Earth and Space Sciences

Permanent URI for this collectionhttps://digital.lib.washington.edu/handle/1773/16348

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Molecular and thermodynamic controls on cell envelope reactivity of a deep-sea hyperthermophilic methanogen: Solute-specific mechanisms governing archaeal envelope chemistry and nutrient acquisition
(2026-04-20) Downey, Autum; Gorman-Lewis, Drew
Methanogenic archaea play an integral role the global carbon cycle, collectively generating well over half of annual methane production on Earth today. Methanogenesis represents one of the most influential and ancient metabolic strategies utilized by life on Earth. Biotic methane production is critically dependent on the bio-availability of nickel (Ni), which acts as an essential enzymatic cofactor in methanogenesis. Despite methanogens dependency on Ni, the processes governing its bioavailability remain limited. Nutrient bioavailability, including Ni, begins at the interface of the cellular envelope and the surrounding environment where non-metabolic, acid-base reactions control solute adsorption for subsequent transport into the cell to be utilized for metabolic processes. In archaea, this interface is dominated by a proteinaceous surface layer (S-layer) whose chemical reactivity and structural response to changing solution conditions remain poorly understood compared to bacterial systems. Here, I thoroughly explore this critical interface using the hyperthermophilic deep-sea methanogen Methanocaldococcus sp. FS406-22 as a model organism. This work employs a multi-technique approach integrating potentiometric titrations, isothermal titration calorimetry, attenuated total reflectance Fourier transform infrared spectroscopy, surface complexation modeling, and geochemical modeling to quantify proton-active surface functional groups at the cell envelope and evaluate the mechanisms governing the local chemical environment. Collectively, these data provide critical insights into how life mitigates outstanding environmental conditions. Ultimately, the scientific contributions herein better inform our understanding of the limitations on habitable conditions, both on Earth (spatially and temporally) and on extraterrestrial worlds Results demonstrate that the Methanocaldococcus sp. FS406-22 cell envelope contains multiple proton-reactive sites consistent with carboxyl, phosphate, and amine functional groups, and that their expression is strongly modulated by solution chemistry. Within a “simulated seawater" (SSW), the presence of divalent cations, particularly magnesium (Mg), reduces the contribution of amine sites to proton buffering at elevated pH and induces an enthalpy-entropy compensation indicative of solution driven surface reorganization. Spectroscopic analyses further reveal that the Mg containing SSW solution promote conformational changes in S-layer proteins, which coincide with altered spectral behavior in carbohydrate and phosphate associated regions attributed to glyco-protein components of the cell envelope. These observations indicate that protein conformation governs the hydration environment and accessibility of proton-active surface sites, linking molecular-scale structural changes to macroscopic surface reactivity. Building on these findings, bulk Ni adsorption experiments show that adsorption onto the cell envelope increases with increasing pH, corresponding to sequential deprotonation of carboxylic acid, phosphate, and amine surface sites in both sodium chloride (NaCl) and SSW solutions at matching ionic strengths. However, total Ni adsorption is maintained despite a reduction in amine-associated proton buffering in SSW, indicating that the amine sites that remain accessible and continue to participate in Ni coordination. Spectroscopic data further reveal protein-associated vibrational responses to Mg and Ni, suggesting that solution-driven protein reorganization (i.e. conformation) preserves Ni binding functionality at the cell envelope. Collectively, this work establishes archaeal cell envelopes as dynamic, chemically responsive interfaces that directly influence trace metal bioavailability. Through an integrated thermodynamic and spectroscopic approach aimed at probing archaeal surface reactivity, this dissertation advances understanding of Ni acquisition by methanogens and highlights the role of protein conformation in regulating nutrient availability. While this work explores only a single methanogen and its coordination with Ni, it provides foundational evidence to warrant the exploration of similar surface mediated mechanisms present in other archaea and trace metal systems. This dissertation has broad implications for constraining methanogenesis productivity, advancing models of trace metal cycling at the microbe-fluid interface, and interpreting biogeochemical signals preserved in the geologic record and planetary environments.
Subduction Zone Blues: Laboratory and Field Constraints on the Rheology and Deformation Mechanisms of Mafic Blueschist at the Subduction Interface
(2026-02-05) Ott, Jason Noel; Condit, Cailey B
Subduction zones are key drivers of plate tectonics, facilitating crustal recycling and deformation. The associated geological hazards of these convergent margins, including megathrust earthquakes and tsunamis, pose significant risks to society. Although these hazards are generated by frictional, seismic slip along the shallow subduction interface, aseismic, ductile deformation plays a critical role by modulating the strength of the subducting slab and transferring stress up-dip to load the seismogenic zone. During subduction, progressive metamorphic transformations of mid-ocean ridge basalt to blueschist and, ultimately, to eclogite alters the chemical and mechanical properties of the subducting crust through changes in its mineralogy, volatile content, density, and rheological strength. Field observations and limited experimental work have offered some insights into the evolution of subducting oceanic crust—such as a strength hierarchy estimating blueschist to be weaker than both gabbro (basalt) and eclogite—but a substantial knowledge gap persists. This thesis addresses this knowledge gap by investigating key questions regarding subduction zone dynamics including: (1) Does blueschist generate an observable seismic signal that can improve seismic imaging of blueschist along the subduction interface? (2) What mechanism(s) accommodates blueschist deformation along the subduction interface down-dip of the seismogenic zone and what are the implications for the seismic-aseismic transition? (3) How do chemical and mechanical changes at the blueschist-eclogite transition influence interface subduction zone dynamics during the subduction and exhumation of HP/LT lithologies? Electron backscatter diffraction (EBSD)-based petrofabric analysis was applied to model the seismic anisotropy generated by a suite of mafic blueschists exhumed from a range of peak P-T conditions relevant to the ductilely deforming interface in active subduction zones. The blueschists displayed a broad range of P-wave anisotropies (AVp) up to 20%, correlating positively with the abundance and deformation-produced crystallographic preferred orientation (CPO) of the sodic amphibole mineral, glaucophane. Modeled AVp magnitudes were commonly ~10%, suggesting a common blueschist seismic anisotropy signal with potential to improve mapping of the extent and deformation of blueschists along the subduction interface. To investigate the underlying deformation that generates observed seismic anisotropies, EBSD techniques—including a novel technique coupling weighted Burgers vector and misorientation analyses—were applied to glaucophane in a lawsonite blueschist from the Catalina Schist that was exhumed from P-T conditions down-dip of the seismogenic zone. The results of this investigation reveal deformation of blueschists at these conditions to be primarily accommodated by the dislocation creep mechanism. A suite of deformation experiments were performed on glaucophane aggregates to derive a constitutive, power-law relationship (flow law) for dislocation creep in glaucophane and, by extension, blueschist. Extrapolation of flow laws for glaucophane dislocation creep and related subduction lithology flow laws to natural conditions predict deformation by dislocation creep in the subducting slab initiates at ~350°C, evolving to grain-size-sensitive mechanisms with increasing temperatures and pore fluid pressures or finer grain sizes. Detailed microstructural and petrological investigation were conducted on an exhumed blueschist-eclogite transition on Vroulidia Beach, Sifnos Island, Greece to investigate the complex interplay between deformation, metamorphism, and aqueous fluids in during the subduction and exhumation of blueschists and eclogites. This investigation demonstrates that fluid-deformation feedbacks promote zones of weakness that further localize deformation and fluid flow to enhance retrogression in such zones during exhumation. Together, these investigations advance our understanding of deformation and metamorphic processes in blueschists, eclogites, and at the blueschist-eclogite transition and subduction zone dynamics.
Systematics, diversity dynamics, and paleobiogeography of early Paleocene mammals from northeastern Montana and the Western Interior of North America
(2026-02-05) Hovatter, Brody Thomas; Wilson Mantilla, Gregory P.
The early Paleocene was a critical interval in the evolution of mammals during which the group underwent a remarkable evolutionary radiation following the Cretaceous/Paleogene (K/Pg) mass extinction ca. 66 Ma. Although the broad patterns of this mammalian radiation are well established, the precise details remain more poorly understood and ongoing work continues to refine our understanding of this important episode in mammal evolution. The Hell Creek Formation and overlying Tullock Member of the Fort Union Formation in northeastern Montana are well known as an excellent study system for examining the evolution of mammals leading up to and across the K/Pg extinction event. Substantial work in this region has focused on the record of mammals immediately before and after the K/Pg boundary, whereas the younger mammal-bearing horizons have received less attention and until recently have produced fewer fossils. Consequently, this has hindered a more complete understanding of the recovery and subsequent diversification of mammals in this area. This dissertation seeks to add to our knowledge of early Paleocene mammal systematics and diversity dynamics by way of three studies that vary in spatiotemporal scale. In Chapter 2, a coauthor and I describe a new assemblage of mammalian dental fossils from the stratigraphically highest mammal-bearing localities from the upper part of the Tullock Member in Garfield County, Montana—the Farrand Channel and Horsethief Canyon local faunas. Both local faunas have been correlated to the early Torrejonian (To1) North American Land Mammal ‘age’ and currently represent the oldest and most northerly occurrences of To1 mammals. These new fossils substantially increase the previously known sample size from the Farrand Channel and Horsethief Canyon local faunas and document as many as 40 distinct taxa, several new occurrences, and likely more than one new species. Further, these fossils help better characterize the age and composition of both local faunas, which are temporally intermediate between the youngest Puercan and oldest Torrejonian faunas known elsewhere. More broadly, we contribute to the limited record of To1 mammals and demonstrate that within less than 1 Ma after the K/Pg boundary mammals were considerably more taxonomically diverse in the Hell Creek region than previously appreciated. In Chapter 3, coauthors and I report new plesiadapiform (putative stem primates) dental fossils from the Farrand Channel and Horsethief Canyon local faunas that record several poorly known taxa and represent the largest and most diverse assemblage of To1 plesiadapiforms known. We describe a new species of purgatoriid plesiadapiform (Ursolestes blissorum, sp. nov.) that represents the largest plesiadapiform known from the early Paleocene. We also document intraspecific variability and one undescribed tooth locus of the oldest known member of the Paromomyidae, Paromomys farrandi. Further, we evaluate plesiadapiform species richness, mean body mass, and body-mass disparity through the Paleocene and reveal unrecognized levels of richness in To1 and a general trend of stable body mass and body-mass disparity, thereby providing new insights into the early evolutionary history of Primates. In Chapter 4, I provide a quantitative assessment of the taxonomic composition of the Farrand Channel and Horsethief Canyon local faunas and the biogeography of early Paleocene mammals from the Western Interior using a newly assembled dataset of North American mammalian occurrences. Further, I compare geographic patterns of mammalian diet and body mass distributions to test for regional differences in community structure that may be indicative of differences in habitat and a driver of compositional differentiation. I find that the Farrand Channel and Horsethief Canyon local faunas are compositionally most similar to other To1 faunas, supporting their previous correlation. I also find evidence of temporal and latitudinal differentiation among early Paleocene faunas. My ecological analyses reveal some geographic patterning but ultimately cannot detect meaningful differences in community structure, likely due to the lack of certain ecological data currently available and sampling gaps in the fossil record. Collectively, these studies add to our knowledge of early Paleocene mammal systematics and help emphasize the importance of continued collecting efforts and specimen-based work.
Disentangling climatic, tectonic, and proxy-system controls on clumped isotope temperature records from lacustrine carbonates: A case-study from the Miocene Bidahochi Formation on the Colorado Plateau, Navajo Nation, AZ, U.S.A.
(2026-02-05) Heitmann, Emma O; Huntington, Katharine W.
Landscapes are shaped by climate and tectonic forces over geologic timescales. To understand these processes, there is a need to reconstruct paleo-environments and their topographic and climatic conditions. The Colorado Plateau is a place where the mechanisms of uplift and the development of the modern climate system are debated, and thus there is a need for proxy records of climate and topography in this region. A relatively novel and potentially powerful proxy for reconstructing topography and climate is the clumped isotope thermometer of lacustrine carbonate (T?47). This T?47 proxy is assumed to record lake water temperature and, in turn, surface temperature which is controlled by both topography and climate. This thesis reconstructed a T?47 record from a Miocene lacustrine basin on the Colorado Plateau, and interpreted the record by integrating multiple lines of geologic evidence, geochemical proxies, and proxy-system models. In Chapter 1 I compiled existing paleo-elevation estimates and geologic constraints for the Colorado Plateau region and reviewed recent advances in paleo-elevation methods. This work concluded that existing estimates permitted multiple hypotheses of Colorado Plateau uplift, and that there was potential to leverage proxy method advancements to refine and re-evaluate a record from the 16-6 Ma Bidahochi Fm. In Chapter 2 I collected field data and micritic lacustrine samples that I characterized sedimentologically and analyzed for their bulk isotope, clumped isotope, and dual-clumped isotope compositions. This work found that the T?47 record preserved a significant cooling trend of ~10 ± 6 °C. In Chapter 3 I used climate (CESM) and proxy-system (PRYSM) models to explore hypotheses to explain the observed proxy record and the sensitivity of T?47 to variable lake conditions. This work found that the proxy record could be explained by climate or topographic change, but likely requires changes within the lake carbonate proxy-system and thus could record a combination of different forcings. Altogether, this thesis presents an integral framework for interpreting paleo-environmental records and highlights the importance of thoroughly understanding proxy-systems. There is great potential for interdisciplinary collaborations on future research of the Bidahochi formation, and in basins similar to it, to reconstruct paleoclimate and tectonic change in the geologic past via collections of multiple proxy datasets and field observations paired with model experiments.
Soils, Salts, and Water: Geochemical and Astrobiological Investigations of Briny Martian Regolith
(2025-08-01) Shumway, Andrew O; Catling, David C
Despite its present cold and dry surface, Mars may once have been able to support life—and perhaps still could. Liquid water is essential for life as we know it, and there is ample physical and chemical evidence proving that water flowed in Mars’ distant past. Today, what H2O remains exists primarily as ice or vapor, but some liquid water may form on the surface through interactions with salts and regolith (i.e., soil). This dissertation explores how salt, soil, and water shape the chemistry, mechanical properties, and astrobiological potential of Mars’ surface.The first part of this thesis investigates modern surface processes through laboratory analog experiments with Martian regolith simulant. Mars’ soil is rich in salts (such as perchlorates) that can form liquid brine, but prior investigations of the formation, stability, and habitability of these brines frequently neglected the soil component. Experimental results presented here demonstrate that regolith has a strong effect on brine properties, increasing the water content and stabilizing perchlorate brine at Mars-relevant conditions. Specifically, we find that regolith can inhibit salt and ice crystallization in brines, which likely also improves the habitability of those brines. These results greatly expand the geographic and temporal extent where potentially habitable brines could exist on Mars. Related experiments test a hypothesized formation mechanism for mysterious seasonal features dubbed Recurring Slope Lineae (RSL). RSL are apparently linked to water because of their appearance during warm seasons, yet simultaneously exhibit characteristics of dry granular flows. This work reconciles these seemingly contradictory observations by testing a semi-dry model of RSL formation, in which seasonal humidity cycles alternatively stabilize and destabilize slopes, periodically triggering mass wasting. Experimental results illustrate a correlation between relative humidity, soil hydration, and slope stability, which supports the hypothesis that RSL are desorption-triggered granular flows. Findings demonstrate that neither liquid water nor high abundances of salt are required to explain RSL, which raises questions about their protected status and presumed astrobiological potential. This thesis also reports novel geochemical measurements of salty Martian regolith made in situ using the Planetary Instrument for X-ray Lithochemistry (PIXL) on NASA’s Perseverance rover. PIXL’s investigations map the elemental chemistry of regolith at a higher spatial resolution than any previous measurement on Mars. We interpret the results to infer the mineralogy and provenance of various regolith components that appear to be contributed from local, regional, and global sources. Abundances of S and Cl, which may form soluble species, deviate from the global trend for Martian soils, which suggests a unique local aqueous history. The results provide compelling new motivation for a future mission to return a sample of Martian regolith—two of which have been cached by Perseverance. As a whole, this dissertation elucidates how water interacts with salty soil, which improves our understanding of the various roles of water on Mars, the potential habitability of the surface, and possible resources and/or hazards for future human explorers.
Developing water-isotope records from stratigraphically disturbed blue ice: observations, limitations, and insights from the Allan Hills of Antarctica
(2025-08-01) Davidge, Lindsey; Steig, Eric J
Polar ice cores contain direct archives of past atmosphere and past precipitation, and water-isotope measurements of polar ice-core samples are used to reconstruct Earth's past climate. Ice cores from stable domes or ice divides -- where post-depositional ice flow results primarily from compression of more recently accumulated snowfall -- have produced continuous climate records as old as 800 ka. Climate records from older ice cores are of interest due to a notable change in Earth's climatic fluctuations around 1 Ma. Extending the ice-core paleoclimate record beyond 1 Ma likely requires measurements from more complicated depositional environments, such as from blue ice areas at the margins at the Antarctic ice sheet. The Allan Hills blue-ice area contains ice as old as 6 Ma at relatively shallow depths (<200 m below the surface), but cores from this region are typically discontinuous and stratigraphically disturbed. In addition, they have likely experienced orders of magnitude more thinning than more typical ice cores, severely limiting their temporal resolution. The development and interpretation of ice-core climate records from blue-ice areas is still in nascent stages, and is only possible due to the development of an absolute dating technique by the analysis of atmospheric 40-Ar (Bender et al., 2008). Age constraints at discrete depths along the core provide information about the average age of the ice, but significant uncertainty remains about the amount of time represented by each measurement. Here, I present new water-isotope records from discontinuous blue-ice cores from the Allan Hills blue ice area of Antarctica, and explore the implications and limitations of those data. This thesis uses high-resolution water-isotope data to explore the integrated history of ice recovered from the Allan Hills. It (1) details advancements in high-resolution water-isotope measurement methodology by continuous-flow analysis, which is the preferred method for analyzing many Allan Hills cores, (2) advances our understanding of water-isotope signal preservation and alteration in blue ice by examining the reproducibility of the water-isotope record at lateral distances of 0.22 to 140 m, (3) demonstrates that glacial-interglacial signals are preserved in some disturbed Allan Hills ice and provides insights into ice accumulation and climate history in this area, and (4) explores differences in water-isotope distillation pathways caused by atmosphere and ocean changes between the present-day and a warmer (i.e. 6-Ma) world.
Insight into Enceladus’s ocean chemistry, habitability, and past from fractionation studies of the erupting plume
(2025-08-01) Fifer, Lucas Michael; Catling, David C
The erupting plume of Enceladus provides an ideal opportunity to investigate the chemistry and astrobiological potential of the subsurface ocean. However, the complexities of the eruption process likely result in a plume that is chemically fractionated and distinct from its ocean source. Chemical fractionation in the plume is not well understood, but it has ramifications for both extrapolation from plume measurements to ocean composition, and for long-term changes to the ocean chemistry due to preferential eruptive loss. In this work we (1) numerically model gas fractionation over the course of an Enceladus plume eruption, including gas exsolution from the ocean, (2) use laboratory experiments to constrain and validate our numerical models of gas exsolution, and (3) use models to investigate the long-term effects of plume eruption on Enceladus’s bulk chemistry and constrain the longevity of plume eruption. We find that Enceladus’s ocean is likely gas- and ammonium-rich and moderately alkaline, with free energy for methanogenesis. We also find that terrestrial models and measurements of mass transfer can generally be applied to carbon dioxide exsolution under Enceladus conditions, but may underestimate mass transfer coefficients of insoluble gases. Finally, we constrain overall timescales of plume eruption to 30–300 Myr and find that Enceladus’s early ocean may have been carbon dioxide-rich and acidic, but was more likely ammonium-rich and basic. Our work advances our understanding of this small, dynamic moon and the nature of its ocean-plume connection, and provides tools for the interpretation of future spacecraft measurements at Enceladus.
Using Landslides Induced by Earthquakes for Paleoseismology, Hazard Prediction, and Spatial Analysis
(2024-10-16) Herzig, Erich Noa; Duvall, Alison
Landslides are a dangerous natural hazard that can kill people and damage property. Mass movement on hillslopes also erodes mountains and shapes landscapes over the longer term. By studying both landslides and their causes, we can better understand how they influence the environment and how to minimize the risk they pose to people and property. Landslides can be caused by shaking from an earthquake. Unlike landslides induced by other forces such as rainfall, earthquake induced landslides all happen at roughly the same time, which can amplify their hazards and impact on the landscape. I use this pulsed nature of earthquake induced landslides to investigate past earthquakes on the Seattle Fault. The Seattle Fault represents a major hazard for the city of Seattle, but has not experienced a major earthquake since approximately 1000 years ago. Because of this, earthquakes on the fault are poorly understood. We date past landslides in the area using a novel roughness based landslide dating technique. By looking for times when more landslides occurred than normal in places that would likely have strong shaking from an earthquake, I find landslide clusters that likely represent past earthquakes on the Seattle Fault. This method can be generalized to other areas prone to landsliding and infrequent but strong earthquakes. Next, I examine different scenarios to predict where a modern Seattle Fault earthquake would induce landslides. By combining the ground motions from 18 different scenario Seattle Fault kinematic earthquake models and a multi-modal landslide hazard model, I find where landslides are likely to occur and what controls these patterns. Over all scenarios, I find that there are always high numbers of modelled landslides (10% of slopes failing with translational landslides, and 4% of slopes failing in rotational landslides) and that these landslides mostly occur in the same places as modern rainfall induced landslides. Between scenarios, water saturation and local geologic strength are more important factors than earthquake properties such as hypocenter and magnitude. Finally, I investigate the assumptions behind the roughness based landslide dating technique used previously. I measure the roughness of earthquake induced landslide inventories from both the 1994 Northridge CA, USA and 2016 Kaikoura, NZ earthquakes. The roughness based dating method assumes that these landslides are all about the same roughness when they occur, however I find that these landslides actually have a wide range of initial roughness values. The range of initial roughness values can be reduced by filtering landslides by geology and landslide size. I also test how landslide deposits smooth over time by comparing repeat lidar surveys of the 2014 Oso, WA, USA landslide over a ten year period. These data show that geology also plays an important role in determining the rate at which the landslide smooths. Overall I found that when different geologic units are isolated and analysed separately, the variation in roughness in large landslides (Area greater than of equal to 10,000 sqm) would lead to a variation in estimated ages of 500 and 200 years. This is similar to the error (200 years) used in Chapter 2 and validates the use of this technique for landslide age estimation.
In-Situ Sampling and Spatially Resolved Measuring Approaches for Optical Surface Exposure Dating for Late Quaternary Applications
(2024-09-09) Bench, Tristan Graham; Stone, John O
Optically stimulated luminescence (OSL) exposure dating utilizes OSL-at-depth signals to extrapolate an exposure age from rock surfaces. Exposure ages are commonly obtained by fitting the forms of luminescence depth profiles, which depend on parameter estimates of light attenuation and defined rates of luminescence bleaching. Current procedures for obtaining these parameters for a rock surface require matching luminescence depth profiles from compositionally and morphologically matched rock surfaces with known exposure ages, which limits the accuracy and applicability of the technique. Further, traditional measuring procedures for depth profiles involve measuring the OSL of millimeter slices from surface core samples, offering poor resolution datasets and limiting parameter and age fitting accuracies. With the aim of improving the accuracy and applicability of OSL surface exposure dating, modified sampling and measuring procedures incorporating controlled exposure experiments and spatially resolved OSL laser scanning measures are performed in a trial study on 11-year exposed quartzite rocks. The first experiment involves trials of controlled exposure sampling approaches, to attempt to reliably determine exposure dating model parameters directly from the rock surface of interest using OSL saturated core samples subjected to controlled light exposures. Measured from wafer derived datasets, natural sunlight controlled exposed parameters were able to produce decadal ages, equivalent in magnitude to that of ages produced from proximal rock sourced parameters. Parameters acquired from simulated light, however, produced centennial to decamillennial ages. Data scatter in the luminescence depth profiles substantially limit parameter and age precision of all techniques, however, warranting more resolute OSL measuring protocols to make more valid conclusions about the use of controlled exposures for rock surface parameterization. Resultingly, OSL scanning measures were trialed on the Lane Mountain samples to record sub-millimeter resolution OSL depth profiles of the core surface samples, to compare if the parameterizations from these higher resolution datasets would be more precise than parameters acquired from lower resolution wafer derived datasets. Scanning electron microscopy with energy dispersive spectroscopy analyses (SEM-EDS) were used additionally on OSL scanned samples to identify and filter out non-quartz OSL anomalies in scan data, with the aim of generating depth profile data which more closely follows model expectations for quartz-OSL exposure dating, and to see how parameterizations are influenced with this modification to data. The use of spatially resolved OSL depth profiles from scan datasets improved parameter extrapolative precision over wafer derived datasets for individual and combined core datasets, although individual datum error was generally higher than in wafer datasets. The use of non-quartz anomaly filtering on spatially resolved scan data improved data error bounds of anomaly affected regions, and improved the data scatter of spatially resolved depth profiles for both individual and combined core datasets, but minorly improved or matched parameter precision to nonfiltered OSL scan data. Once core parameterizations were acquired, age calculations using spatially resolved data were performed using proximal rock sampling techniques. For individual age fits from each core sample, filtered and nonfiltered spatially resolved OSL depth profiles produced inconsistent ages between known age surfaces, but the calculated precision was improved when using scan and scan filtered data. Combined core age fits of each rock surface using scan and scan filtered data produced more accurate age results to 11 years than age fits from individual cores, but each fit type produced comparable fit precision. Ages using controlled exposure experiment techniques were unable to be extrapolated from spatially resolved data, given the scan data from the controlled exposed samples were too low in OSL intensity to acquire viable depth profiles for adequate parameter extrapolation. The promising results in using controlled exposure experiments for parameterization, and the observed precision improvements when using spatially resolved OSL to parameterize rock surfaces, invite the opportunity to apply both sampling and measuring approaches to exposure date quartzite erratic members of the Foothills Erratics Train, which are interpreted to have been deposited during the final retreat stage of the Laurentide and Cordilleran ice sheets and the opening of the ice-free corridor. Inconsistent deposition timelines of the erratics and the opening of the ice-free corridor warrant the application of an alternative surface exposure chronometer more sensitive to millennial timescales, such as luminescence exposure dating, to attempt refining the timeline of Erratics Train deposition. The use of controlled exposure experiments allows for exposure dating trials at the sites, where no known age proximal rocks are available for the erratic surfaces. Further, the use of higher resolution spatially resolved OSL may offer precise age characterizations for decamillennial time. Lab simulated, unidirectional sunlight exposures were used to extrapolate parameters from the erratic surfaces, given difficulties in facilitating natural sunlight exposures, and depth profiles were measured using spatially resolved OSL laser scanning techniques. Age results for the erratics offered sub-annual to centennial exposure ages, providing no realistic insight on the deposition timelines of the Foothills Erratics Train members. However, the results indicate that the parameterizations of OSL depth profile development in the exposure dating model may be too simple, and that other influences in the experimental setup may affect parameter accuracy and precision. For instance, mineralogical variations in OSL may impact fit precision, which are unaccounted for in the model form, and were not able to be considered for this assessment, as X-ray element maps produced for the cores were not usable for filtering assessments. Second, the higher intensity OSL acquired from the controlled exposure samples indicates that the use of unidirectional, single flux rate solar simulated light, which follows model conditions for light exposure, may oversimplify exposure conditions over decamillennial timescales, producing inaccurate exposure ages. Third, the fit approach for parameterization may also not be effective in obtaining accurate parameters for the rock surface, given inconsistencies observed between physically derived parameters and curve fitted parameters both from Foothills Erratics Train and Lane Mountain samples. Fourth, the presence of weathering rinds may impact the feasibility of controlled exposure rock surface parameterization, and surface altered cores should be incorporated in controlled exposed samples to attempt best emulating rock surface exposure conditions. Finally, erosion rates are likely to impact depth profile evolution in the Foothills Erratics Train study, but are not quantified in the model form, potentially causing the younger than expected exposure ages. With continued physical experiments on depth profile evolution, and application trials to test modified parameterizations, the ability to date millennial-decamillennial exposed surfaces can be more effectively evaluated. Specific physical experimental studies on depth profile effects from the light source, such as photon flux variations and angle of illumination variations, as well as sample characteristics light surface coverage, weathering rind impacts, and erosion, can offer improved insight on the effective parameterization of depth profile evolution for exposure dating applications. Still, the use of higher spatially resolved OSL measurements, and the use of controlled exposure parameter sampling, have shown potential in expanding the applicability and precision of the exposure dating technique, and provide more detailed measures of OSL, which can benefit trial experiments aiming to better parameterize depth profiles for exposure dating methods. With these new procedures, continued research on the parameterization of depth profiles for exposure dating can be more effective in execution, and can improve OSL exposure dating to become an established geochronometer.
Identifying the signature and mechanism of long-term permanent strain along the Cascadia coastline, southwestern Washington
(2024-09-09) Stanton, Kelsay; Crider, Juliet
Long-term deformation at a subduction margin (i.e., over many earthquake cycles) implies permanent strain accumulation. This complicates regional strain budgets as well as expectations of earthquake cyclicity based on the elastic rebound model because not all interseismic strain is released during an earthquake. Characterizing the extent, timing, and rate of forearc deformation at a subduction zone is crucial to understanding the strain budget and subduction zone earthquake cycle. Characterizing coastal uplift is also necessary for assessing the possible mechanisms accommodating permanent deformation in the forearc, such as crustal folding or faulting on the overriding plate. Finally, constraining permanent strain accumulation informs tectonic models and provides key information for estimating seismic hazards. Pleistocene coastal uplift is observed at the Cascadia subduction zone, although no prior studies constrained long-term uplift in coastal southwestern Washington. This dissertation presents new mapping and luminescence dating of Quaternary deposits near Grays Harbor and Willapa Bay to show that estuarine deposits record a late Pleistocene average uplift rate of 0.4 Â± 0.1 mm/yr. Uplift rates of this magnitude are consistent with other Pleistocene uplift and incision rates in Cascadia, and when compared to observed interseismic vertical deformation, the rates suggest that about one-tenth of interseismic strain may become permanent. Other locations in Cascadia with similar uplift rates are characterized by crustal folds or faults, but no faults are evident in the onshore Quaternary deposits near Grays Harbor and Willapa Bay. Map-view interpretation and two-dimensional modeling with gravity and magnetic data indicate north- and northwest-trending faults underlie the Quaternary deposits. The modeling suggests two 20-25Â° east-dipping reverse faults. One fault aligns with the active Willapa Bay fault zone, identified previously from offshore seismic-reflection studies, and the other fault aligns with the Raymond fault, previously inferred from geophysical modeling. Uplift recorded in the estuarine deposits is likely accommodated by the Willapa Bay fault zone. The modeling also suggests that faults mapped in regional bedrock with small lateral offsets must also have a significant vertical component of slip. The geophysical modeling combined with previous geologic mapping thus suggests that regional faults may be oblique. Coastal southwestern Washington may be transitional between deformational domains, with faults accommodating both east-directed, subduction-related strain and north-directed strain related to tectonic block rotation. Active subsurface faults may leave a geomorphic signature of deformation. Where geophysical modeling locates the Raymond fault, geomorphic analyses indicate knickpoints in stream profiles that may reflect a relict fluvial system once graded to a base level 85-150m higher than current sea level. Because Quaternary sea level fluctuations cannot account for 85-150 m of base level fall, the base level change likely reflects regional rock uplift, possibly accommodated by the Raymond fault. The Willapa Bay fault zone, however, does not produce surficial lineaments nor measurable differences in stream profile steepness where it projects ashore nor where geophysical maps indicate a change in strike. The geomorphic analyses of longitudinal stream profiles indicate a close relationship between steepened channels, bedrock faults, and lithologic contacts, an observation that supports structural and geophysical models for uplifted fault-bound blocks of basalt. Taken together, long-term permanent uplift in coastal southwestern Washington is at least partially accommodated by long-lived, active crustal faults.
Tracing continental evolution by potassium isotopes
(2024-04-26) Huang, Tianyi; Teng, Fang-Zhen
The Earth’s continental crust has diverse lithologies and preserves a rich geologic history of Earth’s evolution, which is vital for understanding the origin and evolution of our planet. Potassium (K) and its isotopes can provide new insights into the continent formation and evolution. This dissertation focuses on the K isotope system in the continental crust and its application in tracing continental evolution.The dissertation begins with a review of the analytical protocol for high-precision K isotope measurements using MC-ICPMS in our laboratory, which makes it possible to resolve K isotope variations in natural terrestrial samples. Then, the first paper constrains the K isotopic composition of the upper continental crust. A variety of upper crustal samples, including granite, loess, shale, and upper crustal composites, were measured, which display significant K isotopic variations ranging from 0.68 to -0.12‰. Variations in sedimentary rocks reflect continental weathering, whereas variations in granites may reflect either source heterogeneity or magmatic differentiation. Although the upper crust is isotopically heterogeneous, it has a mantle-like average K isotopic composition, with an average δ41K of 0.44 Â± 0.05‰ (2SD, n = 88). The next paper further investigates K isotope behaviors during granitic magmatism to explain the origin of K isotopic variations in granites. Large K isotope fractionations are observed among the granitic minerals, with plagioclase being highly enriched in heavy isotopes compared to other minerals (e.g., K-feldspar, mica, and hornblende). We measured two sets of co-genetic granitoids and their K-bearing minerals. Our results suggest that the differentiated melts may be isotopically light, while plagioclase cumulates are expected to have heavy K isotopic compositions. The third paper aims to investigate the temporal K isotopic variation of the continental crust. We analyzed K isotopes in glacial diamictites from Archean to the present, representing average crustal compositions through time. We found large K isotopic variations in the Archean samples but more homogeneous compositions in younger samples. This change in the magnitude of K isotopic variation is interpreted as reflecting a transition from Na-rich to K-rich continental crust and decreasing weathering intensity over time. Collectively, this dissertation reveals the heterogeneous K isotopic compositions of the upper continental crust, confirms the occurrence of K isotope fractionation during granitic magmatic differentiation, and highlights the importance of K isotopes in tracing continental evolution. The results from this dissertation contribute to the application of K isotopes to trace the Earth’s history.
Nearshore records of natural hazards of the past millennium, western Washington
(2024-04-26) Davis, Elizabeth J; Crider, Juliet; Atwater, Brian
Since earthquakes cannot be predicted, assessment of earthquake and fault-related hazards relieson knowledge of fault behavior in historic and prehistoric times paired with models based on the physics of how faults work. Historic records of earthquakes in many parts of the world are limited; in these places, paleoseismology extends the earthquake record into the geologic past. In this dissertation, I used field geology and geomorphology at three sites in western Washington to identify, evaluate, and date evidence for earthquakes and lay groundwork for future paleoseismic studies. In chapter 1, submerged shorelines across the Seattle Fault Zone, a crustal fault that crosses Seattle, indicate that the fault produced earthquakes as large as M~7.5 only once in the past 11,000 years, a longer recurrence interval than used in current hazard estimates. In chapter 2, liquefaction records from the Duwamish estuary in Seattle contribute to the regional earthquake catalog and demonstrate that liquefaction incompletely records regional earthquakes. In chapter 3, I mapped and dated a low-elevation terrace at Rialto Beach on the Pacific coast of Washington, one of many such terraces along the coast, that has been hypothesized to record tectonic uplift. I found that the terrace is ~600–200 years old and that neither formation by tectonic uplift nor formation by beach progradation can be ruled out. The stratigraphic framework developed at Rialto Beach and presented here can be used to evaluate the other terraces to test the hypothesis that these terraces formed via coseismic uplift during Cascadia subduction zone earthquakes. I also found that deep-seated landslides on the slope above the Rialto Beach terrace pre-date the terrace formation, which has implications for their potential use in paleoseismic studies. Findings from all three of these studies will contribute to growing catalog of paleoseismic data in Cascadia that are used to reconstruct earthquake histories and evaluate behavior of the megathrust and crustal faults.
The legacy of megafloods in the eastern Himalaya: from erosion to deposition
(2024-02-12) Morey, Susannah Marie; Huntington, Katharine W
Glacial lake outburst megafloods are infrequent, high magnitude (discharge ≥10^6 m^3/s) events that are uniquely efficient agents of landscape change, despite being short lived. Much of our understanding of megaflood hydraulics and their subsequent erosion and deposition comes from the study of megafloods in low relief systems. However, recent research suggests that outburst floods will behave differently in steep, mountainous terrain than they would in relatively flat landscapes. The Yarlung-Siang River (YSR) in the eastern Himalaya provides an excellent case study to examine the impact of megaflooding in high-relief topography because of its history of extreme megafloods throughout the Quaternary. In this dissertation, I investigate outstanding questions regarding the legacy of eastern Himalayan megafloods using interdisciplinary methodologies. Hydraulic simulations of an ancient megaflood sourced from the Tibetan Plateau show that megaflood erosional patterns will differ from those of the modern river due to dynamic interactions between flood hydraulics and the rugged mountain landscape which is far above the inundation of the modern or paleo YSR river channel, even in extreme meteoric flooding. If megafloods are a significant contributor to erosion in this region, as many have suggested in prior studies, then simply using annual stream power estimates of erosion is insufficient to capture the impact of megaflooding. Beyond erosion, these simulations revealed an extensive potential for deposition of sediment, both fine-grained (sand and silt) and coarse-grained (decameter scale boulders). Additionally, hydraulic simulations predict that the Siyom River, a large tributary of the YSR, experienced up to 60 km of with megaflood backflooding. Remote topographic analysis and field geomorphology, sedimentology, and geochronology in fluvial terraces in the valley reveal evidence of extensive deep stagnant and turbulent water, which I interpret as megaflood backflooding deposits. Radiocarbon geochronology reveals megaflood deposit ages ~9-11 ka in the Siyom River valley, which coincide with the timing of a paleolake on the Tibetan Plateau thought to be a source of megaflooding down the YSR. The inundation of a tributary during a megaflood introduces complexities not typically considered in predictive models of river evolution in mountain landscapes. Finally, hydraulic simulations indicate a potential for the deposition of ≥4m diameter boulders in boulder bars in the YSR channel during a megaflood, a finding that is consistent with field and remote sensing observations. Using a 1D landscape evolution model that incorporates the impact of immobile boulders on channel evolution, I determined that megaflood-deposited boulders create hundreds of meter-scale knickpoints which can persist for tens of thousands of years along the longitudinal profile of a river that experiences a megaflood. If that river experiences a sequence of megafloods, as is likely at the end of a glacial cycle, then those knickpoints will be refreshed and the change to the channel will compound, leading to a stepped longitudinal river profile—a potential signature of megaflooding in the landscape. Altogether, this dissertation advances understanding of how megafloods function in steep mountain landscapes and outlines the complex legacy of these immense floods, which includes both extensive erosion and deposition.
Geometry and Visualization of Folds
(2024-02-12) Needle, Mattathias David; Crider, Juliet G
Reconstructing the geometry of geologic folds, and subsequently expressing this geometry visually, are necessary for understanding the kinematic and mechanical properties of folds. Novel technology for acquiring 3D geometric data, including drone-based photography and structure-from-motion photogrammetry, enables data collection at fold outcrops that were previously difficult to study in the field. This new technology introduces opportunities to enhance kinematic and mechanical models of folds from field data, but also requires the exploration of new methods for reconstructing and visualizing fold form from 3D point clouds. I present work involving point-cloud data collected from a drone-based photographic survey in the Bear Valley Strip Mine (Shamokin, Pennsylvania), where the 30-meter-high Whaleback Anticline and adjacent folds are exquisitely exposed in three dimensions. While the strip mine has been a popular field trip destination for half a century, it has not been previously possible to make detailed observations throughout the folded surface or making a detailed geometric reconstruction. In Chapter 1, I provide the inspiration and roadmap of this work. In Chapter 2, I propose a new methodology of interpolating fold form with non-uniform rational basis splines to construct a smooth, continuous, mathematically operable fold surface from noisy and discontinuous point-cloud data of the fold train at the Bear Valley Strip Mine. In Chapters 3 and 4, I introduce the Structural Geology Query Toolkit – an open-source software package for building first-person-perspective field-geology simulations in which a student or researcher can visualize and interrogate geologic surfaces, like the folded sandstone surface at the Bear Valley Strip Mine, generated from 3D scans. Finally, in Chapter 5, I leverage 3D models of the Whaleback and the Structural Geology Query Toolkit to determine that mesoscale extensional faults accommodate <6% strain evenly distributed throughout the Whaleback’s surface, which contributes to our understanding of the late-stage kinematics of buckle folding. Although the methods and new tools discussed in this dissertation revolve around data collected at the Bear Valley Strip Mine, they are intended to be applicable to other field sites where 3D scans of outcrops are possible.
Biosignatures, the Origin of Life, and the Early Earth Atmosphere
(2023-09-27) Wogan, Nicholas F.; Catling, David C
The ancient Earth atmosphere is our only example for how a microbial biosphere impacts planetary atmospheres and is therefore a critical asset to the spectroscopic search for life on exoplanets. Additionally, for a subaerial origin of life, the nature of the earliest Earth atmosphere determines the environmental conditions under which life began. However, our understanding of the early Earth is shrouded by deep time; very few clues to its composition, climate and biosphere have been preserved over billions of years. To complement the sparse geologic record, this thesis uses thermodynamic, photochemical, and climate models to better understand the atmospheres of early Earth to inform the search for life on exoplanets and improve our understanding of the origin of life. In Part I of this dissertation, I investigate atmospheric chemical disequilibrium anti-biosignatures, as well as methane and oxygen biosignatures during the Archean (4.0 - 2.5 Ga) and Proterozoic (2.5 - 0.54 Ga) eons. By modeling the change in Earth's atmospheric composition when life first began, I argue that the disequilibrium coexistence of atmospheric H2 and CO2 or CO and water vapor is an anti-biosignature if observed on an exoplanet because these easily metabolized species should be consumed if life was present. Next, I estimate the likelihood of volcanism on an exoplanet mimicking the CH4+CO2 biosignature characteristic of the Archean Earth. I find that significant volcanic methane is unlikely, but, if possible, could be identified by observations of atmospheric CO because volcanoes that produce CH4 should also make CO. The final Chapter in Part I argues that atmospheric oxygen, Earth's most recognizable biosignature gas, was unstable during the Great Oxidation Event (~ 2.4 Ga). I also set a lower limit on O2 levels during the Proterozoic eon, which improves potential detectability of O2 on an exoplanet if it was like the ancient Earth. Part II explores how Earth's Hadean (4.5 - 4.0 Ga) atmosphere may have influenced the origin of life. Specifically, I use atmospheric models to estimate the HCN and HCCCN produced in the Hadean atmosphere in the wake of large asteroid impacts. Both HCN and HCCCN are critical ingredients in "RNA world" origin of life hypotheses. Simulations show that asteroid impacts make transient H2- and CH4-rich atmospheres that persist for millions of years, until hydrogen escapes to space. I find that impacts larger than between 5 x 10^20 to 4 x 10^21 kg (570 to 1330 km diameter) produce sufficient atmospheric CH4 to cause ample HCN and HCCCN photochemical production and rainout to the surface, while smaller impacts produce negligible amounts of origin-of-life molecules. The second chapter of Part II places these results in the context of Earth's impact history. I estimate when 5 x 10^20 to 4 x 10^21 kg impacts most likely occurred on the early Earth to shed light on when life began if it did so in an impact-driven scenario.
Investigating the drivers of glacier retreat in West Antarctica using proxy-data assimilation and numerical modeling
(2023-09-27) O'Connor, Gemma; Steig, Eric
Outlet glaciers in West Antarctica are rapidly retreating and contributing to sea level rise. Ice loss is primarily occurring via wind-driven incursions of warm circumpolar deep water melting the ice shelves that buttress the glaciers. The leading hypothesis is that the current stage of retreat was triggered by a wind-driven change in ocean conditions that occurred in the mid-20th century. However, the short record of observed wind and ocean conditions in this region leaves our knowledge of the mechanisms driving glacier retreat highly uncertain (i.e., what wind/ocean changes occurred, and whether they are characteristic of natural variability or a response to anthropogenic forcing). Here, I use proxy data to reconstruct atmospheric circulation around Antarctica over the full 20th century, revealing century-scale trends and a large westerly event in the 1940s; both are candidates for initiating the current stage of retreat. I investigate the potential for the 1940s westerly event to induce substantial changes in ocean conditions near West Antarctica by using numerical modeling to constrain the sensitivity of ocean circulation to various local wind events. I find evidence that: (1) the circumpolar westerly winds have strengthened over the 20th century, in a zonally asymmetric pattern with the strongest wind trends occurring in the mid-latitude Pacific at approximately 55Â°S; (2) an unusually large and persistent westerly event occurred near West Antarctica from approximately 1938 to 1942, coinciding with the estimated start of glacier retreat. The results from my numerical ocean modeling experiments show that 5-year wind events with a similar pattern are indeed capable of enhancing the transport of warm circumpolar deep water toward the glaciers, but that the ocean is highly sensitive to the pattern of local wind forcing. The studies in this dissertation demonstrate that using nontraditional data sources from proxy records can provide novel constraints and useful insight on the complex atmosphere-ocean-ice processes occurring in this region. The results reveal several unexpected nuances to relationships widely accepted by the scientific community and highlight several specific lines of research that can further advance our understanding of the historical drivers of glacier retreat in West Antarctica.
Assimilating novel geophysical datasets into ice-sheet models: Experiments at the margins and interior of West Antarctica
(2023-08-14) Hoffman, Andrew Osten; Christianson, Knut
This thesis uses novel remote-sensing datasets in conjunction with focused modeling efforts to contextualize present and past changes in the mechanical behavior of the West Antarctic Ice Sheet. In the second and third chapters of the thesis, we explore the controls for sliding beneath Thwaites Glacier using surface velocity and elevation time series and the first swath radar scans of the subglacial topography in the ice-sheet interior. Observations of surface velocity and surface elevation over the last 10 years reveal that a system of subglacial lakes on Thwaites Glacier filled and drained without substantially changing the traction of the glacier to the ice-sheet bed. From swath topography recovered near these lakes, we diagnose the relationship between unresolved topography and basal shear stress inferred from ice geometry and surface velocity. In the fourth chapter, we present data collected at Hercules Dome, where swath radar data reveal bedforms and landscapes consistent with a marine proximal glaciated alpine valley network. These bed features require faster flow speeds for formation than the speeds observed at Hercules Dome today and suggest Hercules Dome may have been a nucleation center for ice-sheet growth preceding glaciation. In Chapter 5, we move downstream of Hercules Dome to Conway Ridge (formerly Ridge A), which sits between the van der Veen and Mercer Ice Streams. Encoded in the layer stratigraphy at Conway Ridge are signals that suggest ice flow around Conway Ridge has slowed as the ice sheet and bed topography isostatically equilibrated. Much of this evidence is linked to buried crevasse patterns that constrain past surface strain rates. These past surface crevasse features motivated the investigation of modern surface features and the focus of the sixthchapter concerned with crevasse area change on Thwaites Glacier.
Sensing impacts to the Earth-ionosphere waveguide from terrestrial and space weather
(2023-08-14) Anderson, Todd Shelby; Holzworth, Robert H
The Earth's lower ionosphere forms the lower-altitude limit of space plasma near the Earth, coexisting with the neutral mesosphere-lower thermosphere at 60-150 km altitude. In-situ measurements of this region are made difficult by the significant neutral density there, preventing long-duration satellite orbits, while at the same time being too high for balloons to reach. The ionization of this region is perturbed by dynamic solar processes. Solar flares and precipitating radiation belt particles enhance low-altitude ionization, causing communication blackouts, and generating ozone-destroying radicals in the mesosphere. In order to quantify the global impacts of solar flare ionization and energetic electron precipitation (EEP) on low-altitude ionization, measurements of the spatial extent of solar flare and EEP signatures are needed. The Earth-ionosphere waveguide, a region formed by the lower ionosphere and conducting Earth surface, allows radio waves in the very low frequency (VLF) band to propagate long distances around the world. This fact is useful for communication and other technological means, and also allows for the location of global lightning by sensing the radio waves emitted by lightning strokes, called sferics. Previous work has analyzed signal propagation in this waveguide from both natural (lightning) and artificial (transmitter) sources, in order to study the response of the waveguide to space weather drivers. However, such efforts have focused on accurately measuring ionosphere conditions either at a small number of specific locations relative to source lightning, or else along long propagation paths between a small number of source-receiver pairs. Here, we present efforts to detect and quantify the spatial signatures of both solar flare and EEP ionization in the Earth-ionosphere waveguide using lightning location data from the World Wide Lightning Location Network. First, we demonstrate a method using only changes in the stroke-to-station propagation path distribution to infer ionization associated with solar flares, and compare the timing of enhanced ionization with geostationary X-ray flux measurements for two X-class solar flares in September 2017. Then, we expand the method to include sferic waveform information, calculate the range-normalized dispersion of each WWLLN sferic, and investigate changes in range-normalized dispersion statistics associated with enhanced >300 keV electron flux detected by POES spacecraft. We find that spatiotemporal averaging of WWLLN propagation paths, and of sferic properties associated with these paths, is an effective proxy for detecting the onset and extent of enhanced ionization during strong solar flares, and may be used to determine changes in the lower ionosphere height associated with EEP. Additionally, the Earth-ionosphere waveguide, a spherical capacitor, plays host to the global electric circuit, a current system carrying charge to the ionosphere from global thunderstorms and other drivers, where it then disperses horizontally and leaks through the fair weather atmosphere to the ground. The fair-weather current density in this circuit has been thought to be spatially invariant, as supported by simultaneous measurements of the electric field and conductivity by separated stratospheric balloons. Were this invariance to be well-established, studies to determine the relative importance of various drivers to the circuit could open the door for long-term observations of global thunderstorm activity and other large-scale atmospheric electrical parameters using small numbers of instrumented stratospheric balloons. In order to test this expectation, we launched two stratospheric balloons in June 2021 that measured electric fields and conductivity in the stratosphere over several days. We found that, in disagreement with previous findings, the two balloons measured vertical return current density that differed by as much as a factor of two. We compared the balloon tracks to WWLLN and spaceborne lightning detections, measurements of cloud-top temperature related to vigorous convection, and precipitation estimates, and found no significant weather near either balloon that could account for the discrepancy in the return current density. This result suggests that fair-weather component of the global electric circuit is not as spatially invariant as previously thought, and further investigation into the causes of this invariance is needed to determine the relationships between atmospheric electrical drivers and the global circuit response.
Exploring past and present ice-sheet dynamics with geophysically-derived temperature and crystal orientation fabric
(2023-08-14) Hills, Benjamin Hale; Christianson, Knut
The greatest physical uncertainty for projections of future sea-level rise is in ice-sheet flow dynamics and the potential realization of theorized instabilities. While knowledge on the precise fate of Earth’s ice sheets is still out of reach, looking to past states provides insight on their potential evolution. Past ice-sheet states, and particularly their flow dynamics, are preserved in present-day ice temperature (Robin, 1955) and preferred crystal orientation fabric (COF); moreover, the future ice dynamics are governed by those same properties through the ice viscosity and the tendency to slide over a temperate bed. Theoretically, ice temperature and COF can both be inferred using ice-penetrating radar measurements, but prior work shows that it is difficult to disentangle these intrinsic properties of the ice column from each other or even from interface properties, such as reflectivity between layers and at the ice-sheet bed. Here, I contribute to the development of methods for ice-penetrating radar power and phase interpretation. I use those methods alongside mathematical models to infer both present-day and historical ice-sheet dynamics for three areas of the Antarctic Ice Sheet: Siple Coast, South Pole Lake, and Hercules Dome. Each of the three regional studies is formulated around a separate scientific problem, and the results for each drive physical interpretation of ice-sheet processes. At the Siple Coast I calculate spatial variations in radar attenuation and use them to show that ice-stream temperatures are colder than previously thought because of their upstream sourcing. At South Pole Lake, I calculate spatial variations in ice-bed reflectivity, depth variations in radar attenuation, and develop a novel ice-temperature model to show that the ice-bed interface is regionally thawed and has been stable at least since the last glacial period, in contradiction to prior studies. At Hercules Dome, I use measurements of the present-day ice dynamics, both surface and englacial velocities, to constrain a model of COF evolution. I then compare the modeled COF against measurements from radar polarimetry to infer stability in the regional ice dynamics since the last glacial period. Together, these three studies demonstrate novel and innovative radar analysis used to theorize dynamic evolution of the Antarctic Ice Sheet. Looking forward, these types of radar measurements will be incorporated into continent-wide modeling studies to constrain ice-sheet dynamics and projections of future sea-level rise.
Investigating the roles of ice dynamics and climate on polar near-surface processes
(2023-08-14) Horlings, Annika; Christianson, Knut
Understanding processes in the snow and firn are important for studies of ice dynamics, ice-sheet mass balance, glacial hydrology, and ice-core interpretation. In this dissertation, I use a combination of modeling, geophysical methods, and analyses of regional climate model output and reanalysis to study the evolution of firn compaction, firn hydrology, and snow accumulation on alpine glaciers and ice sheets in response to changes in ice dynamics and climate. The first study is devoted to investigating one process that is neglected in firn-air content estimates derived from firn-compaction models: enhanced layer thinning due to horizontal divergence. In this study, I implemented a new scheme into the Community Firn Model, a modular model framework that can simulate numerous physical processes in firn. Modeling results showed that horizontal divergence can reduce local firn-air content by up to 41% and may contribute to 16% of surface lowering for some dynamic fast-flowing regions in West Antarctica. I find that omitting transient horizontal divergence in estimates of firn-air content leads to an overestimation of ice loss via satellite-altimetry methods in regions of dynamic ice flow. The second study characterizes the extent of four firn aquifers in the Helheim, Ikertivaq, and KÃ¸ge Bugt glacier basins of southeast Greenland using airborne ice-penetrating radar data from 1993 to 2018. All four firn aquifers first appear and/or show decadal-scale inland expansion during this time period. A relative energy budget calculation using regional climate model output shows that these aquifer expansions are driven by decreasing cold content in the firn since the late 1990s and a recently increasing number of high-melt years. High-melt years are projected to increase on the Greenland Ice Sheet and may contribute to the continued inland expansion of firn aquifers, impacting the ice sheet's surface mass balance and hydrological controls on ice dynamics. The third study is devoted to examining the recent snow-accumulation rates at Hercules Dome, Antarctica, a prospective ice-core site. Only one observation of the snow-accumulation rate exists and accuracy of climate models remains limited there, meriting further study. I use ground-based very high frequency ground-penetrating radar collected during the 2019-2020 and 2022-2023 austral summers to construct the first spatially resolved snow-accumulation dataset over Hercules Dome. The 420-year, 326-year, and 225-year time-averaged accumulation rates are similar and range from 0.10 to 0.15 m/yr ice equivalent across the divide, with greater accumulation to the grid north (EPSG:3031) toward the Filchner-Ronne sector. Analyses of regional climate-model output and climate reanalysis show that the snow-accumulation pattern is likely a result of an orographic effect on the regional scale and wind redistribution on the local scale. The snow-accumulation variability across the divide will be important to consider for future ice-core science at Hercules Dome. This dissertation improves our understanding of a variety of polar near-surface processes that are integral in assessing ice-sheet mass balance in the past, present, and the future.

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