FIELD MEASUREMENTS OF ICE DYNAMICS AND GLACIER
MORPHOLOGY AT BERING GLACIER
Ronald L. Bruhn (1), Richard R. Forster (2), and Andrew L.J. Ford (3)
(1)
Dept. Geology and Geophysics, University of Utah
(2)
Dept. Geography, University of Utah
Salt Lake City, UT 84112-011
Introduction
We are studying the Bering Glacier using remote sensing and field measurements as
part of a comprehensive investigation of the tectonics and glacial geodynamics of the
Saint Elias orogen. Work to date has been funded by the NSF, NASA and with logistical
support from BLM. We also collaborate with Dr. Terry Pavlis at the University of New
Orleans who specializes in tectonics and structural geology.
Summer 2003 measurements
The field research was conducted by Ron Bruhn (Geology and Geophysics), Rick
Forster and Andy Ford (Geography) of the University of Utah. We were based at the
Bureau of Land Management (BLM) Bering Glacier research camp located on the south
shore of Vitus Lake 5 km from the Bering terminus. The ASTER sensor onboard
NASA’s Terra satellite was also tasked to acquire images during our field campaign. The
following sets of measurements were made on the Bering Glacier using one of the two
helicopters based at the camp, and with supplemental help from camp personnel.
Ice Surface Velocities
Ice velocities were derived by reoccupying 19 sites and recording survey grade GPS
measurements along three lateral transects near A – Grindel HIlls, B – Khitrov Hills, and
C – Barkley – Waxell Ridge. Fifteen of the sites were visited three times with the other
four visited twice due to weather conditions. The velocities up-glacier along transect C
are an order of magnitude larger (≈ 1 m / day) than those of transects B and C (0.1 m /
day). We estimate the RMS errors to be +/- 4cm.
Ice Surface Elevations
Ice surface elevations are compared with those measured in 1991 using a precision
altimeter (Molnia and Post, 1995; Trabant and Molina, 1992). The plots of transects A
and B surfaces’ indicate substantial lowering over the 12 years. The surge of 1993/1995
complicates the interpretation of this signal.
Ice Thickness
Ice thickness was measured using a radio echo sounder tuned to 5 Mhz and was most
successful on thin ice near the Medial Moraine Band on the lower west side of the Bering
ice lobe. We believe this thin ice is the due to a sub-ice ridge formed by up-thrusting
along the Aleutian megathrust, the most important tectonic structure in the northeastern
Pacific. Continued activity along this splay of the thrust may be enhanced by rapid
erosion and removal of the upthrust rock by the Bering Glacier. Conversely, this ridge is
also an important feature constraining the flow of ice, accumulation of debris on the
surface of the glacier, and may be a crucial control on channeling water in the subsurface.
Snowline
The position of the snowline was mapped from a helicopter and handheld GPS on the
western branch of the Bagley Icefield. A comparison with an ASTER image acquired on
the 8th August 2003 indicates there is an excellent agreement.
Preliminary results of these measurements were presented in a poster “Glaciological
Field Measurements on the Bering Glacier, Alaska Summer 2003”at the Fall 2003 AGU
in San Francisco. A comparison of glacier elevations from the survey grade GPS
measurements with those of a DEM derived from an ASTER stereo-pair acquired during
the field measurements is the subject of a paper in preparation to be submitted to the
International Journal of Remote Sensing (IJRS).
Proposed Summer 2004 measurements
The two main goals of the 2004 Bering Glacier field measurements are: 1) identifying
sub glacial features related to tectonic structures and 2) continue baseline measurements
of the quiescent phase of the surge cycle.
Specific objectives of the 2004 field campaign:
1) Determine if there are any changes in ice surface velocity and elevation from last
year’s measurements. This will begin to build a baseline for characterization of the
quiescent phase and test the consistency of these values.
2) Measure the longitudinal variation in velocity near the centerline of the Bering from
the “throat” at the Bagley Ice Valley to the near the terminus. This will allow us to
characterize the transition from the fast flow up-glacier to the slow flow down-glacier
measured in 2003.
3) Compare the short-term (daily to weekly) velocities with long-term (monthly to
annual) velocities using feature tracking on repeated satellite images from SAR and
ASTER.
4) Measure the spatial distribution of ablation rate.
5) Estimate ice thickness variations for evidence of sub-ice tectonic features and ice flux
calculations.
6) Compare field measurements of snowline position with those derived from SAR and
ASTER.
Methods for accomplishing the objectives:
1) Repeat the 2003 velocity and surface elevation measurements along the three lateral
transects. Each of the 19 sites will be occupied three times with differential GPS.
Comparisons of 2003 with 2004 ground based velocities can measure changes in shortterm (daily to weekly) motion.
2) A 40 km longitudinal transect near the centerline of the Bering Glacier will be added
to the three lateral transects. There will be 20 sites occupied three times each with the
GPS. The sites will be spaced approximately every 1 km in the vicinity of the velocity
decrease, which was found to be between the upper [C] and middle [B] transverse
transects during 2003. The lower portion of the longitudinal transect will be sampled
every 2-3 km.
3) To measure annual-scale velocity we will use feature tracking on repeated satellite
images from SAR and ASTER. A long time series of SAR data exists beginning in 1992.
We have already ordered this data from ASF and have begun to process it. ASTER
scenes will once again be requested during the field campaign and used to map
displacements since the acquisition of the 2003 scene.
4) Ablation stakes will be set in the ice at each of the GPS location sites along the
transects with the ice surface measured at each visit.
5) Ice thickness will be measured directly at each site using a radio echo sounder (RES)
and a gravity meter. The RES operates at 5 MHz and was purchased for the 2003 field
measurements with funds from the previously mentioned NASA grant. Either a Worden
or Scintrex CG-3M digital gravity meter will be used as an independent measure of ice
thickness and to insure reliable readings in the deeper ice where the RES maybe limited
due to potentially large amounts of englacial water and cavities. Additional RES
measurements will be made in the vicinity of the suspected ice ridge to maps its
orientation and depth below the ice. The gravity meter will be made available courtesy of
the Dept. of Geology and Geophysics (University of Utah).
6) The position of the snowline on the Bering/Bagley Ice Valley will be measured with
GPS. In 2003 this was done by flying along the snow/bare-ice boundary in a helicopter
“tracing” the line with a hand-held GPS during the flight. The same procedure will be
used in 2004 with the addition of two or three landing sites along the snowline. This will
allow us to more accurately tie down the line with the geodetic GPS. We will also
measure the snow depth along profiles approaching the snow/ice transition.
Expected results and future plans
The more detailed and comprehensive 2004 measurements when combined with the
2003 measurements will allow the main goals to be obtained; 1) identifying sub glacial
features related to tectonic structures and 2) continue baseline measurements of the
quiescent phase of the surge cycle.
An important and cost-effective method for continuation of the quiescent phase study
of the Bering is the use of remote sensing data. SAR data of the Bering (regularly
acquired and archived at ASF) is particularly useful owing to its operation in the presence
of clouds and darkness that can be limit the temporal coverage of optical remote sensors.
We will continue our multi-temporal SAR analysis of the Bering quiescent phase using
ground observations and measurements made during the 2003 and 2004 field campaigns
to compare velocity measurements, elevations and snowline positions and improve our
knowledge of the Bering ice surface characteristics. We sill seek continued support for
the study of the Bering quiescent phase through future funding opportunities.