Date of Award

Summer 2025

Document Type

Thesis (Master's)

Department or Program

Earth Sciences

First Advisor

William Leavitt

Abstract

Methane is a potent greenhouse gas and critical energy source for the global economy. Atmospheric methane concentrations play a critical role in global warming, while mitigating methane emissions may provide route to short-term reduction of warming effects. In order to track rates and sources of emissions, it is first necessary to understand the sources and sinks of methane; yet, there are challenges in distinguishing abiotic (water/rock), thermogenic, and biogenic methane. Traditional approaches of using the bulk isotopes of carbon (δ13C) and hydrogen (δD) of methane have limitations due to overlapping signatures and sensitivity to substrate isotope composition. In the last decade, multiply-substituted or “clumped” isotopologues of methane (13CH3D and 12CH2D2) have emerged as additional tools to track methane producing and consuming reactions. It is estimated that biological sources produce more than two thirds of global methane emissions. However, there is still a relatively poor understanding of how biological processes affect clumped isotope fractionation. Here, I investigate how methanogen strain, substrate, and temperature influence methane bulk and clumped isotope signatures. I cultured strains Methanosarcina barkeri and Methanosarcina acetivorans on either acetate, methanol, or monomethylamine (MMA) at either 25°C or 35°C and measured the bulk and clumped isotope compositions of product methane produced over time. I find that the bulk isotope values can reach enriched signatures typically associated with abiotic processes, but herein reflecting closed system distillation. The carbon isotope fractionation factors also show high variance compared to previous literature, reinforcing the limitations of relying on bulk isotopes alone to determine methane origin. Importantly, I contribute new clumped isotope measurements of methane derived from acetate and MMA. These data suggest that methanogenesis pathway, rather than temperature or strain, exerts a dominant control on clumped isotope composition. Overall, my findings can help reveal trends in environmental methane data that have complex formation histories. This study has implications for interpreting methane cycling in the environment and understanding the future impact of methane emissions on Earth’s climate.

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