Subglacial sedimentary basins focus key vulnerabilities of the Antarctic ice-sheet

Antarctica preserves Earth’s largest ice sheet which, in response to climate warming, may lose ice mass and raise sea level by several metres. The ice-sheet bed exerts critical controls on dynamic mass loss through feedbacks between water and heat fluxes, topographic forcing and basal sliding. Here we show that through hydrogeological processes, sedimentary basins amplify critical feedbacks that are known to impact ice-sheet retreat dynamics. We create a high-resolution subglacial bedrock classification for Antarctica by applying a supervised machine learning method to geophysical data, revealing the distribution of sedimentary basins. Sedimentary basins are found in the upper reaches of Antarctica’s most rapidly changing ice streams, including Thwaites and Pine Island Glaciers. Hydro-mechanical numerical modelling reveals that where sedimentary basins exist, water discharge rate scales with the rate of ice unloading and the resulting hydrological instabilities are likely to amplify further retreat and unloading. These results indicate that the presence of a sedimentary bed in the catchment focuses instabilities that increase the vulnerability of the ice streams to rapid retreat and enhanced dynamic mass loss.

Finite-element Modelling of Haupapa/Tasman Glacier's Basal Sliding Events

<p>The rate of ice loss from glaciers and ice caps is a major source of uncertainty in predicting sea level rise out to 2100. Improving the predictive capability of ice flow models will, in part, require a more robust coupling of climate to long-term and short-term variability in glacial discharge. An ongoing concern is the role that surface melting and rainfall plays in accelerating glacier flow. Rapid drainage of surface water to the base of a glacier or ice sheet is thought to elevate basal water pressure, reduce basal friction, and thereby increases sliding speed. Here, we present several rain-induced speed-ups of Haupapa/Tasman Glacier, South Island, New Zealand, recorded by GNSS (Global Navigation Satellite System) instruments. Observed speed-up events involve large vertical offsets (up to ~53 cm) and large horizontal accelerations of up to twenty-four times background velocity. Due to it's pronounced sliding events, Haupapa/Tasman Glacier offers a useful case study for investigating the processes that govern the sliding behaviour of large glaciers prone to increasing meltwater variability as a cause of enhanced mass loss in a warming climate. The observed correspondence of vertical displacement and horizontal acceleration in this study suggests that the rapid growth of water-filled cavities at the bed controls basal motion during speed-ups. However, sliding laws that relate changes in basal velocity to changes in water pressure do not account for cavity growth. To investigate the processes governing a typical speed-up event, we use a finite-element modelling approach combined with a commonly-used sliding law to recreate internal deformation and basal sliding of Haupapa/Tasman Glacier during rain-induced acceleration. In general, we find peak velocities can only be achieved when basal water pressure exceeds ice overburden and velocity at the glacier sides is allowed to exceed that observed by a GNSS unit situated near the margins. The sliding law requires a more complete treatment of cavity growth under rapid water pressure changes to better capture basal acceleration observed at Haupapa/Tasman Glacier.</p>

Late Cenozoic behaviour of two Transantarctic Mountain outlet glaciers

<p>Earth’s climate is undergoing dramatic warming that is unprecedented in at least the last ~2000 years. Outlets of the Antarctic ice sheet are experiencing dynamic thinning, terminus retreat and mass loss, however, we are currently unable to accurately predict their future response. The drivers and mechanisms responsible for these observed changes can be better understood by studying the behaviour of outlet glaciers in the geological past. Here, I use cosmogenic nuclide surface-exposure dating and numerical glacier modelling to investigate the past configurations and dynamics of Transantarctic Mountain outlet glaciers, in the Ross Sea sector of Antarctica. Numerical modelling was first applied to understand the present-day and past behaviour of Skelton Glacier. A suite of sensitivity experiments reveal that Skelton Glacier is most susceptible to atmospheric temperature through its affect on basal sliding near the groundingline. Under past climates, large changes occurred in the lower reaches of the glacier, with basal sliding and bedrock erosion predicted in the overdeepened basins during both the Pliocene and Quaternary. Skelton Glacier was likely much shorter and thinner during Pliocene interglacials, with warm-based sliding that extended along most of its length. Informed by the glacier modelling, I applied surface-exposure dating to constrain past fluctuations in the geometry of Skelton Glacier. The lower reaches of the glacier were likely thicker at the Last Glacial Maximum (LGM), supporting the idea of buttressing by grounded ice in the Ross Sea during glacial periods. The glacier then thinned to near-modern surface elevations by ~5.8 ka before present (BP). Multiple isotope analysis (²⁶Al-¹⁰Be) and exposure-burial modelling indicates that Skelton Glacier has fluctuated between interglacial and glacial configurations probably at orbital frequencies since the Miocene. These data record a total of >10 Ma of exposure and 2.5 Ma of burial. An unexpected outcome is that the average cosmogenic production rate over this time appears to have been at least twice that of today. The long-term dynamics of Transantarctic Mountain outlet glaciers are further explored at Mackay Glacier. Here, geomorphological evidence reveals that glaciers can both erode and preserve bedrock surfaces during the same glacial episode, with basal erosion controlled primarily by ice thickness. Mackay Glacier likely experienced a widespread erosive regime prior to the Quaternary and a polythermal glacier regime during the LGM. Deglaciation following the LGM is constrained with (¹⁰Be) surface-exposure dating at Mackay Glacier. Samples collected at two nunataks, across four transects, reveal glacier thinning of >260 m between the LGM and ~200 years BP. Ice surface lowering was initially gradual, however an episode of rapid thinning is then recorded at ~6.8 ka BP, during a period of relative climatic and oceanic stability. This accelerated surface lowering occurred at a rate commensurate with modern observations of rapid ice sheet thinning, persisted for at least four centuries, and resulted in >180 m of ice loss. Numerical modelling indicates that ice surface drawdown resulted from ‘marine ice sheet instability’ as the grounding-line retreated through a deep glacial trough on the inner continental-shelf. This research provides new geological constraints and quantitative predictions of the past behaviour of Transantarctic Mountain outlet glaciers. The basal conditions and discharge of these glaciers evolved through the Late Cenozoic in response to climate forcing at orbital timescales, but also to topographically-controlled feedbacks at centennial to millennial timescales. Importantly, under enhanced atmospheric warming, these results imply that such outlet glaciers could experience greater ice loss through increased basal sliding and unstable grounding-line retreat into overdeepened basins.</p>

Late Cenozoic behaviour of two Transantarctic Mountain outlet glaciers

<p>Earth’s climate is undergoing dramatic warming that is unprecedented in at least the last ~2000 years. Outlets of the Antarctic ice sheet are experiencing dynamic thinning, terminus retreat and mass loss, however, we are currently unable to accurately predict their future response. The drivers and mechanisms responsible for these observed changes can be better understood by studying the behaviour of outlet glaciers in the geological past. Here, I use cosmogenic nuclide surface-exposure dating and numerical glacier modelling to investigate the past configurations and dynamics of Transantarctic Mountain outlet glaciers, in the Ross Sea sector of Antarctica. Numerical modelling was first applied to understand the present-day and past behaviour of Skelton Glacier. A suite of sensitivity experiments reveal that Skelton Glacier is most susceptible to atmospheric temperature through its affect on basal sliding near the groundingline. Under past climates, large changes occurred in the lower reaches of the glacier, with basal sliding and bedrock erosion predicted in the overdeepened basins during both the Pliocene and Quaternary. Skelton Glacier was likely much shorter and thinner during Pliocene interglacials, with warm-based sliding that extended along most of its length. Informed by the glacier modelling, I applied surface-exposure dating to constrain past fluctuations in the geometry of Skelton Glacier. The lower reaches of the glacier were likely thicker at the Last Glacial Maximum (LGM), supporting the idea of buttressing by grounded ice in the Ross Sea during glacial periods. The glacier then thinned to near-modern surface elevations by ~5.8 ka before present (BP). Multiple isotope analysis (²⁶Al-¹⁰Be) and exposure-burial modelling indicates that Skelton Glacier has fluctuated between interglacial and glacial configurations probably at orbital frequencies since the Miocene. These data record a total of >10 Ma of exposure and 2.5 Ma of burial. An unexpected outcome is that the average cosmogenic production rate over this time appears to have been at least twice that of today. The long-term dynamics of Transantarctic Mountain outlet glaciers are further explored at Mackay Glacier. Here, geomorphological evidence reveals that glaciers can both erode and preserve bedrock surfaces during the same glacial episode, with basal erosion controlled primarily by ice thickness. Mackay Glacier likely experienced a widespread erosive regime prior to the Quaternary and a polythermal glacier regime during the LGM. Deglaciation following the LGM is constrained with (¹⁰Be) surface-exposure dating at Mackay Glacier. Samples collected at two nunataks, across four transects, reveal glacier thinning of >260 m between the LGM and ~200 years BP. Ice surface lowering was initially gradual, however an episode of rapid thinning is then recorded at ~6.8 ka BP, during a period of relative climatic and oceanic stability. This accelerated surface lowering occurred at a rate commensurate with modern observations of rapid ice sheet thinning, persisted for at least four centuries, and resulted in >180 m of ice loss. Numerical modelling indicates that ice surface drawdown resulted from ‘marine ice sheet instability’ as the grounding-line retreated through a deep glacial trough on the inner continental-shelf. This research provides new geological constraints and quantitative predictions of the past behaviour of Transantarctic Mountain outlet glaciers. The basal conditions and discharge of these glaciers evolved through the Late Cenozoic in response to climate forcing at orbital timescales, but also to topographically-controlled feedbacks at centennial to millennial timescales. Importantly, under enhanced atmospheric warming, these results imply that such outlet glaciers could experience greater ice loss through increased basal sliding and unstable grounding-line retreat into overdeepened basins.</p>

Finite-element Modelling of Haupapa/Tasman Glacier's Basal Sliding Events

<p>The rate of ice loss from glaciers and ice caps is a major source of uncertainty in predicting sea level rise out to 2100. Improving the predictive capability of ice flow models will, in part, require a more robust coupling of climate to long-term and short-term variability in glacial discharge. An ongoing concern is the role that surface melting and rainfall plays in accelerating glacier flow. Rapid drainage of surface water to the base of a glacier or ice sheet is thought to elevate basal water pressure, reduce basal friction, and thereby increases sliding speed. Here, we present several rain-induced speed-ups of Haupapa/Tasman Glacier, South Island, New Zealand, recorded by GNSS (Global Navigation Satellite System) instruments. Observed speed-up events involve large vertical offsets (up to ~53 cm) and large horizontal accelerations of up to twenty-four times background velocity. Due to it's pronounced sliding events, Haupapa/Tasman Glacier offers a useful case study for investigating the processes that govern the sliding behaviour of large glaciers prone to increasing meltwater variability as a cause of enhanced mass loss in a warming climate. The observed correspondence of vertical displacement and horizontal acceleration in this study suggests that the rapid growth of water-filled cavities at the bed controls basal motion during speed-ups. However, sliding laws that relate changes in basal velocity to changes in water pressure do not account for cavity growth. To investigate the processes governing a typical speed-up event, we use a finite-element modelling approach combined with a commonly-used sliding law to recreate internal deformation and basal sliding of Haupapa/Tasman Glacier during rain-induced acceleration. In general, we find peak velocities can only be achieved when basal water pressure exceeds ice overburden and velocity at the glacier sides is allowed to exceed that observed by a GNSS unit situated near the margins. The sliding law requires a more complete treatment of cavity growth under rapid water pressure changes to better capture basal acceleration observed at Haupapa/Tasman Glacier.</p>

A unified model for transient subglacial water pressure and basal sliding

Changes in water pressure at the beds of glaciers greatly modify their sliding rate, affecting rates of ice mass loss and sea level change. However, there is still no agreement about the physics of subglacial sliding or how water affects it. Here, we present a new simplified physical model for the effect of transient subglacial hydrology on basal ice velocity. This model assumes that a fraction of the glacier bed is connected by an active hydrologic system that, when averaged over an appropriate scale, is governed by two parameters with limited spatial variability. The sliding model is reminiscent of Budd's empirical sliding law but with fundamental differences including a dependence on the fractional area of the active hydrologic system. With periodic surface meltwater forcing, the model displays classic diffusion-wave behavior, with a downstream time lag and decay of subglacial water pressure perturbations. Testing the model against Greenland observations suggests that, despite its simplicity, it captures key features of observed proglacial discharges and ice velocities with reasonable physical parameter values. Given these encouraging findings, including this sliding model in predictive ice-sheet models may improve their ability to predict time-evolving velocities and associated sea level change and reduce the related uncertainties.

Changing friction at the base of an Alpine glacier

Repeating earthquakes are a global phenomenon of tectonic faults. Multiple ruptures on the same fault asperities lead to nearly identical waveforms characteristic for these seismic events. We identify their microseismic counterparts beneath an Alpine glacier, where basal sliding accounts for a significant amount of ice flow. In contrast to tectonic faults, Alpine glacier beds are subject to large variations in sliding velocity and effective normal stresses. This leads to inter- and sub-seasonal variations in released seismic moment from stick–slip asperities, which we explain with the rate-and-state friction formalism. During summer, numerically modelled effective normal stresses at asperities are three times higher than in winter, which increases the local shear resistance by the same factor. Stronger summer asperities therefore tend to form in bed regions well connected to the efficient subglacial drainage system. Moreover, asperities organise themselves into a state of subcriticality, transferring stresses between each other. We argue that this seismic stick–slip behavior has potentially far-reaching consequences for glacier sliding and in particular for catastrophic failure of unstable ice masses.

Bayesian Inference of Ice Softness and Basal Sliding Parameters at Langjökull

We develop Bayesian statistical models that are designed for the inference of ice softness and basal sliding parameters, important glaciological quantities. These models are applied to Langjökull, the second largest temperate ice cap in Iceland at about 900 squared kilometers in area. The models make use of a relationship between physical parameters and ice velocity as stipulated by a shallow ice approximation that is generally applicable to Langjökull. The posterior distribution for ice softness concentrates around 18.2 × 10−25s−1Pa−3; moreover, spatially varying basal sliding parameters are inferred allowing for the decomposition of velocity into a deformation component and a sliding component, with spatial variation consistent with previous studies. Bayesian computation is conducted with a Gibbs sampling approach. The paper serves as an example of statistical inference for ice softness and basal sliding parameters at temperate, shallow glaciers using surface velocity data.