Echoes of a Cold War

The Kara Sea was an area of radionuclide contamination as a result of nuclear weapons testing, since it is located in close proximity to the nuclear test site on the Novaya Zemlya archipelago. The vertical distribution of ¹³⁷Cs and ²¹⁰Pb in 15 sediment cores from the sea made it possible to reconstruct the chronology of ¹³⁷Cs entry into marine sediments. The reconstruction was based on the age-depth model RUS2023 (Rusakov et al., 2024) for ²¹⁰Pb_ex, taking into account the grain-size composition and sorption capacity of sediments. Our study showed that higher ¹³⁷Cs concentrations in sediments of the Novaya Zemlya Trough correspond to the time of active nuclear weapons testing in 1961–63 (about 9.0 ± 1.6 Bq kg^‒1), as well as in estuarine sediments of the Yenisei Bay in 1986 (85.2 ± 3.2 Bq kg^‒1). The abnormally high flux in 1980s represents the superposition of ¹³⁷Cs fluxes as a result of global fallout (Chernobyl trace) and as a result of the release of radionuclide from regional source located in the Yenisei River catchment. Both of these events are associated with increased the sedimentation and mass accumulation rates of marine sediments. In 1961‒63, this was a consequence of the release of sediment into the atmosphere due to nuclear explosions, and in the 1980s, as a result of an increase in sediment supply with river runoff. Currently, against the backdrop of a decrease in sediment fluxes into the sea, Novaya Zemlya and Yenisei River remain the main sources of ¹³⁷Cs supply to marine sediments. Background values for recent sediments in the Kara Sea are <2.0 Bq kg^‒1.