Rapid fragmentation of Thwaites Eastern Ice Shelf, West Antarctica

Abstract. Ice shelves play a key role in the dynamics of marine ice sheets, by buttressing grounded ice and limiting rates of ice flux to the oceans. In response to recent climatic and oceanic change, ice shelves fringing the West Antarctic Ice Sheet (WAIS) have begun to fragment and retreat, with major implications for ice sheet stability. Here, we focus on the Thwaites Eastern Ice Shelf (TEIS), the remaining pinned floating extension of Thwaites Glacier. We show that TEIS has undergone a process of fragmentation in the last five years, including brittle failure along a major shear zone, formation of tensile cracks on the main body of the shelf, and release of tabular bergs on both eastern and western flanks. Simulations with the Helsinki Discrete Element Model (HiDEM) show that this pattern of failure is associated with high backstress from a submarine pinning point at the distal edge of the shelf. We show that a significant zone of shear upstream of the main pinning point developed in response to the rapid acceleration of the shelf between 2002 and 2006, seeding damage on the shelf. Subsequently, basal melting and positive feedbacks between damage and strain rates weakened TEIS, allowing damage to accumulate. Thus, although backstress on TEIS has likely diminished through time as the pinning point has shrunk, accumulation of damage has ensured that the ice in the shear zone has remained the weakest link in the system. Experiments with the BISICLES ice sheet model indicate that additional damage to or unpinning of TEIS are unlikely to trigger significantly increased ice loss from WAIS, but the calving response to loss of TEIS remains highly uncertain. It is widely recognised that ice-shelf fragmentation and collapse can be triggered by hydrofracturing and/or unpinning from ice shelf margins or grounding points. Our results indicate a third mechanism, backstress-triggered failure, that can occur when ice ffractures in response to stresses associated with pinning points. In most circumstances, pinning points are essential for ice shelf stability, but as ice shelves thin and weaken the concentration of backstress in damaged ice upstream of a pinning point may provide the seeds of their demise.

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Two ice shelf populations revealed in new gravity- derived bathymetry for the Thwaites, Crosson and Dotson ice shelves

10.5194/egusphere-egu2020-19603 ◽

2020 ◽

Author(s):

Tom Jordan ◽

David Porter ◽

Kirsty Tinto ◽

Romain Millan ◽

Atsuhiro Muto ◽

...

Keyword(s):

Critical Role ◽

Ice Sheet ◽

Blind Spot ◽

Airborne Gravity ◽

Ice Shelf ◽

Ice Shelves ◽

West Antarctic ◽

Glacier System ◽

Cavity Thickness ◽

Basal Melting

<p>Ice shelf buttressing plays a critical role in the long-term stability of ice sheets. The underlying bathymetry and cavity thickness therefore is a key to accurate models of future ice sheet evolution. However, direct observation of sub-ice shelf bathymetry is time consuming, logistically risky, and in some areas simply not possible, meaning there is a blind-spot in our understanding of this key system. Here we use airborne gravity anomaly data to provide new estimates of sub-ice shelf bathymetry outboard of the rapidly changing West Antarctic Thwaites Glacier, and beneath the adjacent Dotson and Crosson Ice Shelves. These regions are of especial interest as the low-lying inland reverse slope of the Thwaites glacier system makes it vulnerable to collapse through marine ice sheet instability, with rapid grounding-line retreat observed since 1993 suggesting this process may be underway. Our results confirm a major marine channel >&#8201;800&#8201;m deep extends to the front of Thwaites Glacier, while the adjacent ice shelves are underlain by more complex bathymetry. Comparison of our new bathymetry with ice shelf draft reveals that ice shelves formed since 1993 comprise a distinct population where the draft conforms closely to the underlying bathymetry, unlike the older ice shelves which show a more uniform depth of the ice base. This indicates that despite rapid basal melting in some areas, these &#8220;new&#8221; ice shelves are not yet in equilibrium with the underlying ocean system. We propose qualitative models of how this transient ice-shelf configuration may have developed, but further investigation is required to constrain the longevity and full impact of these newly recognised systems.</p>

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New gravity-derived bathymetry for the Thwaites, Crosson and Dotson ice shelves revealing two ice shelf populations

10.5194/tc-2019-294 ◽

2020 ◽

Author(s):

Tom A. Jordan ◽

David Porter ◽

Kirsty Tinto ◽

Romain Millan ◽

Atsuhiro Muto ◽

...

Keyword(s):

Critical Role ◽

Ice Sheet ◽

Airborne Gravity ◽

Ice Shelf ◽

Ice Shelves ◽

West Antarctic ◽

Glacier System ◽

Qualitative Models ◽

Cavity Thickness ◽

Basal Melting

Abstract. Ice shelves play a critical role in the long-term stability of ice sheets through their buttressing effect. The underlying bathymetry and cavity thickness are key inputs for modelling future ice sheet evolution. However, direct observation of sub-ice shelf bathymetry is time consuming, logistically risky, and in some areas simply not possible. Here we use airborne gravity anomaly data to provide new estimates of sub-ice shelf bathymetry outboard of the rapidly changing West Antarctic Thwaites Glacier, and beneath the adjacent Dotson and Crosson Ice Shelves. This region is of especial interest as the low-lying inland reverse slope of the Thwaites glacier system makes it vulnerable to marine ice sheet instability, with rapid grounding-line retreat observed since 1993 suggesting this process may be underway. Our results confirm a major marine channel > 800 m deep extends to the front of Thwaites Glacier, while the adjacent ice shelves are underlain by more complex bathymetry. Comparison of our new bathymetry with ice shelf draft reveals that ice shelves formed since 1993 comprise a distinct population where the draft conforms closely to the underlying bathymetry, unlike the older ice shelves which show a more uniform depth of the ice base. This indicates that despite rapid basal melting in some areas, these “new” ice shelves are not yet in equilibrium with the underlying ocean system. We propose qualitative models of how this transient ice-shelf configuration may have developed.

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Melting and freezing under Antarctic ice shelves from a combination of ice-sheet modelling and observations

Journal of Glaciology ◽

10.1017/jog.2017.42 ◽

2017 ◽

Vol 63 (240) ◽

pp. 731-744 ◽

Cited By ~ 3

Author(s):

JORGE BERNALES ◽

IRINA ROGOZHINA ◽

MAIK THOMAS

Keyword(s):

Mass Balance ◽

Ice Sheet ◽

Model Parameters ◽

Ice Shelf ◽

Ice Shelves ◽

Continental Scale ◽

Model Set ◽

Set Up ◽

Basal Melting ◽

The Antarctic

ABSTRACTIce-shelf basal melting is the largest contributor to the negative mass balance of the Antarctic ice sheet. However, current implementations of ice/ocean interactions in ice-sheet models disagree with the distribution of sub-shelf melt and freezing rates revealed by recent observational studies. Here we present a novel combination of a continental-scale ice flow model and a calibration technique to derive the spatial distribution of basal melting and freezing rates for the whole Antarctic ice-shelf system. The modelled ice-sheet equilibrium state is evaluated against topographic and velocity observations. Our high-resolution (10-km spacing) simulation predicts an equilibrium ice-shelf basal mass balance of −1648.7 Gt a−1 that increases to −1917.0 Gt a−1 when the observed ice-shelf thinning rates are taken into account. Our estimates reproduce the complexity of the basal mass balance of Antarctic ice shelves, providing a reference for parameterisations of sub-shelf ocean/ice interactions in continental ice-sheet models. We perform a sensitivity analysis to assess the effects of variations in the model set-up, showing that the retrieved estimates of basal melting and freezing rates are largely insensitive to changes in the internal model parameters, but respond strongly to a reduction of model resolution and the uncertainty in the input datasets.

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An improved bathymetry compilation for the Bellingshausen Sea, Antarctica, to inform ice-sheet and ocean models

The Cryosphere Discussions ◽

10.5194/tcd-4-2079-2010 ◽

2010 ◽

Vol 4 (4) ◽

pp. 2079-2101 ◽

Cited By ~ 3

Author(s):

A. G. C. Graham ◽

F. O. Nitsche ◽

R. D. Larter

Keyword(s):

Ocean Circulation ◽

Ice Sheet ◽

West Antarctica ◽

Ice Shelf ◽

Ice Streams ◽

Ice Shelves ◽

Sea Floor ◽

Bellingshausen Sea ◽

Warm Deep Water ◽

Basal Melting

Abstract. The southern Bellingshausen Sea (SBS) is a rapidly-changing part of West Antarctica, where oceanic and atmospheric warming has led to the recent basal melting and break-up of the Wilkins ice shelf, the dynamic thinning of fringing glaciers, and sea-ice reduction. Accurate sea-floor morphology is vital for understanding the continued effects of each process upon changes within Antarctica's ice sheets. Here we present a new bathymetric grid for the SBS compiled from shipborne echo-sounder, spot-sounding and sub-ice measurements. The 1-km grid is the most detailed compilation for the SBS to-date, revealing large cross-shelf troughs, shallow banks, and deep inner-shelf basins that continue inland of coastal ice shelves. The troughs now serve as pathways which allow warm deep water to access the ice fronts in the SBS. Our dataset highlights areas still lacking bathymetric constraint, as well as regions for further investigation, including the likely routes of palaeo-ice streams. The new compilation is a major improvement upon previous grids and will be a key dataset for incorporating into simulations of ocean circulation, ice-sheet change and history. It will also serve forecasts of ice stability and future sea-level contributions from ice loss in West Antarctica, required for the next IPCC assessment report in 2013.

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Modelling the long-term response of the Antarctic ice sheet to global warming

Annals of Glaciology ◽

10.3189/1998aog27-1-161-168 ◽

1998 ◽

Vol 27 ◽

pp. 161-168 ◽

Cited By ~ 29

Author(s):

Roland C. Warner ◽

W.Κ. Budd

Keyword(s):

Global Warming ◽

Mass Loss ◽

Ice Sheet ◽

Ice Shelf ◽

Antarctic Ice Sheet ◽

Ice Shelves ◽

Basal Melting ◽

The Antarctic ◽

Term Response

The primary effects of global warming on the Antarctic ice sheet can involve increases in surface melt for limited areas at lower elevations, increases in net accumulation, and increased basal melting under floating ice. For moderate global wanning, resulting in ocean temperature increases of a few °C, the large- increase in basal melting can become the dominant factor in the long-term response of the ice sheet. The results from ice-sheet modelling show that the increased basal melt rates lead to a reduction of the ice shelves, increased strain rates and flow at the grounding lines, then thinning and floating of the marine ice sheets, with consequential further basal melting. The mass loss from basal melting is counteracted to some extent by the increased accumulation, but in the long term the area of ice cover decreases, particularly in West Antarctica, and the mass loss can dominate. The ice-sheet ice-shelf model of Budd and others (1994) with 20 km resolution has been modified and used to carry out a number of sensitivity studies of the long-term response of the ice sheet to prescribed amounts of global warming. The changes in the ice sheet are computed out to near-equilibrium, but most of the changes take place with in the first lew thousand years. For a global mean temperature increase of 3°C with an ice-shelf basal melt rate of 5 m a−1 the ice shelves disappear with in the first few hundred years, and the marine-based parts of the ice sheet thin and retreat. By 2000 years the West Antarctic region is reduced to a number of small, isolated ice caps based on the bedrock regions which are near or above sea level. This allows the warmer surface ocean water to circulate through the archipelago in summer, causing a large change to the local climate of the region.

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Antarctic glacier-tongue velocities from Landsat images: first results

Annals of Glaciology ◽

10.1017/s0260305500013100 ◽

1993 ◽

Vol 17 ◽

pp. 356-366 ◽

Cited By ~ 33

Author(s):

B.K. Lucchitta ◽

Κ.F. Mullins ◽

A.L. Allison ◽

J.G. Ferrigno

Keyword(s):

Ice Sheet ◽

Ice Shelf ◽

Landsat Images ◽

The West ◽

Antarctic Ice Sheet ◽

West Antarctic Ice Sheet ◽

Ice Shelves ◽

Glacier Tongue ◽

West Antarctic ◽

First Results

We measured the velocities of six glacier tongues and a few tongues within ice shelves distributed around the Antarctic coastline by determining the displacement of crevasse patterns seen on sequential Landsat images. The velocities range from less than 0.2 km a−1 for East Antarctic ice-shelf tongues to more than 2.5 km a−1 for the Thwaites Glacier Tongue. All glacier tongues show increases in velocity toward their distal margins. In general, the tongues of glaciers draining the West Antarctic ice sheet have moved significantly faster than those in East Antarctica. This observation may be significant in light of the hypothesized possible disintegration of the West Antarctic ice sheet.

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Viscous and elastic buoyancy stresses as drivers of ice-shelf calving

Journal of Glaciology ◽

10.1017/jog.2020.35 ◽

2020 ◽

Vol 66 (258) ◽

pp. 643-657 ◽

Cited By ~ 1

Author(s):

Cyrille Mosbeux ◽

Till J. W. Wagner ◽

Maya K. Becker ◽

Helen A. Fricker

Keyword(s):

Ice Sheet ◽

Elastic Model ◽

Ice Shelf ◽

Ice Shelves ◽

Bending Stresses ◽

Iceberg Calving ◽

Basal Melting ◽

The Antarctic ◽

The Impact ◽

Maximum Stresses

AbstractThe Antarctic Ice Sheet loses mass via its ice shelves predominantly through two processes: basal melting and iceberg calving. Iceberg calving is episodic and infrequent, and not well parameterized in ice-sheet models. Here, we investigate the impact of hydrostatic forces on calving. We develop two-dimensional elastic and viscous numerical frameworks to model the ‘footloose’ calving mechanism. This mechanism is triggered by submerged ice protrusions at the ice front, which induce unbalanced buoyancy forces that can lead to fracturing. We compare the results to identify the different roles that viscous and elastic deformations play in setting the rate and magnitude of calving events. Our results show that, although the bending stresses in both frameworks share some characteristics, their differences have important implications for modeling the calving process. In particular, the elastic model predicts that maximum stresses arise farther from the ice front than in the viscous model, leading to larger calving events. We also find that the elastic model would likely lead to more frequent events than the viscous one. Our work provides a theoretical framework for the development of a better understanding of the physical processes that govern glacier and ice-shelf calving cycles.

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The Dynamics of Marine Ice Sheets

Journal of Glaciology ◽

10.3189/s0022143000014726 ◽

1979 ◽

Vol 24 (90) ◽

pp. 167-177 ◽

Cited By ~ 13

Author(s):

Robert H. Thomas

Keyword(s):

Sea Level ◽

Ice Sheets ◽

Ice Sheet ◽

Ice Shelf ◽

Ice Streams ◽

Ice Shelves ◽

The North ◽

West Antarctic ◽

Grounding Line ◽

Ice Sheet Dynamics

AbstractMarine ice sheets rest on land that, for the most part, is below sea-level. Ice that flows across the grounding line, where the ice sheet becomes afloat, either calves into icebergs or forms a floating ice shelf joined to the ice sheet. At the grounding line there is a transition from ice-sheet dynamics to ice-shelf dynamics, and the creep-thinning rate in this region is very sensitive to sea depth; rising sea-level causes increased thinning-rates and grounding-line retreat, falling sea-level has the reverse effect. If the bedrock slopes down towards the centre of the ice sheet there may be only two stable modes: a freely-floating ice shelf or a marine ice sheet that extends to the edge of the continental shelf. Once started, collapse of such an ice sheet to form an ice shelf may take place extremely rapidly. Ice shelves which form in embayments of a marine ice sheet, or which are partially grounded, have a stabilizing influence since ice flowing across the grounding line has to push the ice shelf past its sides. Retreat of the grounding line tends to enlarge the ice shelf, which ultimately may become large enough to prevent excessive outflow from the ice sheet so that a new equilibrium grounding line is established; removal of the ice shelf would allow retreat to continue. During the late-Wisconsin glacial maximum there may have been marine ice sheets in the northern hemisphere but the only current example is the West Antarctic ice sheet. This is buttressed by the Ross and Ronne Ice Shelves, and if climatic warming were to prohibit the existence of these ice shelves then the ice sheet would collapse. Field observations suggest that, at present, the ice sheet may be advancing into parts of the Ross Ice Shelf. Such advance, however, would not ensure the security of the ice sheet since ice streams that drain to the north appear to flow directly into the sea with little or no ice shelf to buttress them. If these ice streams do not flow over a sufficiently high bedrock sill then they provide the most likely avenues for ice-sheet retreat.

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Antarctic ice sheet response to sudden and sustained ice-shelf collapse (ABUMIP)

Journal of Glaciology ◽

10.1017/jog.2020.67 ◽

2020 ◽

Vol 66 (260) ◽

pp. 891-904 ◽

Cited By ~ 1

Author(s):

Sainan Sun ◽

Frank Pattyn ◽

Erika G. Simon ◽

Torsten Albrecht ◽

Stephen Cornford ◽

...

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Sliding Friction ◽

Ice Sheet ◽

Full Potential ◽

Ice Shelf ◽

Antarctic Ice Sheet ◽

West Antarctic Ice Sheet ◽

Ice Shelves ◽

West Antarctic

AbstractAntarctica's ice shelves modulate the grounded ice flow, and weakening of ice shelves due to climate forcing will decrease their ‘buttressing’ effect, causing a response in the grounded ice. While the processes governing ice-shelf weakening are complex, uncertainties in the response of the grounded ice sheet are also difficult to assess. The Antarctic BUttressing Model Intercomparison Project (ABUMIP) compares ice-sheet model responses to decrease in buttressing by investigating the ‘end-member’ scenario of total and sustained loss of ice shelves. Although unrealistic, this scenario enables gauging the sensitivity of an ensemble of 15 ice-sheet models to a total loss of buttressing, hence exhibiting the full potential of marine ice-sheet instability. All models predict that this scenario leads to multi-metre (1–12 m) sea-level rise over 500 years from present day. West Antarctic ice sheet collapse alone leads to a 1.91–5.08 m sea-level rise due to the marine ice-sheet instability. Mass loss rates are a strong function of the sliding/friction law, with plastic laws cause a further destabilization of the Aurora and Wilkes Subglacial Basins, East Antarctica. Improvements to marine ice-sheet models have greatly reduced variability between modelled ice-sheet responses to extreme ice-shelf loss, e.g. compared to the SeaRISE assessments.

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A Model for Holocene Retreat of the West Antarctic Ice Sheet

Quaternary Research ◽

10.1016/0033-5894(78)90098-4 ◽

1978 ◽

Vol 10 (2) ◽

pp. 150-170 ◽

Cited By ~ 249

Author(s):

Robert H. Thomas ◽

Charles R. Bentley

Keyword(s):

Sea Level ◽

Ice Sheet ◽

Ice Shelf ◽

Ice Streams ◽

The West ◽

Antarctic Ice Sheet ◽

West Antarctic Ice Sheet ◽

Ice Shelves ◽

Ross Ice Shelf ◽

West Antarctic

Marine ice sheets are grounded on land which was below sea level before it became depressed under the ice-sheet load. They are inherently unstable and, because of bedrock topography after depression, the collapse of a marine ice sheet may be very rapid. In this paper equations are derived that can be used to make a quantitative estimate of the maximum size of a marine ice sheet and of when and how rapidly retreat would take place under prescribed conditions. Ice-sheet growth is favored by falling sea level and uplift of the seabed. In most cases the buttressing effect of a partially grounded ice shelf is a prerequisite for maximum growth out to the edge of the continental shelf. Collapse is triggered most easily by eustatic rise in sea level, but it is possible that the ice sheet may self-destruct by depressing the edge of the continental shelf so that sea depth is increased at the equilibrium grounding line.Application of the equations to a hypothetical “Ross Ice Sheet” that 18,000 yr ago may have covered the present-day Ross Ice Shelf indicates that, if the ice sheet existed, it probably extended to a line of sills parallel to the edge of the Ross Sea continental shelf. By allowing world sea level to rise from its late-Wisconsin minimum it was possible to calculate retreat rates for individual ice streams that drained the “Ross Ice Sheet.” For all the models tested, retreat began soon after sea level began to rise (∼15,000 yr B.P.). The first 100 km of retreat took between 1500 and 2500 yr but then retreat rates rapidly accelerated to between 0.5 and 25 km yr−1, depending on whether an ice shelf was present or not, with corresponding ice velocities across the grounding line of 4 to 70 km yr−1. All models indicate that most of the present-day Ross Ice Shelf was free of grounded ice by about 7000 yr B.P. As the ice streams retreated floating ice shelves may have formed between promontories of slowly collapsing stagnant ice left behind by the rapidly retreating ice streams. If ice shelves did not form during retreat then the analysis indicates that most of the West Antarctic Ice Sheet would have collapsed by 9000 yr B.P. Thus, the present-day Ross Ice Shelf (and probably the Ronne Ice Shelf) serves to stabilize the West Antarctic Ice Sheet, which would collapse very rapidly if the ice shelves were removed. This provides support for the suggestion that the 6-m sea-level high during the Sangamon Interglacial was caused by collapse of the West Antarctic Ice Sheet after climatic warming had sufficiently weakened the ice shelves. Since the West Antarctic Ice Sheet still exists it seems likely that ice shelves did form during Holocene retreat. Their effect was to slow and, finally, to halt retreat. The models that best fit available data require a rather low shear stress between the ice shelf and its sides, and this implies that rapid shear in this region encouraged the formation of a band of ice with a preferred crystal fabric, as appears to be happening today in the floating portions of fast bounded glaciers.Rebound of the seabed after the ice sheet had retreated to an equilibrium position would allow the ice sheet to advance once more. This may be taking place today since analysis of data from the Ross Ice Shelf indicates that the southeast corner is probably growing thicker with time, and if this persists then large areas of ice shelf must become grounded. This would restrict drainage from West Antarctic ice streams which would tend to thicken and advance their grounding lines into the ice shelf.

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