019 during seven expeditions on the R/V Akademik Boris Petrov, Professor Shtokman, and Akademik Mstislav Keldysh. Resampling of bottom sediment cores at key-points in areas of increased radiocesium activity and subsequent analysis of the radionuclide distribution made it possible to establish positive trends in the change of the radiation state of the regional ecosystem. The total degradation of the Ob radiocesium overactivity zone and quite significant degradation of the Yenisei zone (primarily the most polluted) were revealed. Comparison of current max-positions of specific activity in the vertical distribution of 137Cs in sediment cores made it possible to determine the sedimentation rate in the distal and proximal facies of the Yenisei estuary: 4.7–5.8 and 3.2–3.7 mm/year, respectively. The established changes that have occurred over ~20 years prove the ability of the Kara Sea ecosystem to self-purify. Pollutant diffusion and assimilation by bottom sediments play a significant role in the self-regeneration of the ecosystem. This is most intense in zone of influence of the river–sea geochemical barrier.