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

这项研究的目标是限制控制海洋环境中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气有效性的档案。不幸的是，目前对海水中碘的反应速度和机理几乎没有限制，限制了定量应用。为了解决这一问题，来自密歇根州立大学(MSU)和伍兹霍尔海洋研究所(WHOI)的科学家将使用一种稀有的碘同位素碘-129作为受控海水孵化中的碘化学反应的示踪剂，旨在确定两个终端环境的特定反应速度和机制：作为英国大西洋子午线断面(AMT)年度时间序列的一部分的大西洋中部富氧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内的一名研究生和至少一名本科生，为建设未来由美国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的实地实践培训和经验，在世界卫生组织的广泛分析培训，以及在地球系统模拟现代和古代海面碘-氧相互作用模拟方面的经验。实验限制旨在为科学界包括大气和海洋化学家、环境监管者和地质学家提供与碘相关的化学循环的更广泛的建模。碘酸盐-碘化物的氧化还原潜力具有独特的应用前景，既可以作为现代和过去海洋中类似氧最小区域(OMZ)条件的氧化还原示踪剂，也可以作为调节对流层臭氧水平的海-气交换反应的关键组成部分。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，限制了我们的研究试图解决的应用。具体而言：(1)尽管来自河口和其他来源的大部分海洋投入物都是还原的碘化物，但尚未从实验上观察到海洋碘酸盐的生产，这是被氧化的和最丰富的物种。物质平衡要求海洋中的原位氧化作用广泛存在。氧化剂未知，但由于热力学障碍，它不太可能是氧气(O2)。(2)不受限制的就地过程促使全球，特别是在低纬度地区的光水中大量积累还原的碘，这最终成为对流层臭氧的主要汇。(3)尽管碘是在氧气减少下减少的第一个氧化还原敏感物种之一，但对OMZ中的反应速度和反应机理的限制是有限的。我们将在船上的海水培养实验中使用同位素示踪剂碘-129作为碘化物和碘酸盐，以确定控制这些广泛趋势的碘氧化还原转化的速率和机制。这一方法将部署在海洋碘物种形成中已知的最大梯度--东热带北太平洋氧气最小地带和作为大西洋子午线断面一部分的光层和亚光层水域的纬向断面上。这些巡航的孵化实验将被用来对高和低[O2]下的碘氧化还原转化速率、最强烈的碘氧化还原循环(垂直和空间)的轨迹以及驱动氧化还原转化的机制施加一级约束。CONTROLS将测试氧化剂、生物和非生物过程，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NSF的法定使命，并通过使用基金会的智力优势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.