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

• Published in: Geophysical research letters 2016-12, Vol.43 (23), p.12,165-12,173
• Main Authors: Damsgaard, Anders, Egholm, David L., Beem, Lucas H., Tulaczyk, Slawek, Larsen, Nicolaj K., Piotrowski, Jan A., Siegfried, Matthew R.
• Format: Article
• Language: English
• Subjects: Ablation; Bearing; Climate; Climate change; Computer applications; Contact pressure; Contact stresses; creep; Creep (materials); Deformation mechanisms; Disruption; Dynamics; Experiments; Flow stability; Glaciation; Glaciers; Glaciohydrology; glaciology; Global warming; Grains; granular materials; Hydrostatic pressure; Ice; Ice sheets; Ice streams; Landslides; Mathematical models; Mechanical properties; Mechanics; Mechanics (physics); Melting; Meltwater; Movement; Networks; Nonlinearity; Organizations; Pore water; Pressure variations; Sea level rise; Sediment; Sediment load; Sediment yield; Sediments; Sheets; Solifluction; stick‐slip; Streams; Strength; Stresses; subglacial mechanics; Velocity; Water pressure; Yield point; Yield stress
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2016GL071579

Glaciers and ice streams can move by deforming underlying water‐saturated sediments, and the nonlinear mechanics of these materials are often invoked as the main reason for initiation, persistence, and shutdown of fast‐flowing ice streams. Existing models have failed to fully explain the internal mechanical processes driving transitions from stability to slip. We performed computational experiments that show how rearrangements of load‐bearing force chains within the granular sediments drive the mechanical transitions. Cyclic variations in pore water pressure give rise to rate‐dependent creeping motion at stress levels below the point of failure, while disruption of the force chain network induces fast rate‐independent flow above it. This finding contrasts previous descriptions of subglacial sediment mechanics, which either assume rate dependence regardless of mechanical state or unconditional stability before the sediment yield point. Our new micromechanical computational approach is capable of reproducing important transitions between these two end‐member models and can explain multimodal velocity patterns observed in glaciers, landslides, and slow‐moving tremor zones. Plain Language Summary The mechanical behavior of subglacial sediments is of fundamental importance to ice‐sheet dynamics, ice‐mass loss, and sea‐level rise in a warming climate. The majority of the ice loss from the ice sheets in Antarctica and Greenland occurs through fast‐flow regions, where granular sediments beneath the ice deform and lubricate the ice flow. Due to their critical importance for future ice‐sheet stability, models for subglacial sediment mechanics and their mathematical representation have been intensely discussed. Previous studies of such ice‐bed dynamics have relied on a priori assumptions. We used a more direct approach and studied the 3D re‐organization of the sediment grains with interaction to the interstitial meltwater. Using computational experiments we investigated the mechanics of granular sediments under ice sheets. We found that in such settings granular assemblages can slowly creep when variations in pore‐water pressure cause rearrangements in the grain‐to‐grain contact network. Our experiments reveal the details of how stress in excess of the yield strength incites rapid failure, which increases the ice‐sheet's sensitivity to surface melting. The modeled creep and stick‐slip behavior of granular material mimics observations from active ice streams and this