Browsing by Subject "Glaciology"
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Item Acoustical characterization of glacierized fjords(2022-05-05) Zeh, Matthew Charles; Wilson, Preston S.; Ballard, Megan S; Deane, Grant B; Haberman, Michael R; Hamilton, Mark F; Pettit, Erin CThe rapidly changing cryosphere motivates a better understanding of the physical processes governing ice-ocean boundaries. These processes are no more pronounced than in glacierized fjords where massive glaciers meet the ocean. This underwater acoustic environment is significantly louder than other ice-covered environments. The abundant acoustic information, coupled with the difficulty performing measurements on or near glacier termini, encourage the use of passive acoustic monitoring to observe the system. While progress has been made towards improved understanding of sound in glacierized fjords, there remain considerable gaps in the community's understanding of the temporal variability of the acoustic field and the influence of the acoustic propagation environment. To address these deficiencies, three field experiments were conducted in three glacierized fjords: LeConte Bay at the terminus of LeConte Glacier near Petersburg, Alaska; Hornsund Fjord at the terminus of Hansbreen Glacier in Svalbard, an archipelago in the Arctic Ocean; and Disenchantment Bay at the termini of Hubbard and Turner Glaciers near Yakutat, Alaska. Acoustic data were collected between October 2016 and May 2017 from an underwater hydrophone array moored 500 m from LeConte Glacier. Ambient noise levels (ANL) from recordings were clustered, revealing relationships between ambient noise and the speed of icebergs above the mooring and with calving events. In particular, the local acoustic field demonstrated a uniquely consistent period between late February and early April 2017 where a single cluster dominated observations. The beginning and end of this period coincided with the formation and breakup of a dense pack of icebergs in the fjord. Characterization of the underside of a brash ice surface was obtained using an inference procedure and time difference of arrival and transmission loss data from an acoustic propagation experiment performed in Hornsund Fjord in September 2017. The inferred surface was incorporated into a forward simulation of the environment using BELLHOP, a ray tracing code. The measured data and simulated results were compared, providing insight to the shape and reflection characteristics of brash ice. Nearly continuous acoustic data was collected over fifty-two hours in June 2021 on two hydrophone arrays moored in Disenchantment Bay. The effect of recording duty cycle on sampling the spectral and temporal variability of the acoustic field was analyzed by comparing clustered spectral shapes and observations from full and reduced duty cycle recordings of varying length from 1% to 99% of the full 59-minute recordings. A relationship between duty cycle and relative error in hourly cluster observations was determined, which may inform the sampling procedure used in future deployments.Item Boundary conditions for an active subglacial lake in the David Glacier Catchment, Antarctica(2018-05) Lindzey, Laura Estelle; Blankenship, Donald D.In the 2016-2017 austral summer, the University of Texas Institute for Geophysics (UTIG) and the Korean Polar Research Institute (KOPRI) collaborated to perform a helicopter- based radar and laser altimeter survey on the Lower David Lakes Catchment. This survey ties in with (and fills some of the gaps in) an ICECAP survey of the Drygalski Ice Tongue and the David Glacier grounding zone from 2011 and 2012 [Blankenship et al., 2017] in order to provide a 5 km resolution survey extending from the grounding line past the first two active subglacial lakes identified by Smith et al. [2009]. While this is far from the first aerogeo- physical survey of an active lake system, it is one of the most extensive, and provides higher resolution boundary conditions and basal characteristics that will enable process studies of these enigmatic features.Item Controls on terminus change of marine terminating glaciers in Greenland over the last 40+ years(2022-11-17) Goliber, Sophie Ann; Catania, Ginny A.; Goudge, Timothy; Greenbaum, Jamin; Heimbach, Patrick; Spikes, KyleSince the 1980s, the Greenland ice sheet has been losing ice mass at an increased rate. Our current understanding of the complex physical processes that control dynamic mass loss is incomplete and, therefore, leads to a wide range of possible future contributions to sea level. Ice dynamics, or changes due to changes in ice flux, is dominated by the behavior of fast-moving outlet glaciers in Greenland. These glaciers are changing through melting of the terminus face and/or calving of icebergs; the combination of these processes and ice motion determines the position of a glacier terminus. In understanding how and why outlet glacier termini change over time compared to external forcing and internal glacier dynamics, we are able to move toward a better understanding of marine-terminating glaciers. In this dissertation, I use terminus traces to observe how and why marine-terminating glaciers change in order to better understand the mechanisms behind these complex heterogeneous changes in Greenland. I develop the largest database of manually-traced marine-terminating glacier terminus data for use in scientific and machine learning applications. These data have been collected, cleaned, assigned with appropriate metadata, including image scenes, and compiled so that they can be easily accessed by scientists. Then I use the location of the termini to identify features in the bed topography that inhibit the retreat of glaciers following the onset of ocean warming and widespread glacier retreat in the late 1990s. I find that the slope and lateral dimensions of bed features exhibit the strongest correlation to retreat and that the shape of the bed features allows different styles of terminus retreat, which may be indicative of how different ablation mechanisms are distributed across termini. Finally, I produce a time series of terminus morphological properties for four glaciers in western Greenland to identify the characteristics that are indicative of calving processes with the goal of categorizing glaciers by calving style. I find that a concave shape and low sinuosity are present at glaciers that calve via buoyant flexure, while the opposite is true at glaciers that are dominated by melt-induced calving via serac failure. I also find that glaciers do not persistently fit into single calving styles and may change over time. By studying how the terminus changes over time compared to external forcing and internal glacier dynamics, we are able to move toward a better understanding of marine-terminating glaciers.Item Dynamic changes at tidewater glacier termini in central west Greenland(2018-06-21) Fried, Mason (Mason Joseph), 1987-; Catania, Ginny A.; Holt, John W; Walter, Jacob I; Kim, Wonsuck; Heimbach, PatrickThe Greenland Ice Sheet rapidly lost mass over the last two decades, in part due to increases in ice loss from termini of large tidewater glaciers. Terminus melting and calving can drive glacier retreat and the pattern of ice sheet mass loss through reductions in resistive stresses near the glacier front and, in turn, increases in ice flow to the ocean. Despite their importance to ice sheet mass balance, factors controlling terminus positions are poorly constrained in ice sheet models, which fundamentally obscures sea level rise predictions. In this dissertation, I use a suite of novel observations and techniques to quantify controls on frontal ablation and terminus positions at tidewater glaciers in central west Greenland. Until recently, frontal ablation processes were obscured due to limited observations of submarine termini. Here, I use observations from multibeam echo sonar to show the morphological complexity of the submarine terminus face and identify previously unrecognized melting and calving processes. The terminus features numerous secondary subglacial plume outlets outside of the main subglacial channel system that drive and disperse large submarine melt rates across the glacier front. Submarine melting drives steep, localized terminus undercutting that can trigger calving by connecting to finely-spaced surface crevasses. In turn, large calving events cause the terminus face to become anomalously overcut. Incorporating observed outlet geometries in a numerical plume model, I estimate small subglacial discharge fluxes feeding secondary plume outlets that are reminiscent of a distributed subglacial network. Regional remote-sensing observations reveal that, for most glaciers in central west Greenland, seasonal terminus positions are more sensitive to glacial runoff than ice mélange or ocean thermal forcing. Shallow, serac-failing tidewater glaciers are most sensitive, where subglacial plumes melt the terminus and locally enhance retreat. Glaciers with large ice fluxes and deep termini retreat sporadically through full ice-thickness calving events less dependent on runoff. Together, these results provide process-oriented constraints on the shape of the submarine terminus face, the geometry of subglacial discharge and submarine melting, the influence of environmental forcing mechanisms and the impact that these variables have on terminus positions and dynamics in a warming climate.Item Elevation and volume change of the ice sheets from GLAS : a comparison of methods(2013-12) Felikson, Denis; Schutz, Bob E.This report compares surface elevation change and volume change esti- mates from three methods: repeat track (RT), crossover (CX), and overlapping footprints (OFP). These three methods use different approaches to group- ing elevation point measurements taken at different measurement epochs and estimating elevation change. Volume changes are calculated from elevation changes in the same manner for all three methods but differences in sampling resolution between the methods affect volume change estimates in different ways. The recently reprocessed Release 633 version of elevation measurements from the Geoscience Laser Altimeter System (GLAS), flown on the Ice, Cloud and land Elevation Satellite (ICESat), are used in this analysis. Both elevation changes and volume changes are compared for both the Greenland Ice Sheet (GrIS) and the Antarctic Ice Sheet (AIS). Additionally, uncertainties in the estimates for each method are quantified and compared. Results are separated by drainage systems and by above/below 2000 m surface elevation for the GrIS. For the AIS, results are aggregated to the East, West, and Penin- vi sula regions. Volume change estimates agree well for the three methods for the GrIS, with estimates of -227.75 ± 2.12 km³/yr, -249.30 ± 3.42 km³/yr, and -218.24 ± 7.39 km³/yr for the RT, CX, and OFP methods, respectively. These estimates are similar to those published from previous studies. For the AIS, however, larger discrepancies are found in the estimates. This stems primarily from a large discrepancy in the volume change estimate of the East AIS, where the RT, CX, and OFP methods estimate volume changes of 33.39 ± 1.42 km³/yr, 46.42 ± 5.46 km³/yr, and -2.72 ± 2.12 km³/yr, respectively. It's not entirely clear why this large discrepancy exists in this particular region, and elevation change estimates for a few particular drainage systems in this region are examined. Previously published volume changes for the AIS also show a large scatter and more work must be done to reconcile the various estimates. Finally, the volume change uncertainties reported do not completely account for the discrepancies in most regions. Additional analysis must be done to completely quantify all error sources.Item Flow re-organization of the East Antarctic ice sheet across glacial cycles(2017-09-14) Cavitte, Marie Genevieve P.; Blankenship, Donald D; Ghattas, Omar; Quinn, Terrence M; Sen, Mrinal K; Siegert, Martin J; Young, Duncan AConstraining the stability of the East Antarctic Ice Sheet (EAIS) over glacial-interglacial timescales is important to both understand its sea level contributions in the past and predict its future contributions in a warming world. Constraining how fast the EAIS can deliver ice to the ocean is as important as how much. Total volume fluctuations can be inferred through the use of nunatak exposure ages and ice core dating as well as through ice sheet modeling reconstructions of the Antarctic Ice Sheet as a whole. However, the EAIS’s ice volume fluctuations over long timescales such as 100-kyr glacial cycles and short spatial scales such as single ice flow catchment are less well known. I establish a method for dating internal reflections from ice-penetrating radar data between the Vostok and Dome C ice core sites, and determine the associated uncertainties in depth and age. I constrain the stability of two catchments of the EAIS through the use of internal stratigraphy from ice-penetrating radar, dated using correlated ice cores, combined with 1D ice flow models to reconstruct past accumulation rates. Here, I show that the ice catchment at the South Pole was highly active during the last glacial maximum while the ice dome/divide at Dome C was fairly stable during the entire last glacial cycle. Enhanced flow reaching South Pole implies the EAIS’s interior is much more susceptible to changes than previously thought. The absence of flow re-organization at Dome C for the last glacial maximum in contrast to South Pole shows that flow re-organization can vary from catchment to catchment. In addition, the stability of the Dome C region for the last 128 kyrs is highly promising for the retrieval of 1.5 million-year-old ice. 1D inversions of the deep radar isochrones interpreted above the subglacial relief of the Little Dome C (LDC) surface dome, ~30 km south of Dome C, predict several 1.5 million-year-old ice drilling sites. However, the complicated basal radar internal stratigraphy above the LDC and the presence of subglacial lakes complicate the task of choosing an ice core drill site. The EAIS-wide internal stratigraphy from the extensive modern ice-penetrating radar data now available over the EAIS have improved our understanding of its configuration and stability on multiple scales and timescales, and provide a foundation for understanding East Antarctica’s future role in global sea level change.Item Geometric controls on the inland extent of dynamic thinning for Greenland Ice Sheet outlet glaciers(2018-08-03) Felikson, Denis; Bettadpur, Srinivas Viswanath, 1963-; Catania, Ginny A.; Bui, Tan; Dawson, Clinton; Chen, JingyiThe Greenland Ice Sheet has been losing mass at an accelerating rate since 2003, in part due to changes in ice sheet dynamics. As ocean-terminating outlet glaciers retreat, they initiate thinning that diffuses inland, causing dynamic mass loss from the ice sheet interior. Although outlet glaciers have undergone widespread retreat during the last two decades, the inland extent of thinning and, thus, the mass loss is heterogeneous between glacier catchments. There remains a lack of a unifying explanation of the cause of this heterogeneity and accurately projecting the sea-level rise contribution from the ice sheet requires improvement in our understanding of what controls the upstream diffusion of thinning, initiated by terminus retreat. In this dissertation, I use observations and modeling to identify limits to the upstream diffusion of dynamic thinning for ocean-terminating glaciers draining the Greenland Ice Sheet. I start by using diffusive-kinematic wave theory to describe the evolution of thinning and I calibrate a metric that identifies how far upstream a thinning perturbation can diffuse from glacier termini. This metric is calculable from the observed glacier bed and surface topography and I use it to predict inland thinning limits for the majority of Greenland's outlet glaciers. I find that inland thinning limits often coincide with subglacial knickpoints in bed topography. These are steep reaches of the bed that are located at the transition between the portion of the bed that is below sea level and the upstream portion that is above sea level. I use the predicted thinning limits to help identify individual glaciers that have the largest potential to contribute to sea-level rise in the coming century. Finally, I use higher-order numerical modeling to validate the predicted thinning limits from the first-order kinematic wave model, and to investigate the timing and magnitude of glacier mass loss over the coming century. I find that glaciers that have small ice fluxes but are susceptible to thin far into the interior of the ice sheet have the potential to contribute as much to sea-level rise as their higher-flux counterparts. These lower-flux glaciers are often not discussed in literature but will be significant contributors to sea-level rise by 2100.Item Modes of deformation in ice in dynamic regions: applications to basal crevasses and calving(2015-12) Curry-Logan, Elizabeth Stacia; Lavier, Luc Louis; Catania, Ginny A.; Hesse, Marc; Ghattas, Omar; MacAyeal, DougCalving remains one of the most important yet unresolved aspects of glacier and ice sheet flow. Providing better constraints on global mean sea level rise will depend on our ability to simulate the dynamic flow of ice as it is discharged into the oceans. The work of this dissertation focuses on the important role basal crevasses play in the discharge of ice from glaciers and ice streams and how we can better model the formation and development of these features, particularly with regard to ice rheology during failure. First we make use of a large amount of ice penetrating radar data to image and understand the geometry and location of these features along the grounding line of the Siple Coast, in Antarctica. These data motivate the use of a thin-elastic beam approximation to the stresses that promote failure there, and the model is applied to all grounding lines across Antarctica, producing order-of-magnitude predictions where basal crevasses have already been observed. The simplicity of this model leads to the development of a more complex numerical model capable of visco-elasto-plastic simulation, DynEarthSol3D (DES), which performs the only time-dependent benchmark test designed for higher-order Stokes models. DES performs reasonably well against purely viscous numerical models and executes several experiments with idealized geometries exploring the roles that ice thickness and grounding line curvature play in the formation of basal crevasses in elastoplastic ice. Finally, with the implementation of a ductile-brittle transition zone based on longitudinal strain rate, we model the development of grounding line basal crevasses using visco-elasto-plastic rheology. Here we explore the roles that ice thickness and basal melting play in the formation and development of basal crevasses in ice as it is advected from resting on bedrock to floating in the ocean. We find that the inclusion of an extra measure of weakening to simulate the infiltration of buoyant ocean water in the basal crevasses is a crucial mechanism in developing the failure pattern seen in the floating portions of Thwaites Glacier and other glaciers around the world. The features that we simulate are truly semi-brittle, in that they require both viscous and elastic components of stress and a failure mechanism to develop.Item Ridges on martian debris-covered glaciers : deconvolving structural and climate processes(2017-05-12) Stuurman, Cassie; Holt, John W; Levy, Joe SDebris-covered glaciers on Earth and Mars often exhibit surface ridges transverse to the flow direction of the glacier. The formation mechanism of these ridges is not well established, but some evidence from terrestrial analogues supports the idea that surface ridges may demarcate climate cycles. It is also possible they are induced by thrust faulting, buckle folding, or other compressional mechanisms. This work aims to differentiate climate-related ridges from thrust fault and buckle folding ridges on Mars through mapping, geometric analysis, and flow modelling. We find that martian glaciers exhibit ridges of a variety of amplitudes and wavelengths. Large ridges on martian debris-covered glaciers cannot be explained by buckle folding given their long arclengths and inconsistent inter-ridge spacing, while some smaller ridges are consistent with the expected arclengths for buckle folds on Mars. The smaller ridges also exhibit highly consistent inter-ridge spacing and morphologies similar to terrestrial examples of two-layer buckle folds. Flow modelling results suggest that thrust faulting is a possible explanation for ridges on some debris covered glaciers, but thrust faulting diminishes as temperature decreases. We conclude that it some ridges on martian debris-covered glaciers are likely caused by climate variations however the buckle folding hypothesis is plausible for finely spaced ridges.Item Spatial and temporal evolution of the glacial hydrologic system of the western Greenland ice sheet : observational and remote sensing results(2015-12) Andrews, Lauren Cristy; Catania, Ginny A.; Cardenas, Meinhard Bayani; Hoffman, Matthew J.; Jackson, Charles S.; Mohrig, DavidThe Greenland Ice Sheet is losing mass at an accelerating rate due to a combination of increased surface melting and changes in dynamical behavior, both of which are associated with changing climate. In the ablation zone, seasonal melting results in a dynamic ice-sheet response as supraglacial meltwater reaches the ice–bed interface via moulins and crevasses. Meltwater delivery to the bed increases subglacial water pressure and decreases basal traction, leading to regional ice acceleration. However, these processes and their future evolution are poorly constrained. An improved understanding of the complex relationship between the glacial hydrologic system and ice velocity will ultimately improve predictions of ice-sheet mass change. In this dissertation, I use a suite of techniques to quantify the response of the glacial hydrologic system to changes in melt supply on daily to inter-decadal timescales. Moulins represent the primary englacial connection between the ice surface and its bed. As such, they play a critical role in determining the location of subglacial channels in the ablation zone. I observe inter-decadal persistence in moulin locations, which can result in positive feedbacks that allow for rapid growth at the onset of the melt season and encourage persistence of subglacial channels. These observations suggest that inter-decadal variability in the relationship between supraglacial melt production and ice velocity is caused by altering the rate at which efficient subglacial drainage pathways develop. Further, my observations indicate that daily changes in ice velocity are mirrored by moulin water levels, but this pattern does not hold at seasonal timescales. This relationship suggests that the channelized portion of the subglacial hydrologic system adjusts rapidly to the available meltwater; therefore, long-term trends in ice velocity are the result of increasing hydrologic connectivity of poorly connected regions of the bed, lowering regional subglacial water pressure. Finally, the subglacial hydrologic system experiences variability on multiple timescales, some of which are not accounted for in existing models of this system. By modeling the mechanisms causing both diurnal to seasonal and changes in moulin water level, I further constrain the physical processes impacting mass change in land-terminating regions of the Greenland Ice Sheet.Item The ice content and internal structure of candidate debris-covered glaciers on mars and earth : insights from radar sounding(2018-09-13) Petersen, Eric Ivan; Holt, John W., Ph. D.; Levy, Joseph S.; Catania, Ginny; Grima, Cyril; Mohrig, DavidMartian lobate debris aprons are enigmatic mid-latitude landforms known to contain a significant fraction of water ice preserved at depth beneath a surface debris layer. They are thought to be important records of climate history and potential water resources for manned missions to Mars. However, their internal structure remains poorly constrained and regional variability in their ice purity is unknown. In this dissertation we report on a regional orbital radar sounding survey of lobate debris aprons in Deuteronilus Mensae – the region of highest concentration of lobate debris aprons on Mars – to constrain trends in lobate debris apron composition and possible internal structure. We also present a geophysical survey of Galena Creek Rock Glacier to constrain its internal structure as an analog to Martian lobate debris aprons. We found that the majority of radar observations imaged a basal reflector, from which we determined that the apron body is composed of material with dielectric properties consistent with relatively pure water ice and that there is no evidence for region-wide variability. Combining our compositional results with apron volumes constrained by Levy et al. (2014) sets the regional ice budget at 0.9-1.0 x 10⁵km³, the equivalent of roughly 4x the combined volume of water in the Great Lakes. We additionally showed that non-detection of basal reflectors in 13% of the observations may be attributed to high apron thickness and surface roughness-induced signal loss. In our analog work on Galena Creek Rock Glacier, we imaged its internal structure consisting of a network of englacial debris layers. This internal structure is indicative of intermittent debris and ice accumulation, with debris fall potentially playing a role in enhancing and facilitating ice accumulation. Similar englacial debris layers may exist in Martian lobate debris aprons, but are not imaged by the available orbital radar dataset due to their dip and thickness.Item West Antarctic Ice Sheet retreat during the Last Interglacial(2018-05-02) Muldoon, Gail Ruth; Jackson, Charles S., doctor of geophysical science; Blankenship, Donald D.; Ghattas, Omar; Heimbach, Patrick; Quinn, Terrence M; Young, Duncan AThe Last Interglacial (116 ka - 130 ka) is the most recent time when Earth's climate was as warm or warmer than it is today. It therefore may make a suitable proxy for understanding the impacts of modern climate change. One such impact of immediate relevance to the modern world is that of sea level rise. Global sea level is currently rising at an accelerating pace, threatening lives and economies around the world. Notably, evidence from paleoclimate data suggests global sea level during the Last Interglacial was at least 6.6 m higher than present sea level and perhaps more than 8 m higher. As the Earth adjusts to the rapid onset of modern climate change, we may expect sea level to approach that of the Last Interglacial. Noticeable changes in sea level have a number of sources, including melting glaciers, ice sheets, and ocean thermal expansion. The Antarctic Ice Sheet has been inferred to be the largest contributor to Last Interglacial sea level change, adding between 4.1 and 5.8 m to global sea level during that time. Most of this change is expected to come from the West Antarctic Ice Sheet, which is thought to be prone to marine ice sheet instability. However, uncertain basal boundary conditions and ocean forcing make it a challenge to know how the ice sheet may have lost such a large amount of mass. In this work, I seek to better constrain mass balance and sea level contributions of the Antarctic Ice Sheet during the Last Interglacial and explore evidence from inside the ice sheet itself which may reveal if and how the ice sheet sustained such a large mass balance change during the Last Interglacial. To do so, I use a transient ice sheet model to simulate reconfiguration of the Antarctic Ice Sheet under Last Interglacial conditions and find a stable configuration consistent with estimates of Antarctic contributions to Last Interglacial sea level. I then analyze englacial radar stratigraphy to study the ice dynamics by dating a series of isochronous englacial reflectors which have been mapped through the central West Antarctic Ice Sheet. For comparison to this data, I implement a steady state ice sheet model to simulate englacial isochronous surfaces with various uncertain model boundary conditions.