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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2018)

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.

    Cover page: Theoretical Study of Tellurium Isotope Fractionations in Ore-Forming Systems, and Studies of Doubly Substituted Isotopologues of Methane
    • Thesis
    • Peer Reviewed
    UCLA Electronic Theses and Dissertations (2024)

    Measurements of isotope fractionation have long focused on mass dependent fractionations, though mass independent effects have long been recognized. One mechanism of mass independent fractionation is the nuclear field shift. Field shift isotope fractionation was first proposed by Bigeleisen (1996) and Nomura et al. (1996) as a mechanism explain to patterns of redox-driven uranium isotope fractionation. This thesis aims to model the nuclear field shift using ab initio methods in multiple isotope systems, in relevant natural species, as well as to explore potential applications of field shift fractionation measurements for these systems. The main method used is the DFT-PAW method of Schauble (2013), which makes it possible to determine field shifts in solids and complex aqueous species.In the uranium isotope system, 238U/235U is predicted to be ~1-2‰ higher in U(IV) than coexisting U(VI) species at 25ºC. U(V) species are either intermediate or similar to U(IV) species. Fractionations of up to 0.6‰ are predicted between species in the same oxidation state. In the case of plutonium and neptunium, Pu(V) and Pu(IV) species thought to be relevant to natural systems are found to be isotopically similar to each other, whereas Pu(VI) species will be approximately 2‰ lower in 242Pu/239Pu than Pu(V) or Pu(IV), and Pu(III) species will have approximately 3‰ higher 242Pu/239Pu than Pu(IV) or Pu(V). Np(VI) will be 0.6‰ lower in 237Np/235Np than coexisting Np(V) species, whereas Np(IV) and Np(III) will have approximately 1.3‰ higher 237Np/235Np. In the final study, field shift effects in platinum and iridium are studied, using Mössbauer isomer shifts according to the method of Schauble (2023) in to investigate iridium. Predicted field shifts for both 198Pt/194Pt and 193Ir/191Ir are relatively modest, approximately <0.1‰ at ambient temperatures for most species (e.g., mass dependent fractionation effects likely dominate for these elements at low temperatures). Notably, Pt in an Fe-alloy (as a possible analog for the Earth’s core) is predicted to show an equilibrium fractionation of 0.09‰ at 3000 K relative to PtII substituted in an olivine crystal structure (as a speculative analog for the silicate mantle), which could explain observed superchondritic 198Pt/194Pt found in Archean mantle samples.

      Cover page: Field shift fractionations in novel isotope systems
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2013)
      The rare evaporite minerals hanksite, Na22K(SO4) 9(CO3)2Cl, and tychite, Na6Mg 2(CO3)4(SO4), are excellent case studies for the high-pressure behavior of ionic groups since their structures combine ionic complexity and high symmetry (hexagonal P63/m and cubic Fd3, respectively). Here we investigate the structure and compressibility of hanksite up to 20 GPa in the diamond-anvil cell using Raman spectroscopy and X ray diffraction and of tychite up to 17 GPa in the diamond cell using X ray diffraction and first-principles modeling. At ambient pressure, the Raman spectrum of hanksite has a single sulfate v1 frequency at 992 cm -1 with a lower-frequency shoulder. As pressure is increased, this mode splits into two distinct peaks, which arise from two distinct local environments for the sulfate tetrahedra within the hanksite structure. Below 10 GPa, the mode Grüneisen parameter of the dominant sulfate v1 frequency is 0.27(1); the mode Grüneisen parameter of the lower frequency shoulder is 0.199(7). X ray diffraction data of hanksite indicate a 5% volume drop between 8-10 GPa with no apparent change of symmetry. A Birch-Murnaghan fit to the data below 8 GPa yields an isothermal bulk modulus of 66(1) GPa for hanksite and 85(1) GPa for tychite, with K′ fixed at 4.
        Cover page: Spectroscopic and X-ray diffraction investigation of the behavior of hanksite and tychite at high pressures, and a model for the compressibility of sulfate minerals
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2014)
        The use of carbonate 'clumped isotope' thermometry as a geochemical technique to determine temperature of formation of a carbonate mineral is predicated on the assumption that the mineral has attained an internal thermodynamic equilibrium. If true, then the clumped isotope signature is dependent solely upon the temperature of formation of the mineral without the need to know the isotopic or elemental composition of coeval fluids. However, anomalous signatures can arise under disequilibrium conditions that can make the estimation of temperatures uncertain by several degrees Celsius. Here we use ab initio calculations to examine the potential disequilibrium mineral signatures that may arise from the incorporation of dissolved inorganic carbon (DIC) species (predominantly aqueous carbonate and bicarbonate ions) into growing crystals without full equilibration with the crystal lattice.We explore theoretically the nature of clumping in the individual DIC species and the composite DIC pool under varying pH, salinity, temperature, and isotopic composition, and speculate about their effects upon the resultant disequilibrium clumping of the precipitates. We also calculate equilibrium clumped signatures for the carbonate minerals calcite, aragonite, and witherite. Our models indicate that each DIC species has a distinct equilibrium clumped isotope signature such that, δ47(H2CO3)>δ47HCO3->δ47(equilibrium calcite)>δ47CO32-, and predict a difference between δ47HCO3-andδ47CO32->0.033‰ at 25°C, and that δ47 (aragonite)>δ47 (calcite)>δ47 (witherite). We define the calcite clumped crossover pH as the pH at which the composite δ47 (DIC pool)=δ47 (equilibrium calcite). If disequilibrium δ47 (calcite) is misinterpreted as equilibrium δ47 (calcite), it is possible to overestimate or underestimate the growth temperature by small but significant amounts. Increases in salinity lower the clumped crossover pH and may cause larger effects. Extreme effects of pH, salinity, and temperature, such as between cold freshwater lakes at high latitudes to hot hypersaline environments, are predicted to have sizeable effects on the clumped isotope composition of aqueous DIC pools.In order to determine the most reliable and efficient modeling methods to represent aqueous dissolved inorganic carbon (DIC) species and carbonate minerals, we performed convergence and sensitivity testing on several different levels of theory. We used 4 different techniques for modeling the hydration of DIC: gas phase, implicit solvation (PCM and SMD), explicit solvation (ion with 3 water molecules) and supermolecular clusters (ion plus 21 to 32 water molecules with geometries generated by molecular dynamics). For each solvation technique, we performed sensitivity testing by combining different levels of theory (up to 8 ab initio/hybrid methods, each with up to 5 different sizes of basis sets) to understand the limits of each technique. We looked at the degree of convergence with the most complex (and accurate) models in order to select the most reliable and efficient modeling methods. The B3LYP method combined with the 6-311++G(2d,2p) basis set with supermolecular clusters worked well. © 2013 Elsevier Ltd.
          Cover page: Theoretical constraints on the effects of pH, salinity, and temperature on clumped isotope signatures of dissolved inorganic carbon species and precipitating carbonate minerals
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2017)
          Molecular nitrogen (N2) comprises three-quarters of Earth's atmosphere and significant portions of other planetary atmospheres. We report a 19 per mil (‰) excess of 15N15N in air relative to a random distribution of nitrogen isotopes, an enrichment that is 10 times larger than what isotopic equilibration in the atmosphere allows. Biological experiments show that the main sources and sinks of N2 yield much smaller proportions of 15N15N in N2. Electrical discharge experiments, however, establish 15N15N excesses of up to +23‰. We argue that 15N15N accumulates in the atmosphere because of gas-phase chemistry in the thermosphere (>100 km altitude) on time scales comparable to those of biological cycling. The atmospheric 15N15N excess therefore reflects a planetary-scale balance of biogeochemical and atmospheric nitrogen chemistry, one that may also exist on other planets.
            Cover page: Extreme enrichment in atmospheric 15N15N
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