Ice-shelf Response to Ice-stream Discharge Fluctuations: I. Unconfined Ice Tongues

AbstractIce-stream discharge fluctuations constitute an independent means of forcing unsteady ice-shelf behavior, and their effect must be distinguished from those of oceanic and atmospheric climate to understand ice-shelf change. In addition, ice-stream-generated thickness anomalies may constitute a primary trigger of ice-rise formation in the absence of major sea-level fluctuations. Such triggering may maintain the current ice-rise population that, in turn, contributes to long-term ice-sheet stability. Here, we show that ice-stream-generated fluctuations of an ideal, two-dimensional ice shelf propagate along two characteristic trajectories. One trajectory permits instantaneous transmission of grounding-line velocity changes to all points down-stream. The other trajectory represents slow transmission of grounding-line thickness changes along Lagrangian particle paths.

Download Full-text

Ice-shelf Response to Ice-stream Discharge Fluctuations: II. Ideal Rectangular Ice Shelf

Journal of Glaciology ◽

10.1017/s0022143000009151 ◽

1988 ◽

Vol 34 (116) ◽

pp. 128-135 ◽

Cited By ~ 16

Author(s):

Douglas R. Macayeal ◽

Manfred A. Lange

Keyword(s):

Time Scale ◽

Stream Velocity ◽

Ice Shelf ◽

Pass Filter ◽

Low Pass Filter ◽

Stream Discharge ◽

Discharge Fluctuations ◽

Ice Stream ◽

Low Pass ◽

Velocity Anomalies

Abstract Abstract. Ice-shelf thickness and velocity anomalies resulting from ice-stream discharge fluctuations are calculated for an ideal ice shelf fed by a single ice stream and confined within a rectangular coastal geometry. Ice-shelf response to periodic forcing is found to be linear (thickness and velocity anomalies oscillate at the forcing frequency, and response scales with the forcing). Thickness anomalies are trapped near the ice-stream outlet and propagate down-stream at a slow, advective time-scale. Velocity anomalies tend to be widespread and propagate instantaneously throughout the ice-shelf environment. Ice-shelf response is sensitive to ice-stream fluctuation time-scale in the manner of a low-pass filter; longer forcing time-scales produce more widespread ice-shelf response. If ice-stream velocity and thickness fluctuations are in phase, thickness-anomaly maxima typically occur down-stream of the ice-stream outlet. This effect may determine where ice rumples and rises are likely to form in response to stochastic ice-stream variability.

Download Full-text

Ice-shelf Response to Ice-stream Discharge Fluctuations: II. Ideal Rectangular Ice Shelf

Journal of Glaciology ◽

10.3189/s0022143000009151 ◽

1988 ◽

Vol 34 (116) ◽

pp. 128-135 ◽

Cited By ~ 1

Author(s):

Douglas R. Macayeal ◽

Manfred A. Lange

Keyword(s):

Time Scale ◽

Stream Velocity ◽

Ice Shelf ◽

Pass Filter ◽

Low Pass Filter ◽

Stream Discharge ◽

Discharge Fluctuations ◽

Ice Stream ◽

Low Pass ◽

Velocity Anomalies

AbstractAbstract. Ice-shelf thickness and velocity anomalies resulting from ice-stream discharge fluctuations are calculated for an ideal ice shelf fed by a single ice stream and confined within a rectangular coastal geometry. Ice-shelf response to periodic forcing is found to be linear (thickness and velocity anomalies oscillate at the forcing frequency, and response scales with the forcing). Thickness anomalies are trapped near the ice-stream outlet and propagate down-stream at a slow, advective time-scale. Velocity anomalies tend to be widespread and propagate instantaneously throughout the ice-shelf environment. Ice-shelf response is sensitive to ice-stream fluctuation time-scale in the manner of a low-pass filter; longer forcing time-scales produce more widespread ice-shelf response. If ice-stream velocity and thickness fluctuations are in phase, thickness-anomaly maxima typically occur down-stream of the ice-stream outlet. This effect may determine where ice rumples and rises are likely to form in response to stochastic ice-stream variability.

Download Full-text

Sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions: a model study

Journal of Glaciology ◽

10.3189/172756502781831061 ◽

2002 ◽

Vol 48 (163) ◽

pp. 552-558 ◽

Cited By ~ 47

Author(s):

Marjorie Schmeltz ◽

Eric Rignot ◽

Todd K. Dupont ◽

Douglas R. MacAyeal

Keyword(s):

Shear Stress ◽

Element Model ◽

Shelf Area ◽

West Antarctica ◽

Ice Shelf ◽

Ice Stream ◽

Model Study ◽

Grounding Line ◽

Glacier Velocity ◽

Basal Shear

AbstractWe use a finite-element model of coupled ice-stream/ice-shelf flow to study the sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions. By tuning a softening coefficient of the ice along the glacier margins, and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream, we are able to match model velocity to that observed with interferometric synthetic aperture radar (InSAR). We use the model to investigate the effect of small perturbations on ice flow. We find that a 5.5–13% reduction in our initial ice-shelf area increases the glacier velocity by 3.5–10% at the grounding line. The removal of the entire ice shelf increases the grounding-line velocity by > 70%. The changes in velocity associated with ice-shelf reduction are felt several tens of km inland. Alternatively, a 5% reduction in basal shear stress increases the glacier velocity by 13% at the grounding line. By contrast, softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity. Hence, both the ice-shelf buttressing and the basal shear stress contribute significant resistance to the flow of Pine Island Glacier.

Download Full-text

Can Relict Crevasse Plumes on Antarctic Ice Shelves Reveal a History of Ice-Stream Fluctuation?

Annals of Glaciology ◽

10.3189/s0260305500006364 ◽

1988 ◽

Vol 11 ◽

pp. 77-82 ◽

Cited By ~ 3

Author(s):

D. R. MacAyeal ◽

R. A. Bindschadler ◽

K. C. Jezek ◽

S. Shabtaie

Keyword(s):

Numerical Model ◽

Ice Shelf ◽

Ice Streams ◽

Ice Shelves ◽

Stream Discharge ◽

Ice Stream ◽

History Of

Configurations of relict surface-crevasse bands and medial moraines that emanate from the shear margins of ice streams are simulated, using a numerical model of an ideal rectangular ice shelf to determine their potential for recording a past ice-stream discharge chronology.

Download Full-text

The supply of subglacial meltwater to the grounding line of the Siple Coast, West Antarctica

Annals of Glaciology ◽

10.3189/2012aog60a119 ◽

2012 ◽

Vol 53 (60) ◽

pp. 267-280 ◽

Cited By ~ 56

Author(s):

S.P. Carter ◽

H.A. Fricker

Keyword(s):

Temporal Variability ◽

Freshwater Flux ◽

Spatial And Temporal Variability ◽

Ice Shelf ◽

Ice Streams ◽

Ross Ice Shelf ◽

Lake Volume ◽

Grounding Line ◽

Subglacial Lakes

AbstractRecent satellite studies have shown that active subglacial lakes exist under the Antarctic ice streams and persist almost to their grounding lines. When the lowest-lying lakes flood, the water crosses the grounding line and enters the sub-ice-shelf cavity. Modeling results suggest that this additional freshwater influx may significantly enhance melting at the ice-shelf base. We examine the spatial and temporal variability in subglacial water supply to the grounding lines of the Siple Coast ice streams, by combining estimates for lake volume change derived from Ice, Cloud and land Elevation Satellite (ICESat) data with a model for subglacial water transport. Our results suggest that subglacial outflow tends to concentrate towards six embayments in the Siple Coast grounding line. Although mean grounding line outflow is ~60m3 s–1 for the entire Siple Coast, maximum local grounding line outflow may temporarily exceed 300 m3 s–1 during the synchronized flooding of multiple lakes in a hydrologic basin. Variability in subglacial outflow due to subglacial lake drainage may account for a substantial portion of the observed variability in freshwater flux out of the Ross Ice Shelf cavity. The temporal variability in grounding line outflow results in a net reduction in long-term average melt rate, but temporary peak melting rates may exceed the long-term average by a factor of three.

Download Full-text

Grounding-zone flow variability of Priestley Glacier, Antarctica, in a diurnal tidal regime

10.5194/egusphere-egu21-15197 ◽

2021 ◽

Author(s):

Reinhard Drews ◽

Christian Wild ◽

Oliver Marsh ◽

Wolfgang Rack ◽

Todd Ehlers ◽

...

Keyword(s):

Spatial Scales ◽

Ice Shelf ◽

Ice Streams ◽

Ice Flow ◽

Ice Shelves ◽

Ice Stream ◽

Grounding Line ◽

Temporal And Spatial ◽

Gnss Measurements ◽

Natural Laboratory

<p>Dynamics of polar outlet glaciers vary with ocean tides, providing a natural laboratory to understand basal processes beneath ice streams, ice rheology and ice-shelf buttressing. We apply Terrestrial Radar Interferometry to close the spatiotemporal gap between localized, temporally well-resolved GNSS and area-wide but sparse satellite observations. Three-hour flowfields collected over an eight day period at Priestley Glacier, Antarctica, validate and provide the spatial context for concurrent GNSS measurements. Ice flow is fastest during falling tides and slowest during rising tides. Principal components of the timeseries prove upstream propagation of tidal signatures $>$ 10 km away from the grounding line. Hourly, cm-scale horizontal and vertical flexure patterns occur $>$6 km upstream of the grounding line. Vertical uplift upstream of the grounding line is consistent with ephemeral re-grounding during low-tide impacting grounding-zone stability. On the freely floating ice shelves, we find velocity peaks both during high- and low-tide suggesting that ice-shelf buttressing varies temporally as a function of flexural bending from tidal displacement. Taken together, these observations identify tidal imprints on ice-stream dynamics on new temporal and spatial scales providing constraints for models designed to isolate dominating processes in ice-stream and ice-shelf mechanics.</p>

Download Full-text

History of the Larsen C Ice Shelf reconstructed from sub–ice shelf and offshore sediments

Geology ◽

10.1130/g48503.1 ◽

2021 ◽

Author(s):

J.A. Smith ◽

C.-D. Hillenbrand ◽

C. Subt ◽

B.E. Rosenheim ◽

T. Frederichs ◽

...

Keyword(s):

Sediment Cores ◽

Ice Shelf ◽

Climatic Forcing ◽

Ice Shelves ◽

Future Evolution ◽

Grounding Line ◽

History Of ◽

Two Phases ◽

Line Following

Because ice shelves respond to climatic forcing over a range of time scales, from years to millennia, an understanding of their long-term history is critically needed for predicting their future evolution. We present the first detailed reconstruction of the Larsen C Ice Shelf (LCIS), eastern Antarctic Peninsula (AP), based on data from sediment cores recovered from below and in front of the ice shelf. Sedimentologic and chronologic information reveals that the grounding line (GL) of an expanded AP ice sheet had started its retreat from the midshelf prior to 17.7 ± 0.53 calibrated (cal.) kyr B.P., with the calving line following ~6 k.y. later. The GL had reached the inner shelf as early as 9.83 ± 0.85 cal. kyr B.P. Since ca. 7.3 ka, the ice shelf has undergone two phases of retreat but without collapse, indicating that the climatic limit of LCIS stability was not breached during the Holocene. Future collapse of the LCIS would therefore confirm that the magnitudes of both ice loss along the eastern AP and underlying climatic forcing are unprecedented during the past 11.5 k.y.

Download Full-text

Thinning and Grounding-Line Retreat on Ross Ice Shelf, Antarctica

Annals of Glaciology ◽

10.3189/s0260305500006492 ◽

1988 ◽

Vol 11 ◽

pp. 165-172 ◽

Cited By ~ 6

Author(s):

R. H. Thomas ◽

S. N. Stephenson ◽

R. A. Bindschadler ◽

S. Shabtaie ◽

C. R. Bentley

Keyword(s):

Snow Accumulation ◽

Gravity Potential ◽

Ice Thickness ◽

Ice Shelf ◽

Ross Ice Shelf ◽

Major Activity ◽

Ice Stream ◽

Grounding Line ◽

Ice Stream B ◽

Basal Melting

Detailed measurements of surface topography, ice motion, snow accumulation, and ice thickness were made in January 1974 and again in December 1984, along an 8 km stake network extending from the ice sheet, across the grounding line, and on to floating ice shelf in the mouth of slow-moving Ice Stream C, which flows into the eastern side of Ross Ice Shelf, Antarctica. During the 11 years between surveys, the grounding line retreated by approximately 300 m. This was caused by net thinning of the ice shelf, which we believe to be a response to the comparatively recent, major decrease in ice discharge from Ice Stream C. Farther inland, snow accumulation is not balanced by ice discharge, and the ice stream is growing progressively thicker.There is evidence that the adjacent Ice Stream B has slowed significantly over the last decade, and this may be an early indication that this fast-moving ice stream is about to enter a period of stagnation similar to that of Ice Stream C. Indeed, these large ice streams flowing from West Antarctica into Ross Ice Shelf may oscillate between periods of relative stagnation and major activity. During active periods, large areas of ice shelf thicken and run aground on seabed to form extensive “ice plains” in the mouth of the ice stream. Ultimately, these become too large to be pushed seaward by the ice stream, which then slows down and enters a period of stagnation. During this period, the grounding line of the ice plain retreats, as we observe today in the mouth of Ice Stream C, because nearby ice shelf, no longer compressed by ice-stream motion, progressively thins. At the same time, water within the deformable till beneath the ice starts to freeze on to the base of the ice stream, and snow accumulation progressively increases the ice thickness. A new phase of activity would be initiated when the increasing gravity potential of the ice stream exceeds the total resistance of the shrinking ice plain and the thinning layer of deformable till at the bed. This could occur rapidly if the effects of the shrinking ice plain outweigh those of the thinning (and therefore stiffening) till. Otherwise, the till layer would finally become completely frozen, and the ice stream would have to thicken sufficiently to initiate significant heating by internal deformation, followed by basal melting and finally saturation of an adequate thickness of till; this could take some thousands of years.

Download Full-text

A Coupled Marine Ice-Stream – Ice-Shelf Model

Journal of Glaciology ◽

10.3189/s0022143000005281 ◽

1987 ◽

Vol 33 (113) ◽

pp. 3-15 ◽

Cited By ~ 5

Author(s):

Isabelle Muszynski ◽

G. E. Birchfield

Keyword(s):

Sea Water ◽

Two Dimensions ◽

Scale Analysis ◽

Sliding Velocity ◽

Ice Shelf ◽

Ice Stream ◽

Grounding Line ◽

Basal Sliding ◽

Ice Flows ◽

Shearing Deformation

AbstractEvidence as to the potential roles of marine ice flows in the dramatic climatological changes which have occurred from the late Pleistocene to the present is reviewed, indicating the need for careful modeling studies to evaluate several crucial hypotheses. A scale analysis of the flow of a marine ice stream coupled to a freely floating ice shelf is presented, in two dimensions and ignoring thermodynamic effects. With these limitations, the most important control of the dynamics of the ice stream is associated with first-order buoyancy effects related to the density contrast ∆ρ/ρw between ice and sea-water. It is shown that longitudinal stretching, arising from large gradients in basal sliding velocity, dominates shearing deformation provided the aspect ratio ω2 « ∆ρ/ρw. The buoyancy control is established through the necessity of having continuously varying longitudinal strain-rates in the neighborhood of the grounding line.The scale analysis is the basis for derivation of a simplified model of a fast-flowing ice stream coupled to a freely floating ice shelf. The distance in the ice stream up-stream from the grounding line over which the above dynamic regime extends is estimated and found to be relatively insensitive to the basal sliding velocity and to the rheological constant of ice. A further potentially important feed-back mechanism between ice stream and ice shelf is associated with buoyancy corrections to the longitudinal deviatoric stress field.

Download Full-text