What is the Kamb Ice Stream?
Within the last ~10,000 years the West
Antarctic Ice Sheet lost about half of its volume and concerns have been raised
about the possibility of its further retreat or collapse in the near future.
Such retreat and collapse could raise the global sea level by several meters,
contributing to the near-future sea-level rise due to anticipated global warming.
A large fraction of this mass has been lost through the fast-flowing ice streams
of the Ross Embayment. Yet these ice streams themselves are undergoing change
currently with the stoppage of Ice Stream C (ISC) ~150 years ago and to the
slowdown of the Whillans Ice Stream that has continued for the last several
decades.
In order to evaluate the near-future contribution of the WAIS to sea-level changes
we need to understand important aspects of the ice-stream stoppage process,
such as: the mechanism of ice stream stoppage, whether such stoppage can affect
other ice streams in the near future and whether the stopped ISC may restart
in the near future?
This project
is a collaboration between ourselves at St. Olaf and scientists at the University
of California Santa Cruz. We are using ground-based ice-penetrating radar
and GPS to investigate: (1) along-flow variation in bed properties, (2) deformation
of internal reflectors, (3) temporal and spatial patterns of horizontal and
vertical components of ice-surface velocity. The data will be used to
verify whether ISC is already experiencing a surge and to infer how long the
current inactivity of the ice stream may persist.
Science and Results
Background
The St. Olaf College contribution to the US-ITASE
program is a deep-penetrating radio echo-sounding system that can measure the
bedrock surface beneath the ice as well as internal layers that have unique
electrical properties. Total ice thickness data is useful to researchers
who develop ice-flow models of the ice sheet. The immense size of the
ice sheet makes it difficult to obtain high-accuracy thickness measurements
over the entire continent. The long distance traverses of the US-ITASE
program make it an ideal platform for ground-based radar measurements.
Internal ice reflections are usually related to deposition
of volcanic debris (acids or ash) or dust layers. These layers give researchers
a window into the accumulation and flow history of the ice sheet. Changes
in the layer thickness along the traverse routes may be attributed to changes
in snow accumulation due to climate or geographic changes (crossing a drainage
divide, for example). Changes in the ice flow velocity due to bumps in
the bedrock or changes in the bedrock character cause the ice to thicken or
thin as the ice decelerates or accelerates.
Results, Publications & Data
At this point (spring 2005) we have completed two field seasons on the ice
stream and have some preliminary results from the radar. The first image
below shows a radar profile from a longitudinal section of Kamb Ice Stream.
The bed topography is revealed at a depth of approximately one kilometer and
internal stratigraphy can be resolved throughout most of the ice thickness.
The internal layers are deformed by the ice stream flow and are not conformal
with features at the bed. Similar images acquired by us in this location
in 1987 may enable us to measure three-dimensional strain rates throughout the
ice thickness.
Constant Midpoint Profile
The next image shows the results of a "constant
midpoint profile" where the radar transmitter and receiver have been moved
in increments of 10 meters away from a common center and echoes from this "constant
midpoint" have been stacked across the plot. This kind of profile
enables us to image reflectors from the midpoint, both at the bed and throughout
the ice thickness, from a range of angles (hyperbolas at the left). By
solving simultaneous equations for the travel time to a fixed point over multiple
paths we can accurately determine the average velocity as a function of depth
(the semblance) and ultimately extract dielectric properties of both the internal
reflectors and the bed using the amplitude variation of the echoes..
Instrumentation & Methods
We operate a 3 MHz radio echo-sounding system to
transmit and receive radio waves through as much as 3 km of ice. This
signal frequency translates to a wavelength of about 56 m in ice. The
transmitter (made by the University of Washington) emits pulses at frequency
of 250 Hz. The pulses are emitted by our transmitter antenna, a 40 m dipole
dragged along the snow surface. 135 m in front of the transmitter is an
identical dipole antenna that receives the signal. The signal is amplified
and passed on to an oscilloscope board mounted directly on a field-hardened
PC computer. The scope board identifies the incoming signal, digitizes
it, and sends it to the computer to be stored on the hard drive. The scope
board is extremely fast, sampling the signal at up to 100 million samples/second.
Each sample of the signal is digitized to a 14-bit number meaning that the board
is capable of processing up to 1.6 Gigabytes of signal data every second!
Fortunately we don't record data at such a fast rate,
but use the board's speed to stack the incoming signal to remove much of the
environmental radio noise that hides our reflected signal. We stack the
data by averaging a few thousand of the transmitter pulses and their resultant
echoes from the bedrock and internal layers. The traverse train travels
at about 12 km/hr so we generally stack 1200-1500 traces every 10-12 meters.
Stacking and the fast scope board allow us record a dense profile of traces
while eliminating much of the noise that would make the data difficult to process
and interpret.
More about basic
radio echo-sounding techniques
Field Logistics
The system and operator are towed by a snow machine.
The receiver and operator are housed in a small shelter built on a wooden Komatik
sled. The shelter protects the computer and operator from wind, cold,
and snow. Power for the receiver is provided either by a solar-powered
battery system, or a small gas-powered generator mounted on the back of the
sled. The generator also provides power for battery chargers when needed.
The transmitter batteries are recharged by the solar panels or generator.
The shelter is also equipped with a GPS receiver to locate the data points.
A survival bag is strapped to the front of the shelter in case of emergency.
The transmitter is towed 100 m or more behind the
receiver sled. The transmitter and its battery sit in a small sled and
are protected from the elements by the sled's nylon cover. The antennas
for the transmitter and receiver are housed in strong rubber hoses and tied
to the tow ropes to keep the antennas as straight and parallel as possible.
Photos from Kamb Ice Stream
As a cooperative research project there are
numerous universities, organizations, and individuals involved in bringing the
science together. Many of those groups are listed here. Other links
connect to sites related to polar research, exploration, weather, etc.
Universities & Research Institutes of Kamb
Ice Stream Project
Dr. Slawek Tulaczyk:
University of California, Santa Cruz
Ice Dynamics, GPS surveys
Dr. Ian Joughin
University of Washington
Ice dynamics and remote sensing
St. Olaf Contacts
Dr. Robert W.
Jacobel
CEGSIC Director & US-ITASE Principal Investigator
email: jacobel@stolaf.edu
Office: (507) 646-3124
Fax: (507) 646-3968
Rickard Pettersson
Postdoctoral Researcher
email: rickardpettersson@geo.uu.se
St. Olaf College Undergraduate Researchers
Ian Campbell:
Data acquisition software (2005-2006)
David Osterhouse:
Data Processing (2005-2006)
Support Agencies & Logistics
National Science Foundation:
Federal funding of the US-ITASE program
Raytheon Polar Services Company:
holds the logistics contract for the US Antarctic Program
Weather Conditions in Antarctica
Conditions for McMurdo Station, Amundsen-Scott
(South Pole) Station, Siple Dome (sometimes appears as U.WI ID 8900), and Vostok
Station (Russia).
