Collaborative Research: Using Iodine-Calcium Ratios in Carbonates to Measure Oxygen in Ancient Atmospheres during the Development of Early Life

更广泛的意义（非技术性）：了解地球历史上生命是如何发展的，意味着了解古代地球的环境条件。 塑造生命发展的最关键条件之一是地球大气层的组成。 不像今天的大气中大约20%的氧气，古代地球的大气中没有氧气。 早期生命发展的一个重大转变与25 - 30亿年前开始的大气中氧气的积累密切相关，最终导致了我们地球上今天的生物复杂性。 为了更好地了解大气中氧气与古代生命发展的关系，重要的是要知道有多少氧气存在，特别是在作为早期生命摇篮的浅海中。 为了测量古代海洋中的氧含量，这个研究小组正在开发一种方法，使用化学示踪剂，或记录在石灰岩中的氧指纹，类似于在犯罪现场寻找证据的法医科学家。初步数据表明，碘元素的存在石灰岩中的氧气（与伤口上的防腐剂相同）与地球历史上氧气的存在密切相关。 然而，为了使用碘作为示踪剂可靠地确定古代大气中的氧浓度，有必要了解哪些因素影响海洋中形成的石灰石中的碘含量。 这项研究的研究小组将通过研究现代和近现代海洋泥浆和早期岩石中碘的化学性质来关注这个问题。 这项工作将为现代世界的碘化学提供有价值的信息，以及精炼和校准碘含量作为古代氧的示踪剂。 一种对古代氧气进行指纹识别的新方法对了解古代生命的发展具有重要意义。这项工作不仅可能有助于了解现代生命是如何存在的，而且还具有许多教育影响。 一个新的合作将开始之间的早期职业助理教授，陆尊利（锡拉丘兹大学）和资深教授蒂姆里昂（加州滨江大学）。 该项目将通过培训来自UCR多元化校园的两名研究生和本科生，为建设未来的美国STEM培训劳动力做出贡献。 此外，研究团队计划通过与新的滨江STEM学院和科学博览会指导合作，为年轻学生提供重要的外展服务。技术说明.前寒武纪海洋的浅水沃茨是第一批产生氧气的光合生物的家园，也是早期进化的许多里程碑，如真核生物和最终动物的兴起。 对前寒武纪海洋表面氧化还原条件的可靠测量是理解地球历史上这一关键过渡时期生命进化的关键。 目前，对这些表面沃茨中氧含量的了解有限。 该项目旨在通过开发一种有前途的新代理，即使用石灰岩和白云岩中的碘钙比（I/Ca）来填补这一知识空白。 这种碘的方法是基于两个观察：（1）氧化的碘物种碘酸盐只存在于良好的含氧水中，（2）碘酸盐是碳酸盐沉淀过程中唯一的碘物种。研究小组将在经典现代/近现代浅海碳酸盐环境中对I/Ca的吸收和成岩叠印进行首次系统评估。这些计划包括跟踪成岩作用，从浅埋到深埋在南佛罗里达和巴哈马群岛使用独立良好的约束样本跨越不同的成岩环境和过程，从早期有机矿化到大气和海洋埋藏条件，以石化。这些碳酸盐分析将补充碘吸收和保留在现代有机丰富，页岩前体相的新研究。同化成有机物是现代海洋中最大的碘汇，而这种汇的再矿化和输出回上覆水柱是最大的海洋输入。由于有机质主要吸收还原碘物种，碘化物，I-TOC（总有机碳）的比例在页岩与碳酸盐I/Ca配对应允许歧视I/Ca的趋势（或部分趋势），反映当地的氧化还原位移与更广泛的储层控制。换句话说，代理，适用于非常古老的样本，将采取超越简单的存在-不存在的情况下，对全球条件的问题，通过定量了解碘？海洋尺度的质量平衡。以前的研究已经注意到，在现代环境中，氧化还原控制的局部I/TOC比值的变化，从而使这种关系及其与古碳酸盐中I/Ca比值的相关性复杂化。 作为回应，该团队将评估在两个经典的现代缺氧环境中，黑海和Cariaco盆地的氧化还原梯度中富含有机物的沉积物中碘的吸收和保存。最终，研究小组计划将I/TOC比率与其他不同且已经很好理解的沉积氧条件代理进行交叉校准，这些代理在黑色页岩中存档，并在此过程中强调多代理方法重建地球最早生命环境背景的必要性。

Broader significance (non-technical).Understanding how life developed in Earth's history means understanding the environmental conditions on ancient Earth. One of the most critical conditions to shape the development of life is the composition of Earth's atmosphere. Unlike today's atmosphere, which is approximately 20% oxygen, the atmosphere of the ancient Earth had no oxygen. A major transition in the development of early life has been strongly linked to the accumulation of oxygen in the atmosphere which began 2.5 - 3 billion years ago, ultimately leading to the biological complexity on our planet today. To better understand the connection of atmospheric oxygen to the development of ancient life, it is important to know how much oxygen was present, particularly in the shallow oceans which served as the cradle of early life. To measure oxygen content in ancient oceans, this research team is developing a method to use a chemical tracer, or fingerprint of oxygen recorded in limestones, analogous to forensic scientists who develop methods to find evidence at a crime scene.Preliminary data has indicated that the presence of the element iodine (the same substance used as an antiseptic on wounds) in limestones is strongly correlated to the presence of oxygen in Earth's history. However, to reliably determine oxygen concentrations in the ancient atmosphere using iodine as a tracer, it is necessary to understand what factors influence the iodine content of limestones that form in the ocean. The research team in this study will focus on that question with this work by studying the chemistry of iodine in modern and near-modern marine muds and incipient rocks. This work will provide valuable information on iodine chemistry in the modern world, as well as refining and calibrating iodine content as a tracer of ancient oxygen. A new means to fingerprint ancient oxygen has implications for understanding the development of life in ancient times.Not only can this work potentially contribute to understanding how modern life came to exist, it also has a number of educational impacts. A new collaboration will be initiated between an early-career assistant professor, Zunli Lu (Syracuse University) and senior professor Tim Lyons (University of California Riverside). The project will contribute to building the future US STEM-trained workforce via the training of two graduate students and undergraduates from the diverse campus of UCR. Additionally, the research team plans significant outreach for younger students by working with the new Riverside STEM academy and science fair mentoring. Technical description.The shallow waters of the Precambrian ocean were home to the first oxygen-producing photosynthetic organisms as well as many of the milestones of early evolution, such as the rise of eukaryotes and ultimately animals. Reliable measurements of the redox conditions in the Precambrian surface ocean are key to understanding the evolution of life during this critical transitional period in Earth's history. Currently, knowledge of oxygen levels in these surface waters is limited. This project aims to fill this knowledge gap by developing a promising new proxy, namely using iodine-to-calcium ratios (I/Ca) in limestones and dolostones. This iodine method is based upon two observations: (1) the oxidized iodine species iodate exists exclusively in well-oxygenated water and (2) iodate is the only iodine species incorporated during carbonate precipitation.The research team will conduct the first systematic evaluation of uptake and diagenetic overprints for I/Ca in classic modern/near-modern shallow marine carbonate settings. These plans include tracking diagenesis from shallow to deep burial in South Florida and the Bahamas using independently well constrained samples spanning diverse diagenetic settings and processes ranging from early organic remineralization to meteoric and marine burial conditions to dolomitization.These carbonate analyses will be complemented with a novel study of iodine uptake and retention in modern organic-rich, shale precursor facies. Assimilation into organic matter represents the largest iodine sink in the modern ocean, and remineralization of this sink and export back to the overlying water column is the largest marine input. Because organic matter mostly assimilates the reduced iodine species, iodide, ratios of I-to-TOC (total organic carbon) in shales paired with carbonate I/Ca should allow for the discrimination of I/Ca trends (or portions of trends) reflecting local redox shifts versus broader reservoir controls. In other words, the proxy, as applied to very old samples, will be taken beyond simple presence-absence scenarios toward questions of global conditions through a quantitative understanding of iodine?s ocean-scale mass balance. Previous studies have noted redox-controlled variations in local I/TOC ratios in modern environments, thus complicating this relationship and its relevance to I/Ca ratios in ancient carbonates. In response, the team will assess iodine uptake and preservation in organic-rich sediments across redox gradients in two classic modern anoxic settings, the Black Sea and the Cariaco Basin. Ultimately, the research team plans to cross-calibrate I/TOC ratios against other diverse and already well-understood proxies for depositional oxygen conditions as archived in black shales and, in the process, to highlight the necessity for a multi-proxy approach to reconstructing the environmental backdrop of Earth's earliest life.