| Summary: | Advisors: Justin P. Dodd. Committee members: Mark Frank; Ross Powell. Includes bibliographical references. Includes illustrations and maps. Coupled oxygen (δ¹⁸O) and silicon (δ³⁰Si) isotope variations recorded in diatom silica have been increasingly used to reconstruct paleoenvironments and biogeochemical cycling in marine environments. Variations in δ¹⁸O values in diatom silica are used to reconstruct oceanographic conditions such as water temperature and water mass mixing. Corresponding δ³⁰Si variation in diatom silica record silicic acid utilization by diatoms, which is related to changes in primary productivity, global silicon cycling, and carbon cycle dynamics. Diatom silica is a particularly significant paleoenvironmental proxy in high-latitude environments, such as the Southern Ocean, where diatom blooms are abundant and there is a lack of carbonate microfossils (i.e. foraminifera). An ideal location to employ δ¹⁸O and δ³⁰Si as a paleoceanographic proxy is in a -80-m long Pliocene diatomite unit of the AND-1B marine sediment core collected from beneath the Ross Ice Shelf in McMurdo Sound, Antarctica. The results of this study demonstrate that δ¹⁸O values in diatom silica record early diagenetic conditions and variations in pore water δ¹⁸O values. Calculated δ¹⁸O values of AND-1B sediment pore water values are between -16 and -8‰, which is consistent with the δ¹⁸O values of cryogenically formed brines associated with pore waters in the nearby AND-2A sediment core. Changes in the δ¹⁸O value of the diatom silica/pore water through time likely reflect variability in subsurface brine input into McMurdo Sound during the Pliocene. The diatom silica δ³⁰Si values appear to be well correlated with variations in the LR04 benthic stack δ¹⁸O values as well as reconstructed surface water temperatures based on diatom assemblages in the same interval of AND-1B. The correlation between the δ³⁰Si values and other global proxies suggest the δ³⁰Si values are less susceptible to diagenesis, and the large variations in δ³⁰Si values of ~1.6‰ are most likely the result of significant changes diatom productivity and surface water nutrient utilization. Combined, the δ¹⁸O and δ³⁰Si values in the diatom silica indicate that there were significant variations in the paleoenvironment, diatom productivity, and cryogenic brine formation in the Ross Sea during the Pliocene. M.S. (Master of Science)
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