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史一般来说是已知的：在太古代基本上是缺氧的，在元古代经历了两次上升，在显生宙保持了现代的高氧条件。这种上升的氧合趋势影响了生物圈的共同进化，允许越来越高的氧气需求的生物逐步进化。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缺氧的现代阶段之间，环境氧水平有多高，氧水平的扰动是突然的还是渐进的，单调的还是逐步的，预示性的还是均匀的，大气氧水平的变化是否直接反映在海洋氧合和生物代谢中。本提案旨在开发一套新的环境氧合示踪剂，以硒同位素分异、硒相对于硫的丰度以及整个地质记录中海洋沉积岩中硒氧化还原物种的形式来提高我们对这一历史的理解。此外，质量相关的硒同位素分馏应该被证明是研究早期地球历史生命进化的有用的新生物特征。也有可能发现与质量无关的硒的分离，类似于硫的分离，这有可能为硒气体循环和大气氧气水平随时间的变化提供新的见解。该项目的目的是完善我们的知识，随着时间的推移，环境氧合及其限制硒的生物地球化学循环通过生物代谢途径。为了做到这一点，我们将采用一套最新设计的实验方法，包括硫醇-棉纤维从酸消化的岩石样品中定量提取硒，连续流氢化物产生以选择性地挥发低浓度的硒，以及多收集器电感耦合等离子体质谱法以高灵敏度同时测量硒的多种同位素。为了为解释新的同位素结果提供理论背景，研究人员还将建立硒生物地球化学循环的4盒模型和不同氧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发现和理解的进步，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研究，扩大代表性不足的群体的参与，在这种情况下是妇女参与地球科学。它将加强研究和教育的基础设施，因为资金要求为维护ISOLAB设备的研究技术人员提供工资支持，ISOLAB是一个稳定同位素实验室，由3个部门，2个项目和4名主要科学家共享，并向所有华盛顿大学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，以增进科学理解。该研究计划本质上是跨学科的，涉及地球与空间科学、大气科学和海洋学三个部门的科学家，从而为硒生物地球化学循环的环境综合研究提供了框架。

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.