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）。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的组成。今天的大气中大约有20%是氧气，而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，最终导致了今天我们星球上生物的复杂性。为了更好地理解大气中的氧气与古代生命的发展之间的联系，了解当时存在多少氧气是很重要的，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，使用一种化学示踪剂，或记录在石灰石中的氧指纹，类似于法医科学家在犯罪现场寻找证据的方法。初步数据表明，石灰石中碘元素（用于伤口防腐的同一种物质）的存在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，有必要了解影响海洋中形成的石灰石中碘含量的因素。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成分来关注这个问题。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。一种识别古代氧气的新方法对了解古代生命的发展具有重要意义。这项工作不仅有助于理解现代生活是如何形成的，还具有许多教育意义。一项新的合作将在早期职业助理教授，Zunli Lu（雪城大学）和资深教授Tim Lyons（加州大学河滨分校）之间展开。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河畔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。技术描述。前寒武纪海洋的浅水是第一批产生氧气的光合生物的家园，也是许多早期进化的里程碑，比如真核生物和最终动物的兴起。前寒武纪海洋表面氧化还原条件的可靠测量是了解地球历史上这一关键过渡时期生命进化的关键。目前，对这些地表水中氧含量的了解是有限的。该项目旨在通过开发一种有前途的新替代方法来填补这一知识空白，即使用石灰石和白云岩中的碘钙比（I/Ca）。这种碘法基于两个观察结果：(1)氧化碘只存在于含氧良好的水中；(2)碘酸盐是碳酸盐沉淀过程中唯一加入的碘。研究小组将对经典现代/近现代浅海碳酸盐环境中I/Ca的吸收和成岩叠层进行首次系统评估。这些计划包括跟踪南佛罗里达和巴哈马从浅埋到深埋的成岩作用，使用独立的、约束良好的样品，跨越不同的成岩环境和过程，从早期有机再矿化到大气和海洋埋藏条件，再到白云化。这些碳酸盐分析将与现代富有机质页岩前驱相碘吸收和保留的新研究相补充。同化成有机物是现代海洋中最大的碘汇，而这个汇的再矿化并返回到上覆水柱是最大的海洋输入。由于有机质主要吸收被还原的碘，即碘化物，页岩中与碳酸盐I/Ca配对的I/ toc（总有机碳）比率应该允许区分I/Ca趋势（或部分趋势），反映局部氧化还原变化与更广泛的储层控制。换句话说，应用于非常古老的样本的代理将超越简单的存在-不存在情景，通过对碘的定量理解来解决全球状况的问题。海洋尺度的物质平衡。先前的研究已经注意到现代环境中局部I/TOC比值的氧化还原控制变化，从而使这种关系及其与古代碳酸盐中I/Ca比值的相关性复杂化。作为回应，研究小组将在黑海和卡里亚科盆地两个经典的现代缺氧环境中评估富有机质沉积物中碘的吸收和保存。最终，研究小组计划将I/TOC比率与其他不同的、已经被很好地理解的沉积氧条件的代用物进行交叉校准，并在此过程中强调采用多代用物方法重建地球最早生命的环境背景的必要性。

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.