Selenium biogeochemistry as a deep-time redox proxy and biosignature

地球岩石圈、水圈和大气的环境氧合历史大体上是众所周知的：太古宙基本上是缺氧的，在元古代经历了两次上升，并通过显生宙保持了现代的高氧条件。这种氧合作用上升的趋势通过允许氧气需求量越来越高的生物体逐步进化，影响了生物圈的共同进化。然而，关于这段历史，尚不清楚的是，在基本上缺氧的古代阶段和高度氧化的现代阶段之间，环境氧水平有多高，氧水平的扰动是突然还是渐进的，是单调的还是逐步的，是预示的还是均匀的，或者大气氧水平的变化是否直接反映在海洋氧化作用和生物新陈代谢中。这一建议旨在开发一套新的环境氧化示踪剂，以提高我们对这一历史的理解，包括整个地质记录中海相沉积岩中的硒同位素分馏、相对于硫的硒丰度和硒的氧化还原形态。此外，依赖于质量的硒同位素分馏应该被证明是一个有用的新的生物标志，用于研究地球早期历史上的生命演化。也有可能发现类似于硫的与质量无关的硒的分级，有可能提供关于硒气体循环和大气氧水平随时间变化的新见解。该项目的目标是完善我们对环境氧化随时间的变化及其对通过生物代谢途径进行的硒生物地球化学循环的限制的知识。为此，将采用最近设计的一套实验方法，包括硫醇-棉纤维定量提取酸消化岩石样品中的硒，连续流动氢化物发生法选择性挥发低浓度的硒，以及多收集器电感耦合等离子体质谱同时高灵敏度地测定硒的多个同位素。为了为解释新的同位素结果提供理论背景，研究人员还将开发一个硒生物地球化学循环的4盒模型和一个在不同氧气水平下硒气体行为的一维光化学模型。拟议研究的更广泛影响在于促进发现和理解，同时通过让研究生参与大多数研究活动来促进教学、培训和学习。它将通过一名女研究生积极参与这项研究，扩大代表不足的群体的参与，在这种情况下，妇女参与地球科学。它将加强研究和教育的基础设施，因为需要为一名研究技术员提供工资支持，以维护IsoLab的设备，IsoLab是一个稳定的同位素实验室，由3个部门、2个项目和4名首席科学家共享，并向德克萨斯州所有社区和外部用户开放。还要求提供资金，以便在跨学科会议上广泛交流成果，以增进对科学的理解。该研究计划本质上是跨学科的，涉及地球和空间科学、大气科学和海洋学3个系的科学家，从而为对硒生物地球化学循环进行环境综合调查提供了框架。

The history of Earth's environmental oxygenation of the lithosphere, hydrosphere and atmosphere is known in general terms: essentially anoxic in the Archean, passing through two rises during the Proterozoic and maintaining modern highly oxic conditions through the Phanerozoic. This trend of rising oxygenation has influenced the co-evolution of the biosphere by allowing organisms with increasingly high oxygen demands to evolve progressively. However, what is not well known about this history is how high environmental oxygen levels were between the essentially anoxic ancient stage and the highly oxic modern stage, if perturbations in oxygen levels were sudden or gradual, monotonic or step-wise, foreshadowed or uniform, or whether changes in atmospheric oxygen levels were directly reflected in marine oxygenation and by biological metabolism. This proposal seeks to develop a new set of environmental oxygenation tracers to improve our understanding of this history, in the form of selenium isotopic fractionations, selenium abundances relative to sulfur, and selenium redox speciation in marine sedimentary rocks from throughout the geological record. In addition, mass-dependent selenium isotopic fractionations should prove to be a useful new biosignature for investigating the evolution of life during early Earth history. It is also possible that mass-independent fractionations of selenium, analogous to those of sulfur, will be discovered, with the potential to provide novel insights into selenium gas cycling and atmospheric oxygen levels over time.The objective of the project is to refine our knowledge of environmental oxygenation through time and its constraints on selenium biogeochemical cycling through biological metabolic pathways. In order to do this, a recently-devised set of experimental methods will be employed, involving thiol-cotton fiber to quantitatively extract selenium from acid-digested rock samples, continuous-flow hydride generation to selectively volatilize selenium in low concentrations and multi-collector inductively-coupled-plasma mass-spectrometry to simultaneously measure multiple isotopes of selenium with high sensitivity. To provide a theoretical background for interpreting the new isotopic results, investigators will also develop a 4-box model of the selenium biogeochemical cycle and a 1-D photochemical model of selenium gas behavior at different oxygen levels. The broader impacts of the proposed study lie in the advancement of discovery and understanding while promoting teaching, training and learning by involving a graduate student in most of the research activities. It will broaden the participation of underrepresented groups, in this case women in earth science, by the active participation of a female graduate student in the research. It will enhance the infrastructure for research and education, as funding is requested for salary support for a research technician to maintain equipment in ISOLAB, a stable isotope laboratory shared between 3 departments, 2 programs and 4 principal scientists and open to all of the U.W. community and to external users. Funds are also requested so that results can be communicated broadly at interdisciplinary conferences to enhance scientific understanding. The research plan is inherently interdisciplinary, involving scientists affiliated with 3 departments: Earth & Space Sciences, Atmospheric Sciences and Oceanography, thus providing the framework for an environmentally comprehensive investigation of the selenium biogeochemical cycle.