Master of Science, The Ohio State University, 2020, Earth Sciences

The isotopic composition and mineralogy of modern microbialites provides us with tools useful for interpreting the formation processes and environments of ancient microbialites. Growing in the hypersaline and turbid Storr's Lake on San Salvador Island in The Bahamas today are microbialites with low levels of photosynthesis and high levels of sulfate reduction-in contrast to many of their modern counterparts. Living planktonic, motile microorganisms and suspended algal and bacterial debris create the high turbidity of the shallow lake (<2 m) and rapidly attenuate sunlight in the water column. Within Storr's Lake microbial metabolisms induce precipitation of carbonate within microenvironments of the microbial mats. Both high-Mg calcite (HMC) and aragonite are found within a majority of the microbialites measured leading to the hypothesis that the organomineralization process involves a step where HMC transforms to aragonite. Mineralogy and elemental analysis of a wide sampling of microbialites was undertaken to understand the extent of aragonite within Storr's Lake microbialites. It was found that aragonite occurs at water depths greater than 40 cm within the lake and was present in all but one microbialite measured in this study. New calcium (Ca) stable isotopic analyses from the thermal ionization mass spectrometer using a 42Ca-43Ca double spike provides evidence for exploring the systems fractionating Ca within Storr's Lake water and microbialites. In contrast to geochemical data and previous Mg stable isotopic measurements on the same waters, the Ca stable isotopic value (δ44/40Ca) of water in Storr's Lake is not homogeneous. While the northern sector is primarily influenced by seawater, the southern sector δ44/40Ca is shifted away from seawater to lower values, suggesting internal variability within the lake. In both microbialites measured, δ44/40Ca is strongly correlated to mineralogy and trace elements in the carbonate. To explore the potenti (open full item for complete abstract)

Committee: Elizabeth Griffith PhD (Advisor); Matthew Saltzman PhD (Committee Member); Thomas Darrah PhD (Committee Member) Subjects: Biogeochemistry; Earth; Geobiology; Geochemistry; Geological; Geology; Morphology; Petroleum Geology

Doctor of Philosophy, The Ohio State University, 2006, Geological Sciences

The Silurian rocks within Ohio and Indiana contain carbonates deposited under the mineral-precipitating activities of benthic microbes. Fossil microbes occur with varying degrees of preservation, and an assortment of lithologies contains evidence of microbial influence on the precipitation of micrite and cement. Features indicative of microbial activity include micritic coatings around skeletal grains; laminated, clotted, and micropeloidal fabrics; micrite in forms having independence of gravitational control; and fibrous calcite cement adjacent to well-preserved organic material. The variety of microbial products and their preservational states illustrate that numerous taphonomies and pathways for microbial calcification existed. Fossils of the calcifying microbes Girvanella, Renalcis, and Rothpletzella occur in well-preserved rocks of the Brassfield Formation (Llandovery) cropping out at the Oakes Quarry Park in Fairborn, Ohio. The quarry also contains mottled mudstones and wackestones surrounding cm-scale growth framework cavities containing veneers and pendants of microbially-precipitated micrite. Bioherms in the Waldron Shale (Wenlock) in Bartholomew County, Indiana are considered incipient reef cores and consist of skeletal fossils that encrusted, and were encrusted by, micrite with massive or laminated microtextures. The microfabrics and steep-angled forms of the Waldron Shale bioherms strongly suggest that they developed via synsedimentary lithification of micrite produced by calcifying microbes. Salina Group (Ludlow) rocks that crop out in the Duff Quarry near Huntsville, Ohio contain a paucity of megafauna and an abundance of microbialites indicating that microbial colonies overwhelmingly dominated the biota in that depositional setting. A shallowing-upward sequence of dolomitic boundstones comprises a variety of microbial geometries and textures that developed within a relatively narrow depth range in a peritidal setting. Analysis of the Silurian rocks wi (open full item for complete abstract)

Committee: William Ausich (Advisor) Subjects: Geology