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

This study investigates the activities of radionuclides (¹³⁷Cs, ²¹⁰Pb, ²¹⁴Pb, ^239,240Pu, and ⁴⁰K), carbon isotopes (δ¹³C and ¹⁴C), 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 ¹³⁷Cs 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 ^239,240Pu are similar between the study areas, but the mean value at the Skagerrak site is higher at 1.6 Bq/kg. The transport of ²¹⁰Pb_ex to the bottom is mainly dependent on the number of fine particles (0–20 μm) in the water column, while the transport of ¹³⁷Cs primarily driven by scavenging by marine organics from the water column. In the southern Baltic, a strong correlation indicates that a significant share of ¹³⁷Cs 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 δ¹³C 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.