Theoretical Study of Tellurium Isotope Fractionations in Ore-Forming Systems, and Studies of Doubly Substituted Isotopologues of Methane

• Main Author: Haghnegahdar, Mojhgan
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: eScholarship, University of California 2018
• Subjects: Geochemistry; Biogeochemistry; Atmospheric chemistry; 12CH2D2; 13CH3D; atmospheric methane; Clumped isotopes; methane; tellurium; Canada; Alaska; Siberia
• Online Access: http://www.escholarship.org/uc/item/7bd5c85d

This thesis encompasses three research projects on the stable isotopes of tellurium and multiply substituted isotopologues of methane. In the first project (Chapter 2), I used theoretical techniques to predict tellurium isotope signatures of ore formation processes. In this study, equilibrium mass-dependent isotopic fractionation among Te-bearing species relevant to the formation of precious-metal ores is estimated with first-principles thermodynamic calculations. This work is the first theoretical study on the isotope fractionation of tellurium in ore-forming systems. In the second project (Chapter3), I investigated the potential of 13CH3D and 12CH2D2, the doubly substituted mass-18 isotopologues of methane, as tools for tracking atmospheric methane sources and sinks. Methane is the most abundant organic chemical and the second most important long-lived greenhouse gas in the atmosphere. It also has a significant impact on the chemistry of the troposphere and stratosphere. However, there are large uncertainties in the sources and sinks of methane to the atmosphere, as well as in their variability in time and space. I used electronic structure methods and initial isotopologue abundance measurements, from the UCLA Nu Panorama and other laboratories [Stolper et al., 2015; Wang et al., 2015; Young et al., 2017], to estimate kinetic and equilibrium isotope signatures for 13CH3D and 12CH2D2. Then I predicted the abundances of singly and doubly substituted methane species in atmosphere in different scenarios using a whole-atmosphere box model. This study is the first model ever to describe the atmospheric methane budget that includes both doubly substituted isotopologues, 13CH3D and 12CH2D2.The third project (Chapter 4) focused on measuring 13CH3D and 12CH2D2 in methane samples from natural boreal lakes, collected in Alaska, Canada, and Siberia. Wetlands are among the largest methane sources, and they will need to be characterized as part of the construction of any realistic global budget of 13CH3D and 12CH2D2. In this project all measurements were performed using gas chromatography and a high-resolution gas-source multiple-collector mass spectrometer (the UCLA Nu Panorama). Finally, these data are combined with recent predictions of future emissions growth to model the likely impacts of boreal lakes on the future atmospheric CH4 isotopologue budget up to year 2100. In addition to testing our model atmospheric budget, doubly substituted isotopologue measurements of CH4 appear to provide information about the likely production mechanism for natural CH4 samples.