Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    Sulfur isotopic evolution of Phanerozoic and Ediacaran seawater sulfate
    (2013) Wu, Nanping; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Dynamics of &sigma34S and &sigma33S of oceanic sulfate and sedimentary pyrite over geologic time has been used to understand and extract information about the marine sulfur cycle. My Ph.D project uses this approach to study the evolution of the marine sulfur cycle and focuses on: 1)Providing temporal &sigma34S and &sigma33S records for Phanerozoic and Ediacaran seawater sulfate based on the analysis of carbonate associated sulfate 2) Reinterpreting previously documented variations of sulfur isotope fractionation using new measurements of &sigma33S of oceanic sulfate. The sulfur isotope fractionation between sulfate and pyrite appears to have varied widely, between 25 / to 40 / over the course of Phanerozoic. For the earlier part of Phanerozoic, the values of sulfur isotope fractionation are approximately 35 /. The fractionation then decreases to 25 / during the Carboniferous Period. Following this, the sulfur isotope fractionations progressively increase, reaching approximately 40 / during the Cenozoic Period. 3) Evaluating the connection between sulfate concentration, sulfide re-oxidation and sulfate exchange between water column and marine sediment systems using steady state and also non steady state models. The model results and the data presented here suggest the sulfur cycle in the Ediacaran Oman basin evolved from one that was similar to the global sulfur cycle to a sulfur cycle that was disconnected or partially was disconnected from the open ocean sulfur cycle. It also implies that the sulfate levels drop at this transition due to weakening of vertical bioturbation or weakening of other physical processes that involve in mixing of sulfate in pore water and overlying sulfate in marine settings. The significance of these three directions is the new information that they provide about the evolution of the sulfur cycle. It is relevant to understanding the environmental changes and their connections to sulfur ecosystem evolution for the geological time interval extending from present until latest Precambrian.
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    A Stable Isotope Investigation of Precipitation Nitrate
    (2005-08-15) Cooney, Katherine Suzanne; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The isotopic composition of precipitation nitrate reflects the processes involved in its formation. Silver nitrates prepared from Maryland precipitation were thermally decomposed to gases for mass spectrometric analyses. Nitrate δ15N ranged from -5.1±0.5 to +5.9±0.5 (1σ), δ18O ranged from +42.6±0.5 to +81.9±0.5, and Δ17O ranged from +15.33±0.05 to +31.71±0.05. Precipitation samples were re-analyzed for δ15N and δ18O using a method in which bacteria convert nitrate to N2O for analyses, and d15N ranged from -3.6±0.2 to +7.1±0.2 and δ18O ranged from +61.6±0.3 to +86.8±0.3. Differences between the methods were attributed to organic contaminants in the silver nitrates. Nitrate δ15N, δ18O, and Δ17O were highest in the winter and lowest in the summer. Ion concentrations, storm track data, and the δ34S of precipitation sulfate were used for interpretations. Most likely, δ15N varied due to seasonal changes in the NOx photo-stationary state, and δ18O and Δ17O varied due to changes in oxidation chemistry.