Thesis: S.M. in Geology, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2019 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 88-98). / The Red Sea rift is a nascent ultra-slow spreading ridge superimposed on the Afar plume. Based on high-resolution seismic data, the southernmost (south of the Danakil rift at 17.05°N), southern (17.05-19.75°N), and central (19.75-23.8°N) segments display seafloor spreading that is anomalously magma-rich compared to other ultra-slow spreading centers. In contrast, the northern segment (23.8-28°N) exhibits magma-poor extension along large-offset east- and west-dipping detachments. Sediment-corrected basement depths along the northern Red Sea reveal an axial valley as deep as the Gakkel Ridge (4200-5100 m). South of 19.75°N, plume-supported axial shoaling matches that of adjacent parts of Arabia, Africa, and the Gulf of Aden. Geochemically, the southernmost Red Sea is the locus of plume-ridge interaction. Here, E-MORBs are enriched in alkali, incompatible, and light rare-earth elements. / High mantle potential temperatures (T[subscript p]; 1326±5°C), melting pressures (12±0 kbars) and temperatures (1306±6°C), and fractionation pressures (5.3±1.6 kbars) calculated using the reverse fractional crystallization model of Brown (2019) suggest thickened oceanic crust created by high-degree partial melting of a plume-like source. North of the Danakil rift, T[subscript p] (1307± 11°C) spans a narrow range and is within the range of ambient mantle. The southern Red Sea contains N- to E-MORB depleted in alkali, incompatible, and light rare-earth elements indicating limited mixing with Afar plume material, while the central segment is host to the most depleted magmas along the ridge (La/Sm[subscript N] < 0.8). Within the southern and central regions, fractionation pressures (2.0±1.2 and 4.8±2.1 kbars, respectively) indicate lithosphere (5-15 km) thinner than that of normal ultra-slow spreading ridges (15-35 km). / In the northern Red Sea, high Na₈ and deep pressures of melting (10.4±1.4 kbars) suggest thickened lithosphere, undulations in which induce melt focusing into volcanic deeps. Based on these results, we propose that the Red Sea south of at least 26.5°N is an oceanic spreading center. We find that anomalously magma-rich spreading in the central and southern segments cannot be related to the Afar plume. Instead, the Danakil rift diverts plume-related mantle flow northeast beneath Arabia. Thus, the southern and central Red Sea must be characterized by vigorous mantle upwelling that causes heightened melt production and lithospheric thinning. / by Emilie Elisabeth Bowman. / S.M. in Geology / S.M.inGeology Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2019 / Page 57 blank. Cataloged from PDF version of thesis. / Includes bibliographical references (pages 48-52). / This study aims to contribute to the body of knowledge regarding mercury emissions by reporting on a coastal, urban setting subject to inputs from both anthropogenic and oceanic reservoirs, in order to improve related policy decisions. Mercury can have serious health and ecological consequences, but the chemistry, transport and deposition of gaseous elemental mercury (Hg0) are still not well understood. Estimates of anthropogenic emissions also remain uncertain. To better constrain urban- and regional-scale chemistry and emissions in a specifically coastal environment, concentrations of Hg0 were measured at an urban site in Boston, MA from Aug 2017 to Sept 2018. The recorded concentrations were compared against supplementary records of several additional pollutants and meteorological variables. Concentrations in Boston were found to be relatively low, but follow diurnal and seasonal trends previously observed in other sites in the United States driven by meteorology. Further, back-trajectory and potential source contribution function analysis revealed oceanic re-emission of legacy deposits is a major input of Hg0 to the Boston area, but no influence from specific large anthropogenic point sources was discernible in the data. A one box model was developed to represent the physical processes controlling Hg0 concentrations in Boston in order to replicate concentrations, capture the difference in concentrations from land and ocean sources, and estimate both anthropogenic and oceanic emissions. Results from the box model analysis show the sensitivity of local Hg0 concentrations to varying assumptions of mixing, background flux, meteorology, and emissions, and indicate that oceanic emissions and anthropogenic emissions are likely both higher than current estimates. The results of this study indicate the ocean plays a major role in Hg0 cycling in coastal areas and provides motivation for further improvement of models to better capture local sources and cycling. / by Emma Rutkowski. / S.B. / S.B. Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1991. / Vita. Title as it appears in the M.I.T. Graduate List, Sept. 1991: Stratification and upper ocean dynamics. / Includes bibliographical references (leaves 149-153). / by Rebecca R. Schudlich. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2019 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 141-150). / Seismic imaging techniques rely on a velocity model inverted from noisy data via a non-linear inverse problem. This inferred velocity model may be inaccurate and lead to incorrect interpretations of the subsurface. In this thesis, I combine a fast Helmholtz solver, the field expansion method, with a reduced velocity model parameterization to address the impact of an uncertain or inaccurate velocity model. I modify the field expansion framework to accurately simulate the acoustic field for velocity models that commonly occur in seismic imaging. The field expansion method describes the acoustic field in a periodic medium in which the velocity model and source repeat infinitely in the horizontal direction, much like a diffraction grating. This Helmholtz solver achieves significant computational speed by restricting the velocity model to consists of a number of non-overlapping piecewise layers. / I modify this restricted framework to allow for the modeling of more complex velocity models with dozens of parameters instead of the thousands or millions of parameters used to characterize pixelized velocity models. This parameterization, combined with the speed of the forward solver allow me to examine two problems in seismic imaging: uncertainty quantification and benchmarking global optimization methods. With the rapid speed of the forward solver, I use Markov Chain Monte Carlo methods to estimate the non-linear probability distribution of a 2D seismic velocity model given noisy data. Although global optimization methods have recently been applied to inversion of seismic velocity model using raw waveform data, it has been impossible to compare various types of algorithms and impacts of parameters on convergence. The reduced forward model presented in this paper allows me to benchmark these algorithms and objectively compare their performance to one another. / I also explore the application of these and other geophysical methods to a medical ultrasound dataset that is well approximated by a layered model. / by Gregory Tsiang Ely. / Ph. D. / Ph.D. Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, May, 2020 / Cataloged from student-submitted PDF of thesis. "May 21 2019." / Includes bibliographical references (pages 29-32). / Transmission spectroscopy is a widely-used method for studying exoplanetary atmospheres. However, the differential data analysis techniques that are generally applied to high-resolution ground-based spectroscopic data are only sensitive to narrow spectral features and do not preserve broadband features. This makes them insensitive to the strong Rayleigh scattering slope of HD 189733 b that is due to possible atmospheric aerosols. The Rossiter-McLaughlin (RM) effect provides a way to probe broadband spectral features because its amplitude varies as a function of wavelength according to the effective planet radius. Previously, radial velocity (RV) variations caused by the RM effect were interpreted as being a tentative detection (2.5[sigma]) of the broadband scattering slope of HD 189733 b. We developed a new method that directly models the distortions in spectral lines (rather than the resulting RV variation) and applied this method to the same archival HARPS data that was used in the previous tentative detection. Here we will present this method and the ongoing work necessarily to problem-solve and fully implement it. / by Charlotte Minsky. / S.B. / S.B. Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1998. / Includes bibliographical references. / The Connemara region of the Irish Caledonides is a classical example of a regional-scale high-temperature metamorphic terrain. Its formation was related to intrusion of a compressional continental magmatic arc, for which a protracted thermal evolution was inferred based on a >75 Ma spread in U-Pb, Rb-Sr, and K-Ar mineral dates. Such a history is inconsistent with field observations which suggest a simple relationship between metamorphism and syntectonic magmatism. This study was designed to explore the significance of the large spread in apparent ages using higher resolution U-Pb and 40Ar/ 39Ar geochronometers. The results indicate that arc magmatism, sillimanite-grade metamorphism, anatexis, and late fluid infiltration spanned only about 12 million years. Cooling following the metamorphic peak was actually relatively rapid at 35*C/Ma until about 460 Ma, then 214*C/Ma until 450 Ma. Regional differences in 40Ar/ 39Ar cooling ages of >15 Ma are related to spatial and temporal variations in magmatism, metamorphism, and deformation, rather than differential unroofing of the orogen. 40Ar/ 39Ar dates older than the onset of magmatism or younger than a regional Silurian unconformity represent the combined effects of excess 40Ar contamination, metasomatism, thermal resetting or alteration related to post-orogenic pluton emplacement. This study shows that geochronologic data must be evaluated in the context of careful field mapping, structural and petrologic analysis. Geochronological data from Connemara suggest that arc magmatism related to the Grampian orogeny in this region spanned a brief interval between 475 and 462 Ma and was followed by rapid cooling. The oldest recognized Grampian processes included high P/T metamorphism, followed by intrusion of the Connemara Gabbros into Dalradian metasedimentary rocks, regional-scale ductile deformation, and sillimanite-grade metamorphism between 474.5 and 470.1 Ma. Voluminous -467 Ma quartz diorites only intruded in southern Connemara associated with more localized deformation, anatexis and metasomatism between 468 and 462 Ma. Intrusion of the 462.5 Ma Oughterard Granite marks the end of arc magmatism and contractional deformation at Connemara. The compressional continental magmatic arc at Connemara (the Grampian orogeny) was coeval with continental arc magmatism in Scotland and Newfoundland, and postdates ophiolite formation and obduction along strike in the Appalachian- Caledonian orogen. / by Anke Maria Friedrich. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1998. / Includes bibliographical references (leaves 90-95). / Key uncertainties in the global carbon cycle are reviewed and a simple model for the oceanic carbon sink is developed and described. This model for the solubility sink of excess atmospheric CO2 has many enhancements over the more simple 0-D and 1-D box-diffusion models upon which it is based, including latitudinal extension of mixed-layer inorganic carbon chemistry, climate-dependent air-sea exchange rates, and mixing of dissolved inorganic carbon into the deep ocean that is parameterized by 2-D eddy diffusion. By calibrating the key parameters of this ocean carbon sink model to various "best guess" reference values, it produces an average oceanic carbon sink during the 1980s of 1.7 Pg yr-1, consistent with the range estimated by the IPCC of 2.0 Pg yr~1 ± 0.8 Pg (1992; 1994; 1995). The range cited in the IPCC study and widely reported elsewhere is principally the product of the structural uncertainty implied by an amalgamation of the results of several ocean carbon sink models of varying degrees of complexity. This range does not take into account the parametric uncertainty in these models and does not address how this uncertainty will impact on future atmospheric CO 2 concentrations. A sensitivity analysis of the parameter values used as inputs to the 2-D ocean carbon sink model developed for this study, however, shows that the oceanic carbon sink range of 1.2-2.8 Pg/yr for the 1980s is consistent with a broad range of parameter values. By applying the Probabilistic Collocation Method (Tatang, et al. 1997) to this simple ocean carbon sink model, the uncertainty of the magnitude of the oceanic sink for carbon and hence atmospheric CO2 concentrations is quantitatively examined. This uncertainty is found to be larger than that implied by the structural differences examined in the IPCC study alone with an average 1980s oceanic carbon sink estimated at 1.8 ± 1.3 Pg/yr (with 95% Confidence). It is observed that the range of parameter values needed to balance the contemporary carbon cycle yield correspondingly large differences in future atmospheric CO2 concentrations when driven by a prescribed anthropogenic CO2 emissions scenario over the next century. For anthropogenic CO 2 emissions equivalent to the IS92a scenario of the IPCC (1992), the uncertainty is found to be 705 ppm ± 47 ppm (one standard deviation) in 2100. This range is solely due to uncertainty in the "solubility pump" sink mechanism in the ocean and is only one of the many large uncertainties left to explore in the global carbon cycle. Such uncertainties have implications for the predictability of atmospheric CO2 levels, a necessity for gauging the impact of different rates of anthropogenic CO2 emissions on climate for policy-making purposes. Since atmospheric CO 2 levels are one of the primary drivers of changes in radiative forcing this result impacts on the uncertainty in the degree of climate change that might be expected in the next century. / by Gary Louis Holian. / S.M.

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, May, 2020 / Cataloged from the official PDF of thesis. / Includes bibliographical references (pages [24]-[29]). / Filamentous cyanobacteria that built macroscopic tufted mats in Proterozoic peritidal environments were fossilized by silicification, but the environmental and biological factors that shaped these structures and enabled this type of preservation are not well understood. Recent work has shown that coccoidal cyanobacteria can become fossilized because they can sequester silica from seawater that is undersaturated with respect to silica. Here, we use taphonomy experiments to demonstrate that filamentous cyanobacteria that form tufted and conical mats are also able to mediate the precipitation of silica in seawater that is undersaturated with respect to silica as well. These results show that different marine cyanobacteria and their macroscopic structures have a high potential to be preserved by silicification. We find that the exterior of macroscopic structures such as tufts and pillars are preferentially silicified, and more completely silicified at silica concentrations below saturation in seawater. The interior of these structures may be preserved by later diagenetic minerals. These results are consistent with microfossil assemblages found in early diagenetic chert deposits throughout the Proterozoic. The data suggest abundant Ca-rich sulfated polysaccharides in the exopolymeric substance (EPS), which are likely the site of silicification. Interactions among similar organisms, seawater chemistry, and silica in evaporative environments may account for the two-billion-year long record of exceptionally silicified filamentous and coccoidal cyanobacteria and their macroscopic aggregates in peritidal environments. / by Kyle J. Morgenstein. / S.B. / S.B. Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, May, 2020 / Cataloged from the official PDF of thesis. / Includes bibliographical references (pages 27-32). / Sub-Saharan Africa faces significant challenges to its food security in the coming decades as climate change and rapid population growth strains its agricultural systems. In a region where crops are near exclusively rainfed, precipitation from the West African Monsoon (WAM) plays a significant role in the region's food production. This study aims to add to the limited literature on the relationship between country-level maize yields and the WAM, particularly through the use of high resolution precipitation estimates to characterize the spatiotemporal variability of the monsoon. Multi-year annual precipitation characteristics of the monsoon such as total precipitation, number of non-precipitating days, and timing were derived and aggregated across the maize growing regions of West African countries. Aggregated precipitation metrics were linearly regressed against country-level maize yields that have undergone timeseries analysis to remove trends occurring independently of the WAM. The metrics most correlated with maize yields while maintaining statistically significant slopes were the minimum of total precipitation, standard deviation of the number of non-precipitating days, and the minimum monsoon end date. The strong positive correlations of the minimum of total precipitation and minimum monsoon end date metrics suggest that the worst performing areas in terms of total precipitation and monsoon end date drive down annual country-level maize yields. The positive correlation found using the standard deviation of the number of non-precipitating days is uninterpretable as an instance of Simpson's paradox, as the opposite relationship is discovered in analyses using individual countries. These results show the efficacy of analyzing maize yields against satellite mapped precipitation characteristics of the WAM. / by Janice Shiu. / S.B. / S.B. Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 139-143). / This thesis investigates the effects of eddy-eddy interactions (EEI) and tropical wind variability on sudden stratospheric warming (SSW) formation in an idealized atmospheric GCM. Chapter 2 introduces a method to produce split and displacement SSWs in comparable amounts using either wavenumber 1 or 2 tropospheric heating perturbations. The results are compared to those obtained with wavenumber 2 topographic forcing. It is shown that the fraction of SSWs forced by anomalously strong tropospheric wave flux in the model is similar to that of SSWs in the observed atmosphere, but that the fractions for splits and displacements are different. Furthermore, a large fraction of SSWs occur without significant anomalous tropospheric wave flux, indicating that stratospheric transmission of climatological tropospheric wave flux plays an important role in SSW formation. Chapter 3 investigates the effects of EEI on SSW formation in the model by reproducing the model runs from Chapter 2 with EEI turned off in parts of the atmosphere. It is found that SSW frequencies can be strongly dependent on EEI throughout the atmosphere, but that EEI are required locally for splits and displacements to occur. Significant changes in SSW frequencies are obtained by turning off EEI locally, without changing the lower stratospheric wave forcing. Chapter 3 shows that while SSW formation can be considered a wave-mean flow interaction to first order, higher order processes are required to accurately reproduce both SSW frequencies and dynamics. The wavenumber 2 heating run used in Chapters 2 and 3 produce spontaneous tropical wind oscillations in the stratosphere. Chapter 4 identifies the source of these oscillations, and investigates the effects of the oscillations on the stratospheric polar vortex. Model runs with suppressed tropical wind variability are compared to the control run of Chapter 2. A slight increase in SSW frequency can be found in the model runs with suppressed tropical variability. It is found that upper stratospheric equatorial wind anomalies are strongly correlated with polar vortex strength, and hypothesized that westerly equatorial wind anomalies in the upper stratosphere can reinforce the conditions that lead to an anomalously strong polar vortex. A mechanism explaining this influence is presented. / by Erik Anders Lindgren. / Ph. D.