Deciphering the Ages of Saline Water in the Baltic Sea by Anthropogenic Radiotracers

Abstract The slow water renewal endows the Baltic Sea a strong retention of pollutants/nutrients. Constraining water age is a practical way to depict the transport pathways/timescales for water masses and accompanying soluble substances. Although the water ages in the Baltic Sea have been resolved by 3D ocean models 20 years ago, the simulated results have not been verified. In this work, we exploited two anthropogenic radionuclides (¹²⁹I and ²³⁶U) as an age marker to constrain the ages of inflowing North Sea saline waters into the Baltic Sea. Our results indicate that the Baltic Sea has a highly stratified structure with distinctly different timescales for surface-water and deep-water circulations (3 ± 2 and 20 ± 3 years, respectively), providing the first observation-based proof for the multi-decadal retention of (radioactive) pollutants within the Baltic Sea. This work demonstrates the power of anthropogenic radiotracers in investigating hydrodynamic processes in the Northwestern European coastal areas.