The oxidized iodine species, iodate, is the most pervasive form of iodine in well-oxygenated marine waters and can be tracked in carbonates as a paleo-redox proxy. Despite known marine spatial concentration variations in iodate and the reduced iodide today and temporal gradients across Earth history, the rates and mechanisms of iodate formation remain poorly understood. To quantify rates and pathways of iodine cycling, we performed ship-board tracer experiments in euphotic waters across a latitudinal transect with known gradients in iodine speciation—the Atlantic Meridional Transect spanning from the Falkland Islands to Southampton in the United Kingdom (UK) (45°S to 37°N) during March 2023. We collected samples for incubations and accompanying iodine speciation depth profiles (5-500 meters) from 11 stations along the transect. All incubation sets were spiked with radioactive 129I (t1/2 ~15.7 My) as iodide and were performed at two depths capturing the 7% and 1% light levels, thus tracking the deep chlorophyl maximum (DCM). Quantified rates varied with depth and location, with northern (spring) locales at 1% light showing the highest activity. Importantly, most locations exhibited no significant iodate production. Iodate formation from iodide is inferred from only one location based on increases in iodate 129I/127I ratios. At other locations, decreases in iodate 129I/127I ratios imply that alternative sources—likely the recycling of intermediates—are an important factor for iodate production. Ultimately, our survey reveals significant variability in iodate formation pathways in Atlantic euphotic waters, which have implications for improving models of iodine cycling and refining the paleo-redox proxy. Future research should focus on further elucidating mechanisms and explore seasonal and regional variations that drive iodine cycling dynamics in marine environments.
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