Recursos de colección

DSpace at MIT (104.280 recursos)

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Earth, Atmospheric and Planetary Sciences - Ph.D. / Sc.D.

Mostrando recursos 1 - 20 de 1.321

  1. The marine biogeochemistry of chromium isotopes

    Moos, Simone Beatrice
    In the ocean, chromium (Cr) is a redox-sensitive trace metal. The reduction of Cr(VI) to Cr(III) occurs in oxygen deficient zones (ODZs), and Cr reduction in general has been identified as a significant Cr isotope fractionation mechanism. This thesis presents the first Cr isotope variations (653 Cr) in ODZs of the ocean and adds to the sparse Cr isotope data published for modern seawater. I developed a precise and accurate Cr isotope method for seawater samples. Seawater acidification converts total Cr to Cr(III) which is preconcentrated by Mg(OH) 2 coprecipitation. A three-column anion exchange chromatography scheme separates Cr from isobaric...

  2. Carbon and mineral transformations in seafloor serpentinization systems

    Grozeva, Niya G
    This thesis examines abiotic processes controlling the transformation and distribution of carbon compounds in seafloor hydrothermal systems hosted in ultramafic rock. These processes have a direct impact on carbon budgets in the oceanic lithosphere and on the sustenance of microorganisms inhabiting hydrothermal vent ecosystems. Where mantle peridotite interacts with carbon-bearing aqueous fluids in the subseafloor, dissolved inorganic carbon can precipitate as carbonate minerals or undergo reduction by H2(aq) to form reduced carbon species. In Chapters 2 and 3, I conduct laboratory experiments to assess the relative extents of carbonate formation and CO2 reduction during alteration of peridotite by C02(aq)-rich fluids....

  3. Water and volatile element accretion to the inner planets

    Sarafian, Adam Robert, 1986-
    This thesis investigates the timing and source(s) of water and volatile elements to the inner solar system by studying the basaltic meteorites angrites and eucrites. In chapters 2 and 3, I present the results from angrite meteorites. Chapter 2 examines the water and volatile element content of the angrite parent body and I suggest that some water and other volatile elements accreted to inner solar system bodies by ~2 Myr after the start of the solar system. Chapter 3 examines the D/H of this water and I suggest it is derived from carbonaceous chondrites. Chapter 4, 5, 6, and 7...

  4. An investigation of basin-scale controls on upper ocean export and remineralization

    Black, Erin E
    The biological carbon pump (BCP) helps to moderate atmospheric carbon dioxide levels by bringing carbon to the deep ocean, where it can be sequestered on timescales of centuries to millennia. Climate change is predicted to decrease the efficiency of the global BCP, however, the magnitude and timescale of this shift is largely uncertain and will likely impact some areas of the global ocean more significantly than others. Therefore, it is imperative that we (1) accurately quantify surface export and remineralization of particulate organic carbon (POC) via the BCP over large regions of the global ocean, (2) examine the factors controlling...

  5. Attribution of stratospheric ozone change and associated climate impacts

    Bandoro, Justin
    Recognition of stratospheric ozone depletion as a significant global danger sparked the landmark international agreement of the Montreal Protocol to control the production of ozone depleting substances (ODSs). There are now signs of stratospheric ozone recovery, and it is essential to understand whether the observed historical changes, during both the depletion and recovery eras, are directly the result of secular changes in ODSs, or influenced by other anthropogenic and natural forcings such as greenhouse gases (GHGs) and solar variability. This thesis explores the climate impacts of stratospheric ozone depletion, and how we can attribute, with high confidence, the causes of...

  6. Taphonomic studies of fossil preservation in coarse-grained siliciclastic environments

    Newman, Sharon Audrey
    Ediacaran and early Cambrian sandy and silty sediments commonly preserve microbial fossils and textures as well as the fossils of soft-bodied organisms. The rarity of similar fossils and textures in coarse-grained siliciclastic strata of the later Phanerozoic suggests that a taphonomic window facilitated this mode of fossil preservation. However, environmental and biological factors that promoted fossilization remain unclear. To experimentally identify mechanisms of preservation in siliciclastic sediments, cyanobacteria and soft tissues (scallop adductor muscles) were incubated in silica sand or clay minerals for up to two months. Clay mineral veneers coated both cyanobacterial filaments and the surfaces of soft tissues...

  7. Quantifying emissions of carbon dioxide and methane in central and eastern Africa through high frequency measurements and inverse modeling

    Gasore, Jimmy
    Carbon dioxide (CO2) and methane (CH 4) are the main greenhouse gases, contributing about 81% of the total human induced radiative forcing. Sufficient observations exist to quantify the global budget of carbon dioxide and methane which is necessary for calculating the resulting radiative forcing. Still, more observations are needed to constrain their time evolution and regional budgets which are needed for climate change mitigation policies. Atmospheric observations are particularly scarce on the African continent, despite Africa's significant CO2 emissions from agriculture, biomass burning and land use changes, as well as methane emissions from wetlands. there are very few low frequency...

  8. Toward detection and characterization of exoplanetary magnetic fields via low frequency radio observation

    Knapp, Mary (Mary E.)
    Low frequency radio emission from planets is produced by the interaction of energetic charged particles from the planet's ionosphere and/or the solar wind with the planet's magnetic field. The temporal and spectral characteristics and variability of planetary radio emission encode information about a planet's magnetic field strength and morphology, rotation, and interior. This thesis describes three distinct approaches to detecting radio emission from extrasolar planets ( exoplanets). The first is a novel approach using 'big data' and computer aided discovery tools to mine radio survey images for faint radio emission from the location of nearby stars. The flexible approach described...

  9. The tropopause region thermal structure and tropical cyclones

    Gilford, Daniel Michael
    This thesis is an exploration of two seemingly unrelated questions: First, how do water vapor and ozone variations radiatively influence the thermal structure of the tropopause region? Second, what sets the thermodynamic limits of tropical cyclone intensity across the seasonal cycle? The link between these subjects is tropical cyclone outflow, which often reaches into the tropopause region, allowing the thermal structure there to impact tropical cyclone potential intensity. A radiative transfer model is employed to calculate the radiative effects of the 2000 and 2011 tropopause region abrupt drops -- events in which temperatures, water vapor, and ozone plunge suddenly to...

  10. Age of air and the circulation of the stratosphere

    Linz, Marianna Katherine
    The circulation of air in the stratosphere is important for the distribution of radiatively-important trace gases, such as ozone and water vapor, and other chemical species, including ozone-depleting chlorofluorocarbons. Age of air in the stratosphere is an idealized tracer with unique mathematical properties, which we exploit to derive a theory for the relationship of tracer observations to the stratospheric circulation. We show that the meridional age gradient is a measure of the global diabatic circulation, the total overturning strength through an isentropic surface, and test this time-dependent theory in a simple atmospheric general circulation model. We apply the theory to...

  11. Influence of meltwater on Greenland Ice Sheet dynamics

    Stevens, Laura A
    Seasonal fluxes of meltwater control ice-flow processes across the Greenland Ice Sheet ablation zone and subglacial discharge at marine-terminating outlet glaciers. With the increase in annual ice sheet meltwater production observed over recent decades and predicted into future decades, understanding mechanisms driving the hourly to decadal impact of meltwater on ice flow is critical for predicting Greenland Ice Sheet dynamic mass loss. This thesis investigates a wide range of meltwater-driven processes using empirical and theoretical methods for a region of the western margin of the Greenland Ice Sheet. I begin with an examination of the seasonal and annual ice flow...

  12. Dependence of continental severe convective instability on climatological environmental conditions

    Agard, John Vincent
    Most of Earth's strongest atmospheric convection occurs over the continents, where potential energy is stored over time in metastable vertical profiles, only to be released rapidly by severe storms. In contrast to equilibrium-state convection in tropical ocean regions, there is a relative paucity of research exploring the climate dynamics of continental deep convection. This work makes a contribution to rectifying that deficiency by exploring the physical mechanisms by which convective available potential energy (CAPE) is generated, and their dependence on climatological properties of the Earth's environment. First, a budget of the time tendency of CAPE is used to examine the...

  13. Archean continental crust formation and the rise of atmospheric oxygen

    Bauer, Ann M., Ph. D. Massachusetts Institute of Technology
    This thesis examines critical aspects of the terrestrial environment that have resulted in a habitable planetary surface: the establishment of the continental crust and the progressive rise of an oxygenated atmosphere. The volume of continental crust on the earliest Earth is a critical parameter for constraining the chemical evolution of major terrestrial reservoirs, and radiogenic isotope signatures document this varying geochemical character. Chapter 1 presents a Lu-Hf and U-Pb isotopic characterization of zircons from the 4.0-2.9 Ga Acasta Gneiss Complex (AGC) and documents the magmatic extraction history of this domain, including changes in source compositions. These results are compared with...

  14. The role of roughness in earthquake source physics

    Tal, Yuval, Ph. D. Massachusetts Institute of Technology
    Faults are rough at all scales and can be described as self-affine fractals. This deviation from planarity results in geometric asperities and a locally heterogeneous stress field, which affect the nucleation and propagation of shear rupture. I study this effect numerically at the scale of small earthquakes, in which realistic geometry and friction law parameters can be incorporated in the model. The numerical approach developed in this thesis includes three main features. First, to enable slip that is large relative to the size of the elements near the fault, as well to capture accurately of the variation of normal stress...

  15. The role of wavenumber one and two in the development of sudden stratospheric warmings

    Miller, Andreas Wolfgang
    In this thesis, we investigate the effects of planetary waves one and two on the polar stratosphere during boreal winter. We use MERRA reanalysis data and the FMS shallow-water model to compare and contrast their propagation into the stratosphere, their interactions within the stratosphere, and their effects on the polar vortex. The results have implications for the predictability of sudden stratospheric warmings (SSWs), theories on the developments of vortex splits and the role of zonal winds in the tropics. In Chapter 2, we use correlations and regressions to demonstrate that the tropopause affects wavenumber one amplitudes more than wavenumber two....

  16. Mapping exoplanet clouds and albedo from phase curves and spectra

    Webber, Matthew (Matthew William)
    This thesis uses planetary albedo models to investigate variations in visible wavelength phase curves of exoplanets. We improve upon existing exoplanet giant planet albedo models and incorporate exoplanet general circulation models to analyze the composition and occurrence of clouds on tidally locked exoplanets. We confirm that non-uniform cloud coverage on the dayside of tidally locked exoplanets will affect the magnitude and location of the maximum of the phase curve. We then apply the models to the exoplanet Kepler-7b and consider the effect of varying cloud species, sedimentation efficiency, particle size, and cloud altitude. In the context of Kepler Space Telescope...

  17. Emergent geometries of groundwater-fed rivers

    Yi, Robert Sngho
    Groundwater-fed rivers form stunning geometries over a range of scales. These rivers grow as water from an underground aquifer reemerges and erodes the overlying topography. Both the aquifer and the overlying topography generate flows along diffusive gradients. We study three features produced by these gradients over different scales: the shape of the valley that forms around a single stream, the network-averaged planform stream shape, and the shape of the drainage basin. First, we identify a new feature in stream valleys - a spatially variable diffusivity - that gives rise to a theoretical valley shape that agrees with the shapes of...

  18. Understanding the chemistry of atmospheric particles using single particle mass spectrometry

    Zawadowicz, Maria Anna
    This thesis explores ways in which single particle mass spectrometry can be extended, whether through hardware improvements, or through the use of advanced data processing techniques to provide new kinds of aerosol chemistry measurements. Most of this work has been carried out using the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument, an aircraft deployable mass spectrometer that uses intense (~10 9 Wcm -2 ) UV laser pulses to vaporize and ionize single particles and measures their mass spectra using a time-of-flight mass spectrometer. Near-term and long-term hardware improvements as well as advanced data analysis techniques are explored in order...

  19. Processing strategies optimization and error mitigation of geodetic measurements

    Coccia, Martina
    This doctoral thesis focuses on the application of geodetic techniques and finite element modeling to studying crustal deformation and other processes. In particular, it focuses on optimizing standard processing strategies, reducing the noise in the measurements and mitigating the effects of external processes in order to extract the signal of interest. Geodetic techniques, such GPS and InSAR, are still affected by major sources of errors, such as multipath, atmospheric effects, snow, blockage of the signal by infrastructure that can make difficult the detection of geophysical signal. In this thesis, I analyze three sets of data for which I have used...

  20. Linking microbes and climate : insights into the marine oxygen and nitrogen cycles with microbial metabolic functional types

    Zakem, Emily Juliette
    This thesis posits that understanding the controls on microbially-mediated marine biogeochemical cycling requires a mechanistic description of microbial activity in biogeochemical models. In the work here, the diverse microbial community is resolved using metabolic functional types, which represent metabolisms as a function of their underlying redox chemistry and physiology. In Chapter 2, I use a simple model to predict the limiting oxygen concentration of aerobic microbial growth in an ecosystem. This limiting concentration is in the nanomolar range for much of the parameter space that describes microbial activity in marine environments, and so anticipates the recent measurements of oxygen to...

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