Analysis of ice shelf flexure and its InSAR representation in the grounding zone of the southern McMurdo Ice Shelf

Abstract. We examine tidal flexure in the grounding zone of the McMurdo Ice Shelf, Antarctica, using a combination of TerraSAR-X repeat-pass radar interferometry, a precise digital elevation model, and GPS ground validation data. Satellite and field data were acquired in tandem between October and December 2014. Our GPS data show a horizontal modulation of up to 60 % of the vertical displacement amplitude at tidal periods within a few kilometres of the grounding line. We ascribe the observed oscillatory horizontal motion to varying bending stresses and account for it using a simple elastic beam model. The horizontal surface strain is removed from nine differential interferograms to obtain precise bending curves. They reveal a fixed (as opposed to tidally migrating) grounding-line position and eliminate the possibility of significant upstream bending at this location. The consequence of apparent vertical motion due to uncorrected horizontal strain in interferometric data is a systematic mislocation of the interferometric grounding line by up to the order of one ice thickness, or several hundred metres. While our field site was selected due to its simple boundary conditions and low background velocity, our findings are relevant to other grounding zones studied by satellite interferometry, particularly studies looking at tidally induced velocity changes or interpreting satellite-based flexure profiles.

Download Full-text

Analysis of ice shelf flexure and its InSAR representation in the grounding zone of the Southern McMurdo Ice Shelf

10.5194/tc-2017-13 ◽

2017 ◽

Cited By ~ 1

Author(s):

Wolfgang Rack ◽

Matt A. King ◽

Oliver J. Marsh ◽

Christian T. Wild ◽

Dana Floricioiu

Keyword(s):

Vertical Motion ◽

Elastic Beam ◽

Surface Strain ◽

Beam Model ◽

Ice Shelf ◽

Validation Data ◽

Simple Boundary ◽

Grounding Line ◽

Bending Stresses ◽

Induced Velocity

Abstract. We examine tidal flexure in the grounding zone of the McMurdo Ice Shelf, Antarctica, using a combination of a TerraSAR-X repeat-pass radar interferometry, a precise digital elevation model, and GPS ground validation data. Satellite and field data were acquired in tandem between October and December 2014. Our GPS data show a horizontal modulation of up to 60 % of the vertical amplitude at tidal periods within a few km of the grounding line. We ascribe this to bending stresses and account for it using a simple elastic beam model. The horizontal surface strain is removed from nine differential interferograms to obtain precise vertical bending curves. This processing step allows us to identify a fixed (as opposed to tidally migrating) grounding line position and eliminates the possibility of significant upstream bending at this location. The change in apparent vertical motion due to horizontal strain can lead to a systematic mis-location of the interferometric grounding line by the order of up to one ice thickness, or several hundred metres. While our field site was selected in consideration of the simple boundary conditions and low background velocity the findings are relevant to other satellite-based grounding zone studies, particularly those looking at tidally-induced velocity changes or interpreting satellite-based flexure profiles.

Download Full-text

The use of tiltmeters to study the dynamics of Antarctic ice-shelf grounding lines

Journal of Glaciology ◽

10.1017/s0022143000042799 ◽

1991 ◽

Vol 37 (125) ◽

pp. 51-58 ◽

Cited By ~ 52

Author(s):

A. M. Smith

Keyword(s):

Elastic Beam ◽

Ice Thickness ◽

Elastic Model ◽

Beam Model ◽

Line Position ◽

Stream Data ◽

Ice Shelf ◽

Ice Stream ◽

Grounding Line ◽

Data Coverage

Abstract New tiltmeter data are presented from Doake Ice Rumples on Ronne Ice Shelf, Antarctica. Five sites which showed a tidal ice-shelf flexure have been analysed using an elastic beam model to investigate the variation of flexure amplitude with distance from the grounding line. An earlier study on Rutford Ice Stream which also used an elastic model required an ice thickness much less than that observed. Reworking the Rutford Ice Stream data suggests that this greatly reduced ice thickness is not required, given the current sparse data coverage. The elastic model is used to improve the estimated grounding-line position on Rutford Ice Stream. Some of the difficulties in modelling ice-shelf flexure and locating grounding lines are discussed.

Download Full-text

The use of tiltmeters to study the dynamics of Antarctic ice-shelf grounding lines

Journal of Glaciology ◽

10.3189/s0022143000042799 ◽

1991 ◽

Vol 37 (125) ◽

pp. 51-58 ◽

Cited By ~ 3

Author(s):

A. M. Smith

Keyword(s):

Elastic Beam ◽

Ice Thickness ◽

Elastic Model ◽

Beam Model ◽

Line Position ◽

Stream Data ◽

Ice Shelf ◽

Ice Stream ◽

Grounding Line ◽

Data Coverage

Download Full-text

On the representation of ice-shelf grounding zones in SAR interferograms

Journal of Glaciology ◽

10.3189/172756502781831197 ◽

2002 ◽

Vol 48 (162) ◽

pp. 345-356 ◽

Cited By ~ 16

Author(s):

Bernhard T. Rabus ◽

Oliver Lang

Keyword(s):

Elastic Beam ◽

Ice Thickness ◽

Beam Model ◽

Ice Shelf ◽

Ice Shelves ◽

One Dimensional ◽

West Antarctic ◽

Grounding Line ◽

The One ◽

Differential Interferogram

AbstractWe investigate limitations of the one-dimensional elastic-beam model to detect grounding line and thickness of an ice shelf from a differential interferogram. Spatial limitations due to grounding-line curvature and variable ice thickness are analyzed by comparison with two-dimensional plate flexure. Temporal limitations from the tide-dependent shift of the grounding line are analyzed by superpositions of four tidal flexure profiles representing differential interferograms. (i) At scales greater than one ice thickness, seaward protrusions of the grounding line are well represented by the elastic-beam model, while landward embayments of the same scale produce significant misplacements >10% of the ice thickness. (ii) For reasonable spatial variations of shelf thickness, the elastic-beam model gives reliable estimates of grounding-line position and unfractured mean ice thickness near the grounding line. (iii) For about 20% of superpositions of four tidal flexure profiles, the resulting grounding-line misplacements exceed the physical tidal shift of the grounding line by factors >2. For differential tide levels <10% of a 1 m tide dynamics, a physical shift of the grounding line of 0.3 km per metre of tide can lever misplacements of >2 km. Examples of real interferometric profiles from West Antarctic ice shelves corroborate our results.

Download Full-text

On the interpretation of ice-shelf flexure measurements

Journal of Glaciology ◽

10.1017/jog.2017.44 ◽

2017 ◽

Vol 63 (241) ◽

pp. 783-791 ◽

Cited By ~ 10

Author(s):

SEBASTIAN H. R. ROSIER ◽

OLIVER J. MARSH ◽

WOLFGANG RACK ◽

G. HILMAR GUDMUNDSSON ◽

CHRISTIAN T. WILD ◽

...

Keyword(s):

Elastic Beam ◽

Beam Theory ◽

Beam Model ◽

Line Position ◽

Ice Shelf ◽

Study Region ◽

Grounding Line ◽

Modelling Studies ◽

Bed Slope ◽

Viscoelastic Modelling

ABSTRACTTidal flexure in ice shelf grounding zones has been used extensively in the past to determine grounding line position and ice properties. Although the rheology of ice is viscoelastic at tidal loading frequencies, most modelling studies have assumed some form of linear elastic beam approximation to match observed flexure profiles. Here we use density, radar and DInSAR measurements in combination with full-Stokes viscoelastic modelling to investigate a range of additional controls on the flexure of the Southern McMurdo Ice Shelf. We find that inclusion of observed basal crevasses and density dependent ice stiffness can greatly alter the flexure profile and yet fitting a simple elastic beam model to that profile will still produce an excellent fit. Estimates of the effective Young's modulus derived by fitting flexure profiles are shown to vary by over 200% depending on whether these factors are included, even when the local thickness is well constrained. Conversely, estimates of the grounding line position are relatively insensitive to these considerations for the case of a steep bed slope in our study region. By fitting tidal amplitudes only, and ignoring phase information, elastic beam theory can provide a good fit to observations in a wide variety of situations. This should, however, not be taken as an indication that the underlying rheological assumptions are correct.

Download Full-text

The location of the grounding zone of Evans Ice Stream, Antarctica, investigated using SAR interferometry and modelling

Annals of Glaciology ◽

10.3189/172756409789624292 ◽

2009 ◽

Vol 50 (52) ◽

pp. 35-40 ◽

Cited By ~ 17

Author(s):

Helena J. Sykes ◽

Tavi Murray ◽

Adrian Luckman

Keyword(s):

Stream Flow ◽

Elastic Beam ◽

Tidal Height ◽

Sar Interferometry ◽

Beam Model ◽

West Antarctica ◽

Tidal Model ◽

Tidal Forcing ◽

Ice Stream ◽

Grounding Line

AbstractEvans Ice Stream, West Antarctica, has five tributaries and a complex grounding zone. The grounding zone of Evans Ice Stream, between the landward and seaward limits of tidal flexing, was mapped using SAR interferometry. The width of the mapped grounding zone was compared with that derived from an elastic beam model, and the tidal height changes derived from interferometry were compared with the results of a tidal model. Results show that in 1994 and 1996 the Evans grounding zone was located up to 100 km upstream of its location in the BEDMAP dataset. The grounding line of Evans Ice Stream is subjected to 5 m vertical tidal forcing, which would clearly affect ice-stream flow.

Download Full-text

Measuring the location and width of the Antarctic grounding zone using CryoSat-2

The Cryosphere ◽

10.5194/tc-14-2071-2020 ◽

2020 ◽

Vol 14 (6) ◽

pp. 2071-2086 ◽

Cited By ~ 1

Author(s):

Geoffrey J. Dawson ◽

Jonathan L. Bamber

Keyword(s):

Elastic Beam ◽

Young Modulus ◽

Beam Model ◽

Ice Shelf ◽

Amundsen Sea ◽

Ice Shelves ◽

Elevation Data ◽

Satellite Radar ◽

And Performance ◽

The Antarctic

Abstract. We present the results of mapping the limit of the tidal flexure (point F) and hydrostatic equilibrium (point H) of the grounding zone of Antarctic ice shelves from CryoSat-2 standard and swath elevation data. Overall we were able to map 31 % of the grounding zone of the Antarctic floating ice shelves and outlet glaciers. We obtain near-complete coverage of the Filchner–Ronne Ice Shelf. Here we manage to map areas of Support Force Glacier and the Doake Ice Rumples, which have previously only been mapped using break-in-slope methods. Over the Ross Ice Shelf, Dronning Maud Land and the Antarctic Peninsula, we obtained partial coverage, and we could not map a continuous grounding zone for the Amery Ice Shelf and the Amundsen Sea sector. Tidal amplitude and distance south (i.e. across-track spacing) are controlling factors in the quality of the coverage and performance of the approach. The location of the point F agrees well with previous observations that used differential satellite radar interferometry (DInSAR) and ICESat-1, with an average landward bias of 0.1 and 0.6 km and standard deviation of 1.1 and 1.5 km for DInSAR and ICESat measurements, respectively. We also compared the results directly with DInSAR interferograms from the Sentinel-1 satellites, acquired over the Evans Ice Stream and the Carlson Inlet (Ronne Ice Shelf), and found good agreement with the mapped points F and H. We also present the results of the spatial distribution of the grounding zone width (the distance between points F and H) and used a simple elastic beam model, along with ice thickness calculations, to calculate an effective Young modulus of ice of E=1.4±0.9 GPa.

Download Full-text

The effect of bottom boundary conditions in the ice-sheet to ice-shelf transition zone problem

Journal of Glaciology ◽

10.3189/002214307783258477 ◽

2007 ◽

Vol 53 (182) ◽

pp. 363-367 ◽

Cited By ~ 3

Author(s):

Alexander V. Wilchinsky

Keyword(s):

Transition Zone ◽

Multiple Solutions ◽

Vertical Motion ◽

Separation Mechanism ◽

Line Position ◽

Rigid Substrate ◽

Ice Shelf ◽

Bottom Boundary ◽

Bottom Boundary Condition ◽

Grounding Line

AbstractUniqueness of the transition zone solution is discussed. It is argued that when it is assumed that the stresses are continuous at the grounding line, the ice-shelf solution at the grounding line should possess a zero slope. In order to avoid issues caused by the mathematical singularity of the solution, the same technique as that used to study the transition zone is applied to a similar problem of lifting an elastic sheet from a rigid substrate, which allows a better physical understanding. This exemplifies the effect of the bottom boundary condition of no vertical motion imposed up to a fixed grounding line position, while the forcing parameters, such as the flow rate, vary. Its effect is to produce multiple solutions due to suppression of the separation mechanism.

Download Full-text

Force-perturbation analysis of Pine Island Glacier, Antarctica, suggests cause for recent acceleration

Annals of Glaciology ◽

10.3189/172756404781814429 ◽

2004 ◽

Vol 39 ◽

pp. 133-138 ◽

Cited By ~ 53

Author(s):

Robert Thomas ◽

Eric Rignot ◽

Pannirselvam Kanagaratnam ◽

William Krabill ◽

Gino Casassa

Keyword(s):

Surface Strain ◽

Ice Thickness ◽

Strain Rates ◽

West Antarctica ◽

Ice Shelf ◽

Amundsen Sea ◽

Ice Flow ◽

Velocity Increase ◽

Grounding Line ◽

Force Perturbation

AbstractPine Island Glacier, flowing into the Amundsen Sea from West Antarctica, thinned substantially during the 1990s, its grounding line receded by several km, and its velocity increased by >10% to values approaching 3 km a–1. Here, we use these observations, together with estimates of ice thickness and surface strain rates, to estimate the perturbation in forces resisting ice flow compatible with the observations. The analysis assumes that such perturbations are transmitted far upstream from where they originate, and that creep response to the perturbations can be described by equations similar to those that govern ice-shelf creep. It indicates that observed acceleration between 1996 and 2000 could have been caused by progressive ungrounding within the most seaward 25 km ‘ice plain’ of the grounded glacier. Earlier retreat and thinning of the glacier’s floating ice shelf may have provided the conditions that initiated ungrounding of the ice plain. Our analysis indicates that continued ice-plain thinning at the current rate of about 2 ma–1 will result in a velocity increase by 1 km a–1 within the next 11 years as the ice plain becomes totally ungrounded.

Download Full-text

Interactions between ice and ocean observed with phase-sensitive radar near an Antarctic ice-shelf grounding line

Journal of Glaciology ◽

10.3189/172756506781828502 ◽

2006 ◽

Vol 52 (178) ◽

pp. 325-346 ◽

Cited By ~ 57

Author(s):

Adrian Jenkins ◽

Hugh F.J. Corr ◽

Keith W. Nicholls ◽

Craig L. Stewart ◽

Christopher S.M. Doake

Keyword(s):

Vertical Motion ◽

Ice Thickness ◽

Strain Rates ◽

Ice Shelf ◽

Vertical Strain ◽

Grounding Line ◽

Phase Sensitive ◽

Radar Measurements ◽

Basal Melting

AbstractPrecise measurements of basal melting have been made at a series of 14 sites lying within a few kilometres of the grounding line of the Ronne Ice Shelf, Antarctica, where the ice thickness ranges from 1570 to 1940 m. The study was conducted over the course of 1 year and included a detailed survey of the horizontal deformation, as well as phase-sensitive radar measurements of the vertical displacement of both internal reflecting horizons and the ice-shelf base. Results from the surface survey show that the long-term viscous strain rate is modulated at tidal frequencies by (probably) elastic strains of order 10−5 per metre of tidal elevation. The radar measurements show a similar modulation of the long-term thinning/thickening of the ice shelf, with thickness oscillations up to a few centimetres in range. The long-term trends in ice thickness determined at points moving with the ice-shelf flow are consistent with a steady-state thickness profile. Vertical strain rates within the ice shelf were determined from the relative motion of internal reflectors. At two sites the observations were sufficient to discern the effect of tidal bending about a neutral surface 60% of the way down the ice column. Coincident measurements of horizontal and vertical strain imply a Poisson’s ratio of 0.5, and this combined with the asymmetric bending gives rise to the observed oscillations in thickness. At a number of sites the longterm viscous strain rates were found to be a linear function of depth. For an ice shelf this is an unexpected result. It can be attributed to the presence of significant vertical shear stresses set up close to the grounding line where the ice is still adjusting to flotation. Additional vertical motion arising from firn compaction was observed within the upper layers of the ice shelf. The additional motion was consistent with the assumption that firn density is a function only of the time since burial by steady surface accumulation. With both spatial and temporal fluctuations in the vertical strain rate accurately quantified it was possible to estimate the vertical motion of the ice-shelf base in response. Differences between the calculated and observed motion of the basal reflector arise because of basal melting. Derived melt rates at the 14 sites ranged from −0.11 ±0.31 to 2.51 ±0.10 m a−1, with a mean of 0. 85 m a−1 and a standard deviation of 0.69 m a−1, and showed no signs of significant sub-annual temporal variability. There was no obvious global correlation with either ice thickness or distance from the grounding line, although melt rates tended to decrease downstream along each of the flowlines studied. Previous estimates of basal melting in this region have been obtained indirectly from an assumption that the ice shelf is locally in equilibrium and have included a broad range of values. Only those at the lower end of the published range are consistent with the directly measured melt rates reported here.

Download Full-text