Geophysical Investigations of Rapid Tidewater Glacier Retreat

Shad O'Neel[^1], Daniel E. McNamara^2^ and W.Tad Pfeffer^1^

^1^*Institute of Arctic and Alpine Research, University of Colorado
Boulder, Boulder CO 80309-0450, USA*

*^2^U.S.G.S. Golden, CO, 80401, USA*

IRIS Newsletter Article: DRAFT September 9, 2005

## Glacier Climate Interaction

Although they comprise only 13% of the world\'s mountain glacier area
(3% of total glacierized area of Earth), Alaskan and immediately
adjacent Canadian glaciers supply one of the largest measured
glaciological contributions to global sea level rise (\~0.14 mm yr^-1^,
equivalent to new estimates from Greenland). Retreating tidewater
glaciers dominate the Alaskan sea level contribution due to their
ability to efficiently transfer mass via iceberg calving \[*Arendt et
al.*, 2002\]. During retreat phase, a tidewater glacier may retreat on
the order of 1-2 km yr^-1^ concurrent with dramatic increases in ice
velocity. Catastrophic retreats of this style are thought to be
irreversible \[*Meier and Post*, 1987\] until the terminus retreats to a
position above local sea level. Comparatively, terrestrial glaciers,
whose main mass loss mechanism is surface ablation, generally retreat on
the order of 10 to several hundred m yr^-1^ and are reversible given
adequate mass balance forcing.

Unlike their terrestrial counterparts, the relationship between climate
and tidewater glaciers is complexly nonlinear; once initiated, tidewater
glacier retreats proceed independently of the local climate. This is
nicely illustrated by tidewater glaciers in south central and southeast
Alaska, which all experience roughly the same climate. The tidewater
glaciers of Glacier Bay initiated rapid retreat phases around the turn
of the 20^th^ century, while others, such as Columbia and LeConte
Glaciers \[*Krimmel*, 2001; *O'Neel et al.,* 2003\], are currently
undergoing rapid retreat. Contrarily, Taku and Hubbard Glaciers are
slowly advancing \[*Motyka et al.,* 2003\].

# Rapidly Retreating Columbia Glacier

Columbia Glacier is located in south-central Alaska, approximately 30 km
west of Valdez. The glacier flows 52 km from a maximum elevation of 3050
m down the flanks of the Chugach Mountains into Prince William Sound,
where it terminates at a grounded ice cliff in deep water (Figure 1).
The glacier attained a stable, shallow-water extended position (67 km
long, 1100 km^2^ surface area) at about 1100 A. D. and suffered only
minor length and thickness changes until ca. 1980, after which time
rapid retreat began \[*Meier and Post,* 1987\]. Since then, the terminus
has retreated over 15 km concurrent with thinning exceeding 20 m a^-1^
at low elevations. The retreat is propagated by iceberg calving that
exceeds incoming ice flux at an average rate of 0.74 km yr^-1^ \[*O'Neel
et al.*, 2005\]. Although there has been a recent decline in retreat
rate, which we attribute to a prominent constriction in the channel
(Figure 1, near the 1999 terminus), discharge flux continues to greatly
exceed the mass balance flux. Radio echo sounding measurements suggest
that the glacier will retreat another \~15 km before the bed rises above
sea level.

Columbia Glacier ranks among the world\'s fastest glaciers; surface
speeds commonly exceed 25 m d^-1^. Both observations of large amplitude,
short period velocity variations (Figure 2) and calculations of internal
deformation rates indicate that the flow is predominantly by basal
sliding.

**Field Studies**

In order to study the role of calving in the dynamics of Columbia
Glacier, we made measurements with a variety of geophysical
instrumentation around the lower 20 km of the glacier channel. An array
of ten high-frequency (1 Hz) Mark L22 seismometers and one broadband
(0.33-30Hz) Guralp 40T seismometer were deployed on rock outcrops at the
glacier margin during June 2004 for a period one-year (Figure 3). The
array recorded over 100 Gbytes of continuous data at a rate of 100
sampless^-1^ that included local and teleseismic earthquakes, small
local seismic events generated by the glacier (icequakes), and four
explosions detonated in boreholes on the glacier. Seismometers, power
systems and recording equipment were obtained through the IRIS
consortium's PASSCAL program.

The seismic experiment is one component of a larger integrated approach
that addresses the problem of the role of calving in glacier dynamics.
Additional field studies include:

-   Measurements of ice motion using both high precision GPS (equipment
    supplied by UNAVCO) and optical surveying methods. Measurements of
    surface motion 7 km upstream from the terminus were made from June
    through Sept. 2004 (Figure 2) and motion near the terminus was
    measured for 25 days during June 2005. Additional GPS surveys
    measured short-term-average speeds at several locations and
    established a geodetic control network for photogrammetric analysis.

-   Repeat aerial photography, extending from 1976 to the present used
    to determine glacier geometry and surface velocity fields.

-   Terrestrial time-lapse photography (4 images d^-1^) of the terminus
    (see http://tintin.colorado.edu/group/columbia/TLC.avi), which,
    using single camera photogrammetric techniques, provides a time
    series of volume change near the terminus.

-   Bathymetry, water temperature and conductivity (salinity)
    measurements in newly exposed regions of the fjord to infer channel
    geometry and water discharge rates and patterns emanating from the
    sub-marine terminus.

-   Detailed calving observations including timing, style, location, and
    subjective magnitude of thousands of calving events, used to ground
    truth the seismic records.

-   Hourly surface air temperature and precipitation measurements, used
    to investigate forcing mechanisms for the time distribution of
    calving.

**Integrated Data Analysis**

Three primary goals of this experiment are documenting mechanical
fracture processes involved in calving (including the location, rupture
style, and seismic energy release), understanding the relationship
between ice motion and calving and correlating ice motion and calving
with possible forcing mechanisms. Previous studies have identified three
characteristic seismic waveforms associated with three different
icequake event types \[e.g. *Qamar*, 1988; *Deichmann et al.,* 2000\].
These include: 1. Calving events (low frequency, long duration,
non-impulsive onsets, surface waves); 2. basal sliding (low frequency,
short duration, no surface waves); and 3. surface crevassing (high
frequency, short duration, impulsive onsets). *Qamar*'s (1988) work was
performed at Columbia Glacier, when the glacier extended 10 km further
down fjord, and provides a baseline for our work.

Calving events span a broad range of style and magnitude, ranging from
small isolated pieces of ice dropping from the ice cliff to the water to
large (1 km x 1 km) slabs rising up from great depths below the ocean
surface (Figure 4a). *Qamar* (1988) showed that the duration of a
seismic signal associated with calving correlates with ice volume
however, amplitude does not. The position of their camp made
differentiation of calving style and determination of ice volume
challenging. Using our calving observations and seismic record from the
broadband sensor located near the terminus, we intend to characterize
the seismic waveforms (Figure 4b) associated with the primary calving
styles and formulate an ice volume relationship using event amplitude,
frequency content and duration.

Typically calving is thought of as a process occurring immediately at
the terminus of a tidewater glacier. However, observations of along-flow
strain rates indicate that failure processes may be operating
significantly upstream of the terminus \[*O'Neel et al*., 2003\].
Accurate calculations of icequake locations will allow us to investigate
this idea. We detonated four explosions of varying magnitude at known
locations on the glacier to determine seismic velocities in the local
bedrock, which will allow us to more accurately determine icequake
locations. Rupture style will also be used to differentiate events
generated by calving, crevassing or basal processes.

Our second primary goal is to investigate the relationship between the
rate of calving and ice speed. Early predictions of the retreat grossly
over-estimated retreat rates, primarily because researchers did not
anticipate the large increase in surface speed as the terminus began to
retreat. Classically, the calving rate *U~c~* is a derived quantity,
calculated as the difference between ice speed at the terminus *U~i~*
and change in glacier length *dL/dt*:

$$U_{c} = \ U_{i}\  - \frac{\text{dL}}{\text{dt}}$$

Ice speed and length change are typically measured photogrammetrically
\[e.g. *Krimmel*, 2001\]. Our seismic data allow an independent
determination of the calving rate (and potentially calving flux) that
can be compared to photogrammatic measures of ice speed and length
change.

For our final goal, we will identify statistical relationships between
icequake occurrence (seismicity) and possible forcing mechanisms such as
ice motion, tidal variations, temperature and precipitation.

Preliminary results suggest that calving events generate unique seismic
records (Figure 4) that are distinguishable from earthquakes and other
events generated by the glacier due to crevassing and basal sliding. The
characteristic frequency of calving events is 1-3 Hz (Figure 4).
Submarine calving events contain more low frequency energy than
subaerial events . In addition to calving, two other event types were
recorded; one has a characteristic frequency much lower than calving in
the range of 0.05 -- 0.1 Hz and may be related to basal sliding or
sub-glacial hydraulics, while the second has a higher frequency content
and shorter duration (\< 1 sec) and is related to crevassing.
Preliminary analysis of the GPS and seismic datasets indicates that
overall icequake seismicity increases during speed-up events (Figure 2).
Our ability to identify and separate icequake types by frequency content
will enable us to investigate how calving and retreat are related to
external driving processes.

Columbia Glacier demonstrates that ice loss from calving is a critical
component in global sea level rise. Analogous, but larger and more
rapid, retreat processes are presently underway at several major outlet
glaciers of the Greenland Ice Sheet \[*Joughin et al.,* 2004; *Krabill
et al.,* 2004\]. Rapid retreats result in a substantial new component to
global sea level rise and provide strong motivation for a quantitative
understanding of calving and rapid ice flow which control such
catastrophic retreats. Our multidisciplinary investigations at Columbia
glacier will provide a significant new contribution to our understanding
of mechanical controls on marine terminating glacier instabilities
including but not limited to climate triggering.

**Acknowledgements**

This work was supported by NSF OPP 0327345. We thank Tim Parker, Mary
Templeton, Eliana Arias, and Bruce Beaudoin from PASSCAL for invaluable
help in the field and with data formatting and timing corrections. Jake
Walter, Ellie Boyce, H.P. Marshall and James McCreight provided
essential volunteer field support. Martin Truffer provided active source
timing and developed the single camera photogrammetry code. Thomas Hart
was essential in the blasting efforts. None of the work would have been
possible without the steady hands of Jim Harvey (Air Logistics) and Jan
Gunderson (ERA helicopters). This work was supported by the National
Science Foundation through grant \# NSF-OPP-03.

**References**

Arendt. A. A., K.A. Echelmeyer, W.D. Harrison, C.S. Lingle, and V.B.
Valentine (2002), Rapid wastage of Alaskan glaciers and their
contribution to rising sea level, *Science*, 297(5580), 382-386.

Deichmann, N., Ansorge, J., Scherbaum, F., Aschwanden, A., Bernardi, F.,
and Gudmundsson, G., (2000), Evidence for deep icequakes in an alpine
glacier. *Ann. Glaciol.* 31, 85-90.

Joughin I., W. Abdalati and M. Fahnestock. (2004), Large fluctuations in
speed on Greenland's Jakobshavn Isbrae Glacier, *Nature*, 432,608-610.

Krabill, W. and 12 others. (2004), Greenland ice sheet: increased
coastal thinning, *Geophys. Res. Lttrs,* 31(L24402).

Krimmel, R. (2001). Photogrammetric data set, 1957-2000, and bathymetric
measurements for Columbia Glacier, Alaska. USGS Water-Resources
Investigations Report 01-4089, 40 pp.

Meier, M F. and A. Post. (1987). Fast tidewater glaciers. *J. Geophys.
Res.,* **92**(B9), 9051-9058.

Motyka, R.J. and Echelemeyer, K.E. (2003) Taku Glacier (Alaska, U.S.A.)
on the move again: active deformation of proglacial sediments. *J.
Glaciol.* **49**(164), p. 50-58.

O'Neel, S., K.A. Echelmeyer, and R.J. Motyka (2003), Short-term
variations in calving of a tidewater glacier: LeConte Glacier, Alaska.
*J. Glaciol.,* 49(167),587-598.

O'Neel, S., Pfeffer, W.T., Krimmel, R.M., and Meier, M.F. (2005),
Evolving force balance at Columbia Glacier, Alaska, during its rapid
retreat, *J. Geophys. Res.*

Qamar, A. (1988) Calving icebergs: a source of low-frequency seismic
signals from Columbia Glacier, Alaska. *J. Geophys.
Res.,*93(B6),6615-6623.

**Figures**

![](media/image1.jpeg){width="4.926388888888889in"
height="6.656944444444444in"}

Figure 1. Lower Columbia Glacier as imaged by Landsat thematic Mapper on
September 26, 1999. Stars mark the location of temporary seismic
stations, yellow stars indicate Mark L-22 seismometers and the blue star
shows the location of the Guralp 40-T broadband seismometer. The yellow
w marks the location of weather observations, and the yellow c marks the
time lapse camera. Explosion locations for the active source experiment
are shown with yellow circles, and the location of the GPS receiver is
marked with a yellow X. The 2004-2005 terminus position is roughly shown
as a yellow line. The pre-retreat terminus position is shown in pink.

![](media/image2.jpeg){width="4.415972222222222in"
height="3.4659722222222222in"}

Figure 2. Horizontal velocity measured using high-precision GPS,
approximately 7 km upstream of the 2004 terminus. (yellow X on figure
1).

![](media/image3.jpeg){width="4.090277777777778in"
height="3.057638888888889in"}

Figure 3. Installing the broadband seismometer June 2004. Trimlines in
the background show the pre-retreat ice thickness at this location.
Photo by Tim Parker.

Figure 4. a) Seismogram of the calving event depicted in c. b)
Spectragram for the hour during which the calving event took place. c)
photographs of a large sub-marine calving event, with icebergs emerging
from significant depth below sea level.

![](media/image4.jpeg){width="5.993055555555555in"
height="8.139583333333333in"}

[^1]: *Corresponding author address: Shad O'Neel; INSTAAR, University of
    Colorado, Boulder, 1560 30^th^ St. Campus Box 450, Boulder CO,
    80309, USA (303) 735-8167, email: shad@colorado.edu*