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Laboratory and Field-Based Investigations of Subsurface Geochemical Processes in Seafloor Hydrothermal Systems

Eoghan Reeves, Ph.D., 2010
Jeffrey Seewald, Advisor

This thesis presents four discrete investigations into processes governing the chemical composition of seafloor hydrothermal fluids in a variety of geologic settings.  Chapters 2 and 3 concern the non-biological formation of organic compounds in vent fluids, which has direct implications for the possible emergence of life in hydrothermal environments.  Chapter 2 represents an experimental investigation of CH4 formation under hydrothermal conditions.  The reduction of CO2 to CH4, previously assumed to be kinetically inhibited without mineral catalysts, is shown to proceed on laboratory timescales.  In Chapter 3, the abundance of methanethiol, a crucial precursor for the emergence of chemoautotrophic life, is characterized in vent fluids from ultramafic-, basalt- and sediment-hosted hydrothermal systems.  Assumptions that CH3SH forms by CO2 reduction are not supported by the observed distribution.  Chapter 4 investigates factors regulating the hydrogen isotope composition of hydrocarbons under hydrothermal conditions.  Isotopic exchange between n-alkanes and water is shown to be facilitated by metastable equilibrium reactions with their corresponding alkenes.  In Chapter 5, a survey of the inorganic geochemistry of fluids from hydrothermal systems in the eastern Manus Basin is presented, indicating that fluids there are influenced by acidic magmatic solutions at depth, and subsequently modified by seawater entrainment and mixing-related processes.