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)。了解生命在地球历史上是如何发展的意味着了解古代地球上的环境条件。影响生命发展的最关键的条件之一是地球大气的组成。不像今天的大气，大约有20%的氧气，古代地球的大气没有氧气。早期生命发展的一个重大转变与大气中氧气的积累密切相关，这种积累始于25-30亿年前，最终导致了我们今天星球上的生物复杂性。为了更好地了解大气氧与古代生命发展的联系，重要的是了解当时存在着多少氧，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，即记录在石灰岩中的氧的指纹，类似于法医科学家开发出在犯罪现场寻找证据的方法。初步数据表明，元素碘(用作伤口防腐剂的同一物质)在石灰岩中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测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岩的碘含量。在这项研究中，研究小组将通过研究现代和近现代海泥和初始岩石中碘的化学作用来关注这一问题。这项工作将提供有关现代世界碘化学的有价值的信息，以及提炼和校准作为古代氧气示踪剂的碘含量。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理解现代生命是如何存在的，它还具有许多教育影响。职业生涯早期的助理教授吕尊利(锡拉丘兹大学)和资深教授蒂姆·莱昂斯(加州大学河滨分校)将启动一项新的合作。该项目将通过培训两名来自UCR多元化校园的研究生和本科生，为建设未来由美国STEM培训的劳动力做出贡献。此外，研究团队计划通过与新的河滨STEM学院和科学博览会辅导合作，为更年轻的学生提供重要的拓展。技术描述：前寒武纪海洋的浅水是第一批产氧光合作用生物的家园，也是早期进化的许多里程碑，例如真核生物的兴起，最终是动物的出现。可靠地测量前寒武纪表层海洋的氧化还原条件，是了解地球历史上这一关键过渡时期生命演化的关键。目前，对这些地表水中氧气水平的了解有限。该项目旨在通过开发一种有前景的新替代品来填补这一知识空白，即在石灰岩和白云岩中使用碘与钙的比率(I/Ca)。这种碘方法基于两个观察结果：(1)氧化后的碘物种碘酸盐只存在于富氧水体中，(2)碘酸盐是碳酸盐沉淀过程中唯一存在的碘物种。研究小组将首次对典型的现代/近现代浅海碳酸盐环境中的I/Ca吸收和成岩叠加进行系统的评估。这些计划包括在南佛罗里达和巴哈马群岛从浅埋到深埋追踪成岩作用，使用独立的受限样品，跨越不同的成岩环境和过程，从早期的有机再矿化到陨石和海洋埋藏条件，再到白云化。这些碳酸盐分析将与一项关于现代富含有机物质的页岩前体相中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，这种汇的再矿化并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的碘物种--碘，页岩中I/TOC(总有机碳)与碳酸盐I/Ca的比率应能区分反映局部氧化还原转变和更广泛的储层控制的I/Ca趋势(或部分趋势)。换句话说，应用于非常古老的样品的代理，将超越简单的有无情景，通过对碘？S海洋尺度质量平衡的定量理解，来研究全球条件问题。以前的研究已经注意到现代环境中局部I/TOC比率受氧化还原控制的变化，从而使这种关系及其与古代碳酸盐中I/Ca比率的相关性复杂化。作为回应，该团队将评估在两个经典的现代缺氧环境--黑海和卡里亚科盆地--富含有机物质的沉积物中跨氧化还原梯度的碘吸收和保存。最终，研究小组计划将I/TOC比率与黑色页岩中存档的其他不同的、已经得到充分理解的沉积氧条件代用品进行交叉校准，并在此过程中，强调采用多代用品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.