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1. Speciation and cycling of iodine in the subtropical North Pacific Ocean

   https://doi.org/10.3389/fmars.2023.1272968  

   Ştreangă, Iulia-Mădălina; Repeta, Daniel J.; Blusztajn, Jerzy S.; Horner, Tristan J. (January 2024, Frontiers in Marine Science)

   Iodine intersects with the marine biogeochemical cycles of several major elements and can influence air quality through reactions with tropospheric ozone. Iodine is also an element of interest in paleoclimatology, whereby iodine-to-calcium ratios in marine carbonates are widely used as a proxy for past ocean redox state. While inorganic iodine in seawater is found predominantly in its reduced and oxidized anionic forms, iodide (I⁻) and iodate (IO₃⁻), the rates, mechanisms and intermediate species by which iodine cycles between these inorganic pools are poorly understood. Here, we address these issues by characterizing the speciation, composition and cycling of iodine in the upper 1,000 m of the water column at Station ALOHA in the subtropical North Pacific Ocean. We first obtained high-precision profiles of iodine speciation using isotope dilution and anion exchange chromatography, with measurements performed using inductively coupled plasma mass spectrometry (ICP-MS). These profiles indicate an apparent iodine deficit in surface waters approaching 8% of the predicted total, which we ascribe partly to the existence of dissolved organic iodine that is not resolved during chromatography. To test this, we passed large volumes of seawater through solid phase extraction columns and analyzed the eluent using high-performance liquid chromatography ICP-MS. These analyses reveal a significant pool of dissolved organic iodine in open ocean seawater, the concentration and complexity of which diminish with increasing water depth. Finally, we analyzed the rates of IO₃⁻formation using shipboard incubations of surface seawater amended with¹²⁹I⁻. These experiments suggest that intermediate iodine species oxidize to IO₃⁻much faster than I⁻does, and that rates of IO₃⁻formation are dependent on the presence of particles, but not light levels. Our study documents the dynamics of iodine cycling in the subtropical ocean, highlighting the critical role of intermediates in mediating redox transformations between the major inorganic iodine species. 

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2. The Distribution and Redox Speciation of Iodine in the Eastern Tropical North Pacific Ocean

   https://doi.org/10.1029/2019GB006302  

   Moriyasu, Rintaro; Evans, Natalya; Bolster, Kenneth M.; Hardisty, Dalton S.; Moffett, James W. (February 2020, Global Biogeochemical Cycles)

   Abstract The distributions of iodate (IO₃⁻), iodide (I⁻), nitrite (NO₂⁻), and oxygen (O₂) were determined on two zonal transects and one meridional transect in the Eastern Tropical North Pacific (ETNP) in 2018. Iodine is a useful tracer of in situ redox transformations and inputs within the water column from continental margins. In oxygenated waters, iodine is predominantly present as oxidized iodate. In the oxygen deficient zone (ODZ) in the ETNP, a substantial fraction is reduced to iodide, with the highest iodide concentrations coincident with the secondary nitrite maxima. These features resemble ODZs in the Arabian Sea and Eastern Tropical South Pacific (ETSP). Maxima in iodide and nitrite were associated with a specific water mass, referred to as the 13 °C Water, the same water mass that contains the highest concentrations of iodide within the ETSP. Physical processes leading to patchiness in the 13 °C Water relative to other water masses could account for the patchiness frequently observed in iodide and nitrite, probably reflecting subsurface mesoscale features such as eddies. Throughout much of the ETNP ODZ, iodine concentrations were higher than the mean oceanic value. This “excess iodine” is attributed to lateral inputs from sedimentary margins. Excess iodine maxima are centered within a potential density of 26.2–26.6 kg/m³, a density range that intersects with reducing shelf sediments and is almost identical to the ETSP. Evidently, margin input processes are significant throughout the basin and can influence the nitrogen and iron cycles as well, as in the ETSP. 

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3. Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem

   https://doi.org/10.5194/acp-16-12239-2016  

   Sherwen, Tomás; Schmidt, Johan A.; Evans, Mat J.; Carpenter, Lucy J.; Großmann, Katja; Eastham, Sebastian D.; Jacob, Daniel J.; Dix, Barbara; Koenig, Theodore K.; Sinreich, Roman; et al (January 2016, Atmospheric Chemistry and Physics)

   We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O₃) concentrations, except in polar regions where the model now underestimates O₃ concentrations. Halogen chemistry reduces the global tropospheric O₃ burden by 18.6 %, with the O₃ lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28  ×  10⁶ molecules cm⁻³ are 8.2 % lower than in a simulation without halogens, leading to an increase in the CH₄ lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH₄ by Cl is small (∼  2 %) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼  15–27 %). Oxidation of VOCs by Br is smaller, representing 3.9 % of the loss of acetaldehyde and 0.9 % of the loss of formaldehyde. 

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

   https://doi.org/10.1029/2021GB007300  

   Moriyasu, Rintaro; Bolster, Kenneth M.; Hardisty, Dalton S.; Kadko, David C.; Stephens, Mark P.; Moffett, James W. (February 2023, Global Biogeochemical Cycles)

   Abstract The distributions of iodate and iodide were measured along the GEOTRACES GP15 meridional transect at 152°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 complementary⁷Be 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. 

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5. Stacking‐Fault Enhanced Oxygen Redox in Li ₂ MnO ₃

   https://doi.org/10.1002/aenm.202200427  

   Li, Xiang; Li, Xinhao; Monluc, Lisa; Chen, Benjamin; Tang, Mingxue; Chien, Po‐Hsiu; Feng, Xuyong; Hung, Ivan; Gan, Zhehong; Urban, Alexander; et al (March 2022, Advanced Energy Materials)

   Abstract Lattice oxygen redox yields anomalous capacity and can significantly increase the energy density of layered Li‐rich transition metal oxide cathodes, garnering tremendous interest. However, the mechanism behind O redox in these cathode materials is still under debate, in part due to the challenges in directly observing O and following associated changes upon electrochemical cycling. Here, with¹⁷O NMR as a direct probe of O activities, it is demonstrated that stacking faults enhance O redox participation compared with Li₂MnO₃domains without stacking faults. This work is concluded by combining both ex situ and in situ¹⁷O NMR to investigate the evolution of O at 4i, 8j sites from monoclinicC2/mand 6c(1), 6c(2), 6c(3) sites from the stacking faults (P3₁12). These measurements are further corroborated and explained by first‐principles calculations finding a stabilization effect of stacking faults in delithiated Li₂MnO₃. In situ¹⁷O NMR tracks O activities with temporal resolution and provides a quantitative determination of reversible O redox versus irreversible processes that form short covalent OO bonds. This work provides valuable insights into the O redox reactions in Li‐excess layered cathodes, which may inspire new material design for cathodes with high specific capacity. 

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