This study explores the distribution and dynamics of iodine isotopes in coastal and marine waters of Denmark and the Baltic region. The highest 129I concentrations were observed in the west coast in the North Sea and North of Kattegat, with significantly decreasing trend southward into the Baltic, confirming the Great Belt as the primary pathway for iodine-rich North Sea inflows. Freshwater inflows and limited vertical mixing create stratified iodine distributions, with higher concentrations in deep saline currents and lower concentrations in surface waters. Temporal variability in 129I levels is influenced by climatic factors, including wind patterns, precipitation, and seasonal temperature shifts, affecting water mass exchange and isotopic composition. The findings enhance understanding of iodine transport and its dependence on regional hydrographic and climatic processes.

Department of Nuclear Chemistry Czech Technical University Prague 115 19 Prague 1 Czech Republic

Laboratory of Ion Beam Physics ETH Zürich 8093 Zürich Switzerland

MOE Frontiers Science Center for Rare Isotopes School of Nuclear Science and Technology Lanzhou University Lanzhou 730000 China; Technical University of Denmark Risø Campus 4000 Roskilde Denmark

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