Ice Streams and Subglacial Environment

Ice Streams and Subglacial Environment

1. Ice Stream Behaviour

   • river of ice
     • flow controlled by water presence
   • GrIS
     • forced by climate = melting
   • AIS
     • internal variability
   • Basal heat balance
     • AIS make own melt
     • geothermal flux
     • frictional heating
     • slide at bed
   • formation
     • topographically
     • ^ water = ^ velocity = ^ water
       • +ve feedback system
     • limit to width, lateral stress, sustain water supply
   • shut down
     • dynamic thinning
     • makes basal heat balance -ve
     • narrowing margins
     • water diverted to another stream

2. Basal Sliding

   • frozen at bed
     • no Csliding
     • could = 13m/yr, but what if observed more?
   • Hard-bed sliding
     • thin water layer at base
     • obstacle hold back (basal stress of land act against driving stress of ice
       • regelation
         • melt on upstream of obstacle
         • freeze on downstream
       • enhanced creep
         • localised stress 'softens' ice
     • cavitation
       • cause basal stress to not = driving stress
       • fill with water = less resistance
   • always requires water at base
   • Deformable bed sliding
     • underlain poorly sorted, weak sediment
       • deformation allow fast motion
     • basal stress can't = driving stress
     • limits to driving stress
       • horizontal stresses
         • weak-bed and strong-bed areas interact
         • longitudinal stresses
         • driving stress spread around bed
           • transmit driving stress
       • lateral shearing
         • slow moving margins
         • resistance to shear stress
         • lateral stress transmission

3. Leverett Glacier, Greenland

   • GrIS ice loss through enhanced discharge/surface melting
   • increased surface melting doesn't = acceleration
   • Ice velocity
     • measured via image correlatiom
       • Landsat mission
       • fusion of all images - maximise precision and spatial coverage
     • surface melting increased dramatically
       • unprecedented 2007-12
       • 49.8% ^ melting
   • Conclusion
     • increased melt = efficient basal drainge = winter slow down
     • ^ seasonal variability
     • long-term annual slowdown in acceleration
     • inefficient > efficient drainage
       • cavity > channel
   • correlation significance increase in long-term
     • lack of long-term observational record
   • why not ice sheet thinning as cause?
     • thinning can slowdown
       • how much can be accounted for by this?
         • rest = hydrology
     • need to know about ice geometry
       • thickness
       • surface slope
       • not ice temp
     • = 10-20m thinning
       • not predicted by model

4. Conclusions

   • AIS - cold environment - no surface melt
     • water = frictional heat/geothermal flux
     • change in flow = cold to warm bed (vice versa)
       • internal variability (no CC)
   • GrIS - significant surface melt
     • temporary speed up ice - melt influx
       • long-term = deaccleration