Browsing Geology Theses and Dissertations by Title
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- ItemAdvection-Diffusion Controlled Lithium Isotopic Distribution in Contact Aureoles: A Case Study from the Florence County Pegmatites, Wisconsin(2009) Liu, Xiaoming; Hier-Majumder, Saswata; Rudnick, Roberta L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Stable isotopes are useful tracers of fluid-rock interactions in contact aureole settings. To date, only a few case studies have used Li isotopes to study fluid-rock interactions in contact aureole settings. These studies highlight the very large Li isotopic fractionation that can be generated in these settings via diffusion of Li from the pluton into the country rocks, but none of these studies have generated a complete and detailed section of the contact aureole needed to understand the Li distributions. Here, I report the results from a combination of Li isotope analyses and 2-D advection-diffusion modeling for two detailed profiles through country rocks adjacent to Li-rich pegmatite dikes in the Florence County pegmatite field. The results show that the Li concentration and isotopic distribution in the two contact profiles is consistent with fluid infiltration and diffusion of Li through a grain boundary fluid from the pegmatites into their country rocks.
- ItemAge, composition, and origin of the lower continental crust, northern Tanzania(2008) Mansur, Adam T.; Rudnick, Roberta L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Granulite xenoliths from the East African rift in Tanzania sample the lower continental crust beneath both the stable Tanzanian craton and the adjacent, polymetamorphic Mozambique belt. Protoliths of surface rocks in both terranes formed 2.8 to 2.6 Ga ago, but only the Mozambique belt underwent significant post-Archean metamorphism. Most granulite xenoliths represent mafic magmas formed in a convergent margin at ca. 2660 Ma that were subsequently metamorphosed at lower crustal conditions. However, a small number of xenoliths are more similar to intraplate magmas, suggesting that at least two episodes of lower crustal growth occurred during the Archean. No post-Archean additions to the crust were detected beneath either terrane, but xenoliths from the Mozambique belt record high P-T conditions, suggesting equilibration at the base of orogenically thickened crust. By contrast, xenoliths from beneath the craton equilibrated under conditions typical of unthickened continental crust.
- ItemANALYZING TIME-VARYING SEISMICITY AND AFTERSHOCK BEHAVIOR IN CENTRAL AND EASTERN UNITED STATES(2021) Pearson, Karen M; Lekic, Vedran; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Central and Eastern United States (CEUS) earthquakes are far less common than those in the tectonically active west coast, but the significance is elevated for a few reasons. Due to older, harder, and often denser rocks making up the bedrock geology east of the Rockies, seismic waves can travel much further without losing energy. Poor construction, efficient transmission of seismic waves, and site amplification effects can make even light to moderate earthquakes pose high risk within the CEUS. The CEUS has significant aging infrastructure and some of the highest population densities in the country, which would lead to great economic losses and even the potential for human injury if hazards are not properly identified and communicated. Aftershock sequences are governed by several descriptive statistical “laws,” each with one or more characteristic parameters. These parameters are used to illustrate factors such as the overall productivity, the rate of decay, and the relative frequency of larger and smaller magnitude aftershocks. Variations in these parameters can relate to the geologic region being studied, the tectonic environment, the driving force of seismicity (i.e. induced earthquakes, volcanic, or geothermal-related), and more. This work discusses the aftershock sequences of two unusual CEUS earthquakes from the past five years. The first earthquake I study is a M4.2 earthquake that occurred east of Dover, DE, in late 2017. I continue by studying the aftershocks in the six weeks following a M5.8 earthquake that occurred near Pawnee, OK, in autumn 2016. Both of these earthquakes experienced below-normal aftershock productivities. I explore the challenges of analysis when station coverage is heterogeneous for the period of aftershock analysis. From there, I discuss the limitations of some statistical tests for special cases such as aftershock decay. The work concludes by highlighting additional CEUS earthquakes exceeding M4 that have occurred in the past 20 years and discussing some preliminary analytical findings.
- ItemAnatexis and crustal differentiation: Insights from the Fosdick migmatite-granite complex, West Antarctica(2014) Yakymchuk, Christopher; Brown, Michael; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the Fosdick migmatite-granite complex of West Antarctica, U-Pb geochronology of monazite in migmatites and zircon in granites records two episodes of high-temperature metamorphism, one in the Devonian-Carboniferous and another in the Cretaceous. For the older lower-grade event, whole-rock and zircon isotope geochemistry of granites within the Fosdick complex are interpreted to record crustal reworking during metamorphism associated with continental arc magmatism along the East Gondwana convergent plate margin. By contrast, the geochemistry of correlative granites suites from along and across the same margin indicates a greater proportion of crustal growth. This suggests prominent arc-parallel and arc-normal variations in the mechanisms of crustal reworking versus growth in continental arc systems. Based on garnet Lu-Hf ages, the timing of peak metamorphism in the younger higher-grade event has been determined as c. 116-111 Ma. U-Pb ages of monazite from migmatites and zircon from anatectic granites suggest that exhumation of the complex as a gneiss dome occurred during the interval c. 107-100 Ma. Contemporaneous exhumation of high-grade metamorphic rocks in the Western Province of New Zealand suggests that intracontinental extension preceded the final breakup of New Zealand from West Antarctica by c. 25 Myr. Melt migration through and emplacement within the Fosdick complex during Cretaceous metamorphism was accomplished via a self-organized melt network controlled by the regional stress field and anisotropy of the host rocks. Granites within this network and in sills at shallower crustal levels have microstructures and chemistry consistent with a cumulate origin, and are interpreted to record fractional crystallization during magma ascent and doming related to exhumation. Phase equilibria modeling of open system melting during prograde metamorphism is used to quantify the reduced fertility of source rocks during high-temperature exhumation and later overprinting orogenic events. Quantitative modelling of the dissolution of zircon and monazite during prograde melting demonstrates that accessory minerals are expected to be partially to completely consumed up to the metamorphic peak. New growth of these minerals in migmatite melanosomes is predicted to be limited during cooling, whereas leucosomes and anatectic granites are predicted to contain new zircon and monazite growth.
- ItemAPPLYING GEODESY TO MODEL POSTSEISMIC SLIP OF THE 2016 MW 6.4 MEINONG EARTHQUAKE(2019) Butcher, Rebecca; Huang, Mong-Han; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In regions of rapid convergence such as southwest Taiwan, unmapped active structures at multiple depths increase the uncertainty of seismic hazard estimates. The 2016 Mw 6.4 MeiNong earthquake occurred below the main Taiwan detachment, and may have illuminated some preexisting, but undocumented, fault structures. In this study, I use geodetic measurements to constrain afterslip on the main fault for 15 months following the MeiNong earthquake. The inverted afterslip is concentrated around the peak coseismic slip asperity without significant aftershock correlation, which implies heterogeneous frictional properties on the fault. Additionally, slip model misfit indicates shallower faults that are critically stressed before the earthquake creep due to the MeiNong coseismic stress. My results can help identify active faults located at shallower depth as well as their seismic potential in southwest Taiwan.
- ItemASSESSING FAULT SLIP HAZARD IN TAIWAN USING SPACE GEODESY(2022) Robbins, Kathryn Rose; Huang, Mong-Han; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Taiwan is a geologically complex region due to the continuous collision of the Eurasian Plate and the Philippine Sea Plate. This study aimed to quantify the interseismic crustal deformation of Taiwan and detail the island’s seismic hazard potential using space geodesy. Data were collected between 2016 and 2021 through C-band Copernicus Sentinel-1 synthetic aperture radar imagery and continuous GNSS data from Academia Sinica, Taiwan. I excluded major earthquake events within this time period and generated a dataset consisting of interferometric synthetic aperture radar ground motion velocities with GNSS corrections and interpolated GNSS ground motion velocities. Then, utilizing this dataset, I performed a deformation rate analysis and error analysis. Next, I explored block modeling and used a total variation regularization approach to determine the reference block model that best reduced velocity residuals and minimized the number of independently rotating blocks. Results suggested that the Taipei Basin, Ilan Basin, Western Foothills, and Longitudinal Valley were experiencing increased total strain rate accumulation and, therefore, posed increased seismic hazard.
- ItemAssessing the Oxidative History of Miller Range Martian Meteorites(2016) Dottin III, James Wosley; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Miller Range (MIL) Martian meteorites are oxidized nakhlites. Early studies attribute their oxidation to reduction-oxidation reactions involving assimilated sulfate. I utilize the sulfur isotope and major element composition of the MIL pairs to assess their oxidative history. MIL sulfides display an average sulfur isotope composition that is different from Nakhla sulfate and sulfide. The sulfur isotope differences produce a mixing array between juvenile sulfur and mass-independent sulfur signatures, indicating assimilation of anomalous sulfur into the melt. I estimate an fO2 of QFM (+3.5 ± 0.4) and a sulfur content of 360 ppm ± 12 – 1300 ppm ± 50. With these results, I test the hypothesis of sulfate assimilation through models of charge balance, isotope mixing, and degassing of sulfur bearing compounds. I conclude that sulfate assimilation was significant in the oxidation of the MIL pairs but, additional oxidants were assimilated.
- ItemAuthigenesis, biomineralization, and carbon-sulfur cycling in the Ediacaran ocean(2015) Cui, Huan; Kaufman, Alan J.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fossil record of the Ediacaran Period (635-541 Ma) reveals unprecedented rise of early animal life (metazoan) in Earth history. Coupled with this evolutionary milestone, the Earth’s atmosphere and hydrosphere experienced dramatic redox fluctuations. In order to better constrain the redox architecture of the Ediacaran ocean margin, an integrated chemostratigraphic correlation of the Doushantuo Formation in basin scale was conducted (see Chapter 2). The revised redox model suggests that euxinic conditions on the platforms were mainly restricted in lagoonal settings, which helps us to better understand Ediacaran fossil distributions and fluctuated δ13C records in the Ediacaran strata in South China. One of the most distinct features of the Ediacaran chemostratigraphy is the δ13C negative excursion (i.e. Shuram Excursion, or SE) reported globally, which is the largest known C cycle anomaly in Earth history. In order to understand the biogeochemical processes that gave rise to the SE expressed in the upper Doushantuo Formation, systematic petrographic and geochemical investigations were conducted for the outer shelf sections in the Yangtze block (see Chapter 3). Methane-derived authigenic calcite cements and nodules with extreme 13C-depletion were discovered and interpreted as the first empirical evidence of authigenic mineralization associated with the SE. In light of these novel observations, it is proposed that the globally distributed SE may be formed by widespread syndeposition of authigenic carbonates in a sulfate-methane transitional zone positioned at the sediment-water interface in response to a global seawater sulfate increase. Finally, to provide environmental context for the terminal Ediacaran biomineralization of animals, we conducted a high-resolution elemental and isotopic study of the richly fossiliferous Gaojiashan Member (see Chapter 4). Coincident with the first appearance of Cloudina are significant C-S-Ca-Sr cycle anomalies. It is proposed that the onset of calcarious biomineralization of animals may have coincided with an increase in terrestrial weathering fluxes of sulfate, alkalinity, and nutrients to the depositional basin. Enhanced concentration of Ca ion in seawater may have promoted the calcarious biomineralization of the early animals. Integrated chemo-, bio- and litho-stratigraphy of the Doushantuo and Dengying formations presented in this dissertation emphasized intimate co-evolution of Earth-life system during the Ediacaran Period.
- ItemBRITTLE FAULTING IN POROUS, WATER-SATURATED SANDSTONE DEFORMED UNDER HIGH PORE FLUID PRESSURE(2020) Zega, Zachary; Zhu, Wenlu; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effective stress law is widely used to model the deformation of crustal rock. It states that effective pressure, the difference between confining pressure and pore fluid pressure, dictates rock strength. In low-porosity rock, previous work has shown that the stability of faulting is controlled by the magnitude of pore fluid pressure rather than the effective pressure. To understand the effect of pore fluid pressure on porous rocks, I deformed water-saturated samples of Darley Dale Sandstone (~13-15% initial porosity) at strain rates from 10-4 to 10-6 s-1 while using a constant effective pressure of 10 MPa and pore fluid pressures ranging from 2 to 180 MPa. The results show that the shear strength is well-predicted by the effective stress law, but fault growth at Pf = 180 MPa resulted in pervasive grain comminution and stabilized fault propagation compared to all other lower pressure experiments.
- ItemA carbon and nitrogen isotopic analysis of Pleistocene food webs in North America: implications for paleoecology and extinction(2008-05-05) France, Christine Ann Missell; Kaufman, Alan J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Carbon and nitrogen isotopic reconstructions of North American Pleistocene trophic relationships were used to examine the extinction within terrestrial mammals ~10,000 years ago and distinguish between two potential causal mechanisms - human over-hunting and climate change. Additionally, individual animals were examined for unique isotopic signatures associated with feeding specializations, digestive strategies, and juveniles. Bones representing a comprehensive set of Pleistocene mammalian genera were obtained from three fossil localities: McKittrick Brea, California; Saltville, Virginia; and several sites in Florida. Collagen, a durable bone protein whose carbon and nitrogen isotopic composition reflects dietary input, was extracted from specimens and analyzed for delta-13C, delta-15N, % collagen, %C, %N, and C:N. Radiocarbon dating and amino acid analyses were performed on select sample sets. Results indicated that several specimens contained well preserved collagen, the isotopic values of which indicated both trophic position and vegetation preference. Those samples that contained residual diagenetic proteinaceous material exhibited increased hydrolysis of collagen with time and leaching of disassociated amino acids. Trophic relationships were reconstructed from well preserved specimens for Aucilla River, Florida and the herbivores of Saltville, Virginia, with a less complete reconstruction established for McKittrick Brea, California. The following notable trends emerged: 1) absence of nitrogen isotopic distinction between ruminants and non-ruminants, 2) enriched juvenile nitrogen isotopic signature, 3) distinction of giant ground sloths as omnivores, 4) C4 grass grazers and open C4 grasslands restricted to southern North American latitudes, 5) generalized and opportunistic feeding habits of herbivores, 6) potential prey specializations of carnivores. A noticeable lack of competition and feeding specialists among herbivores suggested a stable base to these late Pleistocene ecosystems, which argues against climatically induced stress on plants. While carnivore specimens were fewer, the apex trophic levels appeared to exhibit a similar lack of competition, which would be expected in a human-driven extinction where carnivores were stressed due to rapidly over-hunted herbivores. The ultimate cause of the late Pleistocene mammalian extinction in North America can not be exclusively attributed to either of these two mechanisms at this point in time; rather, a combination of factors should be considered.
- ItemCharacterization of the regional, crustal, and global distribution and abundance of the heat producing elements and their geoneutrino flux(2019) Wipperfurth, Scott Alan; McDonough, William F; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The amount and distribution of radiogenic power generation from the heat producing elements (HPE) U, Th, and K in the Earth is not well constrained. Compositional estimates of these elements vary by a factor of three in the bulk-Earth and 30 in the mantle after removal of the continental crust contribution. Understanding the total power derived from these elements is critical to understanding the power driving the Earth as they supply fuel to the geodynamo and mantle convection. The decay of HPE's produce particles called geoneutrinos and the measurement of the geoneutrino flux reveals the frequency of decay and the abundances of these elements in the Earth. The total geoneutrino flux can be categorized into three major contributors: the dominant component from the nearest 500 km of continental crust surrounding the detector and slightly smaller sub-equal contributions from the remaining global continental crust and the mantle. The negligible amount of HPE's within the core was tested by a mass-balance of the Th/U derived from Pb isotopes (κ_Pb). Each Earth layer was attributed a κ_Pb from representative samples with associated weighting factor from the estimated mass of U in each reservoir. The radiogenic power in the core from U and Th was constrained to ~0.03 terra-watts (median), emphasizing the core's negligible geoneutrino luminosity. To unravel the contribution from the inaccessible mantle to the signal at a detector one must build a physical and chemical description of the local and global crust. The 50x50 km regional geoneutrino flux surrounding the SNO+ detector (Sudbury, Canada) was modeled. 112 geologic samples were analyzed for their U, Th, K abundances and combined with a 3D physical model of the region. To supplement this, the methodology of Huang et al. (2013) was applied to an updated geophysical model for the bulk-crust to predict the global crustal signal at SNO+ and other detectors. Variable correlation is addressed and uncertainties from density, seismic velocity, crustal thickness, and abundances propagated. This dissertation explores the amount and distribution of HPE's within the Earth and their geoneutrino flux through geochemical and geophysical modeling on regional, crustal, and global scales. Together, the results update our understanding of the Earth's geoneutrino flux and the uncertainties still in the system.
- ItemCharacterization of the spatial differences in hydrological functioning in a tidal marsh, Patuxent River, MD: A framework for understanding nutrient dynamics(2004-12-08) Phemister, Karen; Prestegaard, Karen L.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study investigates spatial variations in sediment hydraulic conductivity (K), network channel shape and horizontal groundwater flux magnitude toward tidal network channels in a freshwater tidal marsh. Results showed the average value of K at zero meters from the creekbank was significantly higher than the K at both 5 and 15 meters from the network channel creekbank. Creekbank gradient did increase with increasing distance from the main channel and some data indicated that channel width-to-depth ratio (F), which is inversely related to creekbank gradient, correlates well with K. In addition, horizontal groundwater flux magnitude at a depth of 11 cm was significantly greater than flux magnitude at 22 cm below the ground surface at the first-order network channel location. Horizontal flux magnitude was also significantly higher from 5 to 0 meters than from 15 to 5 meters from the network channel creekbank at both the first- and second-order channel locations.
- ItemCharacterizing 3-dimensional Melt Distribution and Anisotropic Permeability in Sheared Partially Molten Rocks(2020) Bader, James A; Zhu, Wenlu; Montesi, Laurent; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With increasing shear strain, initially homogeneously distributed melt can segregate into an array of melt-rich bands, flanked by melt-poor regions. To address how the formation of these melt-rich bands affects the transport properties of partially molten rocks, I analyzed X-ray synchrotron microtomographic images of an aggregate composed of 10 vol% basaltic melt and 90 vol% olivine that was sheared to a total strain of 13.3. At 0.16 m per pixel, the spatial resolution of the microtomographic dataset is sufficiently high for quantitative characterization of 3-dimensional melt distribution. The results show that the melt distribution is bimodal: in the melt-poor regions, the total melt fractions range from 0.078-0.100, with no interconnected melt; in the melt-rich regions, the total melt fractions range from 0.116 to 0.178, with the interconnected melt fraction ranging from 0.08 to 0.16. The permeability of the sample was calculated using a digital rock physics approach. Along a melt-rich band, permeability (k) as function of melt fraction (ϕ) and grain size (d) can be expressed as k=(ϕ^3.2 d^2)/12.4. Between melt-rich bands, the permeability is negligible. Thus, the permeability of the sheared partially molten rock is highly anisotropic and negligible in the direction perpendicular to the bands. Grain size measurements were obtained through electron backscatter diffraction. After adjusting for grain size, the permeability of a sheared partially molten rock measured along the direction of melt bands is higher than that of its isotropic counterpart with the same bulk melt fraction. The strong anisotropic permeability provides new insight into the effect of melt band formation on melt migration and melt focusing at mid ocean ridges.
- ItemCharacterizing Chemical Signatures of Life via Mass Spectrometry(2023) Ni, Ziqin; Arevalo Jr., Ricardo; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The search for signs of life beyond Earth has been fueled by a natural curiosity about whether we are alone in the universe. Organic molecules, as the primary chemical components of terrestrial living organisms, are major targets in life detection missions. However, organic compounds have also been found in abundance in meteorites. They can be synthesized via abiotic processes such as lightning strikes, and naturally degraded with time. Searching for chemical signatures of life requires analog experiments to constrain chances of false positive detection and advancement of instrumentation to reduce possibilities of false negative measurements. The capacity to synthesize organics via abiotic mechanisms is influenced strongly by the redox condition. In this dissertation, the redox state of early Earth is estimated using trace element systematics in zircon grains. The Earth's surface is found to have reached a habitable redox condition as early as 4 billion years ago, coinciding with a time when Earth was still routinely bombarded by meteorites. Simulations of such high-speed impacts using high-power laser beams demonstrate the feasibility of synthesizing simple organic compounds from carbonates and nitrogen salts. Laboratory experiments and numerical modeling suggest that crater-forming impacts could have synthesized a considerable concentration of organics on the surface of Earth, Mars, and Ceres. The detection and characterization of organic molecules requires sophisticated analytical instrumentation. Laser desorption mass spectrometry (LDMS) and OrbitrapTM mass analysis are examples of next-generation techniques under development for conducting comprehensive chemical investigations in space. Although a combination of these two techniques enables the determination of the atomic composition of organic molecules, even complex polymers such as peptides, such an approach fails to recognize the 3D structure and sequencing of polymers. To facilitate the ionization and sequencing capability of peptides via LDMS, silicon nanoparticles are incorporated in the substrate as an alternative to conventional organic matrices but with reduced risk of forward contamination. The simultaneous measurement of mass to charge ratio and collision cross-section via Orbitrap mass analysis allows for rapid separation of organic molecules by their class, structure, and composition in sample mixtures. The techniques developed here are valuable for astrobiological exploration beyond Earth.
- ItemCHARACTERIZING THE DURATION, PERIODICITY AND CHEMICAL IMPACT OF FLUID TRANSPORT IN THE SUBDUCTING SLAB: INSIGHTS FROM ISOTOPE GEOCHEMISTRY OF HIGH-PRESSURE METAMORPHOSED OCEANIC CRUST(2021) Hoover, William Floyd; Penniston-Dorland, Sarah C; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Subduction zones are key loci of geochemical cycling and natural hazards on Earth including large earthquakes and explosive volcanic eruptions. Fluids produced during subduction are thought to play a role in all these processes, however, many aspects of fluid transport in subduction zones remain enigmatic. In this dissertation, three types of fluid-related features are examined: 1) an eclogite-facies vein and 2) an eclogite- facies shear zone block and metasomatic rind, both from the Monviso Ophiolite (Western Alps), and 3) two amphibolite-facies mélange blocks and rinds from the Catalina Schist (CA). The mechanisms, episodicity and duration of fluid transport associated with these fluid pathways are investigated with bulk and in situ Li isotope geochemistry, in situ Sr isotope and trace element geochemistry, and quantitative transport modeling. In the eclogite-facies vein, evidence for five distinct locally-derived fluid compositions suggests a complex process of fluid-rock interaction. The unusual geometry of alteration features in the host rock suggests that initial host rock heterogeneity led to the development of reactive porosity channels. A method for in situ measurement of Li isotopes in garnet by secondary ion mass spectrometry is developed to explore the relative chronology of fluid rock interaction preserved in mineral zoning. The equivalence of natural garnet and garnet-like glass reference materials is demonstrated and a correction procedure for instrumental mass fractionation due to MnO and FeO is proposed. The resulting method is highly adaptable and attains 2-4‰ precision at the 20-μm-scale. Application of this method to garnet from the eclogite-facies shear zone block and rind reveals negative ?7Li excursions to values as low as -9‰ that record fluid-driven Li diffusion and rapid garnet growth. Multiple negative excursions within a single garnet require at least four episodes of fluid infiltration in the shear zone. Lastly, the first fluid transport durations for the subduction interface are obtained by inverting Li isotopes profiles from the amphibolite-facies mélange blocks and rinds using an advection-diffusion model. Uniform durations of ~60 years for metasedimentary rock-derived fluid flow near peak metamorphic conditions suggest fluid infiltration was pervasive and episodic, with earlier episodes erased by the expansion of rinds into blocks.
- ItemA CHEMICAL AND ISOTOPIC COMPARISON OF TWO 1ST-ORDER AGRICULTURAL TRIBUTARIES, KENT COUNTY, MARYLAND(2005-05-27) Teerlink, Jennifer; Candela, Philip; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nitrate derived predominantly from agricultural fertilizers results in algal blooms and wide-spread anoxia in estuarine environments. Roughly half the nitrate delivered to the Chesapeake Bay is derived from groundwater (residence time up to 40 years). Two 1st-order tributaries in eastern Maryland were sampled on five dates. Over the study interval, average nitrate, alkalinity, and delta 13C were 207 µM, 212 µM, and -12.1, respectively, in the ditched tributary, and 106 µM, 451 µM, and -9.7, respectively, in the unaltered tributary. Ditching of the western tributary results in discharge of less anthropogenically-altered groundwater. Nitrate, calcium, and magnesium concentrations decrease and 13C abundance of DIC becomes enriched along the reach. The unaltered tributary is stagnant in the headwaters, resulting in consumption of dissolved oxygen and denitrification. Alkalinity correspondingly increases suggesting reduced carbon as the electron source for this microbial process. Alteration of 1st-order tributaries influences the processing and delivery of nutrients.
- ItemClimate 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.
- ItemConstraining Lithospheric Structure across the Continental Borderland using Receiver Functions(2014) Reeves, Zachary Anthony; Lekic, Vedran; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Due to its complex history of deformation, which includes subduction, rifting, and transtensional motions, the California Continental Borderland provides an interesting geological setting for studying how the structure of oceanic and continental plates responds to deformation. We calculate Ps and Sp receiver functions at permanent stations of the Southern California Seismic Network as well as ocean bottom seismometer data gathered as part of the ALBACORE seismic experiment in 2010-2011. Our results indicate that the Outer Borderland has been translated with little to no internal deformation, while the Inner Borderland underwent significant crustal thinning to compensate for the 90 degree clockwise rotation of the western Transverse Range block. We detect an oceanic seismic lithosphere-asthenosphere boundary at 58 km depth west of the Patton Escarpment. Sp common conversion point stacks confirm wholesale lithospheric thinning of the Inner Borderland and suggest the presence of a slab fragment beneath the Outer Borderland.
- ItemCONSTRAINTS ON THE DEPOSITIONAL AGES OF LESSER HIMALAYAN ROCKS IN CENTRAL NEPAL AND THEIR STRUCTURAL IMPLICATIONS(2009) Burgy, Katherine Diane; Martin, Aaron; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The lack of good exposures and paucity of datable horizons in central Nepal has hindered the ability of geologists to piece together a relatively cohesive and straightforward stratigraphic succession within the Lesser Himalaya. U-Pb isotopic analyses of detrital zircons from the Modi Khola valley indicates maximum depositional ages of ~1875 Ma for the Kuncha Formation, ~1800 Ma for the Fagfog Formation, and ~ 1780 Ma for the Kushma Formation. The intrusive 1831 ± 17 Ma Ulleri augen gneiss provides a minimum depositional age bound for the Kuncha. Combined, these data suggest the Kuncha Formation is the oldest member of the Lesser Himalayan series in central Nepal. Additionally, 13C data suggest the Malekhu Formation of the Lakharpata Group was deposited before ca. 1250 Ma. A field mapping comparison based on the redefined stratigraphy indicates the Ramgarh thrust is located >10 km farther south than previously mapped, potentially reducing regional shortening estimates.
- ItemCONSTRAINTS ON THE LITHOSPHERIC STRUCTURE OF MID OCEAN RIDGES FROM OCEANIC CORE COMPLEX MORPHOLOGY(2016) Larson, Mark Oscar; Montési, Laurent GJ; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Mid-oceanic ridge system is a feature unique to Earth. It is one of the fundamental components of plate tectonics and reflects interior processes of mantle convection within the Earth. The thermal structure beneath the mid-ocean ridges has been the subject of several modeling studies. It is expected that the elastic thickness of the lithosphere is larger near the transform faults that bound mid-ocean ridge segments. Oceanic core complexes (OCCs), which are generally thought to result from long-lived fault slip and elastic flexure, have a shape that is sensitive to elastic thickness. By modeling the shape of OCCs emplaced along a ridge segment, it is possible to constraint elastic thickness and therefore the thermal structure of the plate and how it varies along-axis. This thesis builds upon previous studies that utilize thin plate flexure to reproduce the shape of OCCs. I compare OCC shape to a suite of models in which elastic thickness, fault dip, fault heave, crustal thickness, and axial infill are systematically varied. Using a grid search, I constrain the parameters that best reproduce the bathymetry and/or the slope of ten candidate OCCs identified along the 12°—15°N segment of the Mid-Atlantic Ridge. The lithospheric elastic thicknesses that explains these OCCs is thinner than previous investigators suggested and the fault planes dip more shallowly in the subsurface, although at an angle compatible with Anderson’s theory of faulting. No relationships between model parameters and an oceanic core complexes location within a segment are identified with the exception that the OCCs located less than 20km from a transform fault have slightly larger elastic thickness than OCCs in the middle of the ridge segment.