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August 22, 2003
Tides Control Flow of Antarctic Ice Streams
The moon is often accused of causing lunacy, bringing on labor and transforming
werewolves. Now it seems that in reality, the moon, through the tides,
is responsible for the pattern of motion exhibited by ice streams in
the Antarctic, according to a team of geologists from NASA, Penn State
and University of Newcastle, Newcastle Upon Tyne, England.
"My observations from a few years ago were that Ice Stream D in the
West Antarctic was slowing to about half average speed and then speeding
up," says Dr. Sridhar Anandakrishnan, associate professor of geoscience,
Penn State. "I thought that the speeding up and slowing down was tied
to rising and falling of the ocean tides."
The ice streams in West Antarctica move large amounts of ice downward
from the center of the glacier toward the ocean. Most of the glacier
rests upon bedrock and/or rubble on land, but part of the glacier floats
above the ocean. The grounding line, the line where the glacier stops
being grounded and floats, is quite a distance back from the leading
edge of the glacier.
Some ice streams are moving rapidly, some are slowing down and others
have completely stopped moving. Researchers have looked at a number of
ice streams and recently, they discovered that Whillan's Ice Stream exhibits
the most bizarre behavior because it actually stops dead and then slips
for a short time, moving large distances, before it stops again.
"The fact that such a huge lumbering river of ice can be stopped by
a one meter change in the tide underscores how delicate the balance of
forces is at the edge within the ice sheet," said Robert Bindschadler,
lead author of the study and a glaciologist and senior fellow at NASA
Goddard Space Flight Center.
The researchers report in today's (Aug. 22) issue of Science, that there
is a clear association between this stick-slip phenomenon and the ocean
tide.
Anandakrishnan and Bindschadler working with Richard B. Alley, the Evan
Pugh professor of geoscience, Penn State; Matt A. King, University of
Newcastle, Newcastle Upon Tyne, UK; and Laurence Padman, Earth and Space
Research, Seattle, combined data from various ice streams and produced
a model of how the tides control the slip stick of ice stream motion.
They note that If there were no tides at all, slip events would be predicted
to occur approximately every 12 hours.
However, the movement of the ice streams occurs every 18 and then 6
hours. That is, the stream remains still for 18 hours and then slips
for 10 to 30 minutes and halts. Then 6 hours later, the stream slips
again and halts. The first slip after 18 hours corresponds to just short
of high tide and the second slip is when the tide is falling, but is
not low.
"The up stream portion of the ice stream keeps moving all the time,"says
Anandakrishan. "The tide rises and puts pressure upward on the ice stream.
Somewhere in the middle, the ice stream sticks."
Eventually the pressure being exerted on the ice streambed from above
is enough to overcome the sticking point and the stream slips and then
halts. The tide continues to rise and then recede still putting pressure
on the ice stream until once again the ice slips.
"The motion of the ice streams is not as regular during neap tide because
the sea rise is not as high," says Anandakrishnan.
Each day the ocean by the West Antarctic has only one high tide and
one low tide separated by 12 hours. The levels of the tides vary on a
28-day cycle creating spring tides of up to 5 feet and neap tides of
16- to 20-inches separated by 14 days.
The National Science Foundation funded this research.
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Contacts:
Andrea Elyse Messer
Penn State Public Information
(814) 865-9481
aem1@psu.edu
Krishna Ramanujan
NASA Goddard Space Flight Center
(301) 286-3026
kramanuj@pop900.gsfc.nasa.gov |
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Ice Streams in West Antarctica
Pictured are 5 West Antarctic ice streams lettered A-E. Ice Stream B
splits in two. Red areas are fast-moving central ice streams. Blue areas
show slower tributaries feeding the ice streams. Green areas depict slow-moving,
stable areas. Thick black lines outline areas that collect snowfall to
feed their respective ice streams. Original
image. CREDIT: RADARSAT Antarctic Mapping Project
Diagram of Ice
The Antarctic Ice sheet extends outward to the ocean around most of the
continent. In some regions, it actually extends over the surrounding
water, forming floating "ice shelves." Part of the Ice flow occurs in "ice
streams." CREDIT: NASA
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