Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 119-135). / This dissertation contributes to three unresolved problems in planetary science regarding potential dynamo action in asteroids, the Moon, and Mars. First, we examine the physical processes active during asteroid core crystallization. We model inward crystallization as well as crystal settling and the compaction of a possible cumulate inner core. We then explore the potential strength and longevity of a dynamo in the planetesimal's early history. We find that cumulate inner core solidification would be capable of sustaining a dynamo during the bulk of solidification, but that there may be insufficient power for a dynamo in an inward dendritic solidification scenario. Next, we explore the origin of the magnetic field recorded in the lunar crust. Evidence suggests a core dynamo existed from 4.2 to 3.56 Ga, and possibly until near present day. Seismic measurements indicate the lunar core is partially solidified. Latent heat and concentrated light elements at the interface of a solidifying inner core could drive outer core convection. We demonstrate that core solidification can account for the observationally inferred duration of the lunar dynamo. However, it cannot explain the magnitude of the recorded magnetic field. A dynamo may also stop and restart due to heat flow fluctuations as convective vigor falls below the threshold for dynamo action. Finally, we examine the early history of the Martian mantle. The solidification of a magma ocean may result in an unstable density profile prone to overturn. A long-wavelength instability could play a role in the stark contrasts observed between the northern and southern hemispheres of Mars, including the dichotomy in crustal thickness and magnetization. However, we find that cumulate overturn in the Martian scenario would likely have occurred with short wavelengths. In an isoviscous model, thermal convection ensues rapidly after overturn; however, when viscosity is temperature dependent, compositional stability suppresses the onset of convection. / by Aaron Lewis Scheinberg. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2002. / Includes bibliographical references. / This thesis examines the response of alpine landscapes to the onset of glaciation. The basic approach is to compare fluvial and glacial landscapes, since it is the change from the former to the latter that accompanies climatic cooling. This allows a detailed evaluation of hypotheses relating climate change to tectonic processes in glaciated mountain belts. Fieldwork was carried out in the eastern Sierra Nevada, California, and the Sangre de Cristo Range, Colorado, alongside digital elevation model analyses in the western US, the Southern Alps of New Zealand, and the Himalaya of northwestern Pakistan. The evidence presented here suggests that the so-called "chicken-and-egg" hypothesis is overstated in its appeal to glacial erosion as a major source of relief production and subsequent peak uplift. Glaciers in the eastern Sierra Nevada and the western Sangre de Cristos have redistributed relief, but have produced only modest relief by enlarging drainage basins at the expense of low-relieftopography. Glaciers have lowered valley floors and ridgelines by similar amounts, limiting the amount of "missing mass" that can be generated, and causing a decrease in drainage basin relief. / (cont.) The principal response of glaciated landscapes to rapid rock uplift is the development of towering cirque headwalls. This represents considerable relief production, but is not caused by glacial erosion alone. Large valley glaciers can maintain their low gradient regardless of uplift rate, which supports the "glacial buzzsaw" hypothesis. However, the inability of glaciers to erode steep hillslopes as rapidly can cause mean elevations to rise. Cosmogenic isotope dating is used to show that (i) where plucking is active, the last major glaciation removed sufficient material to reset the cosmogenic clock; and (ii) former glacial valley floors now stranded near the crest of the Sierra Nevada are at varying stages of abandonment, suggesting a cycle of drainage reorganisation and relief inversion due to glacial erosion similar to that observed in river networks. Glaciated landscapes are quite distinct from their fluvial counterparts in both landforms and processes. Given the scarcity of purely fluvial, active mountain ranges, it is essential that glacial erosion be considered amongst the processes sculpting active orogenic belts. / by Simon H. Brocklehurst. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1984. / Microfiche copy available in Archives and Science. / Bibliography: leaves 63-64. / by Peter Paul Neilley. / M.S.

Thesis: S.M., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 69-75). / Tropical cyclones (TCs) are among the most expensive and lethal geophysical hazards. Studies suggest that the intensity of TCs will increase due to the thermodynamic effects of anthropogenic greenhouse gas input. In contrast, while aerosols are shown to have an overall cooling effect on global climate, their impact on TCs is not yet well-understood. This paper explores the influence of the sulfate aerosol first indirect effect (AIE) on Atlantic hurricane intensity and genesis. I use a single-column radiative convective model that incorporates the first AIE (aerosol enhancement of cloud reflectivity) through parameterization of cloud droplet number, radius, and optical depth. Cloud droplet number is parameterized using an empirical scheme, while the radius is determined from cloud liquid water content and number concentration moments, and the optical depth scheme is embedded in the original single-column model. The model is run with both the IGAC/SPARC Chemistry Climate Model Initiative (CCMI) historical simulations of sulfate concentrations over the hurricane main development region during hurricane peak season (August-October) and a self-generated inventory of sulfate concentrations based on realistic vertical variability in sulfate levels. The model was run to radiative-convective equilibrium (RCE), then rerun under weak temperature gradient mode (WTG). Runs successfully produce the Twomey or first indirect effect, which states that increased aerosols will increase cloud droplet number concentration, decrease the effective cloud droplet radius, and increase the cloud optical depth. The net effect is increased reflection of radiation from the atmosphere, which theoretically cools the Earth, decreasing the potential intensity and genesis potential of TCs. While model runs produce the expected changes in cloud properties, cloud cover is not sufficient for sulfate concentrations to have a substantial impact on hurricane activity via the AIE when the model is run to RCE. The WTG mode is then implemented with the goal of producing low-lying stratocumulus clouds to increase total cloud cover, but the single-column WTG scheme was not able to produce stratocumulus that did not also produce an overly strong negative feedback. Using the single-column model, one can demonstrate the indirect effect of sulfate aerosols on cloud reflectivity and that sufficient cloud cover is needed to produce a noticeable cooling and change in expected hurricane behavior. A further study of the subject could include parameterization of the poorly-understood cold or mixed-phase clouds, which can include characterization of additional aerosol types. In addition, a two-dimensional model has greater capacity to model phenomena such as low-lying stratocumulus, which could produce a more substantial ambient effect. / by Hao-yu Derek Chang. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 91-101). / Details of the biogeochemical cycles and the dominant mechanisms present in Precambrian remain heavily debated topics. The events of the Late Proterozoic onset to glaciations and what types of early life existed in the Archean are two of the many provoking topics within the Precambrian. We set out to improve the understanding of these geologic intervals by examining stable isotopic signatures and molecular fossils (biomarkers) in Late Proterozoic and Mesoarchean ages sedimentary rocks in Northwestern Territories, Canada and Pilbara, Western Australia, respectively. This thesis presents sulfur, carbon, oxygen and nitrogen stable isotopic data along with distribution of steranes and hopanes biomarkers. Geochemical data is analyzed in the context of elucidating the key biological and environmental factors involved in the Mesoarchean marine biosphere and the Late Proterozoic onset of glaciations. Stable isotopic analysis of the Gorge Creek Group in Pilbara, Western Australia reveals organisms capable of microbial sulfur disproportionation were likely the dominant biological players in Mesoarchean deep-ocean sulfur cycling. Biomarker and isotopic proxies of the Coppercap Formation reveal diverse biological activity directly prior to the Sturtian Glaciation with communities of green and purple sulfur bacteria as well as methanotrophs and cyanobacteria. Possible environmental implications of these communities co-existing are explained in context of changes in ocean chemistry and the diversification of eukaryotic life. / by Katherine S. Thomas. / S.M.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1994. / Includes bibliographical references (leaves 69-70). / by Susan Aileen Evans. / M.S.

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 37-40). / Escarpments are present on passive margins around the world, but their evolution is poorly understood. Some geologists interpret escarpments as stationary features, whereas others have argued that they are retreating inland faster than a kilometer per Myr. I investigate Brazilian escarpments by determining whether or not the river networks on either side of the escarpment are in erosional equilibrium with each other. My approach is based on the premise that rivers on opposite sides of a stationary escarpment would be eroding at the same rate (erosional equilibrium), whereas rivers on opposite sides of a mobile escarpment would be eroding at different rates (erosional disequilibrium). I use a recently developed technique called chi mapping to assess the erosional disequilibrium of river networks along the Brazilian escarpments. For comparison, I also compile erosion rates on either side of the escarpments from cosmogenic "Be measurements in the existing literature, and use these to calculate recent retreat rates of the escarpments, which fall between 4 and 40 m/Myr. I determine that chi mapping and cosmogenic erosion rates agree on the direction of escarpment movement, but disagree on the magnitudes of the retreat rates. I also estimate the percentage of drainage area exchanged by stream capture as the escarpment drainage divide moves across the landscape. Using two different estimation methods, I find that less than 40% of drainage area is exchanged by stream capture, making divide migration the dominant mechanism for drainage basin reorganization. If my estimates of recent escarpment retreat rates are representative of long-term rates, the Brazilian escarpments have retreated up to 5 km since their formation during the Cretaceous rifting event. My analysis shows that the topographic retreat of the Brazilian escarpments could have been driven by drainage basin disequilibrium resulting in divide migration. / by Madison M. Douglas. / S.B.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1998. / Includes bibliographical references (leaves 86-90). / by Carrie T. Friedman. / M.S.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2002. / Includes bibliographical references (p. 113-121). / It is suggested that orbital tuning casts a false light upon the chronology of glaciation and the understanding of the climatic response to orbital variations. By developing a new age-model, independent of orbital assumptions, a significant non-linear response to orbital forcing becomes evident in the [delta] 18 0 record. The new age-model also indicates glacial terminations two through eight are 8,000 years older than the orbitally based estimates. A simple obliquity threshold model is presented which reproduces the timing, amplitude, and observed non-linearities of the [delta] 18 0 record; and supports the plausibility of the new age-model and the inferred non-linear climatic response. / by Peter Huybers. / S.M.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1998. / Includes bibliographical references (leaves 163-173). / A Stoneley wave propagating in a borehole generates a flow of pore fluid in permeable zones intersected by the borehole. In turn, this flow of pore fluid induces a streaming electrical field. This thesis is an experimental and theoretical investigation of the electrical fields induced by Stoneley waves. The main emphasis of this thesis is to understand the electroseismic phenomena that are observed in the field. In the first experiment described in this thesis, we measured Stoneley-wave-induced electrical fields in a borehole drilled through fractured igneous rocks. Analysis of field data confirms that the electrical fields that we measured were induced by fluid flow in fractures. The normalized amplitude of these electrical fields correlated with the fracture density log. In the second experiment, we measured Stoneley-wave-induced electrical fields in several boreholes in vuggy dolomite. In dolomite, the normalized amplitude of the Stoneley-wave-induced electrical field correlates with the porosity of the formation around the borehole. Further, the Stoneley-wave-induced electrical fields have anomalously high amplitudes at an isolated fracture that intersected two boreholes. To explain the experimental results, we developed a theoretical model for the Stoneley-wave-induced electrical fields. According to the model, the normalized amplitude of the Stoneley-wave-induced electrical field is proportional to the porosity and inversely proportional to the pore space tortuosity of a formation around a borehole. Moreover, the amplitude-versus-frequency behavior of this electrical field depends on the permeability of the formation. To further test the theory's prediction, we measured electrical potentials induced by the borehole Stoneley wave in the frequency range from 100Hz to 4kHz. The normalized amplitudes of the Stoneley-wave-induced electrical potentials measured in the field were consistent with the amplitudes predicted by the theory. Also, the amplitude- versus-frequency dependence of the electroseismic signals recorded at the depth of the large fracture roughly followed the trend predicted by the theory. However, the general amplitude-versus-frequency dependence of the electroseismic signals recorded in the field is more complicated than that predicted by the theory. The main contributions of this thesis are: 1. This thesis develops a borehole electroseismic measurement technique and demonstrates that it works in the field. This technique can be used to make electroseismic logging measurements. 2. This thesis investigates an electrical field induced by a borehole Stoneley wave. This electroseismic phenomenon is explained, measured in the field and modeled theoretically. 3. This thesis derives from field data a parameter that describes local electroseismic coupling in a formation around a borehole. This parameter, the normalized amplitude of the Stoneley-wave-induced electrical field, is defined as the ratio of an electrical field amplitude to a pressure amplitude in the Stoneley wave at a certain depth. This thesis demonstrates that the normalized amplitude of the Stoneley-wave- induced electrical field can be used to identify permeable fractures in situ. 4. This thesis uses field electroseismic measurements to quantitatively characterize rock formations around a borehole. Using the theoretical model developed in this thesis, a porosity log for fractured granite is derived from electroseismic field data. / by Oleg Mikhailov. / Ph.D.