Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the
Woods Hole Oceanographic Institution June 2017
Description:
This thesis documents the origin, distribution, and fate of methane and several of its isotopic
forms on Earth. Using observational, experimental, and theoretical approaches, I illustrate how
the relative abundances of 12CH4, 13CH4, 12CH3D, and 13CH3D record the formation, transport, and
breakdown of methane in selected settings.
Chapter 2 reports precise determinations of 13CH3D, a “clumped” isotopologue of methane,
in samples collected from various settings representing many of the major sources and reservoirs
of methane on Earth. The results show that the information encoded by the abundance
of 13CH3D enables differentiation of methane generated by microbial, thermogenic, and abiogenic
processes. A strong correlation between clumped- and hydrogen-isotope signatures in microbial
methane is identified and quantitatively linked to the availability of H2 and the reversibility of
microbially-mediated methanogenesis in the environment. Determination of 13CH3D in combination
with hydrogen-isotope ratios of methane and water provides a sensitive indicator of the
extent of C–H bond equilibration, enables fingerprinting of methane-generating mechanisms,
and in some cases, supplies direct constraints for locating the waters from which migrated gases
were sourced. Chapter 3 applies this concept to constrain the origin of methane in hydrothermal
fluids from sediment-poor vent fields hosted in mafic and ultramafic rocks on slow- and
ultraslow-spreading mid-ocean ridges. The data support a hypogene model whereby methane
forms abiotically within plutonic rocks of the oceanic crust at temperatures above ca. 300 C during
respeciation of magmatic volatiles, and is subsequently extracted during active, convective
hydrothermal circulation. Chapter 4 presents the results of culture experiments in which methane
is oxidized in the presence of O2 by the bacterium Methylococcus capsulatus strain Bath. The results
show that the clumped isotopologue abundances of partially-oxidized methane can be predicted
from knowledge of 13C/12C and D/H isotope fractionation factors alone.
Description:
The research activities documented in this thesis were made possible by grants to my advisor from the U.S.
National Science Foundation (NSF award EAR-1250394), the National Aeronautics and Space Administration
(NASA) Astrobiology Institute (NAI, University of Colorado, Boulder, CAN 7 under Cooperative Agreement
NNA15BB02A), the Department of Energy (DOE, Small Business Innovation Research program, contract
DE-SC0004575), the Alfred P. Sloan Foundation via the Deep Carbon Observatory, and a Shell Graduate
Fellowship through the MIT Energy Initiative.
I completed the bulk of the work in this thesis while being supported by a National Defense Science
and Engineering Graduate (NDSEG) Fellowship awarded through the Office of Naval Research of the U.S.
Department of Defense. The StanleyW.Watson Fellowship Fund provided support during my first summer
term at WHOI.The Charles M. Vest Presidential Fellowship at MIT supported me in the first year of my
Ph.D. studies. I received additional support that year through NSF award EAR-1159318 (to S. Ono and T.
Bosak) and theWalter & Adel Hohenstein Graduate Fellowship of Phi Kappa Phi. The MIT Earth Resources
Laboratory and PAOC Houghton Fund funded my attendance at several conferences.
Keywords:
Methane
;
Chemistry
;
Isotopes
;
Oxidation
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
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