ORCID Profile
0000-0001-5865-3459
Current Organisation
University of St Andrews
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Publisher: American Geophysical Union (AGU)
Date: 09-01-2022
DOI: 10.1029/2021GL096244
Abstract: We introduce a transfer matrix model for radio‐wave propagation through layered anisotropic ice that permits an arbitrary dielectric permittivity tensor in each layer. The model is used to investigate how crystal orientation fabrics without a vertical principal direction affect polarimetric radar returns over glaciers and ice sheets. By expanding the c ‐axis orientation distribution in terms of a spherical harmonic series, we find that radar returns from synthetic fabric profiles are relatively insensitive to the harmonic mode responsible for a nonvertical principal direction however, only for normally incident waves. Consequently, the strength of this mode might be relatively difficult to infer in glaciers and ice sheets, which in turn has implications for the ability to determine the full second‐order structure tensor, needed to infer the local flow regime, flow history, or to represent the directional viscosity structure of glacier ice for ice‐flow modeling.
Publisher: American Geophysical Union (AGU)
Date: 2019
DOI: 10.1029/2018JF004821
Publisher: American Geophysical Union (AGU)
Date: 02-2018
DOI: 10.1002/2017JF004529
Publisher: American Geophysical Union (AGU)
Date: 05-2021
DOI: 10.1029/2020JF006023
Abstract: In airborne radargrams, undulating periodic patterns in litude that overprint traditional radiostratigraphic layering are occasionally observed, however, they have yet to be analyzed from a geophysical or glaciological perspective. We present evidence supported by theory that these depth‐periodic patterns are consistent with a modulation of the received radar power due to the birefringence of polar ice, and therefore indicate the presence of bulk fabric anisotropy. Here, we investigate the periodic component of birefringence‐induced radar power recorded in airborne radar data at the eastern shear margin of Thwaites Glacier and quantify the lateral variation in azimuthal fabric strength across this margin. We find the depth variability of birefringence periodicity crossing the shear margin to be a visual expression of its shear state and its development, which appears consistent with present‐day ice deformation. The morphology of the birefringent patterns is centered at the location of maximum shear and observed in all cross‐margin profiles, consistent with predictions of ice fabric when subjected to simple shear. The englacial fabric appears stronger inside the ice stream than outward of the shear margin. The detection of birefringent periodicity from non‐polarimetric radargrams presents a novel use of subsurface radar to constrain lateral variations in fabric strength, locate present and past shear margins, and characterize the deformation history of polar ice sheets.
Publisher: Copernicus GmbH
Date: 27-08-2021
Abstract: Abstract. The crystal orientation fabric (COF) of ice sheets records the past history of ice sheet deformation and influences present-day ice flow dynamics. Though not widely implemented, coherent ice-penetrating radar is able to detect bulk anisotropic fabric patterns by exploiting the birefringence of ice crystals at radar frequencies, with the assumption that one of the crystallographic axes is aligned in the vertical direction. In this study, we conduct a suite of quad-polarimetric measurements consisting of four orthogonal antenna orientation combinations near the Western Antarctic Ice Sheet (WAIS) Divide ice core site. From these measurements, we are able to quantify the azimuthal fabric asymmetry at this site to a depth of 1400 m at a bulk-averaged resolution of up to 15 m. Our estimates of fabric asymmetry closely match corresponding fabric estimates directly measured from the WAIS Divide ice core. While ice core studies are often unable to determine the absolute fabric orientation due to core rotation during extraction, we are able to identify and conclude that the fabric orientation is depth-invariant to at least 1400 m, equivalent to 6700 years BP (years before 1950) and aligns closely with the modern surface strain direction at WAIS Divide. Our results support the claim that the deformation regime at WAIS Divide has not changed substantially through the majority of the Holocene. Rapid polarimetric determination of bulk fabric asymmetry and orientation compares well with much more laborious s le-based COF measurements from thin ice sections. Because it is the bulk-averaged fabric that ultimately influences ice flow, polarimetric radar methods provide an opportunity for its accurate and widespread mapping and its incorporation into ice flow models.
Publisher: American Geophysical Union (AGU)
Date: 02-2018
DOI: 10.1002/2017JF004297
Publisher: American Association for the Advancement of Science (AAAS)
Date: 14-05-2021
Abstract: High-resolution observations from a 1043-m-deep borehole show highly variable ice properties and heterogeneous deformation.
Publisher: Proceedings of the National Academy of Sciences
Date: 22-02-2022
Abstract: Subglacial drainage systems control ice sheet flow and the quantity of ice discharged into the ocean. However, these systems are currently poorly characterized, from a lack of direct observations. This shortcoming is problematic, as changes in drainage systems can result in a markedly differently ice sheet response. Here, we present a radar-derived record of basal melt rates with colocated borehole observations, showing unexpectedly warm subglacial conditions beneath a large outlet glacier in West Greenland. The record is unprecedented because the observed basal melt rates are several orders of magnitude higher than predictions and previous estimates. Our observations show that the effect of viscous dissipation from surface meltwater input is by far the largest heat source beneath the Greenland Ice Sheet.
Publisher: American Geophysical Union (AGU)
Date: 07-10-2018
DOI: 10.1029/2018GL079787
Abstract: The delivery of surface meltwater through englacial drainage systems to the bed of the Greenland Ice Sheet modulates ice flow through basal lubrication. Recent studies in Southeast Greenland have identified a perennial firn aquifer however, there are few observations quantifying the input or residence time of water within the englacial system and it remains unknown whether water can be stored within solid ice. Using hourly stationary radar measurements, we present observations of englacial and episodic subglacial water in the ablation zone of Store Glacier in West Greenland. We find significant storage of meltwater in solid ice damaged by crevasses extending down to 48 m below the ice surface during the summer, which is released or refrozen during winter. This is a significant hydrological component newly observed in the ablation zone of Greenland that could delay the delivery of meltwater to the bed, changing the ice dynamic response to surface meltwater.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-2178
Abstract: Glaciers and ice streams account for the majority of ice mass discharge to the ocean from the Antarctic Ice Sheet, and are bounded by intense bands of shear that separate fast-flowing from slow or stagnant ice, called shear margins. The anisotropy of glacier ice (i.e. a preferred crystal orientation) stemming from high rates of shear at these margins can greatly facilitate fast streaming ice flow, however it is still poorly understood due to a lack of in-situ measurements. If anisotropy is incorporated into numerical ice sheet models at all, it is usually as a simple scalar enhancement factor that represents the "flow law" that governs the model's rheology. Ground-based and airborne radar observations along two transects fully crossing the Eastern Shear Margin of Thwaites Glacier reveal rapid development of highly anisotropic fabric tightly concentrated around a lateral maximum in surface shear strain. These measurements of fabric strength at the centre of the shear margin are indicative of a horizontal pole configuration, which potentially represents ice that is & #8220 softened& #8221 to shearing in some directions and hardened in others. The resulting flow enhancement revealed by our results suggest that the viscosity of ice is highly variable and regime-dependent, and supports the importance of considering anisotropic flow laws to model the rheology of ice sheets.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-16308
Abstract: The stability of Thwaites Glacier, the second largest marine ice stream in West Antarctica, is a major source of uncertainty in future predictions of global sea level rise. Critical to understanding the stability of Thwaites Glacier, is understanding the dynamics of the shear margins, which provide important lateral resistance that counters basal weakening associated with ice flow acceleration and forcing at the grounding line. The eastern shear margin is of interest, as it is poorly topographically constrained, meaning it could migrate rapidly, causing further ice flow acceleration and drawing a larger volume of ice into the fast-flowing ice stream.& We present initial insights from a 2-year-long seismic record, from two broadband seismic arrays each with 7 stations, deployed across the eastern shear margin of Thwaites Glacier. We have applied a variety of processing methods to these data to detect and locate icequakes from different origins and analyse them in the context of shear-margin dynamics. Preliminary results suggest there is basal seismicity concentrated near the ice-bed interface on the slow-moving side of the margin, as opposed to within the ice-stream itself. Some of the identified seismic events appear to exhibit clear shear-wave splitting, suggesting a strong anisotropy in the ice, which would be consistent with polarization observed in recently published radar studies from the field site. Further analysis of the split shear-waves will allow us to better constrain the region's ice-fabric, infer past shear-margin location, and assess the future stability of this ice rheology. & With such a large quantity of data, manual event identification is unpractical, and hence we are employing machine-learning approaches to identify and locate icequakes of interest in these data. Our results and forthcoming results from upcoming active-seismic field seasons have important implications for better understanding the stability of glacier and ice stream shear margins.&
Publisher: Copernicus GmbH
Date: 03-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-2107
Abstract: & & Glaciers and ice streams channel the majority of ice mass discharge into the ocean, and are modulated by basal slip at the ice-bed interface, deformation within the ice interior, and lateral shear at the margins separating fast- and slow-moving ice. The anisotropy of glacier ice (i.e. ice that deforms preferentially in certain modes and directions) at shear margins greatly facilitates streaming ice, however it is still poorly understood due to a lack of in-situ measurements and is usually incorporated into models as a simple scalar enhancement factor. The resurgence of polarimetric radar techniques to detect bulk fabric anisotropy through exploiting the birefringence of ice crystals has greatly aided quantification of the ice crystal orientation fabric (COF) across the Antarctic Ice Sheet. In our study, we invert these techniques to infer the azimuthal fabric strength at the Eastern Shear Margin of Thwaites Glacier from non-polarimetric airborne radargrams collected during the 2018-19 field season. From these results, we infer the evolution of the crystal orientation fabric across the shear margin, where ice is subjected to varying levels of both pure and simple shear. Our findings suggest the potential of the upper reaches of the ESM having undergone recent inward migration. Together with compatible ground-based polarimetric radar experiments, our study highlights the potential of radar sounding to observe and infer variations in fabric strength from regions of complex flow at multiple spatial scales. Because the bulk COF of ice sheets records the past history of ice sheet deformation and influences present-day ice flow dynamics, accurate measurements of ice fabric strength and orientation not only places constraints on present and past ice flow history, but also aids in the incorporation of anisotropic rheology in ice flow models.& &
Publisher: Copernicus GmbH
Date: 28-09-2020
DOI: 10.5194/TC-2020-264
Abstract: Abstract. The Crystal Orientation Fabric (COF) of ice sheets records the past history of ice sheet deformation and influences present-day ice flow dynamics. Though not widely implemented, coherent ice-penetrating radar is able to detect anisotropic COF patterns by exploiting the birefringence of ice crystals at radar frequencies. Most previous radar studies quantify COF at a coarse azimuthal resolution limited by the number of observations made with a pair of antennas along an acquisition plane that rotates around an azimuth centre. In this study, we instead conduct a suite of quad-polarimetric measurements consisting of four orthogonal antenna orientation combinations at the Western Antarctic Ice Sheet (WAIS) Divide Deep Ice Core site. From these measurements, we are able to quantify COF at this site to a depth of 1500 m at azimuthal and depth resolutions of up to 1° and 15 m. Our estimates of fabric asymmetry closely match corresponding fabric estimates directly measured from the WAIS Divide Deep Ice Core. While ice core studies are often unable to determine the absolute fabric orientation due to core rotation during extraction, we are able to unambiguously identify and conclude that the fabric orientation is depth-invariant to at least 1500 m, equivalent to 7400 years BP (years before 1950), and coincides exactly with the modern surface strain direction at WAIS Divide. Our results support the claim that the deformation regime at WAIS Divide has not changed substantially through the majority of the Holocene. Rapid polarimetric determination of bulk COF compares well with much more laborious s le-based COF measurements from thin ice sections. Because it is the former that ultimately influences ice flow, these polarimetric radar methods provide an opportunity for accurate and widespread mapping of bulk COF and its incorporation into ice flow models.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Tun Jan Young.