OCE-PRF: Development and Application of the Foraminiferal Iodine/Calcium Paleoredox Proxy

有孔虫I/Ca古氧化还原替代物的研制与应用海水中的溶解氧含量不同，与海洋中生命的分布有关。海洋缺氧区(ODZ)的扩大是气候变化的预期结果，因为海洋缺氧区的氧气水平低到足以限制宜居。随着我们努力预测这将如何影响未来的海洋，研究过去气候变化事件中的海洋氧合作用越来越有价值。发生在1500-1700万年前的中新世气候最佳期就是这样一个事件，在此期间，地球平均气温上升到3C，大气中二氧化碳水平上升到大约400-500ppm。多条证据表明，在MCO期间，全球范围内严重的ODZ加剧，但缺乏直接证据。我们将通过实验来验证有孔虫碘-钙(I/Ca)指标，并将其应用于东热带太平洋MCO的沉积物中，该指标使用微观化石的碘含量来估计海洋氧浓度，以研究MCO期间该地区的低氧强度，并改进对未来气候变化情景中ODZ变化的预测。该项目结合了实验室实验、实地工作和分析程序，将纳入国际海洋发现计划的档案以及研究生和本科生的培训和研究经验(包括指导Latinx本科生STEM专业的具体计划)。这项工作由三个相互独立但相互关联的项目组成，以测试、地面真相和应用有孔虫I/Ca代理。有孔虫I/Ca替代物是一种基于被称为有孔虫的原生生物微壳的碘浓度来重建古代海洋中氧浓度的方法。这种替代物之所以起作用，是因为可以并入有孔虫贝壳中的碘物种(碘酸盐)的水平与它们生活的水的含氧量有关。虽然这一替代物已经被描述并用于重建全球变化多个时期的海洋氧浓度的研究，但它尚未使用受控的实验室条件进行实验验证，也没有系统地筛选成岩作用的影响(沉积在沉积物中的化石物质发生的变化)。为此，我们将：(1)对底栖和浮游有孔虫进行实验，以证明有孔虫方解石对碘的吸收，测试某些替代物中存在的系统偏离预测的速率。这些实验包括计划使用碘-129的一种新的放射性示踪剂方法，碘-129是一种稀有的碘同位素，可用于跟踪培养试验中环境中的碘摄取情况。(2)比较不同保存状态的太平洋邻近组合中有孔虫标本的I/Ca值，以检验成岩作用对有孔虫I/Ca的影响。(3)通过测量墨西哥和秘鲁沿海两个关键地区的浮游和底栖有孔虫I/Ca以及稳定的碳、氧同位素，应用有孔虫I/Ca指标重建MCO期间热带东太平洋地区的氧化还原动力学。通过这项工作，我们将加强一个日益流行的代理系统，该系统可用于研究许多不同的区域和全球古气候事件，并限制大气二氧化碳升高与地球历史上类似时期的海洋ODZ之间的联系。这一奖项反映了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。

Development and Application of the Foraminiferal I/Ca Paleoredox Proxy The amount of dissolved oxygen in seawater varies and is linked to the distribution of life in the oceans. Expansion of marine oxygen deficient zones (ODZs), where oxygen reaches levels low enough to limit habitability, are an expected result of climate change. As we work to predict how this will affect future oceans, studying marine oxygenation over past climate change events is increasingly valuable. The Miocene Climatic Optimum (MCO), which occurred ~15-17 million years ago, is such an event, during which the Earth’s temperature warmed up to 3C on average and atmospheric carbon dioxide levels rose to an estimated 400-500ppm. Multiple lines of evidence suggest that severe ODZs around the globe intensified during the MCO, but direct evidence is lacking. We will use experiments to validate and apply the foraminiferal iodine-to-calcium (I/Ca) proxy—which uses the iodine content of microscopic fossils to estimate marine oxygen concentrations—to sediments from the MCO in the Eastern Tropical Pacific to study the intensity of low oxygen in the region during the MCO and improve predictions of ODZ changes in future climate change scenarios. This project combines laboratory experiments, field work, and analytical procedures that will incorporate International Ocean Discovery Program archives as well as training and research experiences for graduate and undergraduate students (including specific plans for mentoring Latinx undergraduate STEM majors). This effort consists of three discrete but interrelated projects to test, ground-truth, and apply the foraminiferal I/Ca proxy. The foraminifera I/Ca proxy is a method for reconstructing oxygen concentrations in ancient oceans based on the iodine concentrations of microscopic shells of protists called foraminifera. This proxy works because the levels of the iodine species (iodate) which can be incorporated into foraminiferal shells is linked to the oxygen content of the water they live in. Although this proxy has already been described and utilized in studies to reconstruct marine oxygen concentrations over multiple periods of global change, it has not yet been experimentally verified using controlled laboratory conditions or systematically screened for an impact of diagenesis (alteration of the fossil material that occurs after it is deposited in the sediments). To this end, we will: (1) conduct experiments on benthic and planktonic foraminifera to document the uptake of iodate into foraminiferal calcite, testing for systematic offsets from predicted rates that exist in some proxies. These experiments include plans for a novel radiotracer approach using iodine-129, a rare isotope of iodine which can be used to track ambient iodine uptake into the test in culture. (2) Compare the I/Ca values of foraminiferal specimens from neighboring assemblages from the Pacific Ocean that have very different preservation states to test for an effect of diagenesis on foraminiferal I/Ca. (3) Apply the foraminiferal I/Ca proxy to reconstruct the redox dynamics of the Eastern Tropical Pacific region during the MCO by measuring planktonic and benthic foraminifera I/Ca along with stable carbon and oxygen isotopes in two key localities off the coasts of Mexico and Peru. Through this work we will strengthen an increasingly popular proxy system that can be applied to study many different regional and global paleoclimate events and constrain links between elevated atmospheric carbon dioxide and marine ODZs relevant to today during an analogous interval in Earth’s history.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.