²¹⁰Pb, ¹³⁷Cs and ⁷Be in the sediments of coastal lakes on the polish coast: Implications for sedimentary processes

In this study we combined radioisotopes (²¹⁰Pb, ¹³⁷Cs and ⁷Be) and hydrodynamic modeling to investigate sedimentary processes in three coastal lakes on the Polish Baltic coast. The research aimed at establishing the depth of sediment mixing and its effects on sediment geochemistry as well as showing the relationship between lake water salinity and radionuclide distribution in the sediment cores. We established that the intensity of mixing displayed appreciable variability throughout the lakes and the thickness of sediment mixing layer was between <2 and 22 cm. The mixing was primarily due to wind-induced waves. The vertical mixing was shown to shift sulfidation of the sediments towards deeper layers. We found that the distributions of radioisotopes, ¹³⁷Cs in particular, in the sediment cores from coastal lakes were strongly affected by the early diagenetic processes, which caused diffusive migration of radionuclides. The inventories of ²¹⁰Pb_ex and ¹³⁷Cs in the lakes were positively related to salinity. The high inventories of both isotopes (3.2–10.9 kBq ·m⁻² for ²¹⁰Pb_ex and 3.0–6.0 kBq·m⁻² for ¹³⁷Cs) in coastal lakes were explained by enhanced sedimentation within estuarine mixing zone and delivery of "additional" ²¹⁰Pb and ¹³⁷Cs to the lakes during saltwater ingressions. The results of this study have implications for the paleolimnology, sedimentology and biogeochemistry of coastal lakes. - Highlights: • The depth of vertical mixing of the sediments of coastal lakes is estimated. • The sediment mixing in coastal lakes is explained in terms of hydrodynamic modeling. • The effect of vertical mixing on sediment geochemistry is shown. • In coastal lakes inventories of radionuclides are controlled by salinity. • The possibility to obtain undisturbed sediment cores in coastal lakes is low.