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

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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.