A 40-year marine record of ¹³⁷Cs and ⁹⁹Tc transported into the Danish Straits: Significance for oceanic tracer studies

This work reports comprehensive time-series datasets for ¹³⁷Cs and ⁹⁹Tc in marine samples from the Danish Straits over the past 40 years, where dynamic inputs from the two European nuclear reprocessing plants Sellafield (SF) and La Hague (LH) and Chernobyl accident are clearly archived. Distinct seasonal variations between ¹³⁷Cs and ⁹⁹Tc are observed in Fucus vesiculosus (F. vesiculosus), which needs to be taken into account when using F. vesiculosus as a bio-monitor to represent the concentration of radionuclides in seawater. Comparable transfer factor (TF) for ⁹⁹Tc from SF to Kattegat between our calculation and earlier studies indicates a relatively steady water mass transport over the past decades. Three distinct events are observed in the temporal evolution of ⁹⁹Tc/¹³⁷Cs activity ratio in F. vesiculosu with the first event corresponding with the increased ⁹⁹Tc discharge from SF, while the other two are very likely related to the major Baltic inflow (MBI) events. The correlation between the ⁹⁹Tc/¹³⁷Cs activity ratio and salinity fits well into the binary mixing line with the North Sea (NS) and the Baltic Sea (BS) as end members. A model simulation indicates that water mass from NS constitutes less than 50% in the surface water and 50–100% for most locations in the bottom water of the Danish Straits. Overall observations show that ¹³⁷Cs and ⁹⁹Tc in marine samples, especially ⁹⁹Tc/¹³⁷Cs isotope ratios, serve as useful oceanic tracers to study different natural processes, such as water mixing and transport dynamics.