Lithospheric thickness in the western United States has been found to be thinner than that of the stable continental interior of North America. Explanations for this thin lithosphere range from active tectonics to the presence of partial melt. Importantly, the western U.S. has been used to argue for a relationship between lithospheric thickness and thermotectonic age. Past studies of Sp receiver functions and tomography models in the western U.S. have led to significant improvements in our understanding of mantle structure and related dynamic and tectonic processes, but significant uncertainty remains as the results have yet to be successfully integrated. In our analysis we calculated Sp receiver functions using data from over 1000 broadband stations from more than 50 temporary and permanent networks, ranging from 31N to 43N and 112W to 126W. The average depth of the lithosphere-asthenosphere boundary (LAB) phase across the region in our study is 65.3 km, but absolute depths vary from roughly 30 to 100 km. In this study we take a systemic approach to generating maps of the seismic lithosphere-asthenosphere boundary inferred from Sp receiver functions and compare them to a commonly used map of physiographic provinces and sections, volcanic centers, and the surface wave tomography model of Shen and Ritzwoller (2016) to assess if, and to what extent, our interpretation of the lithosphere-asthenosphere boundary is consistent with these different models and observables. Comparison of our results to the tomography model reveal good agreement between receiver function LAB depth and the absolute depth range between maximum and minimum velocities in the tomography model, suggesting that receiver functions are in fact imaging the lithosphere-asthenosphere boundary. However, little detailed agreement exists between them, likely due to differences in sensitivity. Comparison of LAB depth to our tectonic proxies yields little correlation and we find that the relationship between physiographic boundaries does not appear to extend to variations in lithospheric thickness in the mantle. Preliminary comparisons between petrologic studies and our picks reveal minimal agreement in LAB depth, but we find variation in LAB depth between volcanic centers when compared to non-volcanic areas.

Earth’s interior has increasingly been shown to be highly heterogeneous and complex. By utilizing four seismic different methods, we seek to better constrain this heterogeneity at multiple depth and areal scales. We begin with Sp receiver function analysis across the Australian continent, which uses direct seismic phases and those converted at boundaries to estimate velocity contrasts at boundaries below a seismometer. Then, using Ps receiver function analysis we expand beyond this to not only examine these velocity contrasts but also test for changes in seismic anisotropy across said boundaries. Next, we examine shear wave splitting both in Australia and at a smaller scale in the Wyoming Craton: this method relies on the fact that when a shear wave encounters anisotropic material, it is split into two orthogonal components, which can be retrieved at the receiver side. Finally, we cross correlate components of a single seismometer to estimate shallow subsurface velocity changes following the Ridgecrest earthquake sequence. Results from the Australian Sp receiver function analysis show a sharp, shallow seismic lithosphere-asthenosphere boundary on the Phanerozoic east coast, with multiple mid-lithospheric discontinuities in the central cratonic portion. Our Ps receiver functions do not match well with simple models of seismic anisotropy changes across boundaries, but do exhibit characteristic backazimuthal variation: this implies more diffuse zones of seismic anisotropy. This is confirmed by our Australian shear wave splitting results, which show fast directions that often cannot be explained through plate-motion-induced shear alone and may require multiple layers of seismic anisotropy. Shear wave splitting in Wyoming also shows complexity in fast direction and delay time, but there appears to be a more systematic change in parameters moving into the Powder River Basin. Finally, velocity perturbations following the Mw 7.1 Ridgecrest sequence generally recover within hours to days, suggesting a largely elastic response. Two stations do not show a recovery within the time period examined, implying a plastic response instead. All results bolster an argument for interior complexity mirroring Earth’s surface.

Chapter 1 is a seismic study of the mantle structure of the passive margin of northeastern North America. This region experienced multiple episodes of rifting and orogenesis in the past, and the study aims to understand how the Wilson Cycle impacts the geometry of the upper mantle seismic structure beneath southern New England using Sp receiver functions. Our findings indicate that structures related to the formation and breakup of Pangea may still be preserved at depth despite more recent magmatic and tectonic influences. Chapter 2 presents an analysis of P-wave attenuation for East Africa, focusing on the Afar, the Eastern Branch, and the Western Branch regions to analyze the extent to which volatiles may explain the presence of the regions’ slow seismic velocities. Our modeled results indicate that a low Qp melt layer, similar to those observed beneath mid-ocean ridges, is sufficient to reconcile previous geochemical and geophysical results. Chapter 3 describes a quasi-experimental education study that tests the relationship between pedagogical methods and students’ spatial skills in an introductory earth science general education course. In our study, we found no difference in the efficacy of the teaching methods or environments tested. We did observe a difference in male and female performance in domain-general spatial skills, with females scoring lower, suggesting that a focus on improving disparities between genders be considered in courses where spatial learning is required.

The tropical Pacific is a significant component of the ocean's climate system and a source of considerable global climate variability, including El Niño. Tightly coupled sea surface and thermocline dynamics influence tropical climate on a variety of spatial and temporal scales. Records of past climate surface and subsurface temperature variability, on long and short times scales, may aid in identifying the radiative and dynamic forcing important to tropical climate during different global climate states. During the Pliocene warm period (3.0-4.3 Ma) and Last Glacial Maximum (LGM, ~0.020 Ma) the global mean state was substantially different and may have influenced the behavior and strength of various processes and mechanisms that determine tropical climate. In this dissertation, I use geochemical techniques, including the novel application of the magnesium-calcite ratio of individual planktonic foraminifera, to investigate the different processes that determine tropical change on different time scales and background climate states.

In the first project, I generated stable isotope and minor element records using a subsurface dwelling planktonic foraminifera from a transect of sites across the Pacific cold tongue. Using subsurface temperatures as a proxy for thermocline depth, my records suggest there was a gradual shoaling of the eastern equatorial Pacific thermocline from the early Pliocene to present day. In the second and third projects, I use the minor element ratio of individual surface and subsurface dwelling foraminifera to monitor the SST and subsurface temperature background state, high-resolution variability and radiative and dynamic processes that determine tropical climate. The results suggest the tropical western Pacific has responded primarily to radiative forcing on glacial-interglacial time scales since the early Pliocene. In contrast, the eastern Pacific has responded to changes in dynamic and radiative forcing during the last glacial period, which influenced the background spatial pattern of the cold tongue region. Additionally, changes in individual foraminifera variability suggest a decrease in El Niño Southern Oscillation during the most recent glacial period in comparison to the Holocene. These records show that different scales of variability (i.e. long-term, glacial-interglacial, and seasonal/interannual) may illuminate the processes and mechanisms important to paleoclimate interpretations on various temporal and spatial scales.