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Ice Stream Behaviour (Ice Stream Water Source (Basal Heat Balance (=…
Ice Stream Behaviour
Ice Stream Water Source
Ice streams generate heat ∴ cause subglacial melt
Basal Heat Balance
= (geothermal flux + frictional heating) - (conductive heatloss)
= (Qg + u * τb ) - Qc
Heat available for melt = heat in - heat out
Melting occurs if:
HEAT IN > CONDUCTIVE HEAT LOSS
Heat Terms
Frictional heat
Related to sliding velocity & basal stress = u x τb
Heat generated by basal sliding
Geothermal
heatflux Qg
Heat from centre of Earth (small amount)
Conductive heatloss Qc
Heat lost to ice interior & atmosphere
Antarctic Ice Stream Behaviour
Exhibit internal variabiliity
i.e. Behaviour change independent of cliamte
e.g. Prominent region of thickening on WAIS
beside Ross Ice Shelf on Siple Coast
Blue on thinning rates map Pritchard et al 2009
Ross Ice Stream
Behaviour
High velocities
(up to 800m/yr)
Suggests:
Weak sediment & pressurised water
Despite low driving stresses
(~10 kPa vs 100 kPa elsewhere)
Ross (Siple Coast) Ice Streams
System of ice streams & tributaries flow into Ross Ice Shelf
= Multiple ice streams separated by ridges
Slow area in centre yet crevassing @ surface
Kamb Ice Stream = shutdown (no longer streaming)
MODIS
Imagery
∴ Rough surface = fast ice stream flow
Ice deforms during fast flow
Crevassing ∴ surface broken up
Lava Flow Analogy
Bowing out structure of flow
When fast flow (weak) flanked by slower flow (stronger)
Lava flow centre = faster
Edges
=
slowed by lateral stresses
Velocity Profile Across Ice Streams
Flow = faster in centre
Velocity proportional to (width)^4
Can solve/predict velocities with numerical model
Velocity v sensitive to width
Ice stream velocity
= controlled by lateral shear
wide
weak bed in stream
strong bed at margins
Ice Stream Stagnation
Ice streams stagnate
if conductive heatloss > heat inputs (Qg+u*τb)
Bed freezes ∴ ice stream stops flowing i.e. stagnates
Boreholes reveal
Sediment & pressuried water at bed of Siple Coast ice
--> facilitates ice stream motion
Water beneath ice streams on Siple Coast
No water from surface
Little supraglacial melt in Antarctica
∴ Ice sheet must make its own by generating heat
Control Velocity Ross Streams
Lateral shear due to strong bed at margins
