Ice flow dynamics forced by water pressure variations in subglacial granular beds

Damsgaard, Anders; Egholm, David Lundbek; Beem, L.; Tulaczyk, S. M.; Larsen, Nicolaj K.; Piotrowski, Jan A.; Siegfried, Matthew R.

doi:10.1002/2016gl071579

Cited by 57 publications

(33 citation statements)

References 82 publications

(123 reference statements)

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“…During slip events ice and till may be less tightly coupled, enhancing hydraulic conductivities and water flow across the grounding line. Granular till modeling suggests that porosity and thus hydraulic conductivity could increase during slip events (Damsgaard et al, ).This hydrological mechanism may also explain preferential dissipation at K 1 . Slip events appear to be set primarily by the diurnal tide, occurring once or twice daily at high tide and low tide (sometimes skipping low tide) (Bindschadler, ; Beem et al, ; Siegfried et al, ).…”

Section: Discussionmentioning

confidence: 99%

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Tidal Pressurization of the Ocean Cavity Near an Antarctic Ice Shelf Grounding Line

et al. 2020

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Mass loss from the Antarctic ice sheet is sensitive to conditions in ice shelf grounding zones, the transition between grounded and floating ice. To observe tidal dynamics in the grounding zone, we moored an ocean pressure sensor to Ross Ice Shelf, recording data for 54 days. In this region the ice shelf is brought out of hydrostatic equilibrium by the flexural rigidity of ice, yet we found that tidal pressure variations at a constant geopotential surface were similar within and outside of the grounding zone. This implies that the grounding zone ocean cavity was overpressurized at high tide and underpressurized at low tide by up to 10 kPa with respect to glaciostatic pressure at the ice shelf base. Phase lags between ocean pressure and vertical ice shelf motion were tens of minutes for diurnal and semidiurnal tides, an effect that has not been incorporated into ocean models of tidal currents below ice shelves. These tidal pressure variations may affect the production and export of meltwater in the subglacial environment and may increase basal crevasse heights in the grounding zone by several meters, according to linear elastic fracture mechanics. We find anomalously high tidal energy loss at the K 1 constituent in the grounding zone and hypothesize that this could be explained by seawater injection into the subglacial environment at high tide or internal tide generation through interactions with topography. These observations lay the foundation for improved representation of the grounding zone and its tidal dynamics in ocean circulation models of sub-ice shelf cavities.Plain Language Summary One of the challenges for sea level rise prediction is understanding how the Antarctic ice sheets and the Southern Ocean interact. Ocean tides are an important component of this interaction, influencing ice shelf melting and the flow rate of grounded ice toward the coast. We report new observations relevant to this interaction: tidally varying ocean pressures where the ice first goes afloat to become an ice shelf. These tidal ocean pressure variations influence tidal currents below the ice shelf, and we propose that they also push seawater beneath the ice inland of the ice shelf and extend fractures at the ice shelf base. This study identifies tidal processes that may affect melt and fracture near the inland edge of ice shelves, a highly sensitive zone for ice dynamics.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Tidal Pressurization of the Ocean Cavity Near an Antarctic Ice Shelf Grounding Line

et al. 2020

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“…Subglacial sediment may respond to runoff supply in a manner that could explain some aspects of ice flow variability ( Figure 5; Dow et al, 2013;Bougamont et al, 2014;Walter et al, 2014;Kulessa et al, 2017). According to this hypothesis, pulses of runoff to the bed create a vertical hydraulic gradient between (high pressure) subglacial water at the ice-sediment interface and the (low pressure) underlying sediment, leading to downward migration of water into the sediment (Boulton et al, 2001;Damsgaard et al, 2016). This increases sediment pore-water pressure, reduces sediment shear strength and increases sediment deformation and thus ice flow (Figure 5; Boulton et al, 2001;Bougamont et al, 2014;Damsgaard et al, 2016).…”

Section: Runoff-induced Variations In Sediment Shear Strengthmentioning

confidence: 99%

“…According to this hypothesis, pulses of runoff to the bed create a vertical hydraulic gradient between (high pressure) subglacial water at the ice-sediment interface and the (low pressure) underlying sediment, leading to downward migration of water into the sediment (Boulton et al, 2001;Damsgaard et al, 2016). This increases sediment pore-water pressure, reduces sediment shear strength and increases sediment deformation and thus ice flow (Figure 5; Boulton et al, 2001;Bougamont et al, 2014;Damsgaard et al, 2016). Subsequent evacuation of water through the efficient drainage system then leads to a reversal of vertical hydraulic gradients, upward migration of water out of the sediment and a reduction in sediment pore-water pressure.…”

Section: Runoff-induced Variations In Sediment Shear Strengthmentioning

confidence: 99%

The Influence of Hydrology on the Dynamics of Land-Terminating Sectors of the Greenland Ice Sheet

et al. 2019

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Coupling between runoff, hydrology, basal motion, and mass loss ("hydrology-dynamics") is a critical component of the Greenland Ice Sheet system. Despite considerable research effort, the mechanisms by which runoff influences ice dynamics and the net long-term (decadal and longer) dynamical effect of variations in the timing and magnitude of runoff delivery to the bed remain a subject of debate. We synthesise key research into land-terminating ice sheet hydrology-dynamics, in order to reconcile several apparent contradictions that have recently arisen as understanding of the topic has developed. We suggest that meltwater interaction with subglacial channels, cavities, and deforming subglacial sediment modulates ice flow variability. Increasing surface runoff supply to the bed induces cavity expansion and sediment deformation, leading to early-melt season ice flow acceleration. In the ablation area, drainage of water at times of low runoff from high-pressure subglacial environments toward more efficient drainage pathways is thought to result in reductions in water pressure, ice-bed separation and sediment deformation, causing net slowdown on annual to decadal timescales (ice flow self-regulation), despite increasing surface melt. Further inland, thicker ice, small surface gradients and reduced runoff suppress efficient drainage development, and a small net increase in both summer and winter ice flow is observed. Predicting ice motion across land-terminating sectors of the ice sheet over the twenty-first century is confounded by inadequate understanding of the processes and feedbacks between runoff and subglacial motion. However, if runoff supply increases, we suggest that ice flow in marginal regions will continue to decrease on annual and longer timescales, principally due to (i) increasing drainage system efficiency in marginal areas, (ii) progressive depression of basal water pressure, and (iii) thinning-induced lowering of driving stresses. At higher elevations, we suggest that minor year-on-year ice flow acceleration will continue and extend further into the interior where self-regulation mechanisms cannot operate and if surface-to-bed meltwater connections form. Based on current understanding, we expect that ice flow deceleration due to the seasonal development of efficient drainage beneath the land-terminating margins of the Greenland Ice Sheet will continue to regulate its future mass loss.

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“…Interactions between ice, water, and subglacial sediment generate an imprint in the geological and geomorphological record. High water pressure combined with fast ice flow promote subglacial erosion, sediment advection, and deformation (Tulaczyk et al, ; Damsgaard et al, , ; Evans, ), which is reflected in such landforms and their deposits as drumlins (Menzies et al, ), mega‐scale glacial lineations (Clark, ), tunnel valleys (Kehew et al, ), and glacial overdeepenings (Cook & Swift, ). Consequently, deciphering the geological and geomorphological signatures should enable reconstruction of the interactions between past glaciers and their beds.…”

Section: Introductionmentioning

confidence: 99%

Groundwater Flow Under a Paleo‐Ice Stream of the Scandinavian Ice Sheet and Its Implications for the Formation of Stargard Drumlin Field, NW Poland

Hermanowski

Piotrowski

2019

JGR Earth Surface

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Subglacial groundwater flow was an integral part of glaciological systems of past ice sheets, but its impact on the origin of active-ice landforms remains unexplored. Using numerical experiments, we attempt to constrain groundwater flow dynamics under a major paleo-ice stream of the southern Scandinavian Ice Sheet and its impact on the formation of the Stargard drumlin field. Flow models show a total reorganization of groundwater dynamics under the advancing ice stream to a depth of up to~200 m. A mosaic of intervening groundwater recharge and discharge areas originates, whereby the same areas may experience multiple shifts in flow directions. A prominent time-and space-transgressive pressure pump recharges groundwater in a subglacial zone up to about 20 km within the ice margin and discharges it in front of the ice sheet. The simulated groundwater flow pattern suggests that drumlins occupying a portion of the ice stream area occur preferentially where groundwater upwells and discharges at the ice/bed interface. Consistent with the geological composition, a possible drumlin-forming mechanism involves an excess of pressurized water under the ice reducing the strength of the subglacial deposits and facilitating glacial erosion, erosion by turbulent meltwater flows or both, streamlining antecedent deposits into drumlin shapes. This study emphasizes the importance of causal feedback between subglacial groundwater flow and the ice sheet dynamics and suggests an impact of the former on the formation of streamlined subglacial landforms in general.

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Ice flow dynamics forced by water pressure variations in subglacial granular beds

Cited by 57 publications

References 82 publications

Tidal Pressurization of the Ocean Cavity Near an Antarctic Ice Shelf Grounding Line

Tidal Pressurization of the Ocean Cavity Near an Antarctic Ice Shelf Grounding Line

The Influence of Hydrology on the Dynamics of Land-Terminating Sectors of the Greenland Ice Sheet

Groundwater Flow Under a Paleo‐Ice Stream of the Scandinavian Ice Sheet and Its Implications for the Formation of Stargard Drumlin Field, NW Poland

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