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.