This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). The atmosphere of the Earth is dominated by two gases, N2 and O2. N2 is virtually inert and has been the most abundant gas since the atmosphere formed. In contrast, O2, which came to comprise 10 to 30% of the volume of the atmosphere over the past ~500 million years, is highly reactive and must be produced continuously by the photobiological oxidation of water. The cycles of these two elements are linked; the fluxes of N are entirely controlled by biological reactions that are strongly influenced by the concentration of oxygen in the environment. The major goal of the proposed research is to elucidate the interactions and feedbacks between the cycles of nitrogen and oxygen from the middle Archean to middle Proterozoic, a period in Earth's history when it permanently tipped from being an anaerobic or microaerobic planet, to one with an atmosphere sufficiently rich in oxygen to permit the formation of a stratospheric ozone layer. The proposed research is based on an understanding of the N isotope composition of sedimentary organic matter, which can be used to characterize the dominant form of N that was assimilated by autotrophic organisms. Specifically, we propose to use the isotope fractionations associated with the successive transfers of N through the O-dependent N-cycle to determine the paleoredox state of the Precambrian ocean, and its correlatation with inferred geological events. We focus on three sample sets: 1) the Mesoarchean to early Proterozoic in South Africa; 2) late Paleoproterozoic in the Animikie Basin and 3) Meosproterozoic Roper Gp. We will also investigate other samples from North America in order to fill temporal gaps between these three samples sets, or to provide spatial coverage and diversity of environments at specific times. Our approach utilizes careful, high precision, analytical techniques combined with a simple, elegant box model of the coupled CNO cycles with an embedded N isotope submodel. All ä15N analyses in the reported in the literature for Precambrian sediments have very high C/N ratios, indicative of N loss, but little attention is given to changes in ä15N that may have resulted. We will analyze N and N isotopes in bulk material, kerogen, and in fixed lattice sites to determine whether the N isotope system is closed. In collaboration with colleagues at JAMSTEC, a subset of samples will be analyzed for porphyrin ä15N. We will also analyze selected samples for phyllosilicate mineralogy and major element chemistry to address possible N leaching by potassic or saline fluids which can affect ä15N. Finally, we will build upon our earlier modeling efforts to develop a model that will use N isotopes in a fully integrated model of the N, O, C, S and Fe cycles as the oxidation state of Earth's surface increased during the Precambrian. Broader Impacts The results of this proposed work will be significant to increasing our understanding how the N and O cycles interacted early in Earth's history. We can use the changes in ä15N of organic matter, in conjunction with other isotope records and from modeling work to determine the progression from a fully anoxic ocean to oxic surface/anoxic deep to a fully oxic ocean state corresponding to nitrate dominance of the fixed N pool. We have developed an outreach program to improve ocean science literacy. However our research is also intended to assist educators who are in the center of debates regarding creationism and intelligent design, with little guidance on evolution. IMCS and Geology run open houses where this research will be highlighted, and it will be part of a summer workshop as part of ESTEEM, a Rutgers-NJ Dept of Education initiative directed at middle schools. This project will provide an undergraduate student with the opportunity to participate in a collaborative study involving different universities and countries. The students will be involved in laboratory work and will have the opportunity to work with professors, researchers, and students at Rutgers and Lehigh Universities, and encouraged to present their work at meetings.
|Effective start/end date||7/15/09 → 12/31/10|
- National Science Foundation (NSF)