The Geochemistry of Methane Isotopologues

David Wang, Ph.D., 2017
Shuhei Ono, Advisor

This thesis documents the origin, distribution, and fate of methane and several of its isotopic forms on Earth.   Precise determinations of 13CH3D, a “clumped” isotopologue of methane, were made on samples collected from various settings on Earth.  The results show that the abundance of 13CH3D differs between methane generated by microbial, thermogenic, and abiogenic processes.  A strong correlation between clumped- and hydrogen-isotope signatures in microbial methane is identified and linked to H2 availability and the reversibility of methanogenesis.  Determination of 13CH3D with hydrogen-isotope ratios of methane and water enables fingerprinting of methane-generating mechanisms and supplies constraints for locating the waters from which migrated gases were sourced.  This concept is applied to constrain the origin of methane in hydrothermal fluids from sediment-poor vent fields hosted in mafic and ultramafic rocks on unsedimented mid-ocean ridges.  The data support a model whereby methane forms within plutonic rocks deep in the oceanic crust at above ca. 300 °C during respeciation of magmatic volatiles, and subsequently leaches during active hydrothermal circulation.  Also presented are culture experiments in which methane is oxidized aerobically by Methylococcus capsulatus.  The results show that the clumped isotopologue abundances of partially-oxidized methane can be predicted from 13C/12C and D/H fractionations alone.