Doctor of Philosophy, The Ohio State University, 2023, Earth Sciences

One of the most important features of our planet is its ability to sustain life in multi million year timescales. This ability largely depends on Earth's internal system in regulating the global carbon cycle. One of the key processes that regulates the global carbon cycle is weathering of silicate rocks. As such, a refined understanding of the silicate weathering system and its role in regulating the global carbon cycle in multi million year timescales is essential in predicting the fate of our planet amid the current and future increase in anthropogenic atmospheric CO2 concentrations. However, characterizing and identifying the direct or indirect evidence of changes in the global carbon cycle in the rock record is not trivial. First, the signals preserved in the rock record are prone to secondary alteration. Second, the traditional geochemical proxies can produce non-unique interpretations. My research is, therefore, aimed at addressing these issues by investigating and constraining the extent of diagenetic processes and elucidating possible links between tectonics and changes in silicate weathering during the Ordovician Period using novel geochemical techniques and numerical modeling. This research is broken down into three separate projects and presented in Chapter 2 through Chapter 4 of this dissertation. The study presented in Chapter 2 uses bulk carbonate measurements of calcium isotopes (δ44/40Ca) and trace element concentrations to constrain the extent of diagenetic processes affecting the δ13C record from the Middle Ordovician. The data and numerical models presented in this study suggest that variations in Sr/Ca and δ44/40Ca from Meiklejohn Peak correspond to changes in the dominant carbonate primary mineralogy and early marine diagenesis. However, the δ13C seem to reflect a primary change in the dissolved inorganic carbon (DIC). The positive shift recorded as the MDICE (Middle Darriwilian Carbon Isotope Excursion) likely represents a transient increas (open full item for complete abstract)

Committee: Matthew Saltzman (Advisor); John Olesik (Committee Member); Derek Sawyer (Committee Member); Elizabeth Griffith (Committee Member) Subjects: Earth; Geology

Doctor of Philosophy, The Ohio State University, 2023, Earth Sciences

Our understanding of the Earth's multimillion-year carbon cycle, with primary implications for the evolution of life, depends on our ability to decipher information encoded in chemical signals of shallow marine sediments. This dissertation attempts to develop our knowledge of two main topics: (1) the causes of multimillion-year global climate change in terms of changes in the sources and sinks of atmospheric carbon dioxide (CO2), and (2) the causes and meaning of post-depositional alteration of shallow marine sediments with respect to chemical proxy records of the evolution of global (and/or local) Earth processes. To this end, this work applies radiogenic strontium and neodymium (87Sr/86Sr, εNd(t)) and stable calcium (δ44/40Ca) isotopic records in bulk carbonate rocks and conodont apatite from Middle–Late Ordovician (Darriwilian–Katian stages; ~470–450 million years ago, abbrev. Ma) sections in the Antelope Range, central Nevada; Clear Spring, Maryland; and the Fjacka and Kargarde sections of the Siljan district, Dalarna province, central Sweden. Bulk rock samples from the tropical Middle–Late Ordovician setting of the Antelope Range, Nevada were analyzed for proxy records of regional and global continental weathering source lithology (87Sr/86Sr and εNd(t)) which were paired with published paleotemperature proxy measurements (δ18O) of conodont apatite from the same section. This paired suite of proxy records is used to test the hypothesis that low-latitude island arc accretion during the Middle–Late Ordovician Taconic Orogeny enhanced the weatherability of Earth's crust, increasing the rate of CO2 removal by the weathering of mafic silicate minerals and producing the global cooling observed in the Middle–Late Ordovician paleotemperature record. These records show coeval inflections in 87Sr/86Sr and εNd(t) values at ~463 Ma that reveal the influence of tectonic uplift and enhanced weathering of mafic ophiolite provinces on the Taconic margin. This change in weather (open full item for complete abstract)

Committee: Matthew Saltzman (Advisor); Audrey Sawyer (Committee Member); Elizabeth Griffith (Committee Member); William Ausich (Committee Member) Subjects: Chemistry; Earth; Geochemistry; Geology

Master of Science, The Ohio State University, 2023, Earth Sciences

Positive δ13C excursions in the Paleozoic are traditionally thought to represent large, global shifts in organic carbon burial. However, these records may not represent global changes in the δ13C of dissolved inorganic carbon (DIC), as there may be influence from local processes or potential for alteration through early marine diagenesis. In this study, I compare bulk carbonate δ44/40Ca with δ13C and Sr/Ca data from the Early Mississippian δ13C excursion (+5.0 to +7.0‰). The presence of covariation in these quantities would suggest that the excursion is a product of diagenesis. This comparison also indicates whether early marine diagenesis was sediment buffered or seawater buffered. I measured δ44/40Ca and Sr/Ca of across this stratigraphic interval in samples from the Confusion Range, Utah, Pahranagat Range, Nevada, and Dinant Basin, Belgium. The Confusion Range and Pahranagat Range sections show relatively little variability in δ44/40Ca (the Confusion Range mean is -1.08‰ ± 0.14‰ 2σ and the Pahranagat Range mean is -1.19‰ ± 0.16‰ 2σ), while there is a wider range in δ44/40Ca in the Dinant Basin (mean value of -1.19‰ ± 0.32‰ 2σ). When these δ44/40Ca data are compared to δ13C and Sr/Ca data, there is no covariation in the Confusion Range and Pahranagat Range sections, while there is covariation in the Dinant Basin section. I propose that this δ13C excursion was not a product of diagenesis, and that the shift in δ44/40Ca in the Dinant Basin was due in part to a change in primary mineralogy (a shift from aragonite to calcite) with possible contributions of changing precipitation rate. These conclusions are further supported by early marine diagenesis modelling. Therefore, the Early Mississippian δ13C excursion represents a primary signal of either local or global changes in δ13C of DIC.

Committee: Matthew Saltzman (Advisor); Steven Lower (Committee Member); Elizabeth Griffith (Committee Member) Subjects: Geochemistry; Geology

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