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

本研究的目的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可用性的档案。不幸的是，目前存在的碘反应速率和机制在海水中，限制了定量应用的限制。为了解决这个问题，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罕见的碘同位素碘-129作为受控海水培养中碘化学反应的示踪剂，旨在确定两种端元环境的特定反应速率和机制：作为联合王国大西洋经向样带（AMT）年度时间序列的一部分，大西洋中部含氧良好的海水和太平洋的低氧区。 该项目将有助于建立未来的美国STEM（科学技术，工程和数学）培训的劳动力，通过一名研究生和至少一名本科生从密歇根州立大学校园的培训。这包括通过两次研究巡航进行的实地实践培训和经验，WHOI的广泛分析培训，以及在现代和古代海面碘氧相互作用的地球系统建模模拟方面的经验。实验限制旨在为科学界提供更广泛的碘相关化学循环建模，包括大气和海洋化学家，环境监管机构和地质学家。碘酸盐-碘化物的氧化还原电位具有独特的可能应用前景，既可以作为现代和过去海洋中氧气最小区（OMZ）等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阶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多数海洋输入物都是还原态碘化物，但海洋碘酸盐的产生，即最丰富的氧化态物质，还有待于实验观察。物质平衡要求原地海洋氧化是普遍的。氧化剂是未知的，但由于热力学障碍，它不太可能是氧气（O2）。(2)不受约束的原位过程驱动了全球透光沃茨中还原碘化物的大量积累，特别是在低纬度地区，这些水域最终成为对流层臭氧的主要汇。(3)尽管碘是第一个氧化还原敏感的物种之一，以减少下降的O2下，在OMZs的速率和反应机制的限制是有限的。我们会使用同位素示踪剂吗？碘-129既是碘化物又是碘酸盐？在船上海水培养实验，以确定率和碘氧化还原转化机制，这些普遍的趋势。这种方法将部署在海洋碘形态的最大已知梯度？东部热带北太平洋氧气最低区和一个纬度样带的透光和亚透光沃茨的一部分，大西洋子午线样带。这些巡航的孵化实验将用于对高和低[O2]下碘氧化还原转化率的一阶约束，最强烈的碘氧化还原循环的位点（垂直和空间），以及驱动氧化还原转化的机制。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NSF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.