Collaborative Research: Tracking Middle-late Paleozoic Global-ocean Redox Conditions Using U Isotopes in Marine Carbonates

这项资助提案的目标是确定在一个独特的地质时期(3亿至4.2亿年前)古代海洋中溶解的氧气的相对数量，当时发生了三次重大物种灭绝，地球明显比今天温暖。了解古代海洋中氧气水平的历史很重要，因为氧气浓度影响所有海洋动物，控制石油和许多经济矿藏的形成和保存，并影响海洋和大气中的重要生化反应。该项目利用铀同位素这一相对较新的地球化学工具，开发了迄今为止最详细的海洋氧浓度变化历史。这笔资金支持下一代地球科学家的培训，将为第一代大学生提供实践和实验室经验，并将支持多名小学科学教师进行真实世界的科学，以获得信心，并为自己的课堂提供新想法。该项目的目标是利用泥盆纪-密西西比海相碳酸盐中的铀同位素开发全球海洋氧化还原曲线。大约1亿年的研究间隔是连接低氧前寒武纪-早古生代海洋和高氧晚古生代-现代海洋之间的桥梁，具有重要意义，因为：1)大气PO2水平翻了一番，2)从温室到冰库气候的转变和海洋环流的加强，3)三次大规模灭绝的发生，其中两次可归因于海洋缺氧，以及4)多次大规模的正向13C漂移。由于铀在海水中的滞留时间较长(约50万年)，海相碳酸盐的铀同位素组成具有记录全球平均大洋氧化还原条件的潜力。样本将从内华达州和犹他州的生物地层学测年序列中采集，间隔从大约10万年/样本到20000年/样本，通过特定的事件间隔。为了评估样品对局部氧化还原的影响，我们将分析对氧化还原敏感的金属、铁形态和Corg：P比率。为了验证复合曲线的全球性，我们将采样并与其他大陆上选定的较短的地层间隔进行比较。

The goal of the funded proposal is to determine the relative amounts of oxygen dissolved in ancient oceans during a unique geologic time interval (300 to 420 million years ago) when three major mass extinctions occurred and the Earth was significantly warmer than today. Understanding the history of oxygen levels in ancient oceans is important because oxygen concentrations influence all marine animal life, control the formation and preservation of petroleum and many economic mineral deposits, and affect important biochemical reactions in the oceans and atmosphere. This project utilizes the relatively new geochemical tool of uranium isotopes to develop the most detailed history of changing ocean oxygen concentrations to date. The funding supports the training of the next generation of Earth scientists, will provide hands-on field and laboratory experience for first-generation college students, and will support multiple primary school science teachers to conduct real-world science to gain confidence and provide new ideas for their own classrooms. The project goal is to develop a global-ocean redox curve using U isotopes from Devonian-Mississippian marine carbonates. The ~100 million year study interval, which bridges the transition between poorly oxygenated Precambrian-Early Paleozoic oceans and well-oxygenated Late Paleozoic-Recent oceans, is important because of: 1) a doubling of atmospheric pO2 levels, 2) a transition from greenhouse to icehouse climate and intensifying ocean circulation, 3) the occurrence of three mass extinctions, two of which are attributed to marine anoxia, and 4) multiple large positive 13C excursions. The U-isotope composition of marine carbonates has the potential to record mean global-ocean redox conditions owing to the long residence time (~500 thousand years) of U in seawater. Samples will be collected from biostratigraphically well-dated successions in Nevada and Utah at spacings ranging from ~100 thousand years/sample to 20 thousand years/sample through specific event intervals. To assess samples for local redox influences, we will analyze redox-sensitive metals, Fe-speciation, and Corg:P ratios. To verify the global nature of the composite curve, we will sample and compare to selected shorter stratigraphic intervals on other continents.