¹²⁷I and ¹²⁹I species and transformation in the Baltic Proper, Kattegat, and Skagerrak Basins

Occurrence of anthropogenic ¹²⁹I in seawater has provided invaluable information about water circulation and exchange rates, but results on ¹²⁹I species (iodide and iodate) are limited and only available for surface water. We here present the first extensive results on ¹²⁹I and ¹²⁷I species in samples of seawater depth profiles, which were collected in August 2006 and April 2007 in the Skagerrak, Kattegat, and Baltic Proper. The results expose ≤10% annual reduction of iodate as ¹²⁹I is transported from the English Channel along the Dutch coast and German Bight into the Skagerrak and Kattegat. The results also suggest strong variability between surface and bottom seawater with respect to the predominant iodine species. Distribution of iodide and iodate of both ¹²⁷I and ¹²⁹I in the Kattegat mainly reflects water mixing process rather than speciation transformation. In water of the Baltic Proper, high ¹²⁷I^-/¹²⁷IO₃^- and ¹²⁹I^-/¹²⁹IO₃^- values suggest effective reduction of iodate with a maximum rate of 8 × 10 ^-7 (¹²⁷IO₃^-) and 6 × 10 ^-14 (¹²⁹IO₃^-) (g/m³.day). The reduction process of iodate seems to be related to decomposition of organic matter and photochemically induced reactions.