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
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Item Climate Change During Intervals Of The Past Millennium In The Southwestern Tropical Pacific(2018) Lopatka, Alex; Evans, Michael N; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Limited observations from the tropical Pacific over the past millennium make it difficult to assess whether different time periods had significant variations in El Nino-Southern Oscillation (ENSO) amplitude and frequency. Composited simulation results from climate models participating in the Paleoclimate Modeling Intercomparison Project suggest no difference in statistical variance and ENSO event frequency for the Medieval Climate Anomaly (MCA), Little Ice Age (LIA), and the modern Industrial Era. ENSO may not be sensitive to external radiative forcings. Unforced variability arising from the coupled ocean-atmosphere system could explain the observed past millennium results. New coral geochemical measurements were collected from Aitutaki, southern Cook Islands in the southwestern tropical Pacific and composited with existing coral geochemical observations from Rarotonga to increase the temporal coverage of climate data over the past millennium. Forward modelling of coral oxygen isotopes as a function of sea surface temperature and the oxygen isotopic composition of seawater suggests this location is sensitive to interannual variations in the position of the South Pacific Convergence Zone (SPCZ) driven by ENSO activity. Analysis of observed interannual \delo\ indicates interannual variations are driven primarily by sea surface salinity but also sea surface temperature forcings. More negative (positive) coral oxygen isotope results indicate warmer/wetter (cooler/dryer) conditions that occur at Aitutaki when La Nina (El Nino) events redistribute the South Pacific Convergence Zone away (towards) the equator. Spatial correlation of the coral \delo\ signal with regional and tropical climate variables support the interpretation that Aitutaki coral \delo\ varies according to changes in the SPCZ and ENSO activity. Results from modern Aitutaki coral oxygen isotopes may be used to interpret coral data collected from earlier periods of time. Paired coral oxygen isotopes and Sr/Ca measurements were made on diagenetically-screened samples radiometrically dated to the Medieval Climate Anomaly. These results, used to calculate interannual oxygen isotopic composition of seawater anomalies, show higher statistical variance in the fossil record relative to the modern Aitutaki/Rarotonga composite record. Singular spectrum analysis shows the first ten reconstructed components explain 79-86\% of the variability in the timeseries. Composited interannual frequency (2-10 year period) components show variable oxygen isotopic composition of seawater throughout the MCA suggesting an active ENSO period. Large variations of 0.6 permil in calculated oxygen isotopic composition of seawater suggest potential decadal shifts in oxygen isotopes from warmer/wetter to cooler/dryer conditions. Long term trends in calculated oxygen isotopic composition of seawater during the earlier MCA from more negative to more positive values suggest a transition from warmer/wetter to cooler/dryer conditions. Together, the results suggest a highly variable MCA period relative to the modern period. This new data may be used in conjunction with other observations for data/model comparisons to better understand hydroclimate variability over the past millennium.Item DIMETHYSULFONIOPROPIONATE (DMSP) AND DMSP-LYASE IN CNIDARIAN ALGAL SYMBIOSES(2010) Yost, Denise Marie; MITCHELMORE, CARYS L; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dimethylsulfoniopropionate (DMSP) is a multifaceted sulfur compound produced by several groups of marine phytoplankton, macroalgae and higher plants. Additionally, DMSP cleaving enzymes (most of which are thought to be DMSP-lyases) are known to exist in many species of marine phytoplankton, macroalgae and bacteria. Endosymbiotic dinoflagellate microalgae (genus Symbiodinium) of reef-building corals produce high intracellular levels of DMSP. The existence of DMSP-lyase(s) in Symbiodinium was, until recently, unknown. The function(s) of the DMSP/DMSP-lyase system in cnidarian-algal symbioses is poorly understood. Chapter one introduces coral symbioses, DMSP and the potential roles of the DMSP/DMSP-lyase system in cnidarian-algal symbioses. Chapter two describes the first evidence for in vivo DMSP-lyase activity in several isolated Symbiodinium strains, revealing varying levels of DMSP and DMSP-lyase activity. These results prompted further characterization of Symbiodinium DMSP-lyases. Enzyme assay optimization and substrate kinetics experiments found the measured activity of DMSP-lyase enzymes to be affected by permeabilization buffers, pH, temperature and potential oxidative stress effects (chapter three). Prior to investigations of field-collected intact corals, methods for DMSP analyses were optimized in the laboratory to address the inherent complexities of the coral holobiont. This work compared several preparation techniques for the analysis of particulate (algae only, DMSPp) and total (coral tissue and algae, DMSPt) DMSP in several species of stony corals (chapter four). Field-collected corals in chapter five showed DMSPp and DMSPt responses when exposed to the oxidative stressor, copper. The second field study (chapter six) describes how DMSPp and DMSPt concentrations within five prominent Bermudian corals changed with water depth. Finally, chapter seven presents a synthesis examination of the potential functional attributes and significance of the DMSP/DMSP-lyase system in cnidarian-algal symbioses. The factors influencing variable DMSP production and accumulation as well as differences in DMSP-lyase activity are discussed in light of methodological limitations, the biology and physiology of symbiont and coral, Symbiodinium phylotype and environmental variables. The results of this research highlight the existence of DMSP-lyases in Symbiodinium and provide insight into the partitioning of DMSP in cnidarian-algal symbioses, furthering our understanding of the production and potential turnover of DMSP while recognizing the limitations inherent in such investigations.