The distribution of marine iodine is closely linked to the cycling of oxygen (O2), and its concentration in oxygenated waters has implications for ozone (O3) destruction as part of a paleoredox proxy. While the distribution of iodine’s major redox species, iodide (I-) and iodate (IO3-), in the surface ocean is well-documented, the rates and mechanisms of I- oxidation to IO3- remain less well-understood. Iodate in ocean waters is incorporated and preserved within marine carbonate minerals, tracing past and present redox processes and past oxygenation. Iodide formed by microbial reduction of IO3- in surface waters destroys O3 and has implications for modeling of climatological cycles. Iodate reduction is thought to occur fairly quickly and leads to a disequilibrium in [I-] in surface waters, but in situ I- oxidation to IO3- is thought to be slow and may only occur in “hotspots” of large biological influence. Due to these slow reported timescales of I- oxidation, ex situ sources of movement, such as upwelling and water mass mixing may have a larger impact on iodine redox species’ distribution that has been previously thought.To calculate rates of I- oxidation to IO3- and quantitatively constrain their distribution, I used three distinct techniques: 1) incubation experiments using the radiotracer 129I, 2) an iodine mass balance of Pacific basin waters, and 3) an Optimum Multi-parameter Analysis (OMPA) across two field areas, 1) the Bermuda Atlantic Time Series (BATS) in the Sargasso Sea and 2) the Pacific Ocean basin at 152°W from Alaska to the Antarctic Ocean, measuring concentrations of iodine’s major redox species I- and IO3-, as well as some intermediates. Samples were collected as part of monthly BATS cruises in the Atlantic and as part of two GEOTRACES cruises (GP15 and GP17-OCE) across the Pacific basin. In the first study, I tested addition of the reactive oxygen species (ROS) superoxide (O2-) and hydrogen peroxide (H2O2) in their role in iodine redox transformations through shipboard radio tracers incubations under ambient conditions as part of the Bermuda Atlantic Time Series (BATS). Incubation trials evaluated the effects of biology, light, and the presence and absence of ROS on I- oxidation over time and at both euphotic and sub-photic depths. I calculated rates of I- oxidation through use of the radiotracer 129I- (t1/2 ~57 Mya) added to all incubations, quantified through measurement of incubations’ 129I/127I ratios of individual iodine species determined using Neptune high-resolution multi-collector ICP-MS (MC-ICP-MS). Rates of I- oxidation were found to be sluggish and under the influence of additionally occurring redox transformations, highlighting limited change in iodine redox chemistry associated with in situ processes. In studies two and three, I measured iodine redox species concentrations in surface and depth profile samples from the GEOTRACES GP15 (2018) and GP17-OCE (2022) cruise transects to complete the first iodine meridional transect of the Pacific Ocean. Together with complimentary tracers (7Be), I performed mass balance calculations quantifying the contributions of iodine species from ex situ sources in the Pacific. In addition, a water mass analysis using GP17-OCE hydrographic data provides insight into the eight water masses present in the South Pacific study region and the physical versus biogeochemical changes that contribute to iodine’s distribution throughout the Pacific basin. Ultimately, my data highlight multiple mechanisms likely responsible for iodine cycling and evolution on a basin scale, with important implications for iodine’s broader use as a paleoredox tracer.
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