Meridional Survey of the Central Pacific Reveals Iodide Accumulation in Equatorial Surface Waters and Benthic Sources in the Abyssal Plain

The distributions of iodate and iodide were measured along the GEOTRACES GP15 meridional transect at 152 & DEG;W from the shelf of Alaska to Papeete, Tahiti. The transect included oxygenated waters near the shelf of Alaska, the full water column in the central basin in the North Pacific Basin, the upper water column spanning across seasonally mixed regimes in the north, oligotrophic regimes in the central gyre, and the equatorial upwelling. Iodide concentrations are highest in the permanently stratified tropical mixed layers, which reflect accumulation due to light-dependent biological processes, and decline rapidly below the euphotic zone. Vertical mixing coefficients (K-z), derived from complementary Be-7 data, enabled iodide oxidation rates to be estimated at two stations. Iodide half-lives of 3-4 years show the importance of seasonal mixing processes in explaining north-south differences in the transect, and also contribute to the decrease in iodide concentrations with depth below the mixed layer. These estimated half-lives are consistent with a recent global iodine model. No evidence was found for significant inputs of iodine from the Alaskan continental margin, but there is a significant enrichment of iodide in bottom waters overlying deep sea sediments from the interior of the basin. Plain Language Summary Iodine is an important element in the oceans' biology and chemistry. The two principal forms are iodate and iodide, which were measured on a surface to seafloor survey from Alaska to Tahiti in 2018 as part of the international GEOTRACES program. Iodate, the stable form in the presence of oxygen, was predominant throughout the transect, but iodide was present at the boundaries, including the Alaskan margin, the abyssal plain, and surface waters. A "hot zone of high iodide comprising up to 50% of the total iodine concentration was detected in tropical surface waters centered on the equator. It reflects accumulation from biological reduction of iodate by phytoplankton coupled with re-oxidation of iodide by a non-light-dependent process. Iodate reduction probably occurs throughout the transect, but winter mixing of surface waters during storms probably smears out this feature in northern waters. This suggests that the turnover between iodide and iodate is longer than seasonal timescales. Indeed, estimates of iodide half-life in the range of 3-4 years were calculated from complimentary shipboard measurements of a short-lived radioisotope, beryllium-7. High iodide in equatorial surface waters is important for atmospheric chemistry as it has a big impact on the global ozone budget.