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
- Basal melt feedback
- 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
- Last disturbed layer represents ice stream shutdown timing
- 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
- Ice spreads due to fast flow (velocity gradient)
--> Thinning occurs to satisfy mass continuity
- Thinning steepens ice temp gradient ∴ bed is less insulated
- 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:
- Velocity = v. sensitive to ice stream width
- Basal heat generated = f(basal stress & velocity)
∴ Narrowing:
Reduces velocities
Reduces meltwater @ bed
Shifts heat balance @ bed to negative
click to edit