The following abstracts come from the linked sources and are relevant to the Thwaites Glacier:
www.igsoc.org/symposia/2013/kansas/proceedings/procsfiles/procabstracts_63.htmContact: Secretary General, International Glaciological Society
67A030
Sensitivity of Thwaites Glacier to ice-shelf melting
Ian JOUGHIN, Ben SMITH
Corresponding author: Ian Joughin
Corresponding author e-mail: ian@apl.washington.edu
Strong thinning as ice streams have sped up along the Amundsen coast produces ice loss well in excess of that from other regions of Antarctica. Much of the increases in speed appear to be caused by the loss of buttressing as ice shelves have thinned in response to warmer ocean water and subsequent loss of basal traction as the grounding line has retreated. We have developed a finite-element implementation of a prognostic shallow-shelf ice-stream/shelf model, which we have applied to Thwaites Glacier, Antarctica. The model uses an improved bed map from data recently acquired as part of operation IceBridge. We have conducted a number of numerical tests to examine the sensitivity of the glacier to increased melting and surface accumulation. For melt rates comparable with present, the glacier continues to lose mass at roughly its present rate. Strong sub-shelf melt produces stepped retreat of the grounding line by >40 km over 250 years. Examination of the annual thinning rates shows rapid evolution of the spatial distribution of loss over periods of several years (i.e. comparable in length to a typical satellite altimetry mission). In particular, with each episode of grounding-line retreat, a pattern of strong thinning initially develops near the grounding line that then diffuses inland over periods of several years. Only with increased surface accumulation and reduced melting does the glacier stabilize. Thus, it is likely that Thwaites Glacier will continue to lose mass over the next several centuries at a rate largely determined by the amount of warm circumpolar deep water that makes its way to near the grounding line.
67A005
The implications of reflector geometry on radar data acquisition
Nicholas HOLSCHUH, Sridhar ANANDAKRISHNAN, Knut CHRISTIANSON
Corresponding author: Nicholas Holschuh
Corresponding author e-mail: ndh147@psu.edu
The structure of internal layers in ice sheets is used to interpret ice-sheet flow dynamics. The goal of radio-echo sounding is to accurately reproduce that layer geometry. Radar data from Thwaites Glacier and the northeast Greenland ice stream (NEGIS) show that layers whose dip angle exceeds a threshold do not produce a coherent signal in the data. This is likely due to destructive interference in trace stacking and off-nadir backscatter. Reduction of signal amplitude due to destructive interference in stacking is a function of radar center frequency, reflector dip angle and stacked trace spacing. As the stacked trace spacing increases over a dipping horizon, the phase difference between component pre-stack traces increases, resulting in a less coherent stack. Airborne data are more prone to this signal loss given the higher velocity acquisition platform. In addition to destructive interference in stacking, dipping reflectors sample off-nadir portions of the antenna radiation pattern, reducing the signal recorded by the receiver. Imaging reflectors from wide angles also results in longer englacial travel times and thus additional englacial attenuation relative to horizontal reflectors at comparable depths. Both of these effects lead to further reduction in reflection amplitude. Here we use signal amplitudes to interpolate the slope field of the internal layers and reconstruct layer geometries in radar data from Thwaites Glacier and NEGIS. Our results show that it is possible to infer layer angle with reasonable uncertainty for most dip angles and thereby also provide useful data on current/past stress state and the basal properties responsible for internal layer folding even when layers are not directly imaged."
67A033
Flow history of Thwaites Glacier inferred from radar-detected flowlines and flowbands
T.J. FUDGE, H. CONWAY, G. CATANIA, D. BLANKENSHIP, K. CHRISTIANSON, I. JOUGHIN, S. KEMPF, D. YOUNG
Corresponding author: T.J. Fudge
Corresponding author e-mail: tjfudge@uw.edu
Patterns in radar-detected internal layers in glaciers and ice streams can often be tracked several hundred kilometers downstream from their origin. Here we use distinctive patterns detected in the onset region of Thwaites Glacier in the Amundsen Sea sector of West Antarctica to delineate flowlines and flowbands. Flowbands in the onset region contain information about flow over the past 700 years, which is the approximate time for ice to flow along the flowband. Our analysis of flow conditions over century scales gives perspective on recent changes observed on Thwaites Glacier. Along the eastern margin, flow measured with GPS between 2009 and 2010 is rotated outward by about 1° compared with the long-term flow direction. However, such small rotation is within the directional uncertainty of the long-term flow (about 3°); it is not clear that this apparent outward rotation is a response to changes at the grounding line. We use two radar-detected flowlines to define a 110 km flowband in the middle tributary. The ratio of fluxes through gates at the downstream and upstream ends of the flowband is calculated from continuity for a range of values for past thinning and accumulation rate along the flowband. For comparison, we use InSAR-derived surface velocities (from 1996) and estimates of accumulation rate, to define the geometry of the present-day flowband and to calculate the present-day thinning rate and flux ratio. The geometry of the modern flowband is closely similar to the long-term average, but the flux ratio is higher than the long-term average. The simplest explanation for the change is that the modern rate of thinning along the flowband (about 0.52 m a–1) is larger than the long-term average. The method does not allow us to determine when in the past 700 years the rate of thinning increased.
67A036
Buried information: constraining bed geometry and material from the Doppler-dependent radar-scattering function
Dustin M. SCHROEDER, Donald D. BLANKENSHIP, R. Keith RANEY, Duncan A. YOUNG
Corresponding author: Dustin M. Schroeder
Corresponding author e-mail: dustin.m.schroeder@utexas.edu
The morphological, lithological and hydrological basal boundary conditions of ice sheets and glaciers can exert strong, even dominating, control on their behavior, evolution and stability. However, the scales at which the physical processes and observable signatures of this control occur are typically smaller than the spatial resolutions achievable using ice-penetrating radar. Further, the strength of calibrated radar bed echo returns is a combination of both the material (i.e. relative permittivity, conductivity) and geometric (i.e. rms height, rms slope, auto-correlation length) properties of the ice–bed interface. This ambiguity in the relative contribution of material and geometric bed properties, along with uncertainty in englacial attenuation from underconstrained ice temperature and chemistry, also makes definitive assessment of basal conditions from echo strengths extremely difficult. To address these challenges in interpreting geometric and material bed properties at glaciologically relevant scales, we present a new algorithmic approach to measuring the radar-scattering function of the ice–bed interface with varying Doppler frequency by performing range-migrated SAR focusing using multiple reference functions spanning different ranges of Doppler frequencies from the bed. We parameterize this scattering function in terms of the relative contribution of angularly narrow specular energy and isotropically scattered diffuse energy. This specularity content of the bed echo is insensitive to englacial attenuation and is a measure of both the angular distribution of returned echo energy and the geometry of the ice–bed interface at the sub-azimuth-resolution scale. We present an application of this technique to a gridded airborne radar survey over the entire catchment of Thwaites Glacier, West Antarctica. We show how the information in the along-track scattering function of the bed can be used to assess the extent and configuration of distributed water across the catchment and detect the transition of the water system from distributed canals to concentrated channels. We also show how this information can be used to constrain the morphology of basal bedforms and infer the distribution of deformable sediments and exposed bedrocks across the catchment. These applications demonstrate the potential to extract rich information from focusable radar-sounding data to constrain the radar-scattering function as well as the material and geometric properties of the bed.
67A040
Firn variability derived from a statistical analysis of airborne ice-penetrating radar over the Thwaites Glacier catchment, West Antarctica
Cyril GRIMA, Dustin M. SCHROEDER, Don D. BLANKENSHIP, Duncan A. YOUNG
Corresponding author: Cyril Grima
Corresponding author e-mail: cyril.grima@gmail.com
A dry firn layer covers most of the Antarctic ice sheet. Firn characteristics are a function of accumulation rate, air temperature and surface winds. As such, they are indicators of ice-sheet accumulation history and mass balance. To date, most of the observational techniques for firn characterization at depths of a meter or more achieve limited geographical coverage (i.e. ice/firn cores, ground-based GPR). During the aerogeophysical campaign of the 2004/05 austral summer the Airborne Geophysical Survey of the Amundsen Sea Embayment, Antarctica (AGASEA) project surveyed a 15 km grid over a 600 km &mult; 400 km area covering the Thwaites Glacier catchment, West Antarctica, with the High-Capability Radar Sounder (HiCARS) system operated by the University of Texas Institute for Geophysics (UTIG) onboard a de Havilland DHC-6 Twin Otter aircraft. The HiCARS system transmits pulses with a 60 MHz (λ = 5 m) central frequency that are chirped over a 15 MHz bandwidth and 8000 W peak power. One resulting data product is a calibrated radar dataset sampled every ~10 m along the survey tracks that have been coherently integrated and range compressed. In this study, we applied a statistical method to the surface echo in order to separate the coherent (specular) and incoherent (scattered) parts of the signal. We use these estimated components with a backscattering model to derive and map the roughness and real part of the surface permittivity. The resulting permittivity values reflect the physical properties of the first 5 m of the firn. We analyze these results in the context of firn density and/or possibly wetness spatial variability. We observe a ~30 km wide vein of high surface permittivities ~100 km inward from the coastline with a northern boundary that matches a prominent slope break for the surface. We discuss the implications of our results for formation climatological context of catchment-wide firn properties in general and the high-permittivity vein in particular.
67A041
Constraining the recent sea-level contributions of Pine Island and Thwaites Glaciers, West Antarctica, using CReSIS ultra-wideband airborne radar systems
Brooke MEDLEY, Ian JOUGHIN, Sarah B. DAS, Eric J. STEIG, Howard CONWAY, Sivaprasad GOGINENI, Alison S. CRISCITIELLO, Joseph R. McCONNELL, Ben E. SMITH, M. R. VAN DEN BROEKE, J.T.M. LENAERTS, D.H. BROMWICH, J. P. NICOLAS
Corresponding author: Brooke Medley
Corresponding author e-mail: bmed@u.washington.edu
One of the largest sources of uncertainty in quantifying the glacial contribution to sea-level rise originates from our lack of understanding of spatio-temporal snow accumulation rates. Traditional in situ measurements of the accumulation rate (i.e. using firn cores, snow pits and stake farms) are time-consuming and inadequately capture the complex spatial variations in regional accumulation. We use ultra-wideband airborne radar data to track near-surface internal horizons to calculate spatio-temporal accumulation rates over Pine Island and Thwaites Glaciers along the Amundsen coast of West Antarctica. Here, we combine data from both CReSIS snow and accumulation radar systems to generate a spatially complete high-resolution gridded map of mean accumulation rate, thereby constraining the total mass input into these dynamic glaciers over the past 25 years. We furthermore find the snow radar is capable of imaging annual horizons, an improvement over the multi-year resolution available using the accumulation radar system. Based on the annual accumulation rates generated from the snow-radar echograms, we find no significant trend in the accumulation rate over much of Thwaites Glacier. These data indicate that the recent substantial increase in Thwaites ice discharge to the ocean has not been balanced inland by additional snow accumulation. This suggests the Thwaites contribution to sea-level rise has increased over the past few decades as regional accumulation rates have not increased to offset the accelerating discharge of this glacier.
67A074
How well can we determine ice thickness? Examples from Thwaites Glacier
Duncan A. YOUNG, Donald D. BLANKENSHIP, Scott D. KEMPF, Chad A. GREENE
Corresponding author: Duncan A. Young
Corresponding author e-mail: duncan@ig.utexas.edu
Ice-sheet models increasingly require high-resolution ice thickness and topographic data to resolve basal hydrology and internal stress fields. Additionally, new technologies for sampling the bed (e.g. RAID) will require good understanding of bedrock topography. Our primary tool for ice thickness determination has been airborne ice-penetrating radar. A variety of different systems have been fielded over ice sheets, with variations in center frequency, power, range, cross track and azimuth resolutions. Given the expense of fielding airborne campaigns, we need to be able to assess the resolutions and uncertainties that can be retrieved with through both legacy datasets and new systems, to target campaigns appropriately. Bed uncertainty quantification for ice-sheet models requires an evaluation of the spatial distribution of uncertainty in the ice thickness data upon which they rely. Ice thickness uncertainties are dominated by errors caused by cross-track reflectors, which bias thickness measurements low. Cross-track uncertainty is anisotropic, meaning that determinations of cross-over uncertainty do not capture our full knowledge of the bed. The grounding zone of Thwaites Glacier in West Antarctica is an area of fast and changing ice; ice flow is fast and the bed is rough. It has been a target of data acquisition both by the AGASEA program of 2004–05 and Operation IceBridge (OIB) between 2008 and 2012. AGASEA fielded a 60 MHz, 15 MHz bandwidth coherent system on a Twin Otter flying at 60 m s–1. OIB fielded a 195 MHz, 10 MHz system on a DC-8 flying at 130 m s–1. Both systems had broad cross-track beam patterns. The surveys were designed to interleave over the grounding-zone region, with one line reflown for intercomparison purposes. Over deeper ice we also have incoherent data from the 1990s with much less along-track resolution, but tighter line spacing. We evaluate the along-track repeatability and orthogonal cross-overs of these three surveys and construct a model for sensor-based uncertainty as a function of basal roughness and sensor configuration.
67A075
Joint seismic- and radar-sounding analysis of the subglacial environment of upper Thwaites Glacier, West Antarctica
Leo E. PETERS, Joseph A. MacGREGOR, Sridhar ANANDAKRISHNAN, Anthony HOCH, Huw J. HORGAN
Corresponding author: Leo E. Peters
Corresponding author e-mail: lep144@psu.edu
Thwaites Glacier is one of the fastest and largest glaciers draining the West Antarctic ice sheet. While much attention has been given to recent retreat, thinning and acceleration near its grounding line, little is known of the subglacial environment of Thwaites Glacier farther inland and how ice dynamics there might respond to coastal changes. Here we present both ground-based seismic- and radar-sounding surveys from upper Thwaites Glacier, characterizing the subglacial environment and its influence upon ice dynamics. During the 2008–2009 Antarctic field season, we collected 60 km of seismic data and 440 km of radar data ~200 km inland of Thwaites Glacier grounding line. These coincident surveys extend 40 km along flow and 10 km across flow. We find large variability in the subglacial environment, even in this slow-flowing region of the glacier (<200 m a–1), with distinct regions of wet unconsolidated sediments and potentially lithified dewatered sediments at the bed. Some of the brightest bed reflections in the radar data are observed across seismically inferred lithified beds, suggesting that in regions where bed roughness varies significantly, bright radar reflections are not indicative exclusively of wet ice-sheet beds. Modeled basal shear stress, seismically inferred basal conditions and radar-inferred small-scale bed roughness are all correlated. Our observations will allow modelers to better conceptualize the subglacial environment and to predict how Thwaites Glacier will respond to ongoing perturbations in ice flow originating near the grounding line.