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Abstract. Bathymetry beneath ice shelves in West Antarctica plays an important role in the delivery and circulation of warm waters to the ice-shelf bottom and grounding line. Large-scale bathymetric estimates can only be inferred through inversion of airborne gravity measurements. However, previous bathymetry inversions have not robustly quantified uncertainty in the bathymetry due to assumptions inherent in the inversion, and typically only produce a single model, making it difficult to propagate uncertainty into ocean and ice-sheet models. Previous inversions have sometimes considered uncertainties in bathymetry models due to background densities but have not quantified the uncertainty due to the non-uniqueness inherent in gravity and geological variability below ice shelves. To address these issues, we develop a method to generate ensembles of bathymetry models beneath the Crosson, Dotson, and Thwaites Eastern ice shelves with independent realizations of background density and geological variability, represented through the Bouguer gravity disturbance. We sample the uncertainty in the unknown geology below the ice shelves by interpolating the Bouguer disturbance using Sequential Gaussian Simulation. Each inversion is efficiently computed using a random walk Metropolis Markov Chain Monte Carlo approach which randomly updates blocks of bathymetry and accepts or rejects updates. Our ensembles of bathymetry models differ from previous estimates of bathymetry by hundreds of meters in some areas and show that the uncertainty in the Bouguer disturbance is the largest source of uncertainty. The different bathymetry models in the ensembles can be used in oceanographic models to place better bounds on sub-ice-shelf melting and future grounding line retreat.
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New gravity-derived bathymetry for the Thwaites, Crosson, and Dotson ice shelves revealing two ice shelf populations
Abstract. Ice shelves play a critical role in the long-termstability of ice sheets through their buttressing effect. The underlyingbathymetry and cavity thickness are key inputs for modelling future icesheet evolution. However, direct observation of sub-ice-shelf bathymetry istime-consuming, logistically risky, and in some areas simply not possible.Here we use new compilations of airborne and marine gravity, radar depthsounding, and swath bathymetry to provide new estimates of sub-ice-shelfbathymetry outboard of the rapidly changing West Antarctic Thwaites Glacierand beneath the adjacent Dotson and Crosson ice shelves. This region is ofspecial interest, as the low-lying inland reverse slope of the ThwaitesGlacier system makes it vulnerable to marine ice sheet instability, withrapid grounding line retreat observed since 1993 suggesting this process maybe underway. Our results confirm a major marine channel >800 mdeep extends tens of kilometres to the front of Thwaites Glacier, while theadjacent ice shelves are underlain by more complex bathymetry. Comparison ofour new bathymetry with ice shelf draft reveals that ice shelves formedsince 1993 comprise a distinct population where the draft conforms closelyto the underlying bathymetry, unlike the older ice shelves, which show a moreuniform depth of the ice base. This indicates that despite rapid basalmelting in some areas, these recently floated parts of the ice shelf are notyet in dynamic equilibrium with their retreated grounding line positions andthe underlying ocean system, a factor which must be included in futuremodels of this region's evolution.
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- Award ID(s):
- 1842064
- PAR ID:
- 10206992
- Date Published:
- Journal Name:
- The Cryosphere
- Volume:
- 14
- Issue:
- 9
- ISSN:
- 1994-0424
- Page Range / eLocation ID:
- 2869 to 2882
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract. Antarctic ice shelves buttress the flow of the ice sheet but are vulnerable to increased basal melting from contact with a warming ocean and increased mass loss from calving due to changing flow patterns. Channels and similar features at the bases of ice shelves have been linked to enhanced basal melting and observed to intersect the grounding zone, where the greatest melt rates are often observed. The ice shelf of Thwaites Glacier is especially vulnerable to basal melt and grounding zone retreat because the glacier has a retrograde bed leading to a deep trough below the grounded ice sheet. We use digital surface models from 2010–2022 to investigate the evolution of its ice-shelf channels, grounding zone position, and the interactions between them. We find that the highest sustained rates of grounding zone retreat (up to 0.7 km yr−1) are associated with high basal melt rates (up to ∼250 m yr−1) and are found where ice-shelf channels intersect the grounding zone, especially atop steep local retrograde slopes where subglacial channel discharge is expected. We find no areas with sustained grounding zone advance, although some secular retreat was distal from ice-shelf channels. Pinpointing other locations with similar risk factors could focus assessments of vulnerability to grounding zone retreat.more » « less
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Abstract Thwaites Glacier is one of the fastest‐changing ice‐ocean systems in Antarctica. Basal melting beneath Thwaites' floating ice shelf, especially around pinning points and at the grounding line, sets the rate of ice loss and Thwaites' contribution to global sea‐level rise. The rate of basal melting is controlled by the transport of heat into and through the ice–ocean boundary layer toward the ice base. Here we present the first turbulence observations from the grounding line of Thwaites Eastern Ice Shelf. We demonstrate that contrary to expectations, the turbulence‐driven vertical flux of heat into the ice–ocean boundary layer is insufficient to sustain the basal melt rate. Instead, most of the heat required must be delivered by lateral fluxes driven by the large‐scale advective circulation. Lateral processes likely dominate beneath the most unstable warm‐cavity ice shelves, and thus must be fully incorporated into parameterizations of ice shelf basal melting.more » « less
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The loss or thinning of buttressing ice shelves and accompanying changes in grounding-zone stress balance are commonly implicated as the primary trigger for grounding-line retreat, such as that observed in Amundsen Sea outlet glaciers today. Ice-shelf thinning is mostly attributed to the presence of warm ocean waters beneath the shelves. However, climate model projections show that summer air temperatures could soon exceed the threshold for widespread meltwater production on ice-shelf surfaces. This has serious implications for their future stability, because they are vulnerable to water-induced flexural stresses and water-aided crevasse penetration, termed ‘hydrofracturing’. Once initiated, the rate of shelf loss through hydrofracturing can far exceed that caused by sub-surface melting, and could result in the complete loss of some buttressing ice shelves, with marine grounding lines suddenly becoming calving ice fronts. In places where those exposed ice fronts are thick (>900m) and crevassed, deviatoric stresses can exceed the strength of the ice and the cliff face will fail mechanically, leading to rapid calving like that seen in analogous settings such as Jakobshavn on Greenland. Here we explore the implications of hydrofacturing and subsequent ice-cliff collapse in a warming climate, by parameterizing these processes in a hybrid ice sheet-shelf model. Model sensitivities to meltwater production and to ice-cliff calving rate (a function of cliff height above the stress balance threshold triggering brittle failure) are calibrated to match modern observations of calving and thinning. We find the potential for major ice-sheet retreat if global mean temperature rises more than ~2ºC above preindustrial. In the model, Antarctic calving rates at thick ice fronts are not allowed to exceed those observed in Greenland today. This may be a conservative assumption, considering the very different spatial scales of Antarctic outlets, such as Thwaites. Nonetheless, simulations following a ‘worst case’ RCP8.5 scenario produce rates of sea-level rise measured in cm per year by the end of this century. Clearly, the potential for brittle processes to deliver ice to the ocean, in addition to viscous and basal processes, needs to be better constrained through more complete, physically based representations of calving.more » « less
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Abstract Basal channels are incised troughs formed by elevated melt beneath ice shelves. Channels often coincide with shear margins, suggesting feedbacks between channel formation and shear. However, the effect of channel position and shape on ice-shelf flow has not been systematically explored. We use a model to show that, as expected, channels concentrate deformation and increase ice-shelf flow speeds, in some cases by over 100% at the ice-shelf center and over 80% at the grounding line. The resulting increase in shear can cause stresses around the channels to exceed the threshold for failure, suggesting that rifting, calving and retreat might result. However, channels have different effects depending on their width, depth and position on an ice shelf. Channels in areas where ice shelves are spreading freely have little effect on ice flow, and even channels in confined regions of the shelf do not necessarily alter flow significantly. Nevertheless, if located in areas of vulnerability, particularly in the shear margins near the grounding line, melt channels may alter flow in a way that could lead to catastrophic ice-shelf breakup by mechanically separating shelves from their embayments.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.5194/tc-14-2869-2020
