The study of radionuclide activities in the sediments of deep areas of Skagerrak and Southern Baltic

This study investigates the activities of radionuclides ( 137 Cs, 210 Pb, 214 Pb, 239,240 Pu, and 40 K), carbon isotopes (δ 13 C and 14 C), and grain size compositions in surface bottom sediments across the southern Baltic and Skagerrak Strait. Our findings reveal significant connections between these parameters and their dependence on sediment transport dynamics. The distribution patterns of natural and artificial radionuclides were studied to understand the factors influencing their transport to the deep areas. The results show that 137 Cs activity in surface bottom sediments (0–5 cm) ranges from 8 to 11 Bq/kg in the Skagerrak area and reaches 220 Bq/kg in the southern Baltic, closer to the vicinity of river discharges. The activity ranges of 239,240 Pu are similar between the study areas, but the mean value at the Skagerrak site is higher at 1.6 Bq/kg. The transport of 210 Pb ex to the bottom is mainly dependent on the number of fine particles (0–20 μm) in the water column, while the transport of 137 Cs primarily driven by scavenging by marine organics from the water column. In the southern Baltic, a strong correlation indicates that a significant share of 137 Cs most likely originates from fluvial input. The particle size distribution reflects calm conditions in the deep waters, while in the Curonian Lagoon reference areas there are pronounced contrasts between stations with strong and calm hydrodynamic conditions. The δ 13 C values in the surface bottom sediments decrease towards river discharge areas, reflecting an increased terrestrial component of the organic fraction, and the number of coarse particles also increases.