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 2010
Description:
This thesis presents the results of four discrete investigations into processes governing the
organic and inorganic chemical composition of seafloor hydrothermal fluids in a variety of
geologic settings. Though Chapters 2 through 5 of this thesis are disparate in focus, each
represents a novel investigation aimed at furthering our understanding of subsurface geochemical
processes affecting hydrothermal fluid compositions. Chapters 2 and 3 concern the abiotic (nonbiological)
formation of organic compounds in high temperature vent fluids, a process which has
direct implications for the emergence of life in early Earth settings and sustainment of present day
microbial populations in hydrothermal environments. Chapter 2 represents an experimental
investigation of methane (CH4) formation under hydrothermal conditions. The overall reduction
of carbon dioxide (CO2) to CH4, previously assumed to be kinetically inhibited in the absence of
mineral catalysts, is shown to proceed on timescales pertinent to crustal residence times of
hydrothermal fluids. In Chapter 3, the abundance of methanethiol (CH3SH), considered to be a
crucial precursor for the emergence of primitive chemoautotrophic life, is characterized in vent
fluids from ultramafic-, basalt- and sediment-hosted hydrothermal systems. Previous
assumptions that CH3SH forms by reduction of CO2 are not supported by the observed
distribution in natural systems. Chapter 4 investigates factors regulating the hydrogen isotope
composition of hydrocarbons under hydrothermal conditions. Isotopic exchange between low
molecular weight n-alkanes and water is shown to be facilitated by metastable equilibrium
reactions between alkanes and their corresponding alkenes, which are feasible in natural systems.
In Chapter 5, the controls on vent fluid composition in a backarc hydrothermal system are
investigated. A comprehensive survey of the inorganic geochemistry of fluids from sites of
hydrothermal activity in the eastern Manus Basin indicates that fluids there are influenced by
input of acidic magmatic solutions at depth, and subsequently modified by variable extents of
seawater entrainment and mixing-related secondary acidity production.
Description:
The thesis research presented here was funded by the National Science Foundation
through grants OCE-0327448, OCE-0136954, MCB-0702677, OCE-0549829, and by the
Department of Energy grant DE-FG02-97ER14746.
Keywords:
Hydrothermal circulation
;
Submarine geology
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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