[meteorite-list] Tides Control Flow Of Antarctic Ice Streams

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:16:36 2004
Message-ID: <200308251538.IAA15106_at_zagami.jpl.nasa.gov>

A'ndrea Elyse Messer
Penn State Public Information August 21, 2003
aem1_at_psu.edu
(814) 865-9481

Krishna Ramanujan
NASA Goddard Space Flight Center
kramanuj_at_pop900.gsfc.nasa.gov
(301) 286-3026

RELEASE: 03-82

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.

For more information and images on the Internet, visit:

      http://www.gsfc.nasa.gov/topstory/2003/0822icestream.html
Received on Mon 25 Aug 2003 11:38:19 AM PDT


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