scholarly journals Tiltmeter Observations From Doake Ice Rumples, Ronne Ice Shelf, Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 222
Author(s):  
A. M. Smith
Keyword(s):  
Flow Direction ◽  
Beam Theory ◽  
Analysis Data ◽  
Ice Thickness ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Tidal Forces ◽  
Grounding Line ◽  
Level Of Agreement

New tiltmeter data from Doake Ice Rumples on Ronne Ice Shelf are presented. The tiltmeters detected flexing of the ice shelf close to the grounding line, due to tidal forces. In earlier studies on Rutford Ice Stream, flow was from grounded ice into the floating ice shelf. In contrast, the area studied on Ronne Ice Shelf exhibits flow from the ice shelf on to grounded ice rumples. The data span 5 km of the up-stream ice shelf and much of the grounded ice. Approximately 30 d of continuous tilt data are available for one site on the ice shelf. Analysis of this record can determine the dominant tidal frequencies present. A number of shorter records cover periods normally of a few days. These were obtained from sites both up-stream and down-stream of the long-term site. They have been used to investigate the variation in tilt amplitude with distance from the grounding line. The approximate position of the grounding line was located from the position of strand cracks and by using surface-elevation and ice-thickness data. The tiltmeters helped to confirm this position. Over grounded ice, tidal flexing is clearly present up to about 2 km from the grounding line. Beyond this it appears to be absent. The signal recorded by tiltmeters on grounded ice farther than 2 km from the grounding line is attributed, at present, to a temperature dependency of the tiltmeters. The flexing of ice shelves at tidal frequencies has previously been treated as an elastic problem. Available models which use beam theory require the use of a time-dependent function and a reduced “effective” ice thickness in the elastic modulus. On preliminary analysis, data from the ice shelf and grounding line of Doake Ice Rumples appear to be consistent with these theories. More detailed analysis and interpretation is required in order to confirm the level of agreement and to determine possible variations due to the “reversed” nature of the flow direction.

scholarly journals Tiltmeter Observations From Doake Ice Rumples, Ronne Ice Shelf, Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 222-222
Author(s):  
A. M. Smith
Keyword(s):  
Flow Direction ◽  
Beam Theory ◽  
Analysis Data ◽  
Ice Thickness ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Tidal Forces ◽  
Grounding Line ◽  
Level Of Agreement

New tiltmeter data from Doake Ice Rumples on Ronne Ice Shelf are presented. The tiltmeters detected flexing of the ice shelf close to the grounding line, due to tidal forces. In earlier studies on Rutford Ice Stream, flow was from grounded ice into the floating ice shelf. In contrast, the area studied on Ronne Ice Shelf exhibits flow from the ice shelf on to grounded ice rumples.The data span 5 km of the up-stream ice shelf and much of the grounded ice. Approximately 30 d of continuous tilt data are available for one site on the ice shelf. Analysis of this record can determine the dominant tidal frequencies present.A number of shorter records cover periods normally of a few days. These were obtained from sites both up-stream and down-stream of the long-term site. They have been used to investigate the variation in tilt amplitude with distance from the grounding line.The approximate position of the grounding line was located from the position of strand cracks and by using surface-elevation and ice-thickness data. The tiltmeters helped to confirm this position. Over grounded ice, tidal flexing is clearly present up to about 2 km from the grounding line. Beyond this it appears to be absent. The signal recorded by tiltmeters on grounded ice farther than 2 km from the grounding line is attributed, at present, to a temperature dependency of the tiltmeters.The flexing of ice shelves at tidal frequencies has previously been treated as an elastic problem. Available models which use beam theory require the use of a time-dependent function and a reduced “effective” ice thickness in the elastic modulus. On preliminary analysis, data from the ice shelf and grounding line of Doake Ice Rumples appear to be consistent with these theories. More detailed analysis and interpretation is required in order to confirm the level of agreement and to determine possible variations due to the “reversed” nature of the flow direction.

Thwaites and Dotson Ice Shelves: Field Site Selection and Early Results of Field Measurements

2020 ◽  
Author(s):  
Erin Pettit ◽  
Atsu Muto ◽  
Christian Wild ◽  
Karen Alley ◽  
Ted Scambos ◽  
...  
Keyword(s):  
Continental Shelf ◽  
Satellite Data ◽  
Ground Penetrating Radar ◽  
Hot Water ◽  
Ice Thickness ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Grounding Line ◽  
Ground Penetrating

<p>As part of the International Thwaites Glacier Collaboration (ITGC) field activity in West Antarctica for the 2019-2020 season, the Thwaites-Amundsen Regional Survey and Network (TARSAN) team drilled boreholes using hot water, deployed long-term instruments, and gathered several ground-based geophysical data sets to assess the ice-shelf stability and evolution.<br><br>The Thwaites Eastern Ice Shelf is an important buttress for a broad (25 km) section of Thwaites Glacier outflow and is restrained at present by a few pinning points at the northwestern edge of the shelf. The grounding line of this buttress has retreated within the last 5 years indicating instability. Recent imagery shows major new rifting and shearing within the ice shelf.<br><br>In the Dotson-Crosson Ice Shelf (a single ice shelf with a rapidly evolving central region that has thinned and ungrounded over the past 80 years), satellite data show significant ice flow speed and direction changes, as well as retreating grounding lines where tributary glaciers start to float and where ice flows over and around isolated bedrock pinning points. A complex geometry of deep seafloor troughs underlie the central ice-shelf area which lies at the convergence of the two major troughs that extend to the continental shelf edge at two widely separated locations (roughly 103°W and 117°W longitude along the continental shelf break).<br><br>We surveyed the central Thwaites Eastern Ice Shelf (‘Cavity Camp’, 75.05°S, 105.58°W) and central Dotson-Crosson Ice Shelf (`Upper Dotson’, 74.87°S, 112.20°W) to the extent possible considering site safety and scientific interest. Cavity Camp is located approximately 17 km down-flow of the 2011 Thwaites Glacier grounding line. Ground-penetrating radar data show the ice thickness near Cavity Camp to be 300m, which is ~200m thinner than in 2007 estimated from hydrostatic assumption using altimetry analysis by other researchers. The seafloor below Cavity Camp is 816m, based on pressure from a CTD profile (a ~540 m water column and ~40m of firn).   <br><br>Across the central Dotson-Crosson Ice Shelf, a network of basal channels creates variable thinning rates from near-zero to over 30 m/yr (estimated in several previous remote-sensing-based studies). Ice thickness near our camp over a subglacial channel is 390m and the ice has been thinning at ~25 m/yr estimated from satellite data. Seafloor elevation at the Dotson site is estimated at -570 m, but seismic surveys suggest that the seabed topography varies considerably beneath Dotson. <br><br>On each ice shelf, we conducted ~200 km of multi-frequency ground-penetrating radar profiles. We also conducted 46 (Thwaites) and 17 (Dotson) autonomous phase-tracking radio echo-sounding (ApRES) repeat point measurements, as well as 37 (Thwaites) and more than 20 (Dotson) active-seismic spot soundings to characterize the sub-ice-shelf cavity shape, thinning rates, basal ice structures, and ocean circulation. We deployed two Automated Meteorology Ice Geophysics Ocean observation Systems (AMIGOS-III stations) on the Thwaites Ice Shelf that include a suite of surface sensors, a fiber-optic-based thermal profiler, and an ocean mooring. Additionally, we deployed four long-term ApRES on the two ice shelves to monitor temporal variability in ice melt.</p>

Simulating detailed spatial patterns of ice shelf basal melt

2021 ◽  
Author(s):  
Erwin Lambert ◽  
André Jüling ◽  
Paul Holland ◽  
Roderik van de Wal
Keyword(s):  
Flow Direction ◽  
Ice Sheet ◽  
Realistic Model ◽  
Western Boundary ◽  
Layer Model ◽  
Ice Shelf ◽  
Amundsen Sea ◽  
Ice Shelves ◽  
Grounding Line ◽  
Ocean Properties

<p>The contact between ice shelves and relatively warm ocean waters causes basal melt, ice shelf thinning, and ultimately ice sheet mass loss. This basal melt, and its dependence on ocean properties, is poorly understood due to an overall lack of direct observations and a difficulty in explicit simulation of the circulation in sub-shelf cavities. In this study, we compare a number of parameterisations and models of increasing complexity, up to a 2D ‘Layer’ model. Each model is aimed at quantifying basal melt rates as a function of offshore temperature and salinity. We test these models in an idealised setting (ISOMIP+) and in a realistic setting for the Amundsen Sea Embayment. All models show a comparable non-linear sensitivity of ice-shelf average basal melt to ocean warming, indicating a positive feedback between melt and circulation. However, the Layer model is the only one which explicitly resolves the flow direction of the buoyant melt plumes, which is primarily governed by rotation and by the basal topography of the ice shelves. At 500m resolution, this model simulates locally enhanced basal melt near the grounding line, in topographical channels, and near the western boundary. The simulated melt patterns for the Amundsen Sea ice shelves are compared to satellite observations of ice shelf thinning and to 3D numerical simulations of the sub-shelf cavity circulation. As detailed melt rates near the grounding line are essential for the stability of ice sheets, spatially realistic melt rates are crucial for future projections of ice sheet dynamics. We conclude that the Layer model can function as a relatively cheap yet realistic model to downscale 3D ocean simulations of ocean properties to sub-kilometer scale basal melt fields to provide detailed forcing fields to ice sheet models.</p>

scholarly journals Numerical simulations of the ice flow dynamics of George VI Ice Shelf, Antarctica

2007 ◽  
Vol 53 (183) ◽  
pp. 659-664 ◽  
Author(s):  
Angelika Humbert
Keyword(s):  
High Spatial Resolution ◽  
Thickness Distribution ◽  
Satellite System ◽  
Flow Dynamics ◽  
Ice Thickness ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Flow Geometry ◽  
Grounding Line

A diagnostic, dynamic/thermodynamic ice-shelf model is applied to the George VI Ice Shelf, situated in the Bellinghausen Sea, Antarctica. The George VI Ice Shelf has a peculiar flow geometry which sets it apart from other ice shelves. Inflow occurs along the two longest, and almost parallel, sides, whereas outflow occurs on the two ice fronts that are relatively short and situated at opposite ends of the ice shelf. Two data sources were used to derive the ice thickness distribution: conventional radioecho sounding from the British Antarctic Survey was combined with thickness inferred from surface elevation obtained by the NASA GLAS satellite system assuming hydrostatic equilibrium. We simulate the present ice flow over the ice shelf that results from the ice thickness distribution, the inflow at the grounding line and the flow rate factor. The high spatial resolution of the ice thickness distribution leads to very detailed simulations. The flow field has some extraordinary elements (e.g. the stagnation point characteristics resulting from the unusual ice-shelf geometry).

scholarly journals Channelization of plumes beneath ice shelves

2015 ◽  
Vol 785 ◽  
pp. 109-134 ◽  
Author(s):  
M. C. Dallaston ◽  
I. J. Hewitt ◽  
A. J. Wells
Keyword(s):  
Turbulent Mixing ◽  
Simplified Model ◽  
Ice Thickness ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Narrow Channels ◽  
One Dimensional ◽  
Grounding Line ◽  
The One

We study a simplified model of ice–ocean interaction beneath a floating ice shelf, and investigate the possibility for channels to form in the ice shelf base due to spatial variations in conditions at the grounding line. The model combines an extensional thin-film description of viscous ice flow in the shelf, with melting at its base driven by a turbulent ocean plume. Small transverse perturbations to the one-dimensional steady state are considered, driven either by ice thickness or subglacial discharge variations across the grounding line. Either forcing leads to the growth of channels downstream, with melting driven by locally enhanced ocean velocities, and thus heat transfer. Narrow channels are smoothed out due to turbulent mixing in the ocean plume, leading to a preferred wavelength for channel growth. In the absence of perturbations at the grounding line, linear stability analysis suggests that the one-dimensional state is stable to initial perturbations, chiefly due to the background ice advection.

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

Geology ◽  
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.

scholarly journals A comparative modeling study of the Brunt Ice Shelf/ Stancomb-Wills Ice Tongue system, East Antarctica

2009 ◽  
Vol 55 (189) ◽  
pp. 53-65 ◽  
Author(s):  
Angelika Humbert ◽  
Thomas Kleiner ◽  
Chris-Oliver Mohrholz ◽  
Christoph Oelke ◽  
Ralf Greve ◽  
...  
Keyword(s):  
Velocity Field ◽  
Feature Tracking ◽  
Thickness Distribution ◽  
Comparative Modeling ◽  
Ice Thickness ◽  
Heterogeneous Structure ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Grounding Line ◽  
Slow Moving

AbstractTwo diagnostic, dynamic/thermodynamic ice-shelf models are applied to the Brunt Ice Shelf/Stancomb-Wills Ice Tongue system, located off Caird Coast, Coats Land, Antarctica. The Brunt Ice Shelf/Stancomb-Wills Ice Tongue system is characterized as a thin, unbounded ice shelf with an atypical and highly heterogeneous structure. In contrast to other ice shelves, a composite mass of icebergs that calved at the grounding line and were then locked within fast (sea) ice exists between the fast-moving Stancomb-Wills Ice Stream and the slow-moving Brunt Ice Shelf. We simulate the present flow regime of the ice shelf that results from the ice-thickness distribution and the inflow at the grounding line with two different models, and compare the model results with feature tracking and InSAR flow velocities. We then incorporate two observed features, a rift and a shear margin, into the models with two different approaches, and demonstrate the effects of variations in numerical values for the shear strength and viscosity in these zones on the simulated velocity field. A major result is that both kinds of implementation of the rifts lead to similar effects on the entire velocity field, while there are discrepancies in the vicinity of the rifts.

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

2006 ◽  
Vol 52 (178) ◽  
pp. 325-346 ◽  
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.

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

2002 ◽  
Vol 48 (162) ◽  
pp. 345-356 ◽  
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.

scholarly journals Marine ice-sheet dynamics. Part 1. The case of rapid sliding

2007 ◽  
Vol 573 ◽  
pp. 27-55 ◽  
Author(s):  
CHRISTIAN SCHOOF
Keyword(s):  
Ice Sheets ◽  
Ice Sheet ◽  
Ice Thickness ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Sea Floor ◽  
Stefan Problems ◽  
Grounding Line ◽  
Ice Sheet Dynamics ◽  
The Stability

Marine ice sheets are continental ice masses resting on bedrock below sea level. Their dynamics are similar to those of land-based ice sheets except that they must couple with the surrounding floating ice shelves at the grounding line, where the ice reaches a critical flotation thickness. In order to predict the evolution of the grounding line as a free boundary, two boundary conditions are required for the diffusion equation describing the evolution of the grounded-ice thickness. By analogy with Stefan problems, one of these conditions imposes a prescribed ice thickness at the grounding line and arises from the fact that the ice becomes afloat. The other condition must be determined by coupling the ice sheet to the surrounding ice shelves. Here we employ matched asymptotic expansions to study the transition from ice-sheet to ice-shelf flow for the case of rapidly sliding ice sheets. Our principal results are that the ice flux at the grounding line in a two-dimensional ice sheet is an increasing function of the depth of the sea floor there, and that ice thicknesses at the grounding line must be small compared with ice thicknesses inland. These results indicate that marine ice sheets have a discrete set of steady surface profiles (if they have any at all) and that the stability of these steady profiles depends on the slope of the sea floor at the grounding line.

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