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

重建过去的氧气：沉积岩，如果解释正确，可以提供地球演化过程中混乱的海洋和大气条件的时间轴。第一次长期的大气氧积累开始于约24亿年前（Ga），经典地称为大氧化事件。尽管这一事件已经被记录了一段时间，但越来越多的证据表明，氧气的变化趋势比以前认为的更加动态。这一时期的高氧水平由几方面的证据所暗示，包括对氧化和还原（氧化还原）反应敏感的元素的高浓度，以及海洋页岩中的高有机碳。最近的研究表明，在~2.05 g0之后，这些氧合趋势发生了逆转，这与长期以来认为氧随时间增加的观点相反。这个脱氧事件尚未得到很好的解决，但可能为理解地表条件如何不足以进行复杂的生物进化提供了重要的联系。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变化。

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 Gacontrary 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.