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.
