scholarly journals Polarimetric radar reveals the spatial distribution of ice fabric at domes in East Antarctica

2021 ◽  
Author(s):  
M. Reza Ershadi ◽  
Reinhard Drews ◽  
Carlos Martín ◽  
Olaf Eisen ◽  
Catherine Ritz ◽  
...  
Keyword(s):  
Spatial Scales ◽  
Ice Core ◽  
Radar Data ◽  
Horizontal Distribution ◽  
Polarimetric Radar ◽  
Ice Flow ◽  
Orientation Tensor ◽  
Flow Models ◽  
Non Linear ◽  
Almost All

Abstract. Ice crystals are mechanically and dielectrically anisotropic. They progressively align under cumulative deformation, forming an ice crystal orientation fabric that, in turn, impacts ice deformation. However, almost all the observations of fabric are from ice core analysis and its interplay with the flow is unclear. Here, we present a non-linear inverse approach that combines radar polarimetry with vertical changes in anisotropic reflection to extract, for the first time, the full orientation tensor. The orientation tensor is routinely used to synthesize fabric information and it is used in anisotropic ice flow models. We validate our approach at two Antarctic ice-core sites (EDC and EDML) in contrasting flow regimes. Spatial variability of ice-fabric characteristics in the dome-to-flank transition near Dome C is quantified with 20 more sites located along a 36 km long cross-section. Local horizontal anisotropy increases under the dome summit and decreases away from the dome summit. We suggest that this is a consequence of the non-linear rheology of ice also known as Raymond effect. On larger spatial scales, horizontal anisotropy increases with increasing distance from the dome. At most of the sites, the main driver of ice-fabric evolution is vertical compression, yet our data show that ice fabric horizontal distribution is consistent with the present horizontal flow. Our method, which uses co- and cross polarimetric radar data suitable for profiling radar applications, can constrain ice-fabric distribution on a spatial scale comparable to ice flow observations and models.

Polarimetric radar-sounding to infer and quantify shear margin ice fabric anisotropy

2021 ◽  
Author(s):  
Tun Jan Young ◽  
Thomas M Jordan ◽  
Carlos Martín ◽  
Dustin M Schroeder ◽  
Poul Christoffersen ◽  
...  
Keyword(s):  
Crystal Orientation ◽  
Past History ◽  
Spatial Scales ◽  
Ice Sheet ◽  
Polarimetric Radar ◽  
Fabric Anisotropy ◽  
Complex Flow ◽  
Ice Flow ◽  
Flow Models ◽  
Fabric Strength

<p>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.</p>

scholarly journals Rapid and accurate polarimetric radar measurements of ice crystal fabric orientation at the Western Antarctic Ice Sheet (WAIS) Divide ice core site

2021 ◽  
Vol 15 (8) ◽  
pp. 4117-4133
Author(s):  
Tun Jan Young ◽  
Carlos Martín ◽  
Poul Christoffersen ◽  
Dustin M. Schroeder ◽  
Slawek M. Tulaczyk ◽  
...  
Keyword(s):  
Past History ◽  
Ice Core ◽  
Ice Sheet ◽  
Vertical Direction ◽  
Surface Strain ◽  
Polarimetric Radar ◽  
Ice Flow ◽  
Antarctic Ice Sheet ◽  
Flow Models ◽  
Core Site

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 sample-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.

scholarly journals Rapid and accurate polarimetric radar measurements of ice crystal fabric orientation at the Western Antarctic Ice Sheet (WAIS) Divide deep ice core site

2020 ◽  
Author(s):  
Tun Jan Young ◽  
Carlos Martín ◽  
Poul Christoffersen ◽  
Dustin M. Schroeder ◽  
Slawek M. Tulaczyk ◽  
...  
Keyword(s):  
Past History ◽  
Ice Core ◽  
Ice Sheet ◽  
Surface Strain ◽  
Polarimetric Radar ◽  
Ice Flow ◽  
Antarctic Ice Sheet ◽  
Flow Models ◽  
Core Site ◽  
Core Rotation

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 sample-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.

Relating polarimetric radar measurements of ice fabric to ice-flow enhancement of Rutford Ice Stream

2020 ◽  
Author(s):  
Thomas Jordan ◽  
Alex Brisbourne ◽  
Carlos Martin ◽  
Rebecca Schlegel ◽  
Dustin Schroeder ◽  
...  
Keyword(s):  
Polarimetric Radar ◽  
Ice Streams ◽  
Ice Flow ◽  
Near Surface ◽  
Flow Models ◽  
Relative Contribution ◽  
Ice Stream ◽  
Glacier Ice ◽  
Flow Enhancement ◽  
The Impact

<p>Lateral shear margins provide resistance to ice flow within ice streams and play an important role in the overall dynamics of ice sheets. The strength and location of shear margins are known to be influenced by both subglacial factors (e.g. bed roughness, meltwater availability) and ice rheology (ice temperature, ice fabric, and damage). Assessing the relative contribution of these factors upon ice-stream flow is complex but can be aided by geophysical measurements (e.g. radar-sounding and seismic imaging) of the ice-stream subsurface. There are, however, ongoing challenges in obtaining geophysical information in an appropriate form to be incorporated into ice-flow models. This is true of ice fabric, and its direction-dependent effect upon ice viscosity is typically neglected in models of ice streams.</p><p>Here we develop a framework to relate ice fabric measurements from polarimetric radar sounding to ice-flow enhancement within ice streams. First, we extend a `polarimetric coherence’ radar method to automate the extraction of ice fabric using quad-polarized data.  Second, using a previously developed anisotropic flow-law formulation, we relate the radar fabric measurements to direction-dependent enhancement factors of glacier ice. We demonstrate the approach using a radar ground survey, collected by the British Antarctic Survey, which traverses between the centre and shear margin of Rutford Ice Stream. The data indicate that a vertical girdle fabric is present in the near-surface of the ice stream (approximately the top 300 m) which azimuthally rotates and strengthens toward the shear margin. We then assess the effect that the girdle fabric has upon shear and compression and the impact upon ice-flow models of Rutford Ice Stream.</p><p> </p><p> </p><p> </p>

scholarly journals Characterizing the glaciological conditions at Halvfarryggen ice dome, Dronning Maud Land, Antarctica

2013 ◽  
Vol 59 (213) ◽  
pp. 9-20 ◽  
Author(s):  
Reinhard Drews ◽  
Carlos Martín ◽  
Daniel Steinhage ◽  
Olaf Eisen
Keyword(s):  
Flow Model ◽  
Ice Core ◽  
Ice Cores ◽  
Radar Data ◽  
Ice Thickness ◽  
Atmospheric Conditions ◽  
Ice Flow ◽  
International Partnerships ◽  
Drill Site ◽  
Dronning Maud Land

AbstractWe present a comprehensive approach (including field data, remote sensing and an anisotropic ice-flow model) to characterize Halvfarryggen ice dome in coastal Dronning Maud Land, Antarctica. This is a potential drill site for the International Partnerships in Ice Core Sciences, which has identified the need for ice cores covering atmospheric conditions during the last few millennia. We derive the surface topography, the ice stratigraphy from radar data, and accumulation rates which vary from 400 to 1670 kg m−2 a−1 due to preferred wind directions and changing surface slope. The stratigraphy shows anticlines and synclines beneath the divides. We transfer Dansgaard–Johnsen age–depth scales from the flanks along isochrones to the divide in the upper 20–50% of the ice thickness and show that they compare well with the results of a full-Stokes, anisotropic ice-flow model which predicts (1) 11 ka BP ice at 90% of the ice thickness, (2) a temporally stable divide for at least 2700–4500 years, (3) basal temperatures below the melting point (−12°C to −5°C) and (4) a highly developed crystal orientation fabric (COF). We suggest drilling into the apices of the deep anticlines, providing a good compromise between record length and temporal resolution and also facilitating studies of the interplay of anisotropic COF and ice flow.

scholarly journals Ice flow modelling at EPICA Dome C and Dome Fuji, East Antarctica

2007 ◽  
Vol 3 (1) ◽  
pp. 19-61 ◽  
Author(s):  
F. Parrenin ◽  
G. Dreyfus ◽  
G. Durand ◽  
S. Fujita ◽  
O. Gagliardini ◽  
...  
Keyword(s):  
Saturation Pressure ◽  
East Antarctica ◽  
Amplitude Variation ◽  
Ice Flow ◽  
Flow Models ◽  
Non Linear ◽  
Elevation Changes ◽  
Flow Effects ◽  
Basal Melting ◽  
Glacial Periods

Abstract. 1-D ice flow models are used to construct the age scales at the Dome C and Dome Fuji drilling sites (East Antarctica). The poorly constrained glaciological parameters at each site are recovered by fitting independent age markers identified on each core. We reconstruct past accumulation rates, that are larger than those modelled using the classical vapour saturation pressure relationship during glacial periods by up to a factor 1.5. During the Early Holocene, changes in reconstructed accumulation are not linearly related to changes in ice isotopic composition. A simple model of past elevation changes is developed and shows an amplitude variation of 110–120 m at both sites. We suggest that there is basal melting at Dome C (0.56±0.19 mm/yr). The reconstructed velocity profile is highly non linear at both sites, which suggests complex ice flow effects. This induces a non linear thinning function in both drilling sites, which is also characterized by bumps corresponding to variations in surface elevation with time.

scholarly journals Surface undulations of Antarctic ice streams tightly controlled by bedrock topography

2012 ◽  
Vol 6 (5) ◽  
pp. 4485-4516 ◽  
Author(s):  
J. De Rydt ◽  
G. H. Gudmundsson ◽  
H. F. J. Corr ◽  
P. Christoffersen
Keyword(s):  
Spatial Scales ◽  
Radar Data ◽  
Surface Velocity ◽  
Ice Thickness ◽  
Ice Streams ◽  
Slip Ratio ◽  
Flow Models ◽  
Bedrock Topography ◽  
Ice Stream ◽  
The Mean

Abstract. Full Stokes models predict that fast-flowing ice streams transmit information about their bedrock topography most efficiently to the surface for basal undulations with length scales between 1 and 20 times the mean ice thickness. This typical behaviour is independent on the precise values of the flow law and sliding law exponents, and should be universally observable. However, no experimental evidence for this important theoretical prediction has been obtained so far, hence ignoring an important test for the physical validity of current-day ice flow models. In our work we use recently acquired airborne radar data for the Rutford Ice Stream and Evans Ice Stream, and we show that the surface response of fast-flowing ice is highly sensitive to bedrock irregularities with wavelengths of several ice thicknesses. The sensitivity depends on the slip ratio, i.e. the ratio between mean basal sliding velocity and mean deformational velocity. We find that higher values of the slip ratio generally lead to a more efficient transfer, whereas the transfer is significantly dampened for ice that attains most of its surface velocity by creep. Our findings underline the importance of bedrock topography for ice stream dynamics on spatial scales up to 20 times the mean ice thickness. Our results also suggest that local variations in the flow regime and surface topography at this spatial scale cannot be explained by variations in basal slipperiness.

scholarly journals Ice fabrics in natural flows: beyond pure and simple shear

2021 ◽  
Author(s):  
Daniel H. M. Richards ◽  
Samuel S. Pegler ◽  
Sandra Piazolo
Keyword(s):  
Simple Shear ◽  
Seismic Anisotropy ◽  
Ice Core ◽  
Smooth Transition ◽  
Simple Shear Flow ◽  
Ice Flow ◽  
Flow Models ◽  
Ice Fabrics ◽  
Cluster Pattern

Abstract. Ice fabrics are key for understanding and predicting ice flow dynamics. Despite its importance, the characteristics and evolution of ice fabrics beyond pure and simple shear flow has largely been neglected. However, 80 % of the flow of ice in Antarctica is outside the regimes of pure and simple shear. We use a new validated numerical model (SpecCAF), which has been shown to accurately reproduce experimentally observed fabrics in both compression and simple shear, to explore the fabrics produced between pure and simple shear, as well as those that are highly rotational. We present a definitive classification of all fabric patterns. We find that intermediate deformations between pure and simple shear result in a smooth transition between a fabric characterised by a cone-shape and a secondary cluster pattern. Highly-rotational fabrics are found to produce a weak girdle fabric. In addition we obtain complete predictions for the strain required for any fabric under a 2D deformation to reach steady state at any given temperature. Use of our data in current ice flow models as well as for ice core fabric and seismic anisotropy interpretation will enhance the communities' ability to predict future ice flow in a changing climate.

scholarly journals An iterative inverse method to estimate basal topography and initialize ice flow models

2013 ◽  
Vol 7 (2) ◽  
pp. 873-920
Author(s):  
W. J. J. van Pelt ◽  
J. Oerlemans ◽  
C. H. Reijmer ◽  
R. Pettersson ◽  
V. A. Pohjola ◽  
...  
Keyword(s):  
Inverse Method ◽  
Thickness Distribution ◽  
Radar Data ◽  
Time Dependent ◽  
Climate Forcing ◽  
Ice Flow ◽  
Flow Models ◽  
Ice Caps ◽  
Inverse Approach ◽  
Bedrock Topography

Abstract. We present and evaluate an inverse approach to reconstruct two-dimensional fields of bedrock topography and simultaneously initialize an ice flow model. The inverse method involves an iterative procedure in which an ice dynamical model (PISM) is run multiple times over a prescribed period, while being forced with space and time-dependent climate input. After every iteration bed heights are adjusted using information of the remaining misfit between observed and modeled surface topography. The inverse method is first applied in synthetic experiments with a constant climate forcing to verify convergence and robustness of the approach. In a next step, the inverse approach is applied to Nordenskiöldbreen, Svalbard, forced with height- and time-dependent climate input since 1300 AD. An L-curve stopping criterion is used to prevent overfitting. Validation against radar data reveals a high correlation (up to R = 0.89) between modeled and observed thicknesses. Remaining uncertainties can mainly be ascribed to inaccurate model physics, in particular uncertainty in the description of sliding. Results demonstrate the applicability of this inverse method to reconstruct the ice thickness distribution of glaciers and ice caps. In addition to reconstructing bedrock topography, the method provides a direct tool to initialize ice flow models for forecasting experiments. Application of the method is not constrained to a single model or glacier, indicating the potential to use the approach to compute the detailed thickness distribution of a single glacier, as well as the volume contained in a set of glaciers and ice caps.

scholarly journals Estimating the basal melt rate at NorthGRIP using a Monte Carlo technique

2007 ◽  
Vol 45 ◽  
pp. 137-142 ◽  
Author(s):  
Susanne L. Buchardt ◽  
Dorthe Dahl-Jensen
Keyword(s):  
Heat Flux ◽  
Monte Carlo ◽  
Ice Core ◽  
Two Dimensional ◽  
Internal Layers ◽  
Large Area ◽  
Geothermal Heat ◽  
Ice Flow ◽  
Flow Models ◽  
Two Dimensional Flow

AbstractFrom radio-echo sounding (RES) surveys and ice core data it can be seen that the ice sheet is melting at the base in a large area in Northern Greenland. The RES images reveal internal layers in the ice. The layers are former deposition surfaces and are thus isochrones. Undulations of the isochrones in regions where the base is smooth suggest that the basal melt rate changes over short distances. This indicates that the geothermal heat flux is very high and has large spatial variability in Northern Greenland. In this study, the basal melt rate at the NorthGRIP drill site in North-Central Greenland is calculated by inverse modelling. We use simple one- and two-dimensional flow models to simulate the ice flow along the NNW-trending ice ridge leading to NorthGRIP. The accumulation is calculated from a dynamical model. Several ice flow parameters are unknown and must be estimated along with the basal melt rate using a Monte Carlo method. The Monte Carlo inversion is constrained by the observed isochrones, dated from the timescale established for the NorthGRIP ice core. The estimates of the basal melt rates around NorthGRIP are obtained from both the one- and two-dimensional models. Combining the estimated basal melt rates with the observed borehole temperatures allows us to convert the basal melt rates to geothermal heat flow values. From the two-dimensional model we find the basal melt rate and geothermal heat flux at NorthGRIP to be 6.1 mma–1 and 129 mWm–2, respectively.

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