Title: Past and present carbon cycle changes: Deducing water-rock reactions in Earth's thin skin

Title: Reconstructing the landscapes of human evolution

Title: Unlocking the paleoclimate archives of soil carbonates with clumped and triple oxygen isotopes

Title: Novel perspectives on the water cycle and critical zone from the late Quaternary through the present

Title: Turbulence, droplets, and hurricanes: Using simulations to look where experiments cannot.
Abstract: In the environment, air and water transport a wide variety of constituents, including nutrients, pollution, droplets, aerosols, dust, and even bugs. Predicting where these things end up, and in what abundance, is a difficult enterprise; this difficulty impacts a huge range of scientific disciplines, and limits our ability to predict future environmental conditions and engineer solutions. In particular, turbulent motions are a highly nonlinear and small-scale phenomena that form the foundation on which environmental transport is based. Making matters worse, often it is hazardous or simply impossible to observe these motions in nature or recreate them in the laboratory.
Here I will focus on one such effort of leveraging high-resolution, high-fidelity simulations to explore complex flows and their accurate representation in coarse-scale models: tropical cyclones and the problem of air-sea interaction. It has long been hypothesized that sea spray generated at the ocean surface plays a large role in the transfer of heat, moisture, and momentum at the air-sea interface. In high winds, it is well-known that spray is produced in abundance, but it is much less clear how spray may mediate air-sea transfer in these conditions. A turbulence and droplet-resolving framework is used as an idealized testbed to examine the assumptions and premises of commonly used bulk spray flux parameterizations. In multiple respects, spray droplets limit their own ability to enhance air-sea heat and moisture transfer due to the complex thermodynamic feedbacks that occur during their exchange with the surrounding air. Ultimately, the primary factors determining whether or not spray can modulate air-sea energy and momentum fluxes are the spray lifetimes and airborne concentrations – both of which are quantities that are largely unknown or uncertain in high-wind conditions.

Title: Extreme weather in current and future climates: perspectives from global climate models.

Abstract: Large ensembles of climate model simulations are used to investigate how the environments supporting extreme weather change in a warming climate. Results indicate that the U.S. will likely experience less frequent, but deeper and more intense convection in the future. How these projected changes might be modified by solar climate intervention will also be discussed.

Title: Investigate lithosphere structure and dynamics in Alaska from subduction margin to continental interior

Abstract: Subduction margins and continental interiors, although are far away from each other, are parts of the integrated lithospheric system. Studying the structure and dynamics of the lithosphere at these settings provides us important insights into how the lithosphere evolves and how different parts of the lithosphere interact. Alaska is an ideal place to investigate these topics. In this talk, I will discuss recent and ongoing projects in my group to study the Alaska subduction zone and the overriding plate through seismic imaging. 

Title: Out in the Field - How to foster safety and inclusion in field sciences

Abstract: Field scientists from diverse backgrounds are becoming more prominent in their fields, but often encounter traditions and academic systems that are hostile to them. From affording necessary gear to pushing back against sexist ideas of who is welcome in field studies, many challenges remain. In this talk, science writer and paleontologist Riley Black will share her experiences on how we can all create a safer and more supportive discipline for those who don’t fit field researcher stereotypes.

Title: Can stratospheric aerosol geoengineering slow antarctic ice loss?

Abstract: Using global climate models, the geoengineering research community has analyzed a multitude of stratospheric aerosol injection (SAI) cases (how much aerosol to inject, at which latitude(s), and during which season(s)) to potentially ameliorate some of the negative consequences of climate change. Yet, research examining the impact of various SAI cases on the Antarctic region is minimal. Here we use a comprehensive set of SAI cases (where the injection begins in 2035 and we consider the 2050-2069 mean) to systematically analyze the following questions: Which SAI cases lead to enhanced upwelling of warm water on the Antarctic continental shelf pertinent to ice shelf basal melt? Which SAI cases lead to warmer surface air temperature or greater precipitation above the continent relevant to surface mass balance?

Our findings show that cases with the injection primarily at the Equator or in the northern hemisphere alters coastal wind stress such that upwelling of warm water onto the continental shelf increases relative to the 1990-2009 mean and is similar to the 2050-2069 mean from the SSP2-4.5 emissions scenario. As such, these cases would enhance basal melt and Antarctic ice loss. Furthermore, these cases result in greater accumulation on the continent which potentially offsets some of the mass loss through ocean thermal forcing. Conversely, SAI cases dominated by injection in the southern hemisphere show less shelf ocean warming than the other cases and relative to SSP2-4.5 along with muted surface accumulation anomalies. This research provides valuable results towards deriving an SAI strategy that limits Antarctic ice loss while meeting broader SAI objectives such as reducing global mean temperature and minimizing change to global precipitation patterns.

Title: Morphology, function, and evolution of lumbar vertebrae in Paleogene mammals

Abstract: Mammals have diversified in locomotion to a greater extent than any other group, from fully aquatic whales and powered flight in bats to the fastest extant runners, cheetahs and pronghorn. Living mammals have a unique region of the spine, the lumbar vertebrae, that plays an important role in locomotion by facilitating flexing and bending in the back. My research asks how this feature evolved with the initial diversification of mammals after the extinction of the dinosaurs and how its evolution relates to changes in locomotion. I surveyed lumbar vertebrae from Paleogene mammals and tested how differences in morphology related to ancestry and locomotor function (66-23 million years ago). I categorized important features for each vertebrae and mapped these over a phylogenetic tree to assess which features evolved quickly and which evolved slowly. I then quantified the shape of the most intact fossils using geometric morphometrics and compared the 3D shape of fossil lumbar vertebrae with that of modern. I found high variation across Paleogene mammals, suggesting that lumbar vertebrae were already a key component to mammalian locomotion earlier than previously thought. Finally, I tested the function of unusually shaped articulations that were common in many Paleogene mammals using digital models of fossil vertebrae to simulate the range of motion. This showed unexpected flexibility in these early mammals. Together this work shows how the morphology and function of lumbar vertebrae have evolved throughout mammals history and how phylogeny impacted this process.

Title: The impacts of hillslope sediment supply on the evolution of bedrock rivers.

Abstract: The evolution of the surface of the Earth is driven by the competition between tectonic forces which move rocks towards the surface and climatic processes which weather and erode these rocks. Bedrock rivers are the critical interfaces through which tectonic and climatic processes directly interact. Bedrock rivers are dynamic features that adjust their slope, width, erosion rate, and sediment transport capacity in response to spatiotemporal trends in both climate and tectonics. Decades of work has sought to understand the behavior of bedrock rivers across the Earth's landscapes; however, the impact of sediment supply from hillslopes to rivers which varies in both space and time is still enigmatic. We use a combination of field geology, remote sensing, and numerical modeling to document and predict interactions between hillslope sediment supply and bedrock river evolution.

This defense will present three projects. First, we used field geology and remote sensing to document sediment release, aggradation, and transport during a historic typhoon in southern Taiwan. We found new feedbacks between tectonics and bedrock river evolution, driven by patterns of hillslope sediment supply. Second, we developed a numerical modeling framework which integrates time-variable sediment supply tied to weather events into a 1-D model of river evolution. We find that this coupling changes which floods do the most work towards landscape evolution. Finally, we use our modeling framework to reproduce biases common in measured rates of bedrock river incision and investigate which climate and tectonic factors control these biases. We find that biases are strongest when climates are highly variable, and rates of rock uplift are low.

Title: Understanding the link between deformation and exhumation in thrust belt systems: Sevier fold-thrust belt, Northeastern Utah

Abstract: The Sevier thrust belt is one of the best-preserved thrust belts in the world and has been the focus of many studies attempting to understand the mechanics of deformation and erosion in thrust belts. The deformation history of eastward propagation of thrust sheets has been interpreted in part from the foreland basin which provides a record of synorogenic clastic material from 120 – 50 Ma. Previous studies have documented rates and kinematics of thrusting using thermochronology of rocks exposed in individual thrust sheets, however, we still do not understand the link between styles of deformation and exhumation rates. We use provenance analysis (sandstone petrography and U-Pb detrital zircon geochronology) to target synorogenic clasts eroded from the Pennsylvanian – Permian strata and deposited in the Cretaceous – Eocene Sevier foreland strata for zircon (U-Th)/He (ZHe) thermochronology. These clasts are derived from strata known to be exposed across all structures in the Sevier and Laramide system and can be used to track exhumation across eastward propagating thrust sheets and the passively uplifting Wasatch Anticlinorium. Our ZHe cooling ages show a younging up-section which correlates to the eastward propagation of thrust sheets. ZHe distributions from clasts have one of three unique thermal histories, representing exhumation on specific a structural feature in the Sevier fold-thrust belt including 1) exhumation at ~125 Ma on the Willard thrust which is observed in clasts deposited from 120 – 90 Ma (Kelvin, Frontier, and Henefer Formation), 2) exhumation at ~90 Ma on the Crawford thrust of clasts deposited from ~85 -75 Ma (Echo Canyon Conglomerate) and 3) exhumation at ~70 Ma possibly from the Uinta-Cottonwood Arch observed in clasts deposited from ~60 – 50 Ma (Wasatch Formation). We utilize lag time calculations to understand the relationship between deformation and exhumation related to orogenic steady state. We observe constant to lag time trends from ~120 – 70 Ma and a increase between 70 and 50 Ma.. Within individual thrust sheets, we observe a relationship between active thrust sheets which are associated with short lag times (fast exhumation rates) and Laramide structures which are associated with long lag times (slow exhumation rates).

Title: Deep learning for tropical cyclone formation detection

Abstract: Searching for dominant large-scale conditions that govern tropical cyclone (TC) formation is essential for understanding TC development. Using different deep neural network architectures, it is found in this study that deep learning could provide some promising capability in predicting TC formation from climate datasets at a certain forecast lead time. To detect TC formation, two common deep learning architectures including the residual net (ResNet) and UNet, well-known machine learning approaches for medical image segmentation applications, are used to examine TC formation during the 2010-2020 TC seasons in the Pacific Ocean. With a set of large-scale environments extracted from the NCEP/NCAR reanalysis and the labels obtained from the best track data, we show that both ResNet and UNet reach the maximum skill at the 18-36h forecast lead time and gradually deteriorated for longer lead times. Moreover, both architectures perform the best when using the domain covering the Pacific Ocean for input training, as compared to a smaller subdomain in the western Pacific. Because of its ability to provide additional information about TC formation location, UNet performs worse than ResNet overall across the accuracy metrics. Our proposed approach presents an alternative direction in applying machine learning to detect TCs beyond the current classification techniques or vortex-tracking methods. Our attempt to apply these deep-learning models to detecting TC formation in the future climate downscaling data showed that there tends to have less TC formation towards the end of the 21st century in both the RCP4.5 and RCP8.5 scenarios. While these results contain significant uncertainties due to the short training data for the deep learning models with the high-resolution model output, our study could present a new approach to examine TC activities in the future climate, using machine learning techniques. This new approach can provide independent cross-validation and new insights beyond the traditional method based on vortex tracking algorithms that we wish to propose in this thesis.

Title: Scale interactions between local moist phenomena and shifts of the large-scale atmospheric circulation

Abstract: The warming of Earth due to human emissions of greenhouse gases will profoundly alter the structure of its atmosphere and the nature of the climate extremes it generates. To prepare for these changes in climate extremes, we will need confidence in the degree of these changes. However, uncertainty currently surrounds these changes at regional scales because future changes in the controlling atmospheric circulations are also highly uncertain. Therefore, improving confidence in future projections of climate extremes should follow a two-pronged approach. The first prong requires determining which changes in extremes are due to the direct effects of warming (“thermodynamic effects”) and which are due to changes in the controlling atmospheric circulations (“dynamics effects”). Here, we develop a new methodology for diagnosing the contributions of thermodynamic and dynamic processes to hydrologic cycle extremes which provides a cleaner separation and allows for further diagnosis of dynamic contributions. We find that changes in the position of subtropical dry zones and the midlatitude jetstreams are dominant sources of dynamic change and uncertainty. Once dynamic contributions to a climate extreme are identified, the second prong requires reducing uncertainty in changes to the controlling circulation. One pathway for this focuses on understanding the controls on these circulations in our current climate. Examining the jetstreams over the Southern Hemisphere and the North Atlantic, we find that latent heat release has a much larger impact on jetstream behaviors than predicted by leading theories, which suggests that improving how latent heating is represented in climate models might increase confidence in future projections. We also find that the latitude of the North Atlantic jet in the current climate is coupled to its speed in a way not previously recognized. Our results imply that reducing uncertainty in the jet’s future latitude will require confident projections of this coupling