UC Berkeley

Stable isotopes have long been used in Earth science to provide quantitative constraintson the formation and modification of environmentally significant molecules. Stable carbon and hydrogen isotopic compositions are commonly used to trace the sources and sinks of methane in a variety of systems including economic hydrocarbon reservoirs, sediments, lakes, the ocean, hydrothermal systems, volcanic systems, and in biology. They are additionally used to constrain the formation, movement, and alteration of light alkanes, such as ethane and propane. This dissertation describes the use of stable isotopes to probe the equilibrium and kinetics of hydrogen isotopic exchange between methane, hydrogen, water, ethane, and propane. Chapter 1 of this dissertation details novel low temperature (≤200°C) experimental constraints on hydrogen isotopic fractionation between methane and hydrogen gas as well as theoretical constraints on methane, hydrogen, water, and carbon dioxide. Prior to this work, assessment of isotopic equilibrium between methane and hydrogen or water in nature was accomplished using extrapolation of high temperature experiments (≥200°C). Observed trends in our compiled natural and culture data support the literature paradigm that the hydrogen and carbon isotopes of methane are tracers of environmental energy conditions of formation or storage. Chapter 2 provides experimental constraint on the temperature dependant rate of hydrogen isotopic equilibration between CH4 and H2O over a range of temperatures relevant to hydrothermal systems. Extrapolating the results of these experiments provides information about the rate of hydrogen isotopic exchange while methane is in contact with water in hydrothermal, thermogenic, or biogenic environments. Chapter 3 concerns low temperature experimental hydrogen isotopic equilibration between ethane and H2 and propane and H2. With this new experimental constraint, we provide further support to recent claims that isotopic equilibrium sets the hydrogen isotopic composition of some natural gases.