Radio-echo soundings were collected on Black Rapids Glacier, Alaska, USA, from mid-May to mid-July 1993 to investigate spring speed-up and summer slowdown including high-speed events associated with three lake drainages. Temporal changes in echo power from all depths were highly correlated, indicating a strong effect from varying amounts of near-surface water. Evaluation of bed reflectivity was corrected for this effect based on the time variation of spatially stable patterns of internal scattering identified using principal component analysis. Hourly time series collected at two fixed locations over the deepest part of two valley cross sections showed no detectable change in bed reflection power (<5%) or phase (<0.05 rad). Reoccupation of fixed locations toward the margins at several-day intervals revealed changes in bed power reflectivity up to 50%, but with no definable relation to lake drainages. Theoretical analyses indicate that changes in reflectivity of <5% from a rock bed constrain basal water thickness changes to centimeter scale or less. Conductive basal till degrades the constraint to decimeter scale or more. Changes in bed reflectivity of 50% indicate probable absence of thick conductive till at such locations, and that the changes were caused by centimeter to decimeter changes in equivalent water thickness.

AbstractWe synthesize previously published remote-sensing observations, radar data and model output to obtain a ~1000 year ice flow history for the Siple Coast ice-stream system in West Antarctica to investigate the timing and magnitude of changes in mass flux. The synthesized history shows significant short-term variability in ice-stream shear margin and grounding line position due to internal variability of the coupled system. The chronology highlights the interplay between adjacent ice streams, which implies that the behavior of any individual ice stream should not be examined in isolation. Furthermore, individual events cannot be fully interpreted without an understanding of the broad-scale, long-term variability in the ice sheet. In the context of this millennium-scale history, we interpret the relatively recent stagnation of Kamb Ice Stream (KIS) as just one stage in the thermodynamic cycle of an ice stream in this region. The changes in mass balance that result from the KIS stagnation may thus be viewed as century-scale 'noise' relative to the longer-term trend. Understanding and characterizing this noise is a necessary step before accurate model-based predictions of ice-sheet mass balance for the next century can be made.

Anisotropy in englacial radar power was measured using 60‐MHz and 179‐MHz copolarized pulse‐modulated radar at 19 sites in central West Antarctica. The study region is a 100 × 300 km2area near the West Antarctic Ice Sheet Divide that separates ice flow toward the Ross and Amundsen Embayments. The frequency dependence of the returned power indicates that most of the radar data are affected by vertical variations in the crystal‐orientation fabric (COF), though the 60‐MHz data are more affected by acidity contrasts in the top 1000 m. Significant polarimetric variations occur at most sites, likely due to effects of the anisotropic COF patterns. More anisotropic variations occur at sites with significant horizontal strain, whereas more isotropic variations occur at sites where vertical compression dominates. Azimuthal shifts with depth of the principal axes of COF were found in shallow ice near the current flow divide and at greater depths over locations of rough bed. The former indicates that the divide has differentially migrated, resulting in a rotation of the principal COF axes. Nevertheless, the regionally consistent radar signatures suggest that the first‐order ice properties in this area have remained constant and that no major changes in the strain configuration or ice topography have occurred for the past five to eight thousand years. We conclude that shallow polarimetric features can be related to the current strain configurations, and that englacial polarimetric features can help constrain current ice rheology and evolution of the ice topography.

This paper concerns the fast motion of snow and ice in the terrestrial environment and the associated hazards. These hazards tend to be very localized in mountain areas that are mostly sparsely populated. Historically their direct impact on people has been relatively small. However, with increased human activity in mountain terrain including habitation, resource extraction, transportation and recreation, it is necessary to recognize the potential impacts from moving ice that could be very serious.

We analyzed depth patterns of geometrically corrected returned power Pc from within the ice of central West Antarctica to develop a proxy for englacial radar attenuation. The depth patterns of [Pc]dB (Pc in the decibel scale) at individual sites were first approximated with least squares vertical gradients of the local mean [Pc]dB for five depth ranges. Variations of these gradients along radar tracks show identifiable trends but have local anomalous features over distances less than ∼5–10 km that are caused by smaller reflection from tilted internal layers above steep beds and other factors. Consequently, extraction of an attenuation proxy from the returned power requires mitigation of reflectivity variations. Next, returned power was synthesized only from bright layers assembled over distances much wider than the local anomalous features. Individual data ensembles show a clear upper (not lower) cutoff in [Pc]dB. The cutoff power decreases with depth linearly between ∼500 and ∼1600 m, which defines the upper envelope gradient. With the aid of attenuation and reflectivity modeling, we concluded that the upper envelope gradient can be an attenuation proxy in the isothermal ice that is expected in the upper half or more of central West Antarctica. The estimated attenuation rate in the upper ∼1600 m varies 5 dB km−1 (one way), equivalent to lateral temperature variations of about 2°C or chemistry variations of up to a factor of 2. This range indicates that the attenuation variations can significantly affect delineation of bed wetness on the basis of contrasts in power returned from the bed.

AbstractVisible and infrared satellite images reveal numerous lineations on the Siple Coast region of West Antarctica. We used 5 MHz ice-penetrating radar to probe the interior and the bed of the ice sheet beneath a lineation at the boundary between Engelhardt Ice Ridge and flat-ice terrain to the south of the Kamb Ice Stream (KIS) outlet. Results show curved reflectors that emerge from the bed beneath 600 m thick ice. The tops of the reflectors extend about 100m into the ice above the bed, where they become almost horizontal. Apparent reflectivity of the horizontal section is about 20 dB less than that of the bed. We conclude that the likely cause of such strong reflection is sea water that was accreted into basal crevasses when the flat-ice terrain was floating. Internal layers are warped downward just downslope from the basal reflectors. It is thought that the downwarping was caused by localized basal melting in the past. The spatial pattern of downwarping suggests that localized basal melting was stronger on the north side than on the south side of KIS; apparently ice/ocean interactions on the two sides of KIS were different.

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In this paper, effects of birefringence on radio waves obliquely propagating though polar ice sheets are examined to facilitate interpretations of bistatic and side-looking radar data. A formalism applicable for arbitrary radar configurations is developed to predict the returned power from within and beneath the ice sheets that have arbitrary alignments of ice crystals (ice fabrics). We applied this formalism to a range of ice fabrics found in ice cores and assessed the effects of birefringence in terms of ray-path configurations, ice fabrics, and radar frequency. Predicted frequency dependence of the bed return power replicates prominent features observed at Greenland NGRIP ice-core site. Results show that birefringence in ice of 1 km or more thickness with strong (weak) fabric can reduce the power returned from the bed 2 dB or more at frequencies higher than 200 MHz (20 MHz) as compared to isotropic ice. This suggests that quantitative interpretation of the power returned from the bed requires careful assessment of birefringence almost everywhere over the ice sheets. Application of this formalism also suggests a radar-frequency range usable for attenuation measurements, possible effects of fabric on synthetic aperture radar processing, and a feasibility of remote sensing of ice fabric.

Upper-air conditions archived in the NCEP-NCAR Reanalysis have been used to investigate changes in precipitation and snowfall over the Patagonia icefields during 1960-99. Apparently, whereas total precipitation has not changed, warming has caused a decrease in the amount falling as snow. Precipitation at a site is taken to be proportional to the product of the relative humidity and the component of the wind in a particular critical direction, both at 850 hPa (similar to 1400 m) at a point over the ocean to the west of the icefields; whether it falls as rain or snow is assumed to depend on whether the temperature at the elevation of the site is above or below +2 degrees C. The critical direction is assumed to be 270 degrees, which is perpendicular to the north-south trending Andes and is also the prevailing wind direction in this zone of strong westerlies. Because of the scarcity of precipitation records on or near the icefields, the constant of proportionality cannot be determined, so the investigation is limited to examining relative changes in those upper air variables. Warming at 850 hPa has been similar to 0.5 degrees C over the 40 years, both winter and summer, with the effects that it has: (1) shifted from snow to rain similar to 5% of the precipitation, the total of which has changed little, and (2) increased annual melt in the ablation areas by similar to 0.5 m w.e. The icefields have been losing mass since at least 1870, so this 40-year trend represents only an acceleration of the longer-term trend of adjusting to climate change since the Little Ice Age. (C) 2006 Elsevier B.V. All rights reserved.

AbstractSurface-based ice-penetrating radar profiles were made across the active north margin (the Snake) of the upper part of Whillans Ice Stream (formerly Ice Stream B, branch B2), West Antarctica, at three locations. Low frequency (about 2 MHz) and the ground deployment of the radar allowed penetration through the near-surface zone of fracturing to detect internal layering and bed reflection characteristics on continuous profiles spanning from the slow-moving ice of Engelhardt Ridge well into the chaotic zone of the shear margin. Internal layers were tracked beneath the chaotic zone, where they are warped but remain continuous. The energy returned from internal layers showed no systematic changes associated with the transition from the undisturbed surface of the slow-moving ice into the fractured surface of the shear margin, thus indicating little effect from the surface crevasses on the penetration of the radar signal. Based on this calibration of the near-surface effects and corrections for path length, spreading and attenuation, we examine the spatial variation of bed reflectivity. Low bed reflectivity found under Engelhardt Ridge extends under the chaotic zone of the margin into fast-moving ice. We argue that the fast motion in a band along the margin is mediated by processes other than deformation of thick dilated till that is the source of lubrication allowing fast motion in the interior of the ice stream.

Ice‐penetrating radar profiles reveal distinctive internal structure within the flat ice terrains that bound the lower reaches of the now stagnant Kamb Ice Stream, West Antarctica; slightly warped but continuous upper layers overlie extensive deep line diffractors located at a uniform depth several hundreds of meters above the bed. On the basis of their orientation and morphology we interpret the line diffractors to be the tips of basal crevasses. At the boundary between these terrains and the inter–ice stream ridges, internal layers are strongly downwarped, and the deepest layers are truncated at the bed. Results from simple kinematic ice flow modeling indicate that the observed layer pattern requires a cumulative basal melt of 120–350 m over a time period that is less than 300 years. Fields of basal crevasses are typically observed in ice shelves; however, isolated basal crevasses can also be found in grounded ice when the subglacial water pressure is close to overburden. On the basis of the presence of these crevasses we argue that these terrains were once floating or near floating in the past. We combine model results with estimates of recent ice thickness change to suggest that floatation was transitory and the region may have become fully grounded roughly 200 years prior to stagnation of Kamb Ice Stream.

Ice‐penetrating radar data confirm the presence of two relict northern margins of Kamb Ice Stream (KIS), West Antarctica in an area called the Duckfoot. Between the two relict margins, deep layers are preserved and buried crevasses remain at a constant depth until roughly 1.5 km from the innermost (youngest) margin suggesting that this outer portion of the ice stream stagnated suddenly (<10 years). Stagnation was likely accomplished through changes in basal conditions (i.e., reduced lubrication) beneath this outer portion of the ice stream and resulted in a narrowing of the ice stream trunk by 27% and a reduction in flow speed ∼200 years prior to complete stagnation. Flowstripe deformation as a result of Duckfoot stagnation provides an estimate of KIS paleo‐velocity of 210 m/a after Duckfoot stagnation.

In this paper we analyze the magnitude and spatial organization of small‐scale surface features (the surface texture) of the Greenland and Antarctic ice sheets. The texture is revealed in shaded relief maps of digital elevation models because surface slopes emphasize short‐wavelength topography. We show that the surface slope components parallel to and perpendicular to the ice flow direction of ice sheets are both qualitatively and quantitatively different from one another. The parallel component variations are larger in magnitude than the perpendicular component variations, and features in maps of the parallel component are elongated perpendicular to the ice flow direction, while features in maps of the perpendicular component are elongated at a diagonal to the ice flow direction. These properties may be explained by a simple model of glacier dynamics in which a linearly viscous slab of ice flows over a random, isotropic, red noise bed. In this model an anisotropic surface results from an isotropic bed because the surface anisotropy derives from the anisotropic transfer of bed topography to the surface by viscous flow dynamics. The modeling results suggest that analysis of surface texture magnitude and anisotropy can be used to identify areas of sliding ice from surface topography data alone and can be used to roughly estimate sliding rates where bed topography is known.

We analyze differences in ICESat elevation estimates at orbital crossover locations to determine short‐term rates of elevation change for small regions within the Ross Embayment of the West Antarctic Ice sheet. A linear regression of crossover elevation difference against time difference gives an estimate of the mean elevation‐change rate during the ICESat mission to date. We observe prevalent elevation change in the south, with uplift in the upstream end of Kamb Ice Stream at 0.24–0.30 m a−1, and thinning in the parts of Whillans Ice Stream, Mercer Ice Stream, and the adjacent Conway and Engelhardt ice ridges at 0.05–0.18 m a−1. These rates of elevation change are too large to be explained by the 0.02 –0.03 m a−1 formal regression error, by seasonal height variations, or by accumulation‐ or densification‐rate variability, suggesting that they reflect real variations stemming from ice dynamics of the region.

A 1995 surge of Variegated Glacier, Alaska, USA, is discussed in the context of its six 20th-century predecessors, especially the previous surge in 1982/83 which was studied in detail. The average time between surge initiations is 15 years. The 1995 surge was considerably weaker than its predecessors, having a single phase or at most a very weak second phase. The 1995 surge confirms that there is a seasonal cycle, with surge initiation in winter and termination in the first part of the melt season, and a correlation between weather and both surge termination date and surge extent. Two days of record high temperature correlated with the termination of the 1995 surge. The most obvious issue is the absence of a strong second surge phase (as there was in the 1982/83 surge) culminating in a surge extent more in line with that of the predecessors. This is considered in the light of a simple criterion for surge initiation and re-initiation which depends upon the evolving basal shear stress.

Satellite images of Kamb Ice Stream (formerly Ice Stream C), West Antarctica, reveal several long, curved linear features (lineations) oriented sub-parallel to the ice-flow direction. We use ground-based radar to characterize the internal layer stratigraphy of these lineations and the terrains that they bound. Some lineations are relict ice-stream shear margins, identified by hyperbolic diffractors near the surface (interpreted to be buried crevasses) and highly disturbed internal layers at depth. Satellite images show another set of lineations outside the relict margins that wrap around the ends of the surrounding inter-ice-stream ridges. Internal layers beneath these lineations are downwarped strongly into a syncline shape. The internal stratigraphy of the terrain between these lineations and the relict margins is characterized by deep hyperbolic line diffractors. Our preferred hypothesis for the origin of this terrain is that it was floating sometime in the past; the deep hyperbolas are interpreted to be basal crevasses, and the strongly downwarped internal layers mark the position of a relict grounding line. Our study shows that lineations and intervening terrains have different internal layer characteristics implying different origins. Differentiation between these features is not possible using satellite images alone.