Siple Coast
Ice Streams

Ice Stream Location

Most ice streams
= topographically controlled

Flow through deep bed troughs i.e. Jakobshavn Isbrae

But Siple Coast troughs = minimal

e.g. Whillans Ice Stream

Partly result of streams' erosion? Unlikely only cause

Location
Control:

Siple Coast Ice Streams location due to

  1. Basal melt feedback
  1. Regulation by lateral stresses?

Reproducing Spacing Observed

With numerical model which considers:

Model simulates observed ice stream pattern on its own accord

  • Ice flow
  • Basal sliding
  • Basal water production
  • Longitudinal / lateral stresses

Shows spontaneous formation & self organisation

Kyrke-Smith
et al 2014

Siple Coast
Ice Stream
Pattern

Why ice streams organised where they are given lack of topographic constraint ?

1. Spontaneous Formation

Positive Feedback

Increased sliding - more friction & heating = POSITIVE FEEDBACK

2. Ice streams self-organise

Ice motion generates basal stress
--> frictional heating --> water --> increased sliding

Initial ice streams = thin & multiple

Not sustainable - haven't drawn down ice surface yet

Ice streams evolve to flow faster

Surface drawdown forms depressions
∴ sourcing more water from adjacent streams

Fewer (3) wider
ice streams

Flow faster since: greater water supply & sufficiently wide

Widen - overcome lateral stresses

Narrow
streams

sufficient separation so don't disturb eachother's flow / hydrology

not fast enough to self sustain via basal melt

slowdown as too narrow ∴ lateral stresses dominate

Kamb Ice Stream (IS C)

Stagnated ~150 years ago

Just as ice streams spontaneously form, they spontaneously shutdown

∴ Stopped carrying snow to coast (Ross Ice Shelf)

Snow still accumulating on top of stagnant ice stream

Hence: thickening signal
--> as snow freezes into ice

Dating Kamb Shutdown

Conway
2002

Radargram Findings:

Continuous radar layers down to ~40 m depth

Ice accumulating from snowfall

Interannual layers

Disturbed layers Below 40 m depth

∴ Disturbed layers evince ice stream

Ice streams deform/crevasse radar layers

  1. Last disturbed layer represents ice stream shutdown timing
  1. Calibrate depth & time using snowfall rate

Method: GPS Survey @ margin Kamb Ice Stream & Ridge

∴ Shutdown ~ 150 years ago

Cause Kamb Shutdown?

1. Cooling by Dynamic Thinning

  1. Ice spreads due to fast flow (velocity gradient)
    --> Thinning occurs to satisfy mass continuity
  1. Thinning steepens ice temp gradient ∴ bed is less insulated
  1. Faster heatloss to ice interior & atmos

4. Basal heat balance becomes negative ∴ bed freezes

2. Ice stream narrowing

Narrowing can lower ice stream velocity

Two-fold effect:

  1. Velocity = v. sensitive to ice stream width
  1. Basal heat generated = f(basal stress & velocity)

∴ Narrowing:

Reduces velocities

Reduces meltwater @ bed

Shifts heat balance @ bed to negative

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