Constraining the oxygen history of Earth’s middle years using redox sensitive tracers

重建过去的氧气：如果解释正确，沉积岩可以提供地球演化过程中动荡的海洋和大气条件的时间线。第一次长寿的大气氧积累始于约24亿年前(Ga)，经典的说法是大氧化事件。虽然这一事件已经被记录了一段时间，但越来越多的证据表明，氧气的动态趋势比之前认为的更多。有几条证据表明，此时的氧气水平很高，包括对氧化和还原(氧化还原)反应敏感的元素的现代高浓度，以及海洋页岩中的高有机碳。最近的研究表明，在~2.05Ga之后，这些氧化趋势发生了逆转--这与长期以来认为氧气随时间增加的观点相反。这种脱氧事件没有得到很好的解决，但可能为理解地表条件如何不适合复杂的生物进化提供重要的联系。它还可以为海洋化学的快速变化提供类似物，让人想起现代气候变化带来的那种变化。 目标：我的研究项目旨在揭示我们呼吸的空气的早期发育阶段。这项研究整合了先进的分析技术，解决了全球性的重大问题，包括地球表面氧气条件刚刚形成的原因和后果。对氧气史的有力记录可以洞察生命的共同进化或阻碍生命进化的因素。“鸡和蛋”的争论仍然存在于文献中，关于大气氧气升高、全球冰川和生物进化的因果关系，包括氧气光合作用的出现。 科学方法：将使用互补的地球化学方法重建过去的环境条件，包括铀、铬和钼的同位素，以及对氧化还原敏感的元素浓度。这项研究将使用这些工具跟踪早期的氧气状况，以阐明海水中缺氧状况的程度。地层学和放射性测年方法将有助于确定这些变化的速度和时间。 影响：地球系统经历了一次大的扰动，破坏了向现代环境条件转变的趋势，但对这种逆转缺乏可能的机械解释。这项研究将调查与过去地质历史的关键时期海洋中氧的增加和减少趋势以及生物可利用元素浓度有关的假说。拼凑出地球通向其当前氧气状态的路径，对于解开地球系统中的生物地球化学反馈至关重要。过去海洋中的反馈机制有可能提供对现代缺氧海水的长期后果的洞察，这些海水随着现代气候变化而增加。

Reconstructing past oxygen: Sedimentary rocks, if interpreted correctly, can provide a timeline of the tumultuous oceanic and atmospheric conditions on the evolving Earth. The first long-lived accumulation of atmospheric oxygen began ~2.4 billion years ago (Ga), classically known as the Great Oxidation Event. Although this event has been documented for some time, mounting evidence points to more dynamic oxygen trends than previously thought. High oxygen levels at this time are implied by several lines of evidence, including high, modern-like concentrations of elements sensitive to oxidation and reduction (redox) reactions and high organic carbon in marine shales. Recent research shows a reversal of these oxygenation trends after ~2.05 Ga—contrary to a long-held view that oxygen increased with time. This de-oxygenation event is poorly-resolved, but might provide important links to understanding how surface conditions were inadequate for complex biological evolution. It could also provide analogues for rapid changes in ocean chemistry reminiscent of that attendant with modern climate change. Objectives: My research program aims to reveal the early developmental stages of the air we breathe. This research integrates advanced analytical techniques that address globally significant questions, including the causes and consequences of Earth’s fledgling surface oxygen conditions. A robust record of oxygen history provides insight into the co-evolution of life or impediments to it. “Chicken and egg” arguments remain in the literature regarding the cause and effect of atmospheric oxygen rise, global glaciations, and biological evolution, including the advent of oxygenic photosynthesis. Scientific Approach: Past environmental conditions will be reconstructed using complementary geochemical methods, including isotopes of uranium, chromium, and molybdenum, coupled with redox-sensitive element concentrations. This research will track early oxygen conditions using these tools to illuminate the extent of anoxic conditions in ocean waters. Stratigraphic and radiometric dating methods will help determine the rate and timing of these changes. Impact: The Earth system experienced a major disturbance that derailed a trend towards modern-like environmental conditions, but possible mechanistic explanations for this reversal are lacking. This research will investigate hypotheses related to both increasing and decreasing oxygen trends and concentrations of bioavailable elements in the oceans during key times in the geologic past. Piecing together Earth’s path to its current oxygenated state is pivotal to unraveling biogeochemical feedbacks in the Earth system. Feedback mechanisms in past oceans have the potential to provide insights into the long-term consequences of modern-day oxygen-starved ocean waters that are increasing in conjunction with modern climate change.