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March
1, 2007 A wedge of sediment,
pushed up by
glacial movement, may be a buffer against moderate sea level rise,
pointing to
ocean temperature rise as the key factor in glacial retreat, according
to two
papers published today (March 1) in Science
Express. "Sediment beneath
ice
shelves helps stabilize ice sheets against retreat in response to rise
in
relative sea level of at least several meters," says Richard Alley, the
Evan Pugh professor of geosciences, The researchers
identified a
sediment wedge beneath the Whillans Ice Stream in Antarctica using
snowmobile-towed radar where ice from the Whillans Ice Stream in West
Antarctica begins to float in the The radar imaged a
miles-long
pile of sediments as thick as 100 feet deposited beneath the "We found this
miles-long
pile of deposited sediment just where the ice stream goes afloat," says
Sridhar Anandakrishnan, associate professor of geosciences, Antarctic glaciers
form over the
Antarctic land mass and glacial ice streams flow toward the oceans.
When the
edge of the glacier flows past the edge of land, that portion of the
glacier
begins to float and forms an ice shelf. Portions of ice shelves
occasionally
calve off and float into the oceans. Previous research suggested that
rising
sea level would push back the grounding line -- the line where grounded
glacier
and ice shelf meet -- shrinking the glaciers. "Our results suggest
that
the grounding line is well above the point at which the ice floats and
will
tend to remain in the same location even though sea level changes,
until sea
level rises sufficiently to overcome the effect of the sediment wedge,"
says Anandakrishnan. "We determined the grounding line location from
the
drop in ice surface elevation, which was 33 feet over only about 2
miles." According to the
researchers,
"the grounding-line position has probably been stable near the present
position for a millennium." Anandakrishnan and
colleagues
note that the wedge depicted by radar imaging closely matches wedges
found
beyond the floating The Science Express
paper,
"Effect of Sedimentation on Ice-Sheet Grounding-Line Stability,"
suggests
reasons why the sediment wedge provides stability against the increase
or
decrease of a few meters or more of sea-level change. The researchers used
three
different ice-flow models to first model the configuration
approximating the
Whillans Ice Stream and the adjacent The response of
these models to
instantaneous sea-level rise, both with and without the sediment wedge,
showed
that without the sediment wedge, the ice shelf forms at the point where
the ice
thins; however, with the sediment wedge, the ice shelf forms where the
bed
falls away. "In all three
models,
sea-level rise without a wedge causes grounding-line retreat," says
Alley.
"With the wedge, the ice over the wedge thickens to above flotation
mass
so that small increases in sea-level cause only small grounding line
retreat
which never reaches the point where the ice over the wedge floats." However, large
sea-level increase
could push the grounding-line much farther back, allowing the ice above
to
float and the glacier as a whole to retreat. Further calculations
indicate that
a sea-level rise of more than 33 feet may be required to force the ice
to
retreat from the wedge. "Our results,
together with
recent evidence that ice shelves respond sensitively to
ocean-temperature
changes and quickly propagate the response inland, point to greater
importance
of other environmental variables, and especially sub-ice-shelf
temperatures," says Alley. The researchers
caution that sea
level may be the primary control on the ice sheet if other variables
that
affect ice sheets more quickly, such as water temperature under ice
shelves,
remain stable. "Common climatic
forcing,
including an increase in ocean temperatures, which can have very large
and very
rapid effects on ice sheets, is more likely to cause Antarctic glacial
retreat," says Alley. Floating ice shelves
around "Recent discoveries,
including the changing lakes beneath the ice that flows into the
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