scholarly journals Post-stagnation behavior in the upstream regions of Ice Stream C, West Antarctica

2001 ◽  
Vol 47 (157) ◽  
pp. 283-294 ◽  
Author(s):  
S. F. Price ◽  
R. A. Bindschadler ◽  
C. L. Hulbe ◽  
I. R. Joughin
Keyword(s):  
Wave Theory ◽  
Kinematic Wave ◽  
West Antarctica ◽  
Thickness Change ◽  
Current Trends ◽  
Ice Stream ◽  
Bulge Region ◽  
Potential Gradients ◽  
Diffusion Rates ◽  
Ice Stream B

AbstractThe region where two active tributaries feed into the now stagnant Ice Stream C (ISC), West Antarctica, is thickening. In this region, we observe a correlation between faster ice flow (the tributaries) and elevated topography. We conclude that stagnation of ISC resulted in compression and thickening along the tributaries, eventually forming a “bulge” on the ice-sheet surface. Modern hydraulic potential gradients would divert basal meltwater from ISC to Ice Stream B (ISB). These gradients are primarily controlled by the bulge topography, and so likely formed subsequent to trunk stagnation. As such, we argue against “water piracy” as being the cause for ISC’s stagnation. Kinematic-wave theory suggests that thickness perturbations propagate downstream over time, but that kinematic-wave speed decreases near the stagnant trunk. This and modest diffusion rates combine to trap most of the tributary-fed ice in the bulge region. Using interfero-metric synthetic aperture radar velocity measurements, we observe that half of the ice within ISC’s southern tributary flows into ISB. That flow pattern and other observations of non-steady flow in the region likely result from stagnation-induced thickening along upper ISC combined with a longer period of thinning on upper ISB. If current trends in thickness change continue, more ice from upper ISC will be diverted to ISB.

scholarly journals Surface Features of Ice Stream B, Marie Byrd Land, West Antarctica

1986 ◽  
Vol 8 ◽  
pp. 168-170 ◽  
Author(s):  
P.L. Vornberger ◽  
I.M. Whillans
Keyword(s):  
Aerial Photographs ◽  
Relative Importance ◽  
West Antarctica ◽  
Ice Streams ◽  
Longitudinal Compression ◽  
West Antarctic Ice Sheet ◽  
Surface Features ◽  
West Antarctic ◽  
Ice Stream ◽  
Ice Stream B

Aerial photographs have been obtained of Ice Stream B, one of the active ice streams draining the West Antarctic Ice Sheet. A sketch map made from these photographs shows two tributaries. The margin of the active ice is marked by curved crevasses and intense crevassing occurs just inward of them. Transverse crevasses dominate the center of the ice streams and diagonal types appear at the lower end. A “suture zone” originates at the tributary convergence and longitudinal surface ridges occur at the downglacier end. The causes of these surface features are discussed and the relative importance of four stresses in resisting the driving stress is assessed. We conclude that basal drag may be important, longitudinal compression is probably important at the lower end, and longitudinal tension is probably most important near the head of the ice stream. Side drag leads to shearing at the margins, but does not restrain much of the ice stream.

scholarly journals Radar reflections reveal a wet bed beneath stagnant Ice Stream C and a frozen bed beneath ridge BC, West Antarctica

1998 ◽  
Vol 44 (146) ◽  
pp. 149-156 ◽  
Author(s):  
C. R. Bentley ◽  
N. Lord ◽  
C. Liu
Keyword(s):  
Radar Data ◽  
Ice Thickness ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Stream ◽  
Crossover Analysis ◽  
Automated Processing ◽  
Basal Reflection ◽  
Field Season ◽  
Ice Stream B

AbstractDigital airborne radar data were collected during the 1987-88 Antarctic field season in nine gridded blocks covering the downstream portions of Ice Stream B (6km spacing) and Ice Stream C (11 km spacing), together with a portion of ridge BC between them. An automated processing procedure was used for picking onset times of the reflected radar pulses, converting travel times to distances, interpolating missing data, converting pressure transducer readings, correcting navigational drift, performing crossover analysis, and zeroing rémanent crossover errors. Interpolation between flight-lines was carried out using the minimum curvature method.Maps of ice thickness (estimated accuracy 20 m) and basal-reflection strength (estimated accuracy 1 dB) were produced. The ice-thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features. The reflection-strength map shows pronounced contrasts between the ice streams and ridge BC and between the two ice streams themselves. We interpret the reflection strengths to mean that the bed of Ice Stream C, as well as that of Ice Stream B, is unfrozen, that the bed of ridge BC is frozen and that the boundary between the frozen bed of ridge BC and the unfrozen bed of Ice Stream C lies precisely below the former shear margin of the ice stream.

scholarly journals Seismic-Reflection Profiling of a Widespread Till Beneath Ice Stream B, West Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 210 ◽  
Author(s):  
Sean T. Rooney ◽  
D. D. Blankenship ◽  
R. B. Alley ◽  
C. R. Bentley
Keyword(s):  
Seismic Reflection ◽  
Ross Sea ◽  
High Porosity ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Flow ◽  
West Antarctic ◽  
Ice Stream ◽  
Seismic Reflection Profiling ◽  
Ice Stream B

Seismic-reflection profiling has previously shown that, at least at one location. Ice Stream Β in West Antarctica rests on a layer of till a few meters thick (Blankenship and others 1986). Analyses of both compressional- and shear-wave seismic reflections from the ice–till boundary confirm the results of those earlier studies, which showed that the till is water-saturated and has a high porosity and low differential pressure. We conclude that this till is basically homogeneous, at least on a scale of tens of kilometers, though some evidence that its properties vary laterally can be discerned in these data. We propose that the till is widespread beneath Ice Stream Β and probably also beneath the other West Antarctic ice streams. Our seismic profiling shows that the till is essentially continuous beneath Ice Stream Β over at least 12 km parallel to ice flow and 8 km transverse to flow. Beneath these profiles the till averages about 6.5 m thick and is present everywhere except possibly on isolated bedrock ridges parallel to ice flow. The till thickness on these bedrock ridges falls to less than 2 m, the limit of our seismic resolution, but there is evidence that the ridges do not impede ice flow substantially. The bedrock beneath the till is fluted parallel to flow, with flutes that are 10–13 m deep by 200–1000 m wide; we believe these flutes are formed by erosion beneath a deforming till. We also observe an angular unconformity at the base of the till, which is consistent with the idea that erosion is occurring there. The sedimentary record in the Ross Embayment looks very similar to that beneath Ice Stream B, i.e. a few meters of till resting unconformably (the Ross Sea unconformity) on lithified sedimentary rock, and we postulate that the Ross Sea unconformity was generated by erosion beneath a grounded ice sheet by a deforming till.

scholarly journals Thickness changes on Whillans Ice Stream and Ice Stream C, West Antarctica, derived from laser altimeter measurements

2003 ◽  
Vol 49 (165) ◽  
pp. 223-230 ◽  
Author(s):  
Vandy Blue Spikes ◽  
Beáta M. Csatho ◽  
Gordon S. Hamilton ◽  
Ian M. Whillans
Keyword(s):  
Average Rate ◽  
West Antarctica ◽  
Laser Altimeter ◽  
Equivalent Thickness ◽  
Thickness Change ◽  
Airborne Laser ◽  
Ice Stream ◽  
Whillans Ice Stream ◽  
Elevation Changes ◽  
Global Sea Level

AbstractRepeat airborne laser altimeter measurements are used to derive surface elevation changes on parts of Whillans Ice Stream and Ice Stream C, West Antarctica. Elevation changes are converted to estimates of ice equivalent thickness change using local accumulation rates, surface snow densities and vertical bedrock motions. The surveyed portions of two major tributaries of Whillans Ice Stream are found to be thinning almost uniformly at an average rate of ∼1 m a−1. Ice Stream C has a complicated elevation-change pattern, but is generally thickening. These results are used to estimate the contribution of each surveyed region to the current rate of global sea-level rise.

scholarly journals Delineation of a catchment boundary using velocity and elevation measurements

1998 ◽  
Vol 27 ◽  
pp. 140-144 ◽  
Author(s):  
S. F. Price ◽  
I. M. Whillans
Keyword(s):  
Field Measurements ◽  
Surface Velocity ◽  
Data Sampling ◽  
West Antarctica ◽  
Ice Surface ◽  
Ice Stream ◽  
Elevation Data ◽  
Boundary Location ◽  
Ice Stream B

The determination of catchment boundaries is a major source of uncertainty in net balance studies on large ice sheets. Here, a method for defining a catchment boundary is developed using new measurements of ice-surface velocity and elevation near the Ice Stream B/C boundary in West Antarctica. An objective method for estimating confidence in the catchment boundary is proposed. Using elevation data, the resulting mean standard deviation in boundary location is 13 km in position or 6000 km2 in area. Applying a similar uncertainty to both sides of the Ice Stream Β catchment results in a catchment-area uncertainty of 9%. Much larger uncertainties arise when the method is applied to velocity data. The uncertainty in both cases is primarily determined by the density of field measurements and is proportionally similar for larger catchment basins. Differences in the position of the velocity-determined boundary and the elevation-determined boundary probably result from data sampling. The boundary positions determined here do not support the hypothesis that Ice Stream Β captured parts of the Ice Stream C catchment.

scholarly journals Micro-earthquakes beneath Ice Streams Β and C, West Antarctica: observations and implications

1993 ◽  
Vol 39 (133) ◽  
pp. 455-462 ◽  
Author(s):  
S. Anandakrishnan ◽  
C. R. Bentley
Keyword(s):  
Fault Plane ◽  
Source Parameters ◽  
West Antarctica ◽  
Ice Streams ◽  
Rapid Movement ◽  
Plane Analysis ◽  
Ice Stream ◽  
The Difference ◽  
Ice Stream B ◽  
Active Flow

Abstract Micro-earthquakes have been monitored at two locations on Ice Stream Β and one on Ice Stream C using a seismographic array built specifically for that purpose. Subglacial micro-earthquakes arc 20 times more abundant beneath Ice Stream C than beneath Ice Stream B, despite the 100 times more rapid movement of Ice Stream B. Triangulation shows the foci beneath Ice Stream C, like those beneath Ice Stream B, to be within a few meters of the base of the ice, presumably within the uppermost part of the bed, and fault-plane analysis indicates slips on horizontal planes at about a 30° angle to the presumed direction of formerly active flow. Source parameters, computed from spectra of the arrivals, confirmed that the speed of slip is three orders of magnitude faster beneath Ice Stream C than beneath Ice Stream Β which means that a five orders-of-magnitude greater fraction of the velocity of Ice Stream C is contributed by the faulting, although that fraction is still small. We attribute the difference in activity beneath the two ice streams to the loss of dilatancy in the till beneath Ice Stream C in the process that led to its stagnation.

scholarly journals The marginal shear stress of Ice Stream B, West Antarctica

1997 ◽  
Vol 43 (145) ◽  
pp. 415-426 ◽  
Author(s):  
Miriam Jackson ◽  
Barclay Kamb
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Time Scale ◽  
Shear Zones ◽  
Hot Water ◽  
West Antarctica ◽  
Ice Stream ◽  
Flow Law ◽  
Ice Stream B

AbstractTo ascertain whether the velocity of Ice Stream B, West Antarctica, may be controlled by the stresses in its marginal shear zones (the “Snake” and the “Dragon”), we undertook a determination of the marginal shear stress in the Dragon near Camp Up B by using ice itself as a stress meter. The observed marginal shear strain rate of 0.14 a−1is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a depth of 300 m in the Dragon, obtained by using a hot-water ice-coring drill. The test-specimen orientation relative to the stress axes in the tests is chosen on the basis ofc-axis fabrics so that the test applies horizontal shear across vertical planes parallel to the margin. The resulting marginal shear stress is (2.2 ± 0.3) × 105Pa. This implies that 63–100% of the ice stream’s support against gravitational loading comes from the margins and only 37–0% from the base, so that the margins play an important role in controlling the ice-stream motion. The marginal shear-stress value is twice that given by the ice-stream model of Echelmeyer and others (1994) and the corresponding strain-rate enhancement factors differ greatly (E≈ 1–2 vs 10–12.5). This large discrepancy could be explained by recrystallization of the ice during or shortly after coring. Estimates of the expected recrystallization time-scale bracket the ∼1 h time-scale of coring and leave the likelihood of recrystallization uncertain. However, the observed two-maximum fabric type is not what is expected for annealing recrystallization from the sharp single-maximum fabric that would be expected in situ at the high shear strains involved (γ ∼ 20). Experimental data from Wilson (1982) suggest that, if the core did recrystallize, the prior fabric was a two-maximum fabric not substantially different from the observed one, which implies that the measured flow law and derived marginal shear stress are applicable to the in situ situation. These issues need to be resolved by further work to obtain a more definitive observational assessment of the marginal shear stress.

scholarly journals Crevasse patterns at the onset to Ice Stream B, West Antarctica

2001 ◽  
Vol 47 (156) ◽  
pp. 29-36 ◽  
Author(s):  
S. F. Price ◽  
I. M. Whillans
Keyword(s):  
Satellite Imagery ◽  
Water Production ◽  
West Antarctica ◽  
Ice Stream ◽  
Water Production Rate ◽  
Ice Stream B ◽  
And Storage ◽  
Possible Cause ◽  
Over Time ◽  
Streaming Flow

AbstractSequential satellite imagery and modeling are used to investigate crevasse patterns at the head of Ice Stream B tributary B1b. The crevasses, informally called the “chromosomes”, form at the upstream limit to B1b’s northern shear margin and chaotic crevasse zone. We find that the onset to crevasse formation, and by inference the onset to streaming flow, has migrated upstream over time at a mean rate of 230(16) m a−1. A possible cause for that migration is changes in net basal friction due to changes in basal water production rate and storage.

scholarly journals Mass Balance of Ice Stream B, West Antarctica

1988 ◽  
Vol 11 ◽  
pp. 187-193 ◽  
Author(s):  
I. M. Whillans ◽  
R. A. Bindschadler
Keyword(s):  
Mass Balance ◽  
Catchment Area ◽  
Accumulation Rate ◽  
Katabatic Wind ◽  
Stream Velocity ◽  
Beta Activity ◽  
West Antarctica ◽  
Gross Beta ◽  
Ice Stream ◽  
Ice Stream B

The mass balance of the drainage area of Ice Stream B, West Antarctica, is calculated from new measurements of both discharge and accumulation rate. The discharge is computed for a transverse section near the lower end of the ice stream. Velocities have been obtained for 787 sites, using repeated photogrammetry, with ground control by Transit (doppler) satellite tracking. Thicknesses have been obtained by radio echo-sounding. The uncertainties in the discharge calculations are only about 3%. Net accumulation is derived from profiles of gross beta activity and from identification of the 1954–55 and 1964–65 nuclear-bomb strata. The major uncertainties are associated with the identification of the catchment area and with the accumulation rate. Accumulation rate varies locally, probably due to the interaction of katabatic wind with local slope, and many spot measurements are needed to obtain a good regional mean. The integrated input is 21.4 ± 5.2 km3 a−1, and the output is 30.0 ± 1.0 km3 a−1. The deficit is thus 8.6 ± 6.2 km3 a−1, which corresponds to a mean thinning rate of 0.06 m a−1 ± 0.04 m a−1. The difference from earlier estimates is mainly due to the refined catchment area and accumulation. The imbalance is significant but smaller than previously calculated: for balance the accumulation rate or catchment area would need to be about 39% larger or the ice stream velocity would need to be 28% slower.

scholarly journals Seismic-Reflection Profiling of a Widespread Till Beneath Ice Stream B, West Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 210-210
Author(s):  
Sean T. Rooney ◽  
D. D. Blankenship ◽  
R. B. Alley ◽  
C. R. Bentley
Keyword(s):  
Seismic Reflection ◽  
Ross Sea ◽  
High Porosity ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Flow ◽  
West Antarctic ◽  
Ice Stream ◽  
Seismic Reflection Profiling ◽  
Ice Stream B

Seismic-reflection profiling has previously shown that, at least at one location. Ice Stream Β in West Antarctica rests on a layer of till a few meters thick (Blankenship and others 1986). Analyses of both compressional- and shear-wave seismic reflections from the ice–till boundary confirm the results of those earlier studies, which showed that the till is water-saturated and has a high porosity and low differential pressure. We conclude that this till is basically homogeneous, at least on a scale of tens of kilometers, though some evidence that its properties vary laterally can be discerned in these data. We propose that the till is widespread beneath Ice Stream Β and probably also beneath the other West Antarctic ice streams.Our seismic profiling shows that the till is essentially continuous beneath Ice Stream Β over at least 12 km parallel to ice flow and 8 km transverse to flow. Beneath these profiles the till averages about 6.5 m thick and is present everywhere except possibly on isolated bedrock ridges parallel to ice flow. The till thickness on these bedrock ridges falls to less than 2 m, the limit of our seismic resolution, but there is evidence that the ridges do not impede ice flow substantially. The bedrock beneath the till is fluted parallel to flow, with flutes that are 10–13 m deep by 200–1000 m wide; we believe these flutes are formed by erosion beneath a deforming till. We also observe an angular unconformity at the base of the till, which is consistent with the idea that erosion is occurring there. The sedimentary record in the Ross Embayment looks very similar to that beneath Ice Stream B, i.e. a few meters of till resting unconformably (the Ross Sea unconformity) on lithified sedimentary rock, and we postulate that the Ross Sea unconformity was generated by erosion beneath a grounded ice sheet by a deforming till.

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