Discussion Synopsis: Monday, 11/4 –
Numerical ice streams
M.R. Bennett, M. R. Ice streams as the arteries of an ice sheet: their mechanics, stability
and significance, Earth-Sci. Rev., 61,
309–339, 2003.
Premise: Review
of ice streams research
The paper presented work from the Siple Coast of Antarctica, which has several ice streams.
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The Siple Coast ice streams are responsible for 90% of the ice
and sediment flux.
o Why do the Siple
Ice streams exist?
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Heat
flux is significant in Antarctica (rifting), which helps melt the bed
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Presence
of weak Cretaceous sediments with high pore-water pressure that can deform
under low driving stress.
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What
are ice streams?
o Ice streams are areas of fast moving ice
flow within an ice sheet that are responsible for discharging the majority of
ice and sediment.
o These features are not fully understood,
but their motion is thought to be controlled by their lateral margins and by
weak deformable sediments along the basal margin.
o Ice streams exhibit a fluid like motion,
i.e. fastest flow occurs in the middle of the ice stream.
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Ice
stream behavior is highly variable in time
o E.g., due to stick-slope motion, ice and
water piracy, and erosion of bed sediments.
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The
basal conditions are key in understanding their behavior.
o Basal conditions are not fully
understood, but the primary idea is that sediments along the base have low
effective stress, which allows them to easily deform.
o Borehole measurements have shown that
pore-water pressures along the basal slip surface are near lithostatic.
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There
is an ongoing debate about the rheological properties of the basal sediments.
Laboratory experiments show that the sediments deform plastically. Modelers
prefer to use viscous-flow laws with a non-linear rheology
because this it allows them to predict changes in velocity that coincide with
changes in driving stress.
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Ice
streams may play a role in ice sheet collapse, which can lead to Heinrich
events.
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How
much has our understanding of ice streams changed in the 10 years since this
paper came out?
o New data from boreholes in ice streams
record the bed properties.
o Seismic data provides better constrains of
the bed.
o Work on the stick-slip motion at the ends
of the glacier and glacial quakes show they are triggered by diurnal tides.
o Still arguing about the bed rheology
o Still worry about the collapse of the ice
sheet
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Why
not investigate the Laurentide Ice sheet to
understand ice sheet collapse?
o Glacial geomorphology and glaciologists
do not agree
o It is possible that ice streams were not
dominant at the Laurentide ice sheet because heat
flux is low through the craton.
o Glaciers sit softly at the bed i.e. small
driving stress, and therefore do not leave clear evidence of ice streams.
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What
to do next?
o Need more measurements of the lateral and
basal margins.
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In-class
experiment
o While the paper discussion was going on,
Alan and Dustin set up an in-class experiment. The experiment consisted of two
containers: 1) filled with salt water, 2) filled with fresh water. A piece of
ice was put into each container and dyed with food coloring. The melting ice
provided a fresh water flux into each container.
o Results
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The
ice melted faster in the fresh water container because it was able to mix.
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The
salt-water container had stratified layers due to density differences between
the dense salty water and fresh influx of water. Because the water was not able
to mix as well, the colder fresher water stayed in the top layer and slowed the
melting of the ice chunk.