Spatial variation and species transformation of ¹²⁹I and ¹²⁷I in the Central Arctic Ocean

The spatial distributions of anthropogenic ¹²⁹I and natural ¹²⁷I in iodide (I⁻) and iodate (IO₃^-) of anthropogenic ¹²⁹I and natural ¹²⁷I were investigated in the upper central Arctic Ocean. The results show that the molecular ratios of reductive iodide (I⁻) to oxidative iodate (IO₃^-) ranged from 0.228 to 0.560 for ¹²⁷I and 0.383 to 0.827 for ¹²⁹I in the Polar Mixed Layer (PML). Greater ¹²⁷I⁻ concentrations and markedly higher I⁻/IO₃^- ratios for both ¹²⁹I and ¹²⁷I were found in the PML over the Lomonosov and Alpha Ridges. This suggests that the in-situ formation of I⁻ occurs as a result of enhanced reductive environments at these locations due to bacterial-mediated decomposition of dissolved organic matter (DOM) that likely originates from the inflow of river water into the central Arctic Ocean. Ratios of ¹²⁹I⁻/¹²⁹IO₃^- and ¹²⁷I⁻/¹²⁷IO₃^- decrease gradually along the main transport pathway of the Atlantic water, revealing that I⁻ is gradually oxidized back to IO₃^- at a slow net rate within the Atlantic Water Layer. Our study provides insights into the marine geochemical cycling of iodine and raises the potential for iodine species, notably of ¹²⁹I, to be used as indicators of changes in the redox condition of surface water in the Arctic Ocean, which may occur due to the predicted impacts of climate change.