UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
5 results
Search Results
Item RE-OS AND OXYGEN SYSTEMATICS OF VARIABLY ALTERED ULTRAMAFIC ROCKS, NORTH CAROLINA(2020) Centorbi, Tracey; Walker, Richard J.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study focuses on the origin and modification of six ultramafic bodies located in the Blue Ridge Province of North Carolina. The bodies consist mainly of harzburgites and dunites with associated chromites. Some of the bodies are associated spatially and genetically with mafic lithologies while others are fault bounded. All the bodies in the study are characterized by variations in their initial Os isotopic compositions, assuming a formation age of 490 Ma (187Os/188Osinitial 0.1114 to 0.1360). Most of the initial 187Os/188Os ratios are chondritic to subchondritic and can be explained by Re depletion during a partial melting event prior to ophiolite formation. By contrast, some initial 187Os/188Os ratios, particularly for those bodies in the Tallulah Falls formation, are suprachondritic suggesting the addition of radiogenic Os during a melt percolation or melt/rock reaction event, most likely during the event that led to the formation of the bodies. Oxygen isotopic δ18O values of the bodies range from +4.85 to +7.60 which overlap with and extend above mantle estimates. The cause of the higher values remains unresolved, but serpentinization and contamination by large amounts of crustal material can be excluded. It is concluded that the six bodies in this study have a common history as the residues of mantle partial melting, with chemical compositions and isotopic systematics similar to Phanerozoic ophiolite peridotites associated with the same collisional event, as well as modern abyssal peridotites. Nevertheless, Os isotopic characteristics indicate different processes acted within the bodies despite their relatively close spatial association.Item DISSOLVED OXYGEN AND NUTRIENT CYCLING IN CHESAPEAKE BAY: AN EXAMINATION OF CONTROLS AND BIOGEOCHEMICAL IMPACTS USING RETROSPECTIVE ANALYSIS AND NUMERICAL MODELS(2013) Testa, Jeremy Mark; Kemp, William M; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hypoxia, or the condition of low dissolved oxygen levels, is a topic of interest throughout aquatic ecology. Hypoxia has both realized and potential impacts on biogeochemical cycles and many invertebrate and vertebrate animal populations; the majority of the impacts being negative. It is apparent that the extent and occurrence of hypoxic conditions has been on the rise globally, despite a handful of reductions due to management success stories. Efforts to curb the development of hypoxia are well underway in many aquatic ecosystems worldwide, where oxygen levels are a key target for water quality management. Long-term increases in the volume of seasonal bottom-water hypoxia have been observed in Chesapeake Bay. Although there is evidence for the occurrence of low oxygen conditions following initial European habitation of the Chesapeake watershed, as well as direct observations of anoxia prior to the mid 20th century large-scale nutrient load increases, it is clear that hypoxic volume has increased over the last 50 years. Surprisingly, the volume of hypoxia observed for a given nutrient load has doubled since the mid-1980s, suggesting the importance of hypoxia controls beyond nutrient loading alone. I conducted a suite of retrospective data analyses and numerical modeling studies to understand the controls on and consequences of hypoxia in Chesapeake Bay over multiple time and space scales. The doubling of hypoxia per unit TN load was associated with an increase in bottom-water inorganic nitrogen and phosphorus concentrations, suggesting the potential for a positive feedback, where hypoxia-induced increases in N and P recycling support higher summer algal production and subsequent O2 consumption. I applied a two-layer sediment flux model at several stations in Chesapeake Bay, which revealed that hypoxic conditions substantially reduce coupled nitrification-denitrification and phosphorus sorption to iron oxyhydroxides, leading to the elevated sediment-water N and P fluxes that drive this feedback. An analysis of O2 dynamics during the winter-spring indicate that the day of hypoxia onset and the rate of March-May water-column O2 depletion are most strongly correlated to chlorophyll-a concentrations in bottom water; this suggests that the spring bloom drives early season O2 depletion. Metrics of winter-spring O2 depletion were un-correlated with summer hypoxic volumes, however, suggesting that other controls (including physical forcing and summer algal production) are important. I used a coupled hydrodynamic-biogeochemical model for Chesapeake Bay to quantify the extent to which summer algal production is necessary to maintain hypoxia throughout the summer, and that nutrient load-induced increases in hypoxia are driven by elevated summer respiration in the water-column of lower-Bay regions.Item Oxygen Measurement During Cell Culture: From Multiwell Plates to Microfluidic Devices(2011) Thomas, Peter Chung; Forry, Samuel P; Raghavan, Srinivasa R; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Oxygen is an important regulator of normal cell behavior. Proper supply of oxygen is required to maintain ATP production, while perturbation of oxygen supply alters cell behavior and leads to tissue damage and cell death. In vivo, cells are exposed to a mean partial pressure of oxygen between 0.03 to 0.09 atm that is tissue specific. In contrast, conventional cell cultures are routinely performed at an atmospheric oxygen level of 0.21 atm. The disparity between in vivo and in vitro oxygen levels have been shown to affect cell viability, growth and differentiation. Continuous measurements and control of oxygen levels are thus critical to maintaining proper cell behavior. Current methods of oxygen measurement are invasive, difficult to integrate with microscopy and lack imaging capabilities. To improve the current state of measurements, we have developed a new non-invasive oxygen sensor for in vitro cell culture. The sensor was prepared by incorporating a porphyrin dye, Pt(II) meso-Tetra(pentafluoro-phenyl)porphine (PtTFPP), into gas permeable poly(dimethylsiloxane) (PDMS) thin films. The response of the sensor to oxygen followed the linear Stern-Volmer equation and demonstrated an order of magnitude higher sensitivity compared to other sensors (KSV = 548 ± 71 atm-1). A multilayer design created by sandwiching the PtTFPP-PDMS with a thin film of Teflon AF followed by a second layer of PDMS effectively mitigated against cytotoxicity effects and provided a suitable substrate for cell attachment. To demonstrate the utility of the sensor, oxygen measurements were made continuously with NIH 3T3 mouse fibroblast cells. The oxygen levels were found to decrease as a result of oxygen consumption by the cells. Using Fick's law, the data was analyzed and a per-cell oxygen consumption rate for the 3T3 fibroblasts was calculated. In addition, cells were clearly visualized on the sensor demonstrating the ability to integrate with phase-contrast and fluorescence microscopy. Next, human hepatocellular carcinoma HepG2 were cultured on the oxygen sensor and continuous oxygen measurements showed a drastic decrease in oxygen level such that the cells were exposed to hypoxic conditions within 24 h. The per-cell oxygen consumption rate for HepG2 was determined to be 30 times higher than the 3T3 fibroblasts, confirming the high metabolic nature of these cells. At high densities, oxygen flux measurements showed an asymptotic behavior reaching the theoretical maximum of the culture condition. When the oxygen diffusion barrier was reduced, the oxygen flux increased, demonstrating insufficient oxygenation for HepG2 at these densities. In routine culture, HepG2 adhere to their neighboring cells which results in formation of cell clusters. Oxygen measurement confirmed the presence of oxygen gradient across the cell clusters with the lowest oxygen levels observed in the middle. Finally, we successfully integrated the oxygen sensor into microfluidic systems. The sensor provided real-time non-invasive measurements of oxygen levels on-chip. To regulate the oxygen levels in the device, water with different dissolved oxygen concentrations was used instead of gas. This method successfully mitigated the problems of pervaporation associated with previous devices. Physiologically relevant oxygen levels and oxygen gradients were easily generated on the device and the results showed excellent agreement with numerical simulations.Item Plant-sediment Interactions and Biogeochemical Cycling for Seagrass Communities in Chesapeake and Florida Bays(2007-12-17) Nagel, Jessica; Kemp, William M; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Seagrasses are prominent, productive components of shallow coastal ecosystems worldwide. The role of seagrasses in biogeochemical cycling varies widely across ecosystems, and this is due in large part to the complex interactions and feedbacks among processes controlling dynamics of carbon, oxygen, nutrients, and dissolved organic matter (DOM). This dissertation examines the importance of the keystone seagrass species, Thalassia testudinum, to biogeochemical cycling at the community and ecosystem levels in Florida Bay. The research presented here also describes the consequence of disturbances, such as shifts in species composition and seagrass dieback, on biogeochemical processes in both Florida and Chesapeake Bays. In Florida Bay, T. testudinum was shown to stimulate sediment microbial activities and benthic production of oxygen, inorganic nitrogen, and DOM relative to adjacent benthic communities without seagrass but containing benthic microalgae. Strong diel patterns in net fluxes of these solutes in both communities underscore the importance of photosynthesis. Ecosystem-level production (P) and respiration (R) rates were also enhanced in T. testudinum communities. Clear seasonal and regional variations in P and R were evident across Florida Bay, with lowest rates reported in the northern regions. Seagrass dieback had a negative effect on sediment nitrification rates and net ecosystem production (P-R) at one site in Florida Bay, and loss of seagrass habitat may result in significant changes to biogeochemical budgets within this system. In mesohaline Chesapeake Bay, the ephemeral submersed plant species, Ruppia maritima was also shown to stimulate organic production, nutrient cycling, and sediment biogeochemical processes compared to benthic communities without seagrass; however, the more persistent native species, Potamogeton perfoliatus, had an even greater impact on these processes. Collectively, the results of this research reveal the potential significance of seagrass to biogeochemical cycling in Chesapeake and Florida Bays and suggest that disturbances, such as seagrass dieback or shifts in species composition, may substantially alter biogeochemical budgets within these systems.Item Disorder and Doping in the Oxygenated Electron-doped Superconductor PCCO(2006-08-29) Higgins, Joshua Scott; Greene, Richard L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis is composed of two parts: the first part deals with the high temperature electron-doped superconductor Pr_(2-x)Ce_(x)CuO_(4-delta); the second part deals with the diluted magnetic semiconductor Ti_(1-x)Co_(x)O_(2-delta). It is not clear why oxygen reduction and cerium doping are necessary to obtain superconductivity in the electron-doped Pr_(2-x)Ce_(x)CuO_(4-delta). I investigated the effects of oxygenation in this material using resistivity and Hall measurements. For various oxygen contents, I was able to determine that there is a separable doping and a disorder contribution to the superconducting transition temperature. I was able to quantitatively separate out these two effects and show that these two effects are opposite with regards to changes in T_(c) for overdoped thin films. The disorder component is roughly twice as large as the doping component. This analysis is also shown to be self consistent in demonstrating that the doping component of oxygen variation follows the trends of Cerium doping. For the diluted magnetic semiconductor Ti_(1-x)Co_(x)O_(2-delta), I investigated the intrinsic nature of the ferromagnetism observed in thin films. Hall effect measurements were used as the technique because ferromagnetic materials exhibit an anomalous Hall effect, which is due to an interaction between the charge carriers and the magnetic moments. I found that low carrier concentration anatase phase films did not exhibit an anomalous Hall effect, whereas high carrier concentration rutile phase films do. The presence of the anomalous Hall effect at this point cannot be attributed to an intrinsic ferromagnetism as cobalt clusters are observed in these films.