Antarctic Ice Streams Research Articles

Spatiotemporal variations of snowmelt in Antarctica derived from satellite scanning multichannel microwave radiometer and Special Sensor Microwave Imager data (1978–2004)

We derived the extent, onset date, end date, and duration of snowmelt in Antarctica from 1978 to 2004 using satellite passive microwave scanning multichannel microwave radiometer (SMMR) and Special Sensor Microwave Imager (SSM/I) data. A wavelet‐transform‐based method was developed to determine and characterize melt occurrences. About 9–12% of the Antarctic surface experiences melt annually. This is more than twice the surface melt extent measured in Greenland. Seasonally, surface melt primarily takes place in December, January, and February and peaks in early January. Regression analysis over the 25 year period of study reveals a negative interannual trend in surface melt. Nevertheless, the trend inference is not statistically significant. Large year‐to‐year fluctuations characterize the interannual variability. Extremely high melt occurred in the 1982/1983 and 1991/1992 summers, while extremely weak melt occurred in the 1999/2000 summer. A strong correlation with air temperature suggests that the melt index can serve as a diagnostic indicator for regional temperature variations. Periodic melting has been observed over Ross Ice Shelf, Ronne‐Filchner Ice Shelf, the West Antarctic ice streams, and outlet glaciers in the Transantarctic Mountains. The Antarctic Peninsula, West Ice Shelf, Shackleton Ice Shelf, Amery Ice Shelf, and the ice shelf along the Princess Ragnhild Coast experienced the most persistent and intensive melt and should be closely monitored for their stability in the future, given the recent disintegration of the Larsen Ice Shelf A and B.

East Antarctic ice stream tributary underlain by major sedimentary basin

Marine and rift sediments exert a fundamental control on ice stream flow in the West Antarctic Ice Sheet, and hence on its mass balance and stability. In contrast, most ice streams in the much larger East Antarctic Ice Sheet are thought to be relatively stable features resting on till, perhaps underlain by crystalline rock. Any geological controls on East Antarctic Ice Sheet enhanced flow remain largely unknown. We present aerogeophysical evidence indicating that a region of enhanced ice flow in the interior of the East Antarctic Ice Sheet is underlain by subglacial sediments ∼3 km thick and that these are influencing the flow regime of the overlying ice. We show that subglacial sediments are important in modulating ice dynamics, not just for the West Antarctic Ice Sheet, but also for its much larger neighbor, and suggest that the sedimentary basin identified here may contain information on the Neogene glacial history of this part of the East Antarctic Ice Sheet.

Subglacial conditions during and after stoppage of an Antarctic Ice Stream: Is reactivation imminent?

Borehole observations from the base of the West‐Antarctic Ice Sheet (WAIS) reveal the presence of a 10 to 15 m thick accretionary basal ice layer in the upstream area of Kamb Ice Stream (KIS). This ice layer has formed over a time of several thousand years by freeze‐on of subglacial water to the ice base and has recorded during this time basal conditions upstream of its current location. Analysis of samples and videos sequences from boreholes drilled to the bottom of KIS confirms that KIS‐stoppage was due to basal freeze‐on and that relubrication of the ice stream is well underway. These results further suggest that ice stream cyclicity may be shorter than expected (1000s of years) and that a restart of KIS may be imminent within decades to centuries.

Siple Coast Ice Streams in a General Antarctic Ice Sheet Model

Abstract In an earlier paper by Verbitsky and Saltzman, a vertically integrated, high-resolution, nonlinearly viscous, nonisothermal ice sheet model was presented to calculate the “present-day” equilibrium regime of the Antarctic ice sheet. Steady-state solutions for the ice topography and thermodynamics, represented by the extent of the areas of basal melting, were shown to be in good agreement with both observations and results obtained from other three-dimensional thermodynamical equations. The solution for the basal temperature field of the West Antarctic Siple Coast produced areas at the pressure melting point separated by strips of frozen-to-bed ice, the structure of which looks very similar to ice streams A–E. Since the possible response of the Siple Coast basal temperature pattern to global warming and to associated changes in the snowfall rate is not obvious, a special sensitivity study was conducted. Results of such a study suggest that increased precipitation rate and associated intensification of ice advection can effectively “shut down” West Antarctic ice streams.

Analysis techniques for coherent airborne radar sounding: Application to West Antarctic ice streams

Analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment. Echoes from a general interface consist of both specularly reflected and diffusely scattered contributions. Specular reflection results from smooth uniform interfaces, whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media. This article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data. Reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of Whillans Ice Stream and Ice Stream C, West Antarctica. Additionally, reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over Ice Stream C. Analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material. Echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates. Comparison of low‐ and high‐resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and/or scattering interface. Combining the results from these independent analyses provides classification of the subglacial environment. Classified regions include smooth seawater, smooth saturated sediments, accreted marine ice, rough bottom crevasses, mixed conditions with partial subglacial water, and dry frozen conditions.

Recent dramatic thinning of largest West Antarctic ice stream triggered by oceans

A growing body of observational data suggests that Pine Island Glacier (PIG) is changing on decadal or shorter timescales. These changes may have far‐reaching consequences for the future of the West Antarctic ice sheet (WAIS) and global sea levels because of PIG's role as the ice sheet's primary drainage portal. We test the hypothesis that these changes are triggered by the adjoining ocean. Specifically, we employ an advanced numerical ice‐flow model to simulate the effects of perturbations at the grounding line on PIG's dynamics. The speed at which these changes are propagated upstream implies a tight coupling between ice‐sheet interior and surrounding ocean.

Inferring surface heat flux distributions guided by a global seismic model: particular application to Antarctica

We present a method that uses a global seismic model of the crust and upper mantle to guide the extrapolation of existing heat-flow measurements to regions where such measurements are rare or absent. For any chosen spatial point on the globe, the procedure generates a histogram of heat-flow values determined from existing measurements obtained from regions that are structurally similar to the target point. The inferred histograms are based on a “structural similarity functional”, which is introduced to quantify the structural analogy between different regions. We apply this procedure world-wide using the global heat-flow data base of Pollack et al. [Rev. Geophys. 31 (1993) 267] guided by an update of the 3-D shear velocity model of the crust and uppermost mantle of Shapiro and Ritzwoller [Geophys. J. Int. 51 (2002) 88]. The method results in an inferred probability distribution for the heat flux for each geographical region of interest. These distributions are strongly non-Gaussian, but are well approximated by the log-logistic distribution which is completely specified by two parameters. The inferred distributions agree well with observed distributions of heat flux taken in 300-km radius circles regionally in numerous locations. Particular attention is drawn to the inferred surface heat flux distributions across Antarctica, where direct measurements are rare but information about heat flow may be needed to help understand the dynamics of the Antarctic ice sheets and ice streams. Mean heat flow in West Antarctica is expected to be nearly three times higher than in East Antarctica and much more variable. This high heat flow may affect the dynamics of West Antarctic ice streams and the stability of the West Antarctic Ice Sheet.

West Antarctic ice-stream discharge variability: mechanism, controls and pattern of grounding-line retreat

AbstractWest Antarctic ice streams show pronounced flow variability in their downstream reaches, with changes stranding formerly fast-flowing ice and redirecting discharge. A simple model, in which the temperature gradient in basal ice provides control of fast sliding in the downstream reach, can explain this behavior. Downstream thinning steepens the temperature gradient near the bed, increasing upward heat flow and the tendency toward basal freezing. The basal temperature gradient is steepest and the tendency toward basal freezing the strongest in ice that has experienced the most rapid downstream thinning, that is, the fastest-flowing ice. The most ‘successful’ rapid outflows are regions where basal water from elsewhere is likely to be consumed. Freezing here leads to episodic slow-downs and redirections of flow, the history of which appears in satellite imagery as ice rises, distorted streaklines, and margin jumps created when discharge migrates to areas with more favorable basal conditions. One compelling consequence of this process is that it makes catastrophic collapse less likely; if discharge currents are forced to slow when they become too fast (thin), then there may be an upper bound on the retreat rate and discharge flux of the West Antarctic ice sheet (WAIS) ice-stream system under the present climate.

Measurement of glacier geophysical properties from insar wrapped phase

A method is presented for calculating longitudinal glacier strain rates directly from the wrapped phase of an interferometric synthetic aperture radar (InSAR) interferogram assuming the ice flow path is known. This technique enables strain rates to be calculated for scenes lacking any velocity control points or areas within a scene where the phase is not continuously unwrappable from a velocity control point. The contributions to the error in the estimate of the strain rate are evaluated, and recommendations for appropriate SAR and InSAR parameters are presented. An example using Radarsat-1 InSAR data of an East Antarctic ice stream demonstrates the technique for calculating longitudinal strain rate profiles and estimating tensile strength of ice (186-215 kPa) from locations of crevasse initiation. The strain rate error was found to be 17% corresponding to a tensile strength of ice error of 5.3%.

Tidally controlled stick-slip discharge of a West Antarctic ice.

A major West Antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter. Acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less. Slip propagates at approximately 88 meters per second, suggestive of a shear wave traveling within the subglacial till. A model of an episodically slipping friction-locked fault reproduces the observed quasi-periodic event timing, demonstrating an ice stream's ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level.

Response of subglacial sediments to basal freeze‐on 1. Theory and comparison to observations from beneath the West Antarctic Ice Sheet

We have constructed a high‐resolution numerical model of heat, water, and solute flows in sub‐ice stream till subjected to basal freeze‐on. The model builds on quantitative treatments of frost heave in permafrost soils. The full version of the Clapeyron equation is used. Hence, ice‐water phase transition depends on water pressure, osmotic pressure, and surface tension. The two latter effects can lead to supercooling of the ice base. This supercooling, in turn, induces hydraulic gradients that drive upward flow of pore water, which feeds the growth of segregation ice onto the freezing interface. This interface may progress into the till and form ice lenses if supercooling is sufficiently strong. Hence, the ice segregation process develops a stratified basal ice layer. In our model, a high basal temperature gradient (∼0.054°C m−1) triggers ice stream stoppage, and the loss of basal shear heating leads to relatively high basal freeze‐on rates (∼3–5 mm a−1). In response, the subglacial till experiences comparatively rapid consolidation. Till porosity can decrease from 40% to <25%, and till strength can increase from ∼3 kPa to >120 kPa, approximately within one century. Basal supercooling arising from redistribution of solutes and ice‐water interfacial effects amounts to ca. −0.35°C below the pressure‐melting point. Fine‐grained till is in our model associated with widely spaced, thick ice lenses. Coarse‐grained till yields thinner ice lenses that are more closely spaced. Our model results compare favorably, although not in all details, with available observational evidence from borehole studies of West Antarctic ice streams.

Glacial erosion beneath ice streams and ice‐stream tributaries: constraints on temporal and spatial distribution of erosion from numerical simulations of a West Antarctic ice stream

Geologic evidence such as subglacial troughs and grounding zone wedges indicate that soft‐bedded, West Antarctic ice streams are capable of eroding, transporting and depositing large volumes of debris at high rates (˜100 m3 yr‐1 per meter width). In order to understand the dynamics of ice streams and the geologic effects of their activity, it is important to understand the physical mechanisms that control these high rates of sub‐ice‐stream sediment generation and transport. Here, we use a numerical model of Ice Stream C run over c. 8500 model years to quantify the effects of a recently proposed, till‐ploughing mechanism of till formation and redistribution beneath ice streams (Tulaczyk et al. 2001; Clark et al. in press). Our results show that this ‘transport‐limited’ mechanism, in which till transport rates scale with ice velocity and erosion rates with spatial gradients of velocity, is consistent with existing constraints. For instance, our model results predict significantly higher (˜0.6 mm yr‐1) average erosion rates beneath ice‐stream tributaries, which are underlain by deep subglacial troughs, than beneath ice‐stream trunks (˜0.2mm yr‐1), whose subglacial troughs have a significantly smaller relief. We would not obtain this satisfactory result if subglacial erosion was parametrized in the model using the more traditional approach of scaling erosion rates with ice velocity (what we call the ‘production‐limited’ parametrization). Because of the requirement of ice continuity, the magnitude of ice velocity generally increases downstream in polar ice streams, and so do the production‐limited erosion rates. Pending further investigations, we propose that geologic and geomorphic effects of soft‐bedded ice streams should be quantified using some form of a ‘transport‐limited’ parametrization of subglacial erosion rates, e.g. the till‐ploughing mechanism.

Laser profiling over Antarctic ice streams: methods and accuracy

AbstractWe assess the accuracy and precision of the U.S. National Science Foundation’s Support Office for Aerogeophysical Research (SOAR) laser profiling system for mapping topography and detecting surface elevation changes of West Antarctic ice streams. The procedures used to process, calibrate and validate the laser, navigation and global positioning system (GPS) data are presented. The primary objective is to produce surface elevations with the best possible resolution. Repeat surveys of a grid of lines over Whillans Ice Stream and Ice Streams C and E were conducted in the 1997/98 and 1999/2000 seasons. The procedure has been calibrated using special test flights conducted over areas that have been surveyed with precise geodetic GPS equipment mounted on snow-mobiles. After calibration, agreement between the two surfaces is ±10 cm rms. The accuracy and precision of the procedure have been evaluated at points where laser flight-lines cross over one another. The accuracy of the system is found to range from 0.09 to 0.22 m.

Subglacial thermal balance permits ongoing grounding-line retreat along the Siple Coast of West Antarctica

AbstractChanges in the discharge of West Antarctic ice streams are of potential concern with respect to global sea level. The six relatively thin, fast-flowing Ross ice streams are of interest as low-slope end-members among Antarctic ice streams. Extensive research has demonstrated that these “rivers of ice” have a history of relatively high-frequency , asynchronous discharge variations with evolving lateral boundaries. Amidst this variability, a ∼1300 km grounding-line retreat has occurred since the Last Glacial Maximum. Numerical studies of Ice Stream D (Parizek and others, 2002) indicate that a proposed thermal-regulation mechanism (Clarke and Marshall, 1998; Hulbe and MacAyeal, 1999; Tulaczyk and others, 2000a, b), which could buffer the West Antarctic ice sheet against complete collapse, may be over-ridden by latent-heat transport within melt-water from beneath inland ice. Extending these studies to Ice Stream A, Whillans Ice Stream and Ice Stream C suggests that further grounding-line retreat contributing to sea-level rise is possible thermodynamically However, the efficiency of basal water distribution may be a constraint on the system. Because local thermal deficits promote basal freeze-on (especially on topographic highs), observed short-term variability is likely to persist.

Acoustic impedance and basal shear stress beneath four Antarctic ice streams

AbstractThe acoustic impedance of the subglacial material beneath 7.2 km profiles on four ice streams in Antarctica has been measured using a seismic technique. The ice streams span a wide range of dynamic conditions with flow rates of 35–464 m a–1. The acoustic impedance indicates that poorly lithified or dilated sedimentary material is ubiquitous beneath these ice streams. Meanacoustic impedance across each profile correlates well with basal shear stress and the slipperiness of the bed, indicating that acoustic impedance is a good diagnostic not only for the porosity of the subglacial material, but also for its dynamic state (deforming or non-deforming). Beneath two of the ice streams, lodged (non-deforming) and dilated (deforming) sediment coexist but their distribution is not obviously controlled by basal topography or ice thickness. Their distribution may be controlled by complex material properties or the deformation history. Beneath Rutford Ice Stream, lodged and dilated sediment coexist and are distributed in broad bands several kilometres wide, whileon Talutis Inlet there is considerable variability over much shorter distances; this may reflect differences in the mechanism of drainage beneath the ice streams. The material beneath the slow-moving Carlson Inlet is probably lodged but unlithified sediment; this is consistent with the hypothesis that Carlson Inlet was once a fast-flowing ice stream but is now in a stagnant phase, which could possibly be revivedby raised basal water content. The entire bed beneath fast-flowing Evans Ice Stream is dilated sediment.

Dilatant till layer near the onset of streaming flow of Ice Stream C, West Antarctica, determined by AVO (amplitude vs offset) analysis

AbstractA powerful seismic technique that exploits the phase of the ice-bottom reflections shows that soft till is widespread beneath a West Antarctic ice stream very close to the onset of streaming flow. The amplitude vs offset (AVO) method measures the change in amplitude of the reflection as a function of increasing angle of incidence. For a decrease in acoustic impedance with depth, the reflection phase is negative at low angles of impedance but positive at intermediate angles. The change in phase by 180° is an obvious and robust measure of the relative acoustic impedance contrasts. This technique is only usable when there is a change in phase vs offset, conditions which obtain for “UpB-type” tills (high water pressures and porosity, low compressional- and shear-wave velocities, similar to those observed at Upstream B camp). I have applied this technique to the far upstream regions of Ice Stream C and find that a dilatant ( and presumably deforming), relatively thick (meters) till layer has formed beneath the ice stream within tens of km of the region identified as the transition from inland flow to ice-stream flow. These results suggest that the onset of rapid basal motion is linked to the formation of this deforming subglacial layer.

Glacial erosion beneath ice streams and ice-stream tributaries: constraints on temporal and spatial distribution of erosion from numerical simulations of a West Antarctic ice stream

Glacial erosion beneath ice streams and ice-stream tributaries: constraints on temporal and spatial distribution of erosion from numerical simulations of a West Antarctic ice stream

The role of lateral and vertical shear in tributary flow toward a West Antarctic ice stream

AbstractNarrow lateral shear margins are the most distinctive visual feature of the West Antarctic ice streams. Large shear stresses within these layers support the majority of the gravitational driving stress within a fast-flowing ice stream. The present contribution looks upstream, to the tributaries that feed ice-stream onsets, and considers the effects of both horizontal and vertical shear on their flow. Numerical and direct simulations of vertical and horizontal shear are used. Vertical shear, simulated using an anisotropic flow law, is of particular interest. We conclude that by isolating overlying ice from large-amplitude variations in bed elevation,“vertical shear margins” play an important role in sustaining relatively rapid tributary flow.

Changes in west Antarctic ice stream velocities: Observation and analysis

We have produced a map of velocity covering much of the Siple Coast ice streams. The map confirms earlier estimates of deceleration on Whillans Ice Stream. Comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams. Force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams. We have used a theoretical analysis to examine the controls on fast flow. This analysis suggests that ice plains (very wide ice streams) are inherently unstable. This instability may be responsible for the current deceleration on the Ice Plain of Whillans Ice Stream and the shutdown of Ice Stream C 150 years ago. Thinning‐induced reductions in driving stress may also explain some of the observed deceleration, particularly in upstream areas. The active portions of Ice Stream C coincide well with the areas where we estimate that melt should be taking place. Current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain. Uncertainty remains about the basal conditions on Ice Stream D, while the basal resistance on Ice Stream E is large enough to ensure basal melting.

Switch of flow direction in an Antarctic ice stream.

Fast-flowing ice streams transport ice from the interior of West Antarctica to the ocean, and fluctuations in their activity control the mass balance of the ice sheet. The mass balance of the Ross Sea sector of the West Antarctic ice sheet is now positive--that is, it is growing--mainly because one of the ice streams (ice stream C) slowed down about 150 years ago. Here we present evidence from both surface measurements and remote sensing that demonstrates the highly dynamic nature of the Ross drainage system. We show that the flow in an area that once discharged into ice stream C has changed direction, now draining into the Whillans ice stream (formerly ice stream B). This switch in flow direction is a result of continuing thinning of the Whillans ice stream and recent thickening of ice stream C. Further abrupt reorganization of the activity and configuration of the ice streams over short timescales is to be expected in the future as the surface topography of the ice sheet responds to the combined effects of internal dynamics and long-term climate change. We suggest that caution is needed when using observations of short-term mass changes to draw conclusions about the large-scale mass balance of the ice sheet.