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

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 timescales for water masses and accompanying soluble substances. Although the water ages in the Baltic Sea have been resolved by 3D ocean models 20 year ago, the simulated results have not been verified. In this work, we exploited two anthropogenic radionuclides (I and U) to constrain the ages of inflowing North Sea saline waters into the Baltic Sea by using the Tracer Age and Transit Time Distribution approaches. Our results indicate that the Baltic Sea has a highly stratified structure with a more diffusive bottom circulation and a more advective surface circulation. The circulation timescale for the saline water in the Baltic Sea was estimated to be >30 years from the bottom of Arkona Basin to the surface of central Baltic Sea. This work demonstrates the power of anthropogenic radiotracers in investigating circulation timescale and mixing processes in the Baltic Sea and provides the first observation-based proof for the multi-decadal retention of (radioactive) pollutants within the Baltic Sea.