Date of Award

Spring 1-1-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Alexis S. Templeton

Second Advisor

Lang Farmer

Third Advisor

Stephen Mojzsis

Fourth Advisor

Peter Kelemen

Fifth Advisor

John Spear

Abstract

Continental serpentinizing systems, such as the Samail Ophiolite in Oman, are excellent locations to study low-temperature water/rock reactions leading hydrogen production. Hydrogen is a strong electron donor that, along with electron acceptors such as CO2 or SO42-, can sustain microbial life. I performed experimental and analytical work with partially serpentinized peridotite from Oman to determine that Fe(II)-bearing brucite is a key reactive mineral leading to hydrogen production. At 100°C and fluid compositions similar to those found in shallow serpentinizing aquifers, we experimentally observed the destabilization of Fe(II)-brucite to produce hydrogen and Fe(III)-bearing magnetite. These reactions occur quickly over a period of several months. H2 production is associated with CO2 reduction into low weight molecular acids, and perhaps solid carbonaceous material. This experimental work was paired with a field study of drill cuttings obtained from subsurface rocks in contact with serpentinizing fluids rich in H2 and CH4 at depths ranging from 17m to 262m. Mineralogical and geochemical analysis of drill cuttings showed a similar trend of decreased brucite abundance and increased magnetite formation as the rock became more altered near the surface. CH4 sampled from subsurface fluids has δ13C values as positive as +3‰. Although these δ13C values would traditionally be interpreted as abiotic, we believe it is possible microbial activity may contribute to the CH4 isotope values because of the presence of methanogens and methanotrophs in the subsurface fluids, as determined by 16S rRNA analysis. Thus, we isolated Methanobacterium was from the same subsurface fluids in Oman to investigate the carbon isotope fractionation factors associated with methanogenesis at high pH, probing if microbiology could be responsible for the unique isotope values found in the fluids. Carbon isotope fractionation during methanogenesis by Methanobacterium under conditions of carbon-limitation at pH ~9 is suppressed, leading to δ13CCH4 values as high as -28‰. Such δ13CCH4 values may be mistakenly interpreted as abiotic based on current classifications of “biotic” versus “abiotic” methane isotopic signatures. Thus, we need to carefully evaluate the carbon cycling pathways operating in serpentinizing systems and not automatically assume abiotic or biotic origins for CH4 until the system is well characterized.

Available for download on Wednesday, May 16, 2018

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