Rates and pathways of iodine speciation transformations at the Bermuda Atlantic Time Series

Abstract

The distribution of iodine in the surface ocean – of which iodide-iodine is a large destructor of tropospheric ozone (O3) – can be attributed to both in situ (i.e., biological) and ex situ (i.e., mixing) drivers. Currently, uncertainty regarding the rates and mechanisms of iodide (I-) oxidation render it difficult to distinguish the importance of in situ reactions vs ex situ mixing in driving iodine’s distribution, thus leading to uncertainty in climatological ozone atmospheric models. It has been hypothesized that reactive oxygen species (ROS), such as superoxide (O2•−) or hydrogen peroxide (H2O2), may be needed for I- oxidation to occur at the sea surface, but this has yet to be demonstrated in natural marine waters. To test the role of ROS in iodine redox transformations, shipboard isotope tracer incubations were conducted as part of the Bermuda Atlantic Time Series (BATS) in the Sargasso Sea in September of 2018. Incubation trials evaluated the effects of ROS (O2•−, H2O2) on iodine redox transformations over time and at euphotic and sub-photic depths. Rates of I- oxidation were assessed using a 129I- tracer (t1/2 ~15.7 Myr) added to all incubations, and 129I/127I ratios of individual iodine species (I-, IO3-). Our results show a lack of I- oxidation to IO3- within the resolution of our tracer approach – i.e., <2.99 nM/day, or <1091.4 nM/yr. In addition, we present new ROS data from BATS and compare our iodine speciation profiles to that from two previous studies conducted at BATS, which demonstrate long-term iodine stability. These results indicate that ex situ processes, such as vertical mixing, may play an important role in broader iodine species’ distribution in this and similar regions.

Iodide, iodate & dissolved organic iodine in the temperate coastal ocean

“…Relative to the estimated global deep ocean dI T concentration of ~450-480 nM (Tsunogai and Sase, 1969;Barker and Zeitlin, 1972;Elderfield and Truesdale, 1980;Jickells et al, 1988;Nakayama et al, 1989;McTaggart et al, 1994;Campos et al, 1996;Tian et al, 1996;Farrenkopf et al, 1997;Moriyasu et al, 2020;Moriyasu et al, 2023;Schnur et al, 2024) inner shelf waters were depleted (x ̅ =400 ± 30 nM, range 314-477 nM; Table 2). Nevertheless, the average dI T concentration of both inner and outer shelf seawater was within the range of reported Atlantic surface waters dI T concentrations (≈435 nM, Elderfield and Truesdale (1980) and ; 390 ± 20 nM, He et al (2013)).…”

“…In oceanic seawater, iodine is classified as a biophilic element (Goldschmidt, 1954) and is primarily in the forms of thermodynamically stable iodate (IO 3 -), iodide (I -; ~25% of the total dissolved iodine (dI T ) in surface waters) and iodine associated with dissolved organic material (typically representing ~5% of dI T ; Elderfield and Truesdale, 1980;Wong, 1980;Luther et al, 1991;Wong, 1991;Chance et al, 2014;Fuge and Johnson, 2015;Luther, 2023). The central tenet of iodine marine biogeochemistry is that transformations between soluble chemical species are biologically mediated (Tsunogai and Sase, 1969;Amachi et al, 2007), though chemical processes cannot be ruled out (Schnur et al, 2024). This tenet derives from observations of temporal changes in iodine speciation in ocean waters concluding that from spring through to autumn iodide accumulates and iodate is removed, and over winter the reverse occurs (Truesdale, 1978;Jickells et al, 1988;Campos et al, 1996;Tian et al, 1996;Truesdale and Jones, 2000;Truesdale and Upstill-Goddard, 2003;Waite et al, 2006;Chance et al, 2010;Satoh et al, 2019a;Shi et al, 2023).…”

Characterizing the marine iodine cycle and its relationship to ocean deoxygenation in an Earth System model

“…That said, our cGENIE estimate is consistent with a multitude of other constraints that indicate that Ioxidation to IO3undergoes extremely slow kinetics. The Ioxidation rates calculated through indirect methods including mass balance and seasonal iodine speciation changes (Tsunogai, 1971;Campos et al, 1996;Truesdale et al, 2001;He et al, 2013;Edwards and Truesdale, 1997;Žic et al, 2013;Moriyasu et al, 2023) or through radiogenic tracer spiked incubations (Hardisty et al, 2020;Schnur et al, 2024;Ştreangă et al, 2024) have a wide range of variation from 1.5nM/yr to 670nM/yr. The lifetime in cGENIE is 50 years, which can be approximately converted to the zeroth order rate of <9 nM/yr, falling in the lower end of the previous studies.…”

“…Dissimilatory IO3reducing bacteria, as well as abiotic reduction with sulfide and dissolved Fe, have been identified within ODZs (Farrenkopf et al, 1997;Councell et al, 1997;Jiang et al, 2023). In addition, slow oxidation-reduction kinetics (Tsunogai, 1971;Hardisty et al, 2020;Schnur et al, 2024) imply the likelihood that in situ iodine signals could be integrated across large-scale physical oceanography processesincluding ocean currents and mixing between water masses (Hardisty et al, 2021), and meaning that iodine species reflect regional rather than local redox conditions (Lu et al, 2020b). Non-redox related processes, such as phytoplankton-mediated IO3reduction and organic matter remineralization also exerts controls on iodine speciation in the water column (Fig.…”

“…Iodide oxidation to IO3is a 6-electron transfer and other ROS, such as superoxide, are only thermodynamically favorable to catalyze partial oxidation to intermediates (Luther, 2023). These ROS species have heterogenous distributions and ambient ocean concentrations that are typically relatively low compared to iodine, supporting the likelihood of temporally or spatially isolated high Ioxidation rates despite of overall extremely slow rates (Schnur et al, 2024).…”