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ItemObservations of Englacial and Subglacial Hydrology at Sermeq Kujalleq (Store Glacier) from Autonomous Phase-Sensitive Radio Echo Sounding(Georgia Institute of Technology, 2022-05-03) Clavette, ReneeLiquid water at the basal interface of a glacier reduces effective pressure and basal friction thus increasing ice motion. The timing of surface melt reaching the bed is poorly constrained, however. To more accurately predict changes in mass balance across the Greenland Ice Sheet, we require a better understanding of how surface melt drainage varies spatially and temporally. Previous studies have found it possible for liquid water to be stored englacially at Sermeq Kujalleq (Store Glacier) for up to several months. Englacial water storage not only would delay surface melt from reaching the bed, but if storage persists through the winter, it may impact how a glacier responds to surface melt in the subsequent melt season. In general, we lack direct winter observations mainly due to the logistical difficulties of obtaining them. Through autonomous phase-sensitive radio echo sounding (ApRES), a ground-based radar system that can make continuous measurements for up to a year, we can address this challenge. In this thesis, I present ApRES data spanning an 11-month long period from Sermeq Kujalleq. Analyses of englacial radar return power and basal reflectivity indicate a dynamic hydrologic system exists interseasonally and liquid water is potentially stored englacially throughout the winter months. This further emphasizes the need to consider winter-time hydrology and quantify englacial water storage to improve mass balance and ice flow models in Greenland.
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ItemEVIDENCE FOR AN OLIGOCENE YARLUNG-BRAHMAPUTRA CONNECTION AND CROSS SUTURE DRAINAGE FROM DETRITAL ZIRCON U-Pb AND εHf PROVENANCE ANALYSIS, NORTHERN INDO-BURMAN RANGES(Georgia Institute of Technology, 2022-05-03) Villasenor, GabrielThe reorganization of continental-scale river systems is a fundamental control on the size and distribution of sedimentary basins along tectonically active plate boundaries. In the eastern Himalaya, river reorganization has been proposed as an explanation for modern drainage patterns and abrupt changes in foreland basin lithostratigraphy. Widespread application of detrital zircon (U-Th)/Pb geochronology by LA-ICP-MS identifies < 300 Ma Transhimalayan zircons sourced from Tibetan plutonic rocks in Miocene sedimentary deposits within the Siwalik and Indo-Burman basins. The presence of these Transhimalayan plutonic zircons implies the integration of the Yarlung River drainage system across the India-Eurasia continental suture prior to the Middle Miocene. We report that new ages acquired from detrital zircons of late Oligocene deposits of the northern Indo-Burman Ranges (IBR) have a percentage of < 300 Ma from < 20% in marine deposits to > 30% in terrestrial-coastal deposits. However, potential Gangdese pluton ages only constitute 2% of the total population, whereas population of this source is abundant in Miocene deposits. Zircon age populations are consistent with ancestral river drainage in the northern and eastern Transhimalayan plutons of the Bomi-Chayu and Dianxi-Burma region. Utilization of single grain εHf isotope analysis of ages < 300 Ma discriminates plutonic source of zircons, which provides paleo-river reconfiguration of the eastern Himalayan River systems. εHf values are coupled with (U-Th)/Pb ages; our data shows clustered between -20 to 20 for ages < 300 Ma. Clusters of points are located in the Lohit Plutonic Complex, Bomi-Chayu, and Dianxi-Burma plutons. εHf and (U-Th)/Pb values of collected samples points to a shift in provenance in coastal-terrestrial deposits of the late Oligocene. We interpret this shift as an integration of the Yarlung River in the Miocene causing an expansion of a localized drainage area to continental-scale.
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ItemConstrain Paleo pO2 Using One Dimensional Reactive Transport Chromium/Iron Weathering Model(Georgia Institute of Technology, 2021-08-02) Xu, PengxiaoQuantitatively constraining atmospheric oxygen level (pO2) in the Proterozoic has been a long time struggle in the study of paleoclimate and biogeochemistry. An accurate reconstruction would benefit our understanding towards Earth’s early history and complex life evolution. The oxidative weathering of (Cr) and iron (Fe) in soil column is controlled ultimately by oxygen , and thus the oxidation and retention of Cr and Fe are typically linked to pO2, making it a promising approach to estimating pO2 in Earth’s history. However, existing models of Cr mobilization during weathering as a function of atmospheric pO2 are oversimplified, and a surprisingly limited amount of work has been done exploring the dynamics of Fe oxidation/retention in a reaction-transport framework that allows for the explicit representation of redox fronts in the weathering realm. Here, we develop a simple two-phase reaction-transport model to explore the impact of atmospheric pO2 on the oxidative mobilization of Cr and the potential for retention of Fe during weathering. The model is meant to represent the key processes regulating Cr, Mn, and Fe redox cycling in a weathering environment forming from an idealized igneous basalt protolith in a way that is simple, transparent, and computationally tractable. Because many key parameters are uncertain, we subject the model to a stochastic resampling routine with the goal of identifying the overarching controls on relative Cr mobilization as dissolved Cr(VI) and retention of Fe as Fe(III) across a wide range of parameter space. Results suggest that atmospheric pO2 must be extremely low in order to prevent significant Cr(VI) release and allow for significant Fe loss during weathering, likely well below ~1% of the present atmospheric level (PAL). This value supports the idea that the emergence of complex life was delayed by low pO2 in the Proterozoic, given the minimum oxygen level to support these forms of life is ~3×10-3 PAL.
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ItemDYNAMICAL IMPACT OF THE MEKONG RIVER PLUME IN THE SOUTH CHINA SEA(Georgia Institute of Technology, 2021-05-04) Zeng, XiyuanNear the ocean surface, river plumes influence stratification, buoyancy and transport of biological tracers, nutrients and pollutants. The extent to which river plumes influence the overall circulation, however, is generally poorly constrained. This work focuses on the South China Sea and quantifies the dynamical impacts of the Mekong River plume, which is bound to significantly change in strength and seasonality in the next 20 years if the construction of tens of dams moves ahead as planned. The dynamic changes induced by the freshwater fluxes introduced by the Mekong River are quantified by comparing submesoscale permitting and mesoscale resolving simulations with and without riverine input into the basin between 2011 and 2016. In the summer and early fall, when the Mekong discharge is at its peak, the greater stratification causes a residual mesoscale circulation through enhanced baroclinic instability. The residual circulation is shaped as an eddy train with cyclones and anticyclones. Submesoscale fronts are responsible for transporting the freshwater offshore, for shifting eastward the development of the residual mesoscale circulation, and for further strengthening the residual eddy train. Overall, a greater northward transport characterizes the circulation in presence of riverine input. The significance of the mesoscale-induced and submesoscale-induced transport associated with the river plume is especially important in August and September, in the second half of the summer monsoon season, when primary productivity has a secondary maximum. Significant circulation changes, and therefore productivity changes, should be anticipated if the Mekong plume is modified by human activities.
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ItemUnderstanding the role of formic acid in fine and coarse particle mode(Georgia Institute of Technology, 2020-07-21) Min, JustinFormic acid partitioning was investigated at two sites in the southeast U.S., Yorkville, GA and Jefferson Street (JST). Previous studies in the southeast have shown that observed formic acid partitioning to fine particles did not agree with predicted partitioning due to very low predicted fine particle pH. This thesis looks to understand the possible causes for this discrepancy. The abundances of nonvolatile cations (NVCs) observed in particulate matter (PM) lead to the investigation of NVC complexation with formic acid to possibly provide additional insight on formic acid partitioning. Bulk particle pH and particle liquid water content were estimated using the ISORROPIA II thermodynamic model. Formic acid partitioning and pH results showed that assuming PM1.0-2.5 was externally mixed from PM1.0 at JST improved the agreement between the measured and predicted formic acid partitioning. This was only for PM1.0-2.5 because the partitioning pH was closer to neutral. However, it could not resolve the disagreement with PM1.0, suggesting that there may be different chemical processes and sources of formic acid that may be contributing to the fine and coarse mode. Increasing particle water concentrations also affected predicted partitioning of formic acid but were not sufficient to agree with observed partitioning. Correlations also indicate that formic acid in the southeast may be associated with mineral dust and biomass burning, suggesting a need to understand the source contributions of formic acid in more detail.
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ItemA comprehensive study of Titan's magnetic pile-up region during the Cassini era(Georgia Institute of Technology, 2020-04-28) Chen, ChenWhile Saturn's largest moon Titan is devoid of an intrinsic magnetic field, draping of the giant planet's magnetospheric field around Titan's ionosphere generates a pile-up region at the moon's ramside. In this study, we analyze Cassini magnetic field data from all 126 Titan flybys to systematically characterize the extensions and magnitudes of the field enhancements at its ramside. Along each flyby trajectory, the segments where Cassini crossed the piled-up magnetic field are determined, and altitude profiles of the pile-up strength are also generated. We investigate the dependency of the extension and strength of the pile-up region on various parameters, such as the Saturn local time and the magnetospheric environment to which Titan is exposed. In this way, we generate a comprehensive picture of Titan's ramside magnetic barrier during the Cassini era. Our major findings are: (1) The extensions of the observed pile-up reveal an asymmetry between the Saturn-facing and Saturn-averted sides caused by the large gyro-radii of ionospheric pick-up ions. (2) The observed pile-up radial extensions are nearly independent of Titan's orbital position. (3) The lower boundary of the pile-up region is located at an altitude of 0.35 Titan radius, denoting the average altitude of the ionopause. (4) Considering the error bars, the altitude profiles of the pile-up strength show a stable picture, i.e., the radial profile of the field magnitude is independent of Saturn's magnetospheric environment and local time.
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ItemSignatures of the El Nino-southern oscillation on rainfall and cave dripwater oxygen isotopes in N. Borneo(Georgia Institute of Technology, 2019-12-11) Ellis, Shelby AnnOxygen isotope (δ18O) records of speleothem carbonates are a critical terrestrial paleoclimate archive, providing insight into past hydroclimate variations and past changes in atmospheric circulation. Specifically, time series of rainfall and cave dripwater oxygen isotopes (δ18O) provide site-specific assessments of climate or non-climate related signals recorded in stalagmite δ18O used for such reconstructions. However, modern paired multi-year δ18O time series of rainwater and dripwater are limited in the tropical latitudes, an area known to contain regionally-specific atmospheric complexities acting on rainfall δ18O. Furthermore, karst drainage pathways vary significantly within the same cave system, altering the original climate-driven δ18O rainfall signal. In this thesis, I present an extended multi-year study of rainfall and cave dripwater δ18O time series from Gunung Mulu National Park in Northern Borneo to quantify the cloud-to-cave transformation process spatially and temporally across the Mulu karst, building on work previously presented by Moerman et al., 2013 and Moerman et al., 2014. Chapter 1 will broadly cover topics related to how stable water isotopes in rainfall, cave dripwaters, and stalagmites can detect ENSO-driven shifts in the hydrological cycle, building off almost a decade’s worth of modern (Cobb et al., 2007; Moerman et al., 2013; 2014; Partin et al., 2013a) and paleoclimate (Carolin et al., 2013; 2016; Chen et al., 2016; Meckler et al., 2012; Partin et al., 2007; 2013a) observations from a well-established research site in Northern Borneo, Sarawak, Malaysia. Chapter 2 quantifies the rainfall-to-cave dripwater transformation of isotopic climate-signals in the Mulu karst from continuous observations over the last ~12 years. These time series are the longest-running daily rainfall δ18O time series (2006 – 2018) and longest tropical biweekly dripwater δ18O time series (2007 – 2018) globally. Vadose zone mixing translates ENSO-related variations in rainfall δ18O to three monitored cave dripwater δ18O sites. Using two simple modeling techniques, we generated an ensemble of different modeled dripwater time series directly corresponding to local rainfall δ18O, estimating Mulu water takes ~3 to 18 months to transit through the karst. This transit time provides context for what resolution of climate signals can be potentially recorded in local stalagmites employed for hydroclimate reconstructions. Overall, this thesis supports previous interpretations of using the amount effect framework for Mulu stalagmite δ18O records through the multi-year, paired local rainfall and dripwater δ18O time series. This research clearly demonstrates paired rainfall and cave δ18O observations can support more minute interpretations of highly-resolved paleo-ENSO stalagmite records.
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ItemRemote triggering of microseismicity in Antarctica by large distant earthquakes(Georgia Institute of Technology, 2019-07-30) Ji, MingyuRecent studies suggested that seismic waves from significant earthquakes could trigger shallow seismic events and deep tremor in tectonically active regions at the long-distance range. Dynamic stresses carried by teleseismic waves can promote failure on the critically stressed faults at depth and trigger local microseismicity. This phenomenon is also known as remote triggering. However, only a few remote triggering studies have been done in Antarctica, mostly due to sparse network coverage. Since 2007, a new generation of broadband sensors has been deployed as permanent stations throughout Antarctica (POLENET project). This, together with other permanent and temporary seismic stations, provides critical infrastructure to investigate the remote triggering effect in Antarctica. In this study, we examine remotely triggered seismicity following large distant mainshocks since 2000 with visual inspection on raw waveforms and spectrograms. We identify evidence of possible triggered local events at various stations during or immediately following the surface waves of the 2010 Mw8.8 Maule, 2012 Mw8.6 Indian Ocean and 2016 Mw7.8 Kaikoura earthquakes. We also focus on how large distant earthquakes since 2000 triggered seismic activity around Mt. Erebus using broadband station IU.SBA, and find an apparent triggering threshold of 4-6KPa. Besides, we examine the spatial distribution of remote triggering with tectonic background and ice movement of Antarctica. Our results are consistent with previous inferences that most triggered events occur within West Antarctica and Antarctic Peninsula with rapid glacier movement and tectonic activity as compared to stable East Antarctica.
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ItemProminent contribution of hydrogen peroxide to intracellular reactive oxygen species (ROS) generated upon exposure to naphthalene secondary organic aerosols (SOA)(Georgia Institute of Technology, 2019-07-26) Saavedra, Gabriela G.Multiple studies have found an association between exposure to particulate matter (PM) and adverse health endpoints. One of the suggested mechanisms in which inhalable particles exert damage is by inducing the overproduction of reactive oxygen and nitrogen species (ROS/RNS). Hydrogen peroxide is one type of ROS that has been implicated in pathological disorders induced by PM exposure. It has also received increasing attention owing to its dominant role in cellular signaling, metabolic processes, and oxidative stress. However, its biological role upon exposure to PM remains unclear. Secondary organic aerosols (SOA) make up a substantial fraction of ambient fine PM and play a role in the proinflammatory effects of the particles. In this study, the contribution of hydrogen peroxide to intracellular ROS/RNS production upon exposure to water-soluble components of SOA generated from the photooxidation of naphthalene in the presence of NOx (PM samples) was investigated using the general oxidative stress indicator carboxy-H2DCF and catalase as a hydrogen peroxyde scavenger. The intracellular ROS/RNS response with and without the addition of catalase to the PM samples was measured, where the presence of catalase substantially suppressed ROS/RNS response. The hydrogen peroxide produced by water-soluble components in the naphthalene SOA extracted in phosphate buffer solution (PBS) was quantified and ranged from 9.04 ± 0.16 to 11.32 ± 0.27 μM, corresponding to a hydrogen peroxide yield of 3.1 to 3.8 ng/µg. The measured hydrogen peroxide was product of interactions between quinone compounds and peroxide compounds in naphthalene SOA and PBS. Additionally, cells exposed to PM samples released hydrogen peroxide at a rate of 0.21 ± 0.01 to 0.26 ± 0.03 pmol/min/104 cells, which was associated with the mediation of immune responses and/or oxidative stress induced by naphthalene SOA exposure. These findings confirmed that hydrogen peroxide was the main ROS produced by cells exposed to naphthalene SOA and that it was the driver of the PM-induced ROS/RNS response, although this contribution can vary depending on the specific SOA precursors and formation conditions. Findings in this study also showed that, in addition to the hydrogen peroxide produced by cells upon exposure to PM samples, the hydrogen peroxide produced by the PM samples upon interaction with the extracting solution could have diffused into the cell and contribute to the intracellular ROS/RNS response. Although future studies are needed to estimate the contribution of both sources of hydrogen peroxide to the intracellular ROS/RNS response, this study highlights that the diffusion of extracellular ROS/RNS into the cells could represent one of the pathways in which exposure to PM leads to oxidative stress.
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ItemLandslides on Ceres(Georgia Institute of Technology, 2018-04-30) Chilton, HeatherI analyze landslides on Ceres using several quantitative approaches to constrain the composition and structure of the top few kilometers of Ceres’ crust. I focus on a subset of archetypal landslides classified morphologically as thick, steep-snouted “type 1” (T1) flows and thin spatulate “type 2” (T2) flows (Schmidt et al., 2017) to explore the landslides’ mechanical properties, and supplement with comparison with all landslides combined. The results confirm earlier observations showing that T1 landslides are typically found poleward of 70° latitude and T2 mostly equatorward of 70° latitude. Measurements of landslide drop height and runout length imply effective friction coefficients lower than common friction coefficients in any of Ceres’ identified or suggested non-ice surface materials, including saturated clays. Measurements of the volume and area of landslide scars suggest that T1 landslides can fail to greater depths than T2 specifically and most landslides overall for a given scar area, consistent with depth-limited failure in landslides below 70° latitude. These results are consistent with a layer of lower shear strength material overlying a stronger layer in Ceres’ outer shell at low to mid latitudes, and a single layer without an overlying weak layer at polar latitudes. Combining these observations with known constraints on Ceres’ near-surface composition, I propose that Ceres’ crust at low to mid latitudes consists of a topmost layer with an ice content in excess of the near surface that thins out at high latitudes, and which overlies a somewhat stronger and more ice-rich layer.