Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1997. / Vita. / Includes bibliographical references. / by Daniel M. Sigman. / 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 26-29). / Trace elements (Mg/Ca and Sr/Ca) and stable isotopes ([delta]13C and [delta]18O) were measured in two coeval stalagmites, LC3 and BT1, from the Lehman Caves, Nevada. BT1 spans 388 to 384 ka. The LC3 record is split into two parts due to a hiatus observed through fluorescent imaging. The section prior to the hiatus spans 411 ka to 402 ka. The post-hiatus section of the stalagmite has a single age of 383 ka; an age model cannot be constructed for this part of the record. These stalagmites span Marine Isotope Stage 11 (MIS 11), a long interglacial that occurred around 424-374 ka. Comparison with more recent stalagmite records has shown prior calcite precipitation to be the dominant control on Mg/Ca and [delta]13C in the cave. The trace element and stable isotope records obtained point to an arid climate in the Great Basin during MIS 11. / by Ashling Neary. / S.B.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2000. / Includes bibliographical references (p. 41-42). / A finite difference code is used to investigate acoustic waves in a borehole environment. The wave response to a logging-while-drilling (LWD) geometry is modeled in a fast formation. Helical waves circling the tool are shown to asymptote to the Stoneley wave velocity, giving confirmation of the fluid velocity in the borehole. Parameter studies for simpler borehole geometries show that the Stoneley wave, in soft formations where no shear arrival is present, can be used to invert for the shear velocity of the rock. A Beowulf parallel computer is used to implement the finite difference code showing the efficiency of cluster computing in a discretized space. / by Victoria Alice Briggs. / S.M.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 164-169). / As the impacts among land cover change, future climates and ecosystems are expected to be substantial (e.g., Feddema et al., 2005), there are growing needs for improving the capability of simulating the dynamics of vegetation structure across the global landscape as accurately as possible. In order to serve these needs, Dynamic Global Vegetation Models (DGVMs) are used to describe the current status of vegetation structure and biogeography as well as estimate their future dynamics, either with prescribed climates or coupled to climate models. Yet, current DGVMs generally assume ubiquitous availability of seeds and do not generally consider seed dispersal mechanisms and plant migration processes, which may influence the impacts of vegetation structural changes on the climate system (i.e., change in albedo, runoff, and terrestrial carbon sequestration capacity). For the first time, this study incorporates time-varying winddriven seed dispersion (i.e., the SEED configuration) as a dynamic constraint to the migration of natural vegetation in the Community Land Model (CLM)-DGVM. Compared to estimates of satellite-derived tree cover, simulations by this model configuration shows significantly improved representation of boreal forests in Western Siberia and temperate forests in Eastern Europe. The prevailing wind pattern, along with the existing vegetation structure in nearby grid cells, alters the competition dynamics of the trees in these regions by filtering unrealistic plant functional types through adjustment of establishment rates. The SEED configuration was applied to project future vegetation structures under two climate mitigation scenarios (No-policy vs. 450ppm CO 2 stabilization) for the 21st century. The simulation results indicate that regional changes of vegetation structure under changing climates are expected to be significant. In the high latitudes, regions such as Alaska and Siberia are expected to experience substantial shifts of forestry structure, characterized by expansion of needle-leaf boreal forest and shrinkage of C3 grass Arctic. In the mid-latitudes, temperate trees are likely to expand in South America, South Africa, and East Asia at the expense of C3 grass during the latter part of the 21' century. In the Tropics, the most notable degree of change is in the composition of tropical trees and C4 grasses in the Amazon and in Africa. / (cont.) The vulnerability assessment suggested by this study shows that vegetation structures in Alaska, Greenland, Central America, southern part of South America, East Africa and East Asia are susceptible to changing climates, regardless of the two climate mitigation scenarios. Regions such as Greenland, Tibet, South Asia and Northern Australia, however, may substantially alleviate their risks of rapid change in vegetation structure, given a robust greenhouse gas stabilization target. The impacts of future vegetation change on radiation budget cannot be neglected. The results of this study suggest that depending upon the climate mitigation scenarios, vegetation change may enhance or mitigate the anticipated warming trend of the 21St century. Proliferation of boreal forests in the high latitudes to amplify the warming trend (i.e., a positive feedback to climate) if no mitigation policy is implemented. In contrast, under the 450ppm scenario, changes in vegetation structure may reduce the rate of warming, which is a negative feedback to climate. A series of hydrologic processes including interception of rainfall by forest canopy, evapotranspiration, and runoff are also influenced by modifications in vegetation structure. The magnitude of the runoff response by the vegetation change is not projected to exceed the direct response of hydrology to climate change (i.e., changes in precipitation); however, the spatial pattern of changes in runoff associated with vegetation changes indicates that vegetation change may in some regions offset or lessen increases in runoff due to enhanced precipitation under climate warming. Reduction of terrestrial productivity and a conservative estimate of vegetation carbon storage (-8PgC/yr and 24PgC, respectively under the no policy scenario) in the 21st century may be due to ignoring the CO₂ fertilization effect and partially applying the new SEED configuration to project future vegetation structures. The SEED configuration developed in this study may serve to more comprehensively represent future vegetation structure across the global landscape and therefore may provide a tool to better assess the impacts of natural vegetation dynamics on the climate system. This model configuration may also provide outputs that can be used to assess the impacts of climate change on the goods and services that ecosystems provide to society. / by Eunjee Lee. / Sc.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 115-119). / Atmospheric observations and atmospheric observation-based global emission estimates are presented for the five high molecular weight perfluorocarbons (PFCs): decafluorobutane (C 4 F 1 0 ), dodecafluoropentane (C5 F1 2 ), tetradecafluorohexane (C 6 F14 ), hexadecafluoroheptane (C 7F 16 ) and octadecafluorooctane (C8 F 18 ). Their atmospheric histories are based on measurements of 36 Northern Hemisphere and 46 Southern Hemisphere archived air samples, collected between 1973 and 2011, using two of the "Medusa" cryogenic preconcentration gas chromatography-mass spectrometry instruments, which are part of the Advanced Global Atmospheric Gases Experiment (AGAGE). A new calibration scale was prepared for each PFC, with estimated accuracies of 6.8% for C4Fi0 , 7.8% for C5F12 , 4.0% for CF 14 , 6.6% for C7FE16 and 7.9% for CF8i . Based on our observations, the 2011 globally averaged dry air mole fractions of these high molecular weight PFCs are: 0.17 parts-per-trillion (ppt, i.e., parts per 1012) for C4 F 10 , 0.12 ppt for C5 F 1 2 , 0.27 ppt for CF 1 4 , 0.12 ppt for CFE16 and 0.09 ppt for CF 18 . Newly measured infrared absorption spectra are presented for C7F 16 and CF 1 8 , and using these, their radiative efficiencies and global warming potentials (GWPs) are estimated. We find that the radiative efficiency of C8 F 18 at 0.57Wm- 2 ppb-' is similar to that of trifluoromethyl sulfur pentafluoride's (SF 5 CF 3 ), which has the highest radiative efficiency of any known atmospheric species (Forster et al., 2007). Using their radiative efficiencies, the 2011 observed globally averaged atmospheric mole fractions of the above five high molecular weight PFCs combine to contribute a global average radiative forcing of 0.35 mW m-2, which is 6% of the total anthropogenic PFC radiative forcing (Montzka et al., 2011; Oram et al., 2012). Global emissions for C4 Fio, C5F 12 , C6 F 14 , C7 F 16 and CF 18 were estimated from the observations using a 3-dimensional chemical transport model and a Bayseian inverse method that included a constraint on the annual growth rate of their emissions, consistent with the knowledge of the relevant industries emitting them (Rigby et al., 2011). The derived so-called "top-down" emission estimates show that global emission rates were largest in the 1980s and 1990s for C4 F 10 and C5 F 12 and in the 1990s for C6F 14 , C7F16 and C8F1 . After a subsequent decline, emission rates have remained relatively stable, within ±20 % year-to-year variation, for the last 5 years. Using their calculated 100-year time horizon GWPs, the high molecular weight perfluorocarbons studied here contributed up to 15.4 % of the total PFC emissions expressed in carbon dioxide (C0 2)-equivalents in 1997 and 6 % of the total PFC emissions in 2009. Furthermore, we compare our atmospheric observation-based global emissions to the available so-called "bottom-up" inventories, which are based on production information and end usage. Bottom-up emission estimates are available from the Emission Database for Global Atmospheric Research version 4.2 (EDGARv4.2) for C4F1o, C5 F1 2, C6 F 1 4 and C7F16 , and emission inventories of C4Fio, C5 F1 2 and C6 F14 are also reported to the United Nations' Framework Convention on Climate Change (UNFCCC) by Annex 1 countries that have ratified the Kyoto Protocol (European Commission, Joint Research Centre (JRC)/Netherlands Environmental Assessment Agency (PBL), 2009; United National Framework Convention on Climate Change Secretariat, 2011). The atmospheric observation-based emission estimates are 20 times larger than EDGARv4.2 for C4F10 and over three orders of magnitude larger for C5F1 2 . In contrast, the top-down emission estimates for C6F14 largely agree with the bottom-up estimates from EDGARv4.2. Moreover, the top-down C7 F16 emission estimates are comparable to those of EDGARv4.2 at their peak in the 1990s, albeit with significant underestimation by EDGARv4.2 for the other time periods. There are no bottom-up emission estimates for C8Fi8 , thus the emission rates reported here are the first for this gas. In general, the emission inventories for C4Fio, C5F1 2 and C6F 14 reported to the UNFCCC are five to ten times lower than those estimated in this study from observations. This underreporting to the UNFCCC may be due to only Annex 1 countries reporting inventories and also that some of these countries report a total PFC mixture in C0 2-equivalents, instead of individual PFC emissions rates. / by Diane Jean Ivy. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1988. / Includes bibliographies. / by Tien-when Lo. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2007. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references (p. 196-205). / This thesis advances our understanding of how transport properties of turbidity currents are mediated by interactions with seafloor topography, specifically channelized surfaces. Turbidity currents are responsible for crafting the morphology of continental margins. Unfortunately, very few direct observations exists defining turbidity current interactions with submarine channels and canyons because infrequent occurrence, great water depths, and high current velocities make measurements difficult to obtain. To overcome this problem, I utilize reduced scale laboratory experiments, remote sensing of the seafloor and subsurface deposits, and numerical analysis of transport processes. I focus on resolving the topography and composition of the evolving water-sediment interface with additional measurements that characterize the sediment transport and flow fields. I begin by quantifying interactions between turbidity currents and channel-bounding levees. Levees are the primary elements of self-formed channels and act to confine flows within channels, thereby increasing transport efficiency. I quantify the morphology and growth of levees in a submarine channel network offshore Borneo. Levee deposit trends are interpreted using laboratory observations and a morphodynamic model describing levee growth. Channel and levee deposits resulting from interactions between turbidity currents and sinuous submarine channels are then studied using reduced-scale laboratory experiments. Measurements of current superelevation in channel bends are used to illustrate the importance of current runup onto the outer banks of channel bends. This runup resulted in focused overbank flow and production of thick, coarse, steep levees at these sites. / (cont.) Additional laboratory experiments illustrate the importance of current-channel bend interactions to the runout length of turbidity currents. I observed enhanced mixing in channel bends that reduced proximal deposition rates in sinuous channels compared to straight channels. I hypothesize that a wholesale vertical mixing of suspended sediment within turbidity currents at channel bends is a necessary condition for the construction of submarine channels greater than 100 km in length. Finally, I document the deepening of submarine canyons under net depositional conditions using an industry-grade seismic volume from the continental slope offshore Borneo. Interpretation of seismic horizons suggests deposition resulted from sheet-like turbidity currents, highlighting the importance of unconfined currents to the evolution of seascapes. / by Kyle M. Straub. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2008. / Includes bibliographical references (p. 83-91). / Large volumes of sediment can be released into a river when an avulsion carves a new channel in the landscape. Gilbert (1917) described the evolution of a similar pulse of material from mining along the Sacramento River, California as a sediment wave. Sediment waves are transient accumulations of sand and gravel that locally increase the elevation of the bed and reduce the transport capacity of the channel, and diffuse and translate down the channel over time. The Suncook River in Epsom, New Hampshire, avulsed in May 2006. This event created a new channel and mobilized approximately 100,000 m3 of sand into the river in a period of 12 to 24 hours (Perignon, 2007; Wittkop et al., 2007). In April 2007, a new channel formed through a meander bend downstream of the site of the first avulsion, where sediments mobilized the year before had increased the bed elevation by one meter. We propose that the material released in 2006 is traveling down the channel as a sediment wave, increasing the elevation of the bed and driving avulsions. The purpose of this study is to model the evolution of a sediment wave in the Suncook River in order to understand how it can increase the risk of floods and avulsions in the system over time. We developed a mathematical model using the equations of Lisle et al. (1997, 2001) to observe the evolution of the sediment wave under bankfull conditions. We found that the wave evolved mostly through diffusion and showed minimal translation downstream. These findings suggest that the risk of avulsions will be contained near the center of mass of the sediment wave, which we place near the site of the 2007 meander cutoff. Likewise, the diffusive nature of the wave implies that the river could reach a new equilibrium profile with no restoration work and without significantly affecting populated areas downstream. / by Mariela C. Perignon. / S.M.

Thesis: Ph. D. in Planetary Sciences, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2018. / 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 225-256). / 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 in this thesis produced upper limits of rv lOOx Jovian radio flux for a large sample of nearby stars and known exoplanet systems. The sensitivity is sufficient that large radio bursts from nearby stars or planets could have been detected if they took place during the survey observation(s). The framework developed here can be used for automated exoplanet radio emission searches in future radio survey data. The second approach described herein is a blind survey of the nearest Northern hemisphere stars across a broad range of frequencies in order to detect as-yet unknown planets or set tight constraints on radio emission from the stars and possible substellar companions. The survey approach used here is novel because it makes no assumptions about which stars are most likely to host radio emitting planets and it covers frequencies from 30 MHz to 4 GHz. This survey produced a detection of multiple rv50% circularly polarized flares from the M dwarf binary system Ross 614 as well as limits at the lOx Jovian flux level for the remaining stars observed. The limits attained from this survey are the first published at 1-4 GHz for these objects and the only available radio limits for a newly discovered cool (T9) brown dwarf. The limits from this survey place a preliminary constraint on the magnetic field of the brown dwarf at <350 G. The third approach focuses on known exoplanet systems and targets key orbital phases where intense radio emission is predicted. In the case of eccentric hot Jupiter HD 80606 b, radio flux from the planet is expected to increase by a factor of up to 3000 compared to the quiescent flux as the planet passes within 6 Rof its host at periastron due to high density stellar wind impinging on HD 80606 b's putative magnetosphere. Data obtained from LOFAR LBA is used to set the lowest limits to date on radio flux from HD 80606 b near planetary periastron. The same concept of orbital phase targeting is used to optimize an observing strategy for recently-discovered multiple planet host TRAPPIST-I. In the case of TRAPPIST-1, the quadrature phases of planets TRAPPIST-1 b and TRAPPIST-1 c are targeted to maximize the chance of observing Io-Jupiter like planetary modulation of stellar radio emission. The quadrature phase targeting approach is new to this field. The thesis concludes with a discussion of the benefits of space-based observation for exoplanetary radio searches. Ground-based observations are limited by the plasma frequency of the ionosphere, so planets with Earth-like magnetic fields cannot be observed. Telescopes on the ground also suffer from ionospheric phase errors that are difficult to fully calibrate. Space-based observation does not suffer from the effects of the ionosphere and can therefore support lower frequency observations than ground-based instruments. A novel instrument, the vector sensor, optimized for space-based radio interferometry is introduced. New algorithms for all-sky vector sensor imaging have been developed and tested in simulation and on sky data with encouraging results. Finally, the prospects for detecting Earth- or Jupiter-analogs in the solar neighborhood, either from the ground or from space, are assessed. Very large space-based arrays are required to detect either Jupiter or the Earth at 10 pc; at least 105 -106 antennas are needed for sufficient sensitivity. / by Mary Knapp. / Ph. D. in Planetary Sciences

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1994. / Includes bibliographical references (p. 119-123). / by Michael Cottman Morgan. / Ph.D.