Master of Science (MS)
Geology and Geophysics
Microbially-mediated karstification through the production of metabolic byproducts has been well-documented in cave environments, but less is known about deep karstic settings. This research aimed to distinguish between microbial and geochemical influences on carbonate dissolution in the Edwards Aquifer, a prolific karst aquifer in central Texas, specifically from a transect of six wells across a transition from fresh to saline water in New Braunfels. For the first time, a portion of the aquifer’s bacterial diversity was examined from molecular 16S rRNA gene sequence analyses, which revealed that Alphaproteobacteria, Gammaproteobacteria, and Betaproteobacteria dominated the aquifer, with rare Bacteroidetes, Nitrospirae, and Firmicutes taxonomic groups. Local geochemical conditions for each well (H2S levels, TDS, and depth) accounted for ~83% of the genetic variability among wells. In general, putative chemoorganotrophic microorganisms were prevalent in all wells, but chemolithoautotrophs associated with sulfur oxidation were prevalent only in sulfidic wells. Sterile and reactive in situ microcosms containing experimental calcite and dolomite fragments were deployed in the transect wells for ~1 month. Most of the mineral fragments in the microcosms (~64%) had statistically significant mass loss, although fluids were saturated with respect to calcite (SI= +0.6 to +0.14) and dolomite (SI= +0.18 to +0.43). There was greater mass loss in reactive microcosms having higher surface cell densities on colonized mineral surfaces. These results suggest that microbial colonization establishes geochemical disequilibrium between mineral surfaces and bulk aquifer fluids, regardless of the metabolic potential of the microorganisms. Microbially-mediated carbonate dissolution is possible in both fresh and saline water deep karstic zones of the aquifer.
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Gray, Cassie Jo, "The Influences of Microbial Diversity on Carbonate Geochemistry Across a Transition from Fresh to Saline Water in the Edwards Aquifer, Texas" (2010). LSU Master's Theses. 4268.