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 21-23). / Calcite and aragonite speleothems have been used for the past few decades to provide paleoclimate information, but in speleothems of mixed mineralogy the signal of mineral changes can complicate understanding of climate signals also present. This mineral signal must therefore be examined and controlled for. In this work, ICP-MS (inductively-coupled plasma mass spectrometry) analysis of trace element (strontium and magnesium) incorporation into calcium carbonate provides additional evidence for a shift in CaCO3 polymorphs identified via XRD (xray diffraction) analysis of speleothem AB-2 from Anjohibe Cave, Madgascar. The ICP-MS Sr/Ca data qualitatively supports the Sr/Ca data collected by the XRF core scanner, exhibiting a sharp decrease in value across the identified mineral transition. A corresponding increase in Mg/Ca revealed by the ICP-MS provides further evidence for a change from aragonite to calcite at this location. This mineralogical change occurred between 870-880 CE (+/- 13 years) is nearly concurrent with a shift in [delta]13C isotopes that was identified in previous work on this speleothem and attributed to an ecological shift in dominant photosynthetic pathways (Bums et al 2016). A second, control section was identified as pure calcite by XRD analysis, but revealed Mg/Ca and Sr/Ca ratios characteristic of mineralogical transitions, suggesting that there may have been layers of aragonite that have recrystallized to calcite since deposition. / by Kaylee Brent. / S.B.

Thesis (Ph. D. in Atmospheric Chemistry)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2003. / Includes bibliographical references (p. 238-249). / The global-scale emissions and reactivity of dimethylsulfide (CH3SCH3, DMS) make it an integral component in the atmospheric sulfur cycle. DMS is rapidly oxidized in the atmosphere by a complex gas-phase mechanism involving many species and reactions. The resulting oxidized sulfur-bearing products are hygroscopic and interact with aerosols through condensation and secondary aerosol formation. Predictions of the impacts of DMS chemistry on aerosols and climate are inhibited by the poorly understood DMS oxidation mechanism. This thesis diagnoses the gas-phase connections between DMS and its oxidation products by simulating comprehensive DMS chemistry (approximately 50 reactions and 30 species) using three atmospheric models of varying size and complexity. A diurnally-varying box model of the DMS cycle in the remote marine boundary layer is used to identify important DMS-related parameters and propagate parameter uncertainties to the sulfur-containing species. This analysis shows that the concentrations of DMS and sulfur dioxide (SO2) are sensitive to relatively few parameters. Moreover, the concentrations of DMS and SO2 are found to have factor of 2 uncertainties caused primarily (more than 60% of the variance) by uncertainties in DMS emissions and heterogeneous removal, respectively. In contrast, the concentrations of other products, such as sulfuric acid (H2SO4) and methanesulfonic acid (CH3SO3H, MSA), are found to be sensitive to many parameters and have larger uncertainties (factors of 2 to 7) resulting from multiple uncertain chemical and non-photochemical processes. The DMS oxidation mechanism is quantitatively assessed using a one-dimensional column model constrained by high-frequency aircraft measurements from the / (cont.) First Aerosol Characterization Experiment (ACE-1). From this analysis, the baseline mechanism predicts DMS and SO2 concentrations in statistical agreement with the observations, yet it underestimates MSA concentrations by a factor of 10⁴ to 10⁵. These differences for MSA are statistically very significant and indicative of missing gas-phase reactions in the DMS mechanism. To reconcile these differences, five hypothetical MSA production paths are individually tested which greatly improve the model predictions to within a factor of 2 to 3 of the observations. Overall, the best improvement occurs when MSA is produced from the oxidation of methanesulfinic acid (CH3S(O)OH). Furthermore, the boundary layer model predictions of H2SO4 show improve ment after an SO2-independent sulfuric acid production channel is added to the mechanism. The DMS cycle is simulated in a global three-dimensional chemical transport model using, for the first time, comprehensive DMS oxidation chemistry. Four model cases are considered, which include two new comprehensive mechanisms and two parameterized schemes of 4 to 5 reactions taken from previous global sulfur models. The mole fractions of DMS, SO2, H2SO4, and MSA are compared between these four cases and with observations from the ACE-1 and PEM-Tropics A campaigns. Among the four cases, the calculated mole fractions of DMS and SO2 are largely invariant, while those for H2SO4 and MSA exhibit order-of-magnitude differences ... / by Donald David Lucas. / Ph.D.in Atmospheric Chemistry

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1984. / Microfiche copy available in Archives and Science. / Bibliography: leaves 154-159. / by Mark David Handel. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1995. / Includes bibliographical references (p. 83-190). / by Pascal LeGrand. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1995. / Includes bibliographical references. / by Peter Puster. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1993. / Includes bibliographical references (leaves 55-63). / by Wenwei Pan. / M.S.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2015. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 196-210). / In this dissertation, I investigate the structure and dynamics of the Western Hellenic Subduction Zone (WHSZ) by using two complementary seismic imaging methods and interpreting the resulting images with models that describe the dynamics of retreating subduction. First, I produce high-resolution seismic images across northern and southern Greece using a two-dimensional teleseismic migration method. These images show subducted oceanic crust beneath southern Greece and subducted continental crust beneath northern Greece, with the relative position of the two crusts indicating ~70 km of additional slab retreat in the south relative to the north, a result consistent with the predicted relationship between slab buoyancy and retreat rates in recent geodynamic models. Second, I develop a three-dimensional receiver function imaging method, test it with synthetic data, and use it to constrain along strike variations in lithospheric structure. I find a continuous slab Moho across northern and southern Greece between ~40 and 80 km depth, with a gentle, trench-parallel component of dip accommodating the observed differential slab retreat. The overriding Moho is deepest beneath the northern Hellenides (35-40 km) and shallowest beneath the Aegean Sea (25-30 km). It also exhibits several characteristics consistent with a retreating subduction model: (1) it is asymmetric when viewed perpendicular to the trench, not symmetric as has been found in previous studies, (2) the location of its leading edge closely tracks the 70 km depth contour of the slab Moho, (3) a well-developed Moho is not observed below the peak topography of the Hellenides, and (4) it exhibits Moho depth fluctuations that are much larger than those predicted assuming surface topography is locally compensated by Airy- Heiskanen isostasy (>+/-4 km). Finally, I combine the seismic-based constraints with those from geologic data and geodynamic models to better understand how the overriding lithosphere is built and deformed during slab retreat. In northern Greece, the overriding crust is found to be predominately built by accretion of slab-derived continental blocks, while in southern Greece the present-day subduction of an oceanic slab domain has caused previously accreted continental blocks to rapidly extend, yielding an asymmetric, valley-shaped pattern in the top of the crystalline basement. / by Frederick D. Pearce. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2001. / Includes bibliographical references. / The response of bedrock channels to external forcings is investigated in this thesis. The approach is to test and constrain a theoretical model for bedrock-channel incision based on shear stress using field data. The primary study area is a series of 21 small, coastal drainage basins in northern California, USA with known, varying rates and history of rock uplift. The initial application of a simple form of the model to the stream profiles suggests that (1) the channels are eroding at rates approximately equal to uplift rates (i.e. steady-state fluvial incision), and (2) erosion processes are proportionally more effective in the high-uplift-rate zone, with factors in addition to channel gradient responding to tectonic forcing. These results lead to the rest of the study, in which some of the assumptions of the simple model are rejected in order to explain the second observation. A more sophisticated model that includes both a stochastic distribution of floods and a threshold shear stress to initiate bedrock erosion predicts that a greater part of the distribution of flood events will exceed the threshold in steeper channels. Therefore, higher-gradient channels have proportionally higher erosion rates, as is observed in the high-uplift-rate streams of the California field site. The shear-stress model is tested and constrained through a detailed, field-based analysis of topography, lithology, stream morphology and regional hydrology to isolate those factors that respond to tectonics. The stochastic model is able to incorporate the observed variation in stream discharge due to orographic enhancement of precipitation by high topography associated with high uplift rates. / (cont.) This increase in discharge appears to play a second-order role in setting the erosional effectiveness of the high-uplift zone. Other factors, including channel width, lithologic resistance and sediment flux, do not appear to vary in an important way with uplift rate, although this conclusion is based on analyses that have some limitations. The importance of thresholds is underscored by a direct calculation of critical shear stress during a rare bedrock-incision event in a low-erosion-rate creek in New York state (= 100-200 Pa). This event, the only one that caused significant bedrock plucking at the site in an -40-year period, is consistent with a low erosion rate, with few events that exceed the threshold. In contrast, similar 'z values are exceeded during high-frequency flood events in the steep, rapidly eroding California streams. Inclusion of an erosion threshold accounts for the observed relationship between channel gradient and rock-uplift rate in the California site. In summary, by using field examples, the shear-stress bedrock-incision model with a stochastic distribution of flood events and an erosion threshold is demonstrated to be an effective and powerful tool for exploring relationships amongst climatic, tectonic and surficial processes. In the final section of this thesis, a numerical modeling study couples the shear-stress model for onshore fluvial incision with a simple rule for offshore wave-based erosion of bedrock to explore the response of uplifting streams to eustatic fluctuations ... / by Noah P. Snyder. / Ph.D.

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 65-66). / Changes in the volatile distribution on Pluto's surface and in its atmosphere are expected to occur over its orbital path due to varying surface insolation[14]. To investigate these changes, a model was created to synthesize light curves of Pluto, given the viewing geometry and surface albedo distribution. Using an initial surface albedo distribution based on images taken by New Horizons, changes in the light curve mean magnitudes and amplitudes over time were compared to the smallest magnitude changes detectable by a variety of telescopes. The model predicts that yearly observations on a large ground-based telescope, such as the 6.5-meter Magellan telescopes, could observe magnitude changes due to both changes in viewing geometry and surface albedo changes. The model can be compared to future observations to estimate how much surface albedo change is necessary to produce the observed light curves, and can therefore be used to link observational data to physical changes on Pluto's surface and the methods of volatile transport responsible for those changes. / by Megan Mansfield. / S.B.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1999. / Includes bibliographical references (v. 2, leaves 394-410). / This thesis presents the analysis and interpretation of four microearthquake clusters near geothermal and petroleum reservoirs. One cluster was induced during a controlled hydraulic fracture experiment conducted by Atlantic Richfield Corporation (ARCO). The other three clusters occurred near geothermal fields in western Italy during condensate reinjection. Analysis of events induced during ARCO's Deep Well Treatment and Injection project demonstrated "real-time" hypocenter modeling for hydraulic fracture diagnostics. The injection produced a hydraulic fracture that exhibited strong spatial asymmetry. Analysis of the results suggest that the phenomenon of "screenout" may be one mechanism contributing to the observed seismicity's spatial and temporal characteristics and the direction of the asymmetric failure may be related to heterogeneities in formation depositional structures. A swarm of events in 1993 near the northern boundary of the Larderello Geothermal field occurred just below an important regional seismic reflector known as the "K" horizon. The distribution of events suggests that seismic slip on steeply dipping, northwest-southeast striking Tortonian age normal faults could account for the observations and that high pressure fluids circulating in the fracture zone of the "K" horizon cause stress changes and slip on localized patches of the faults. Due to the relative depth of this swarm, compared to reservoir depths, this cluster was attributed to the natural geodynamics of this complex area. Analysis of a set of events from the LAGO seismogenic zone in the Larderello Geothermal field again showed a spatial association with the seismic "1(11 horizon and a lateral NW-SE linear distribution that correlated well with the direction of regional extension. Finally, this thesis reports the results of the relative event relocation for a 1997 swarm of events from the Monte Amiata geothermal field. The hypocenters are located very near a northwest-southeast striking fault at depths correlated with the deeper of two actively producing fracture zones of the Piancastagnaio field. / by Shirley A. Rieven. / Ph.D.