Download Glacial unloading: Forcings, responses and processes Larsen, C.F.

Glacial unloading: Forcings, responses and processes
Larsen, C.F.
Abstract
In a warming global climate, retreating glaciers are expected to cause a sea-level
rise. However, in areas near retreating glaciers, unloading causes uplift of the land that is
much larger than global sea-level rise. This leads to complex patterns of relative sea-level
changes in the polar and sub-polar regions where concentrations of coastal glaciers are
found. A unique case study of such glacial unloading and attendant sea-level change is
feasible in Glacier Bay, Alaska. At the end of the Little Ice Age a glacier system covering
~600 km2 rapidly retreated within Glacier Bay. This unloading event has caused the most
rapid ongoing rebound on Earth, uplifting an area of ~105 km2 at rates of up to 4 cm yr-1.
The scale of this deglaciation and rebound is similar to those that may occur in other
coastal regions of southern Alaska and the Canadian Arctic Archipelago in a warming
climate.
We propose to characterize the unloading and uplift of the Glacier Bay region
with a multidisciplinary study of post-glacial forcings, responses, and processes. In an
ongoing study of uplift here we have determined the time scale of the current rebound
episode (~250 yrs), a limited distribution of total uplift (up to 6.1 m), and continuing
rapid rates of uplift. Our proposed work will build upon these results with the following
approaches:
• GPS measurements of regional horizontal and vertical crustal velocities,
• Tide gauge measurements of mean sea level change,
• Little Ice Age ice load determination from historic terminus positions and studies
of air photos for geomorphic indicators of ice extent,
• Determination of ongoing ice mass-balance changes from airborne laser altimetry
and the upcoming ICESat mission.
• Incorporation of these observations into a post-glacial rebound model.
Results of this study will significantly improve the existing knowledge of the relationship
between deglaciation and rebound, and may aid future studies and predictions of
deglaciation-related sea-level changes within the arctic and sub-arctic regions.
Additionally, this work may lead towards a calibrated unloading response, allowing the
use of uplift data as a proxy for ice mass-balance measurements.
1
Glacial unloading: Forcings, responses and processes
Larsen, C.F.
Introduction
The relationship of glaciers to global sea-level rise as they diminish in a warming
climate is becoming better known (Dyurgerov and Meier, 2000). The current contribution
of small mountain glaciers is estimated to be responsible for 25% of worldwide sea level
rise (Meier, 1984). Systems with high rates of mass exchange, such as the temperate
glaciers of the Gulf of Alaska coastal mountains, have the potential for rapid and
significant contributions to global sea-level rise. Accompanying a rapid shift in the mass
balance of such a system, the underlying crust will experience an equally rapid change in
loading. Initially the Earth will behave as an elastic solid with immediate uplift. If the
unloading is large enough, a visco-elastic response may then occur over thousands of
years (Turcotte and Schubert, 1982). The magnitude, distribution and rate of uplift are
dependant on the magnitude and distribution of the load; close to the load, the rate of
uplift generally will be much faster than current rates of sea-level rise. For coastal regions
with retreating glaciers, impacts from the apparent sea-level fall could range from new
coastal navigation hazards and diminished harbors, to biasing any global sea-level rise
calculations from the affected regions.
To predict how global warming will change sea-level in these regions, we must
understand post-glacial rebound. The earth’s response to continental-scale ice-age
deglaciation is well studied; this is not the case for deglaciation of the scale that is likely
to occur within the arctic and sub-arctic over the next century or so as a result of global
warming. Although the initial elastic response to ongoing deglaciation has been described
(Clarke, 1977; Ivins and James, 1999; Sauber et al., 2000), the transition from elastic to
visco-elastic has not. Also, most studies of crustal loading rely upon load estimates rather
than measured changes in loading. Such deficiencies in the current knowledge of postglacial rebound can be addressed by a study of the Glacier Bay uplift region, SE Alaska
(Fig 1). A tidewater glacier system, completely filling Glacier Bay and covering ~600
km2, rapidly retreated 250 yrs ago at the end of the Little Ice Age (LIA), resulting not
only in the vacation of >1 km thick ice from the bay but also in significant draw-down
and retreat of the icefields that fed the tidewater system. A broad, elongate region of over
105 km2 is now uplifting with a peak rate of 4 cm yr-1 (Fig. 1).
Our proposed study of post-glacial rebound in the Glacier Bay uplift region has
several advantages over previous rebound studies. Direct observations of terminus
positions and heights, dating back to 1794, allow inference of the LIA ice load in Glacier
Bay. The Glacier Bay uplift region has had well-distributed sea-level-change
measurements throughout the past 120 years. This a priori information should constrain
existing earth models and properties such as mantle viscosity. Another boon is the
magnitude of the uplift here allows for accurate uplift determinations over shorter time
intervals between mean sea-level measurements. Also, the timing is such that we may be
observing the transition from elastic to visco-elastic response now. The work we propose
here will allow measurements of both the causes (ice unloading history) and effects
(uplift history) to be included in a single earth-model of post-glacial rebound.
2
Glacial unloading: Forcings, responses and processes
Larsen, C.F.
Proposed Research
We propose a multidisciplinary project to fully describe the unloading and uplift
histories of the Glacier Bay region since the LIA. In an ongoing project, we have been
studying uplift in the region using techniques that offer information over long time scales
(102-103 yrs) using geomorphology and dendrochronology, and over the past 5-10 yrs
using GPS-geodesy (Larsen et al., in press). The work proposed here will continue and
build upon our results of the ongoing study, adding the following goals:
• Determine regional uplift and horizontal strain patterns with GPS.
GPS derived crustal velocities are essential to our project for several reasons. Because the
measurements are space-based, they are not affected by unloading-induced local geoid
changes as sea-level derived measurements of uplift are; nor are they biased by eustatic
sea level rise. Once fully re-occupied, our deformation array will provide uplift data with
much better spatial coverage of the peak uplift region than can be found from sea level
measurements (Fig. 1). Fieldwork to re-occupy the SE Alaska array is funded and
scheduled for this upcoming summer, and we are separately pursuing field-logistics
funding for future re-occupations.
Tsirku
2.7
3.1
au
Glacier
Bay
Juneau
*
1.5
1.1
.7
1.1
ki
Sti
.3
ne
da
1.5
da
na a
Ca l ask
A
Brady
Alaska
Ca
na
e
Jun
1.9
*1.9Skagway
2.3
2.3
Sitka
*
.7
*
Petersburg
Study Area
.3
Figure 1; Location map showing study area. Uplift rates are shown with a contour interval of 0.4
cm/yr (Hicks and Shofnos, 1965). Locations of GPS points are shown as open diamonds,
temporary tide gauges sites where mean sea-level changes can be measured are shown as black
circles. Approximate locations and extent of the primary ice fields (Brady, Tsirku, Juneau and
Stikine) are shown as ovals. Principle cities are indicated.
3
Glacial unloading: Forcings, responses and processes
Larsen, C.F.
• Analyze decadal scale sea-level changes derived from temporary tide gauges.
This goal will be accomplished using the temporary tide gauge analysis of Hicks and
Shofnos (1965), and extending their study over the now greater time span and denser
distribution of available site-occupations. No fieldwork is required for this research- all
the data exist but have not yet been analyzed for sea level changes. The National Ocean
Service (NOS) has been charting the waterways of SE Alaska for more than a century.
Temporary tide gauges are primarily installed for this task, and mean sea-level changes
are but an incidental use. A temporary tide gauge records sea-level over the course of two
or more monthly tidal cycles. The elevation of the gauge is surveyed relative to a local
network of benchmarks. Mean sea-level change can be accurately found if the procedure
is repeated after twenty or more years. Mean sea-level, referenced to the local
benchmarks, is calculated for each occupation, and the uplift can be determined from the
change in mean sea-level (taking eustatic rise into account). Preliminary research into the
NOS tidal observation database indicates ~50 stations have sufficient data to determine
sea-level change (Fig. 1)
• Determine the LIA-maximum ice load extent and thickness using historic
observations and geomorphic indicators.
Geomorphic indicators of ice extent such as trim lines and lateral moraines can be
identified in the upper reaches of Glacier Bay on high altitude aerial photography,
available at the Geo-Data Center at UAF. Historic observations of ice extent in Glacier
Bay are generally limited to terminus position and height. The use of geomorphic
indicators and glacier flow laws allow us to extend thickness and coverage limits
imposed by these historic observations to the upper regions of the glacial system at the
LIA maximum.
• Determine ongoing mass balance changes from ICESAT and airborne laser
altimetry data.
Permanent tide gauges at Skagway and Juneau show no sign of uplift rates slowing down
(Savage and Plafker, 1991; Larsen et al., in press). Uplift from both elastic and viscoelastic processes should slow significantly following the end of the unloading sequence.
This continuing rapid uplift suggests some ongoing unloading. Clarke (1977) found that
25% to 35% of the uplift observed from 1910 to 1960 could be attributed to glacial
unloading occurring during that same time period. Without some knowledge of ongoing
mass balance changes, we will not have a complete set of forcings for rebound modeling.
Clarke’s inverse-modeling results predict that the most important ongoing changes in ice
load are likely occurring at the four primary icefields in the region- Brady, Juneau, Tsirku
and Stikine (Fig. 1). The principal areas of these icefields are characterized by open,
gentle terrain punctuated by dramatic nunataks. For example, the Brady Icefield has an
area of ~300 km2 with an average surface slope of <2°. Mass balance estimates of these
icefields have been made by a comparison of airborne laser altimetry data with elevation
data from 1960’s USGS maps (K. Echelmeyer and C. Lingle, unpublished data). The
laser altimetry data are typically of only the centerline profiles of these icefields. A
denser spatial coverage and more current mass balance measurements will be possible
with data from the upcoming ICESat mission. In addition to having large areas of gentle
surface slopes compatible with the 70 m footprint and ~7 km line spacing (at 58°N) of
the ICESat measurements, these icefields exhibit much greater dynamic changes in mass
balance than the polar ice sheets for which ICESat was designed.
4
Glacial unloading: Forcings, responses and processes
Larsen, C.F.
• Rebound modeling with both unloading and uplift constrained.
Incorporating the wealth of information on forcing and responses attained by the above
goals, this will be the first rebound model with both a priori loading information and
accurate uplift-magnitude and -rate measurements for a deformation event of this scale.
We will be able to focus our modeling efforts on the mechanism of post-glacial rebound,
rather than constraining the various trade offs between estimated load magnitudes and the
material properties of mantle viscosity, flexural rigidity and crustal elastic thickness. Eric
Ivins has graciously provided us with his post-glacial rebound code, developed for a
similar regional scale (Ivins and James, 1999). Starting with this existing code will
provide a tremendous advantage in development time, and allow us to immediately
evaluate current post-glacial rebound models within our observed unloading and uplift
history framework.
The proposed work will enable future studies to use uplift rates and magnitudes as
a proxy for glacial mass balance. The combined uplift history and a priori load
information will significantly constrain post-glacial rebound modeling. In effect, it will
be possible to calibrate the earth’s response to unloading, allowing measurements of
uplift to provide estimates of glacier mass balance. These estimates will correspond to
average mass balance changes of entire glacial systems. Such mass balance
measurements, while critical and fundamental for models of climate change, are difficult
and costly to take directly.
Relationship to ongoing research
The proposed research will compliment our ongoing multidisciplinary study of
post-glacial rebound in northern SE Alaska (P.I. Motyka, NSF award #9870144), which
has been Larsen’s primary thesis support. Under this project (funded through August
2001) we have measured uplift rates throughout the region using three techniques that
offer information over varying time scales: dendrochronology, permanent tide gauge
analysis and GPS geodesy (Larsen et al. in press). We have characterized raised
shorelines at eight sites within the region. The discovery of Mt. Edgecumbe tephra
deposits upslope of these shorelines at 5 of the sites indicates that they represent the
greatest sea-level extent since at least 11250 radiocarbon yrs BP (Beget and Motyka,
1998). Dendrochronology is used to date elevations of uplifted shorelines by finding the
age of the oldest tree at any given elevation (similar to Begin et al., 1993). With tree age
data from elevations between the maximum sea-level extent and present day shoreline,
we can study sea-level change from the onset of uplift to the present day. Tree age and
elevation data from the emerged land below the raised shorelines show that the uplift
episode which raised these shorelines dates back to approximately 1750 AD, with the
majority of uplift occurring after 1800 AD. Total post-1750 AD uplift ranges from 2.7 to
6.1 m at these sites. We have established a regional GPS deformation array with over 70
points throughout the uplift region. Preliminary results indicate an overall vertical motion
pattern similar to that found with sea-level data (Hicks and Shofnos, 1965). Most of the
Pacific Plate-North America velocity difference is found on the western edge of the study
area, with little tectonic stress apparent in the majority of the uplift region.
5
Glacial unloading: Forcings, responses and processes
Larsen, C.F.
Relationship to Previous Studies
Hicks and Shofnos (1965) studied uplift in SE Alaska using data from permanent
tide gauges at Yakutat, Skagway, Juneau, Sitka and Ketchikan, along with data from
repeated temporary tide gauge occupations at 27 sites distributed throughout SE Alaska.
A regional pattern of uplift surrounding Glacier Bay was found, with a peak rate of 4.0
cm/yr near the mouth of the bay (Fig. 1). One component of the research proposed herein
will repeat and extend this study with recently acquired data. Many of the original sites
used in this study have since been reoccupied with tide gauges by NOS. Also, new sealevel change determinations can now be made at approximately 33 additional sites (Fig.
1); the data from these additional sites will allow for a more accurate resolution of sealevel change this region. Furthermore, as 40 yrs have passed since the measurements of
Hicks and Shofnos, it will be possible to determine temporal variations in the uplift
pattern and rates. Measurements of an evolving regional uplift of this scale have never
been made before.
As noted earlier, Clarke (1977) found that 25% to 35% of the uplift found by
Hicks and Shofnos (1965) could be due to ongoing ice mass wastage. Is this elastic effect
an ongoing, constant process, or has the regional distribution and amount of glacial
thinning changed over the last half century? Limited data from glaciers in Alaska and
Washington State indicate that glacial thinning has occurred at increasing rates over the
last 40 years (Dyurgerov and Meier, 2000). It is likely that four large icefields in our
study area (Fig. 1) have also experienced increasing rates of thinning over the last 50
years (K. Echelmeyer and C. Lingle, unpublished airborne laser altimetry data). Ongoing
glacial unloading could help explain the apparent lack of relaxation in the uplift trends
found at Juneau from permanent tide gauge analysis (Larsen et al., in press). It may be
that our observations of continuing high uplift rates are due to a trade off of dying elastic
effects from the LIA unloading with increasing elastic effects from ongoing ice mass
balance changes. However, a study of ongoing glacial thinning in the study area is
required to test this hypothesis. Limited airborne altimetry data exists for the primary
icefields; these data should be greatly augmented by the upcoming ICESat mission.
In the time since the Hicks and Shofnos study, several articles have been
published proposing a tectonic source or contribution to the uplift (Hudson et al., 1982;
Barnes, 1984; Horner, 1983). However, results from our ongoing Glacier Bay uplift
project indicate a strong correlation between the timing of the onset of regional uplift and
the retreat of ice in Glacier Bay (Larsen et al. in press). Furthermore, preliminary results
from a new regional scale GPS deformation array indicate that most of the area’s
horizontal strain is concentrated on the western edge of the uplift pattern. While a
tectonic uplift component has not been completely ruled out, it now appears to be likely
only a fraction of the total signal (Larsen et al. in press). Many of the 70+ sites in the SE
Alaska GPS array (Fig. 1) have yet to have a second occupation; we are currently seeking
additional funding for field logistics to continue this effort.
6
Glacial unloading: Forcings, responses and processes
Larsen, C.F.
References
Barnes, D.F. (1984). No measurable gravity change at Glacier Bay regional uplift area. US Geological
Survey Circular, 939, pp. 88-90.
Beget, J.E. and R.J. Motyka (1998). New dates on late pleistocene dacitic tephra from the Mount
Edgecumbe volcanic field, southeast Alaska. Quaternary Research, 49, pp 123-125.
Begin, Y, D. Berube, M. Gregoire (1993). Downward migration of coastal conifers as a response to recent
land emergence in eastern Hudson Bay, Quebec. Quaternary Research, 40, pp 81-88.
Clarke, J.A. (1977). An inverse problem in glacial geology: The reconstruction of glacier thinning in
Glacier Bay, Alaska between A.D. 1910 and 1960 from relative sea-level data. Journal of Glaciology, 18,
pp 481-503.
Dyurgerov, and M.B., M.F. Meier (2000), Twentieth century climate change: Evidence from small glaciers.
Proceedings National Academy of Sciences, USA, 97, 4, pp. 1406-1411
Hicks, S.D. and W. Shofnos (1965). The determination of land emergence from sea-level observations in
southeast Alaska. Journal of Geophysical Research, 70, pp 3315-3320.
Horner, R.B. (1983). Seismicity in the St. Elias region of northwestern Canada and southeastern Alaska.
Bulletin of the Seismological Society of America, 73, 4, pp 1117-1137.
Hudson, T, K. Dixon and G. Plafker (1982). Regional uplift in southeastern Alaska. US Geological Survey
Circular, 844, pp. 132-135.
Ivins, E.R. and T.S. James (1999). Simple models for late Holocene and present-day Patagonian glacier
fluctuations and predictions of a geodetically detectable isostatic response. Geophysical Journal
International, 138, pp 601-624.
Larsen, C.F., R.J. Motyka and J.T. Freymueller (In press). Post glacial rebound in Southeast Alaska: Rates,
dates and magnitudes. In Proceedings of IAG International Symposium on Gravity, Geoid and
Geodynamics 2000, Banff, Canada, July 2000, Springer-Verlag, New-York. Current draft (as sent out for
review) is available at http://www.aeic.alaska.edu/input/chris
Meier, M.F. (1984). Contribution of small mountain glaciers to global sea-level. Science, 226, pp 14191421.
Motyka, R.J., Principle Investigator, (1998) NSF Award Number 9870144, Active Tectonics Division.
Sauber, J., G. Plafker, B.F. Molnia and M.A. Bryant (2000). Crustal deformation associated with glacial
fluctuations in the eastern Chugach Mountains, Alaska. Journal of Geophysical Research, 105, pp 80558077.
Savage, J.C. and G. Plafker (1991). Tide gage measurements of uplift along the south coast of Alaska.
Journal of Geophysical Research, 96, pp 4325-4335.
Turcotte, D.L. and G. Schubert (1982). Geodynamics. John Wiley and Sons, New York, pp245-248.
7