School of Earth Sciences - Theses
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ItemDeep seawater circulation through oceanic crust stimulates the subseafloor biosphere( 2017)The deep subseafloor hosts the largest prokaryotic biomass on Earth, with cell abundances 3-4 orders of magnitude higher than the overlying ocean. Oxidant concentrations in the subseafloor are consumed sequentially by respiring microorganisms. However, studies show that seawater can flow through hydrologically conductive basalt and deliver oxidants upwards into overlying sediments. Our knowledge of how these fluids may influence biogeochemical cycles both within the oceanic basement and deeply-buried sediments is limited. Recent investigations have focused on more easily accessible sites (e.g. hydrothermal vents, mid-ocean ridges, ridge flanks, and continental margins), while buried subducting oceanic crust is less well studied. This thesis aims to address, through 16S rRNA and functional gene-level investigations, the question of how recirculating crustal fluids have affected deeply buried microorganisms in the subducting Philippines Sea plate. Through amplicon sequencing and functional gene analyses (dsrAB, mcrA) of a deep sediment anaerobic oxidation of methane zone, an inverted redox ladder is revealed that supports sulfate reduction at a greater depth than methanogenesis. Subseafloor microorganisms are intrinsically linked to the production of natural resources (e.g. natural gas, oil and metal ore); therefore, scientists need to understand how these biogeochemical systems run. Moreover, the reinvigoration of bacterial sulfate reduction coupled to hydrogen oxidation in sediments buried ~480 meters below the seafloor (mbsf) may explain the observed low methane concentrations at this site. Geochemical investigations at the sediment-basement interface (SBI) identified the presence of bioavailable metals released via basement alteration. These interpretations were supported by geochemical modelling and an abundance of 16S rRNA genes closely related to metal-cycling bacteria at the SBI. Additionally, substantial water rock reactions drove an increase in salinity, which corresponded with an increase in Halobacteria in the basement. Together these findings indicate the basement aquifer is delivering oxidants and microorganisms through the SBI of the subducting sea plate and influencing in situ microbial ecosystems. These communities offer insights into potential adaptations for microbial survival in deeply buried sediments. Lastly, this thesis extends the current subseafloor habitability database to regions where the ocean crust is being consumed by subduction.