Collaborative Research: Experimental constraints on the rates and mechanisms of iodine redox transformations in seawater

本研究的目的是限制海洋环境中控制碘循环的化学和生物反应。海水碘在碳、溶解氧和臭氧的循环中起着关键作用，并被假设也会影响锰和氮的元素循环。沉积岩中碘的组成也被认为是古代海水氧可用性的档案。不幸的是，目前对海水中碘的反应速率和机理的限制很少，限制了定量应用。为了解决这个问题，来自密歇根州立大学（MSU）和伍兹霍尔海洋学研究所（WHOI）的科学家将使用一种罕见的碘同位素碘-129作为碘化学反应的示踪剂，在受控的海水孵育中设计，以确定两个末端成员环境的特定反应速率和机制：作为英国大西洋经向样带（AMT）年度时间序列的一部分的富氧大西洋中部海水和太平洋的低氧区。该项目将通过培训密歇根州立大学校园的一名研究生和至少一名本科生，为建立未来美国STEM（科学技术，工程和数学）培训的劳动力做出贡献。这包括通过两次研究巡航进行实地实践培训和经验，在WHOI进行广泛的分析培训，以及在现代和古代海面进行碘-氧相互作用的地球系统建模模拟的经验。实验约束的目的是为包括大气和海洋化学家、环境监管机构和地质学家在内的科学界提供更广泛的碘相关化学循环建模信息。碘酸盐-碘化物的氧化还原电位在现代和过去的海洋中作为氧最小带（OMZ）样条件的氧化还原示踪剂，以及调节对流层臭氧水平的海气交换反应的关键组成部分，具有独特的应用前景。然而，目前对海水中碘氧化还原转化的一级速率和机制的了解有限，限制了应用，这是我们的研究试图解决的问题。具体而言：(1)尽管来自河口和其他来源的大多数海洋输入都是由还原物种碘化物组成，但海洋碘酸盐的产生，氧化和最丰富的物种，尚未通过实验观察。物质平衡要求海洋就地氧化是普遍存在的。氧化剂是未知的，但由于热力学障碍，它不太可能是氧（O2）。(2)在全球范围内，特别是在低纬度地区，不受约束的原位过程推动了光水体中还原性碘化物的大量积累，最终成为对流层臭氧的主要汇。(3)尽管碘是在氧气下降条件下最先还原的氧化还原敏感物质之一，但对OMZs中反应速率和反应机制的限制是有限的。我们要用同位素示踪剂吗？碘-129既是碘化物又是碘酸盐？在船上进行海水培养实验，以确定控制这些普遍趋势的碘氧化还原转化的速率和机制。该方法将在海洋碘物种形成的最大已知梯度上部署。东热带北太平洋氧最低带和作为大西洋经向样带一部分的光和亚光水域的纬度样带。这些航行的孵育实验将用于对高[O2]和低[O2]下碘氧化还原转化速率、最强烈的碘氧化还原循环位点（垂直和空间）以及驱动氧化还原转化的机制进行一级约束。控制将测试氧化剂，生物与非生物过程，以及与类似氧化还原循环（如锰和氮）的相互作用和比较。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The goal of this study is to constrain the chemical and biological reactions controlling the iodine cycle in the marine environment. Seawater iodine plays a key role in the cycling of carbon, dissolved oxygen, and ozone, and has been hypothesized to also influence the elemental cycles of manganese and nitrogen. The composition of iodine in sedimentary rocks has also been proposed as an archive of ancient seawater oxygen availability. Unfortunately, few constraints currently exist on iodine reaction rates and mechanisms in seawater, limiting quantitative applications. To remedy this, scientists from Michigan State University (MSU) and Woods Hole Institute of Oceanography (WHOI) will use a rare iodine isotope, iodine-129, as a tracer of iodine chemical reactions in controlled seawater incubations designed to determine specific reaction rates and mechanisms from two end-member environments: well-oxygenated mid-Atlantic seawater as part of the United Kingdom-based Atlantic Meridional Transect (AMT) annual time series and low oxygen zones in the Pacific Ocean. The project will contribute to building the future United States STEM (Science Technology, Engineering and Mathematics)-trained workforce via the training of one graduate student and at least one undergraduate student from the campus of MSU. This includes hands-on field training and experience through two research cruises, extensive analytical training at WHOI, as well as experience in Earth system modeling simulations of iodine-oxygen interactions at the modern and ancient sea surface. The experimental constraints are designed to inform broader modeling of iodine-related chemical cycles for scientific communities including atmospheric and marine chemists, environmental regulators, and geologists. The redox potential of iodate-iodide is uniquely poised for probable applications as both a redox tracer of Oxygen Minimum Zone (OMZ)-like conditions in modern and past oceans as well as a critical component of air-sea exchange reactions regulating tropospheric ozone levels. However, a currently limited understanding of the first-order rates and mechanisms of iodine redox transformations in seawater limits applications, which our research seeks to address. Specifically: (1) Marine iodate production, the oxidized and most abundant species, has yet to be observed experimentally despite the fact that most marine inputs from estuarine and other sources consist of the reduced species, iodide. Mass balance demands that in situ marine oxidation is widespread. The oxidant is unknown, but it is unlikely oxygen (O2) due to thermodynamic barriers. (2) Unconstrained in situ processes drive significant accumulation of reduced iodide in photic waters globally, particularly at low latitudes, which ultimately act as a major tropospheric ozone sink. (3) Constraints on rates and reaction mechanisms in OMZs are limited despite iodine being amongst the first redox-sensitive species to reduce under declining O2. We will employ an isotope tracer?iodine-129 as both iodide and iodate?in shipboard seawater incubation experiments to determine the rates and mechanisms of iodine redox transformations governing these widespread trends. This method will be deployed across the largest known gradients in marine iodine speciation?the Eastern Tropical North Pacific oxygen minimum zone and a latitudinal transect of photic and sub-photic waters as part of the Atlantic Meridional Transect. Incubation experiments from these cruises will be used to place first order constraints on the rates of iodine redox transformations at high- and low-[O2], the loci of most intense iodine redox cycling (both vertically and spatially), as well as the mechanisms driving redox transformations. Controls will test oxidants, biotic versus abiotic processes, as well as interactions and comparisons with similar redox cycles such as manganese and nitrogen.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.