Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2006. / Cataloged from PDF version of thesis. Pages 77-78 missing from original thesis. / Includes bibliographical references (pages 76-84). / A theory for changes in ozone chemistry during late Proterozoic global glaciations is developed. The possible significance of temperature, humidity, nitrogen oxides, reactive chlorine, lightning frequency, surface deposition and albedo as altered constraints on ozone processes is discussed. An elementary box model is developed by the author to make first order judgments regarding the significance of chemistry changes on ozone concentrations and its climactic effect. A one dimensional photochemical-transport model (Kasting, 1995) was used to more precisely determine the effects of global glaciations on ozone concentrations up to 5 hPa in several latitude bands. Reduced NO₂ availability in the stratosphere seems to dominate ozone's response (positive anomalies) in the stratosphere. Low temperatures, low humidity, reduced lighting frequency and altered chlorine and nitrogen chemistry collectively reduce ozone presence in the troposphere, however the overall sign of the tropospheric ozone anomaly depends heavily on poorly characterized deposition rates. With output from the one-dimensional photochemistry model, a time-varying ozone concentration field was assembled for the entire planet and used in snowball runs of the General Circulation Model (NCAR Community Atmosphere Model 3.0). These runs were compared to a controlled snowball run with a modern ozone field to discern the climactic significance of altered ozone. Results suggest that ozone concentrations during global glaciations might directly produce global average surface radiation anomalies of -1.5 ~ 1.5 W/m² , resulting in global average surface temperature anomalies of -0.5 ~ 0.5°K. Magnitude and sign uncertainties result from poorly known deposition rates for ozone over frozen surfaces and the simplicity of the modeling technique. The indirect effect of increasing stratospheric ozone, i.e. a reduction in atmospheric oxidative capacity, may result in positive anomalies of other green house gasses and is discussed as an area for further research. / by Solomon M. Hsiang. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2006. / Includes bibliographical references. / The (U-Th)/He thermochronmeter is the foremost tool available to geoscientists for the purposes of constraining the thermal evolution of the crust below 250°C. However, the potential of the technique is far from fully explored. The existing de facto (U-Th)/He age standard, the fluorapatite of Cerro de Mercado (Durango, Mexico), does not yield reproducible dates, an observation confirmed at several laboratories. A series of experiments combining analytical and numerical techniques suggests that variations in U and Th measured in a single sectioned crystal could result in several percent apparent age variation due to unequal alpha recoil exchange across internal zone boundaries. Heterogeneities may also play a role in the variability observed in the diffusive behavior of He in strongly zoned minerals, such as monazite. He diffusion experiments on monazite indicate that the closure temperature for (U-Th)/He thermochronometry is between 206°C and 286°C, but varies from grain to grain, even within suites of crystals obtained from one rock. This may be due to compositional variations in the host monazite. Typically heterogeneous monazite crystals may have different closure temperatures, and single crystals may preserve large portions of the cooling history of a rock. / (cont.) None the less, monazite appears to have great potential for (U-Th)/He chronometry, and because of the high U and Th (and therefore 4He) concentrations observed in many monazites, it is a prime candidate for microanalytical (U-Th)/He. A microanalytical protocol for (U-Th)/He chronometry has been developed by applying the laser microprobe to polished crystals or crystal fragments of monazite. Agreement with conventional (U-Th)/He ages is to within -1.3%, with previously unavailable spatial resolution of -30pm. Large numbers of laser microprobe (U-Th)/He ages can be generated more rapidly than by conventional means, and this technique allows the dating of grains that would not be acceptable for conventional (U-Th)/He. Application of laser microprobe (U-Th)/He to monazites from Nanga Parbat, Pakistan, yields highly reproducible cooling ages within and between single crystals. Mean ages of 0.746 ± 0.049 Ma and 0.753 ± 0.036 Ma from two crystals, and the direct observation of negligible (U-Th)/He age gradients, support minimum mean cooling rates of > 325 K/m.y., and minimum mean rock uplift rates of > 4 km/m.y.. Laser microprobe (U-Th)/He of zircon produces no geologically useful dates, a consequence of the poor U and Th measurements provided by LA-ICPMS. / (cont.) Future efforts should employ better calibrated LA-ICPMS approaches or next-generation electron microprobe techniques, which can provide more accurate U and Th concentrations than LA-ICPMS, even for the low concentrations found in zircon. / by Jeremy Welles Boyce. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2006. / Includes bibliographical references. / Numerical and analytical models of orogenic growth suggest that erosion can focus deformation in active convergent orogens, leading to a debate over the relative influence of climatic and tectonic forcing on erosion. In this thesis, geologic and observations, thermochronology, and thermo-kinematic numerical modeling are combined to quantify variations in long-term erosion in the Nepal Himalaya. Detrital mineral thermochronology is explored as a tool for quantifying tectonic and geomorphic process rates at a variety of spatial and temporal scales. The assumptions upon which catchment-wide erosion rate estimates based on detrital data depend are evaluated using statistical comparisons of 40Ar/39Ar ages from Nepal and catchment area-elevation distributions. Bedrock 40Ar/39Ar data indicate that erosion rate estimates from detrital thermochronology yield the same rate as the traditional approach of analyzing bedrock cooling ages over a range of elevations. Both bedrock and detrital 40Ar/39Ar data suggest a major acceleration of erosion rates at the Himalayan range front, and apatite fission-track data are used to pinpoint the timing of this acceleration to between 2.5 and 0.9 million years ago. / (cont.) Three-dimensional thermal modeling indicates that the effects of topography, erosion, and rock exhumation pathways on age-elevation relationships do not compromise! this conclusion. While the time frame for this change corresponds to that of an important destabilization of global climate, there is no evidence for a change in tectonic forcing during this interval. These timing constraints support the hypothesis that climate can exert a first-order control on erosion in the evolution of orogens. If climate strongly influences long-term erosion and erosion and deformation are coupled through gravitational feedbacks, then a persistent style of deformation would be expected where monsoon precipitation and erosion have been focused at the Himalayan front for millions of years. Implications of such feedbacks for the steady-state evolution of the range are explored in a detailed analysis of the structural configuration at this position. Future studies of the strength of feedbacks among climate and tectonic forcing, erosion, and deformation are warranted. Creative applications of detrital thermochronology may be used in such studies to constrain landscape response time to climatic and tectonic perturbations. / by Katharine W. Ruhl. / Ph.D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1989. / Includes bibliographical references (leaves 65-68). / by James Jay Klavetter. / Sc.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2006. / Includes bibliographical references. / I investigate postseismic and interseismic deformation using two-dimensional models of a vertical strike-slip fault in an elastic layer over a viscoelastic region. Central to this, I derive an analytic framework to include general linear viscoelastic theologies into models of interseismic deformation. Models with multi-viscous viscoelastic rheologies predict multiple phases of postseismic and interseismic deformation. For instance, with bi-viscous Burgers rheologies there is transient deformation early in the seismic cycle, while the deformation is steady later in the cycle. Only layered models with multi-viscous rheologies predict multiphase postseismic deformation. Fault models containing a steady component of relaxation, evolve to mature states after a sufficient number of fault ruptures. The mature surface deformation is unrelated to the steady deformation at depth and depends only on the rupture history and locking depth of the fault and the theologies of the model. With periodic ruptures, the mature deformation is cycle invariant. If the recurrence time or magnitude of the ruptures then change, the interseismic velocities vary significantly from cycle to cycle as they evolve toward new cycle invariant velocities. / (cont.) The time for a model to equilibrate to an invariant state is proportional to the time scale of the steady component of viscoelastic relaxation, and the effect of prior fault activity is only negligible in models with weak theologies. The interseismic deformation observed both before and after the 2004 Parkfield, CA, and 1999 Izmit, Turkey, earthquakes can be approximated by a model with multi-viscous viscoelastic rheologies. The deformation at Parkfield is approximated using a rheology with possibly two transient viscous phases (relaxation times about one and 3-12 months) and a steady viscous phase with a time scale much longer than 20 years. The deformation at Izmit is described by a model with a Burgers rheology with transient and steady relaxation times of three years and about 500 years or more. / by Eric Andreas Hetland. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1990. / Vita. / Includes bibliographical references (p. 281-300). / by Marilyn Mitchell Wolfson. / Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 57-58). / Two numerical models, one kinematic and one dynamic, were created and compared in their ability to predict trajectories of atmospheric parcels over eight days. While kinematic models are more widely used due to their accuracy, dynamic models can be used pedagogically to visualize the balance of forces in the atmosphere. The kinematic model used gridded wind speed data from the Global Forecast System (GFS) to predict parcel flow, while the dynamic model calculated wind speeds from advection equations using geopotential height fields from GFS. The trajectories of ensembles of parcels were simulated from five launch locations. The spread of parcels from each location was calculated along with the deviation from reference trajectories. The dynamic model performed comparably to the kinematic model, despite the presence of inertial oscillations in some computed trajectories at mid- and high- latitudes which are likely to be physically unrealistic. The dynamic model was more sensitive to changes in spatial resolution than the kinematic model. Dynamic trajectory models were shown to be accurate enough to be used as a tool to visualize the interplay of forces acting in the atmosphere. / by Allison Schneider. / S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references. / In this thesis we further develop concepts in inverse scattering, which enable higher resolution imaging with data from dense networks and arrays. We apply the new methods to studies of the crust beneath south Tibet and the core-mantle boundary (CMB) region beneath Central America and East Asia. First, we develop a new method, which we call passive source reverse time migration (RTM), for subsurface imaging with teleseismic array data. Multi-component array data are first propagated backward by solving the elastic wave equation. After polarization separation, a modified cross-correlation imaging condition between P and S wave constitutes is applied to obtain an inverse scattering transform. From synthetic experiments it is evident that for complex geological structures RTM is superior to traditional receiver functions analysis, such as common conversion point (CCP) stacking. Two preprocessing steps are required for RTM application on sparsely sampled teleseismic dataset: source normalization and trace interpolation. The source radiation pattern, especially the polarity of traces, is corrected by multi-channel cross-correlation technique. The unknown source signature is then estimated by principle component analysis and deconvolved from raw data by Wiener deconvolution. Curvelet interpolation with sparsity promotion is employed to interpolate irregularly and sparsely sampled traces into regular and dense grids. Synthetic and real data examples demonstrate that for typical teleseismic acquisition geometry, with 50% to 85% missing traces, the curvelet-based interpolation works remarkably well. The application on Hi-CLIMB array data in Tibetan plateau reveals clear and continuous Moho discontinuity at the depth of about 70 km, as well as fine crustal structures. Second, we use a high-frequency approximation of inverse scattering, generalized Radon transform (GRT), to probe the lowermost mantle beneath Central America and East Asia. Inverse scattering of about 130,000 ScS traces and 120,000 SKKS traces reveals multiple reflectors above the conventional D" region. This result is inconsistent with expectations from a pure thermal response of a single isochemical post-perovskite transition but can be explained with post-perovskite transitions in differentiated slab materials. Our results imply that the lowennost mantle is more complex than hitherto thought and that the presence of interfaces and compositional heterogeneity beyond the D" region. / by Xuefeng Shang. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1993. / Includes bibliographical references (p. 175-183). / by Delaine Rebecca Thompson. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1986. / Microfiche copy available in Archives and Science. / Includes bibliographies. / by Tianqing Cao. / Ph.D.