C22A-01
Exploring glacier dynamics with subglacial water pressure pulses: Evidence for self- organized criticality?
Because basal hydraulic conditions strongly influence the flow and stability of glaciers and other ice masses, basal water pressure ranks as perhaps the most commonly-measured subglacial parameter. Although most subglacial water pressure records exhibit slowly-varying values, work by Kavanaugh and Clarke [2000, 2001] has suggested the occurrence of high-magnitude pressure excursions (or "pulses") that are sufficiently brief to escape detection. Here I present initial results of an effort to record and characterize these events. During the summer of 2005, an interface board was used to continuously monitor the output of a pressure transducer installed in Trapridge Glacier, Yukon, Canada. During a 231 day period, over 7000 pressure pulses were recorded, with magnitudes reaching nearly 3 times the flotation value. Comparison of the pressure pulse record with a number of other instruments indicates that these pulses are generated by stress transients that compress the water within the borehole; calculations suggest that these transients are as large as 75 times the nominal driving stress. Both the magnitudes and interevent times for these pulses are well-fit by power-law distributions that are remarkably similar to those exhibited by earthquake systems. These similarities suggests that the ice--bed interface of a glacier behaves much like an earthquake fault. The spatial and temporal scale invariance indicated by these power-law distributions also raise the possibility that the glacier bed self-organizes to a critical state; this would have implications for ice-flow phenomena such as glacier surging, ice streaming, and the rapid response of ice sheets to climate forcings.
C22A-02
Analysis of seismic waveforms generated during iceberg calving events, Jakobshavn Isbræ, Greenland
Iceberg calving events at Jakobshavn Isbræ occur every week during summer and involve the detachment and overturning of ice blocks that are 0.25-2.00 km3 in size. The energy released during the events, which can exceed 1015 J, produces large amplitude and far-reaching ocean and seismic waves. Seismograms from the events last as long as the calving events (30-60 min), contain energy over a wide range of frequencies (0.01 Hz – 50 Hz), and are recorded teleseismically. There is also an increase in the number of seismic and acoustic emissions (sometimes for over 24 hrs) following large calving events. Observations of icebergs overturning during periods in which the calving face is quiet produce similar seismograms, suggesting that much of the seismic energy from calving events can be attributed to ocean waves loading and unloading the coast. The steep walls of the proglacial fjord and glacier terminus and the unusually dense melange of icebergs help to couple the fjord to the solid earth; thus, waves generated in the fjord can easily be transmitted to the solid earth and detected hundreds of kilometers away. Seismic energy can also be released during calving events in a number of other ways, including but not limited to (1) fracturing of ice, (2) avalanching of debris and collapsing of seracs, (3) outbursts of subglacial water, (4) icebergs scraping the fjord bottom or terminal cliff, (5) icebergs pushing off of the terminus, and (6) acceleration of the glacier in response to a loss of back forces. We assess the importance of these sources by synthesizing data from a local seismometer and the Global Seismic Network, acoustic recordings, measurements of sea level (recorded every 5 s), and high-rate timelapse photography (one photo every 10 s).
C22A-03
The coupling between outlet glaciers and their catchments
The importance of outlet glacier acceleration on the long-term mass balance of ice sheets hinges on the
duration and magnitude of the acceleration, which in turn will be dependent on the external forcing and the
presence of positive or negative feedbacks on dynamical changes once they are initiated. Recent studies
have shown that periods of substantial increases in ice flow speed and rapid thinning within marine-
terminating outlets are a dynamic response to unstable retreat of ice fronts grounded on inland-sloping beds
or disintegration of floating tongues. This implies that the dynamics of outlet glaciers can be very sensitive to
the hydrostatic conditions at their fronts and a relatively small initial amount of thinning may be all that is
needed to trigger retreat and acceleration.
The first crucial question is, therefore, what causes the initial thinning. Increased ice front calving due to
ocean warming is a possibility. Another potential and relatively unexplored cause is variability in ice flux into
the outlets from the catchments. Since the area of an outlet glacier catchment is typically 4 orders of
magnitude larger than the ice front, very small changes in the Surface Mass Balance (SMB) are equivalent to
much larger changes in ice thickness at the front. For values typical of southeast Greenland outlet glaciers,
a 1% variation in annual SMB is the equivalent of ~5 m/yr of ice thickness change at the front. The actual
relationship between variations in SMB and outlet glacier thickness will be controlled by the advection of
perturbations in SMB to the front, which will impact the lag time between a perturbation and its expression at
the front, the temporal smoothing of the signal, and the degree to which the perturbation is modulated
through diffusion.
An equally important question is how the inland ice responds to rapid thinning within the outlets. Dynamic
thinning should increase the surface slope, resulting in an increase in inland ice speed and flux that may
serve as a negative feedback on further outlet glacier retreat. The efficacy of this feedback will be dependent
on the rate at which flux increases relative to loss at the front, which is controlled by ice sheet geometry and
rheology.
Here we use observations of the ongoing dynamic evolution of outlet glaciers and reanalysis SMB data to
constrain a perturbation model linking variations in catchment and outlet glacier dynamics. We use this model
to assess the potential role of variations in catchment SMB in triggering large-scale outlet glacier changes.
We also examine the dynamic response of the ice sheet to rapid outlet thinning and the role of the ice sheet
in controlling further outlet retreat.
http://www.bprc.osu.edu/GDG/
C22A-04
An estimate of the glacier ice volume in the Swiss Alps
With the ongoing climate warming, there is a major concern in the (partial) disappearance of mountain glaciers. Predictions on the disappearance of glaciers necessarily need the present amount of ice volume as initial condition. For transient modelling of glacier coverage, the ice thickness distribution has to be known. In this study a recently presented method to estimate the ice thickness distribution for alpine glaciers is used to estimate the ice volume present in the Swiss Alps. The method is based on mass conservation: the mass balance distribution of a glacier has to be balanced by the ice flux divergence and the resulting surface elevation change. This allows to calculate the mass turn over of the glacier, from which the ice thickness distribution can be inferred by principles of ice flow dynamics. Integrating available ice thickness measurements, the ice volume and the ice thickness distribution for all glaciers in the Swiss Alps with a surface area larger than 3 km2 is determined. The resulting data set is used to calibrate a Bahr-type volume-area scaling relation of the form V=c · Aγ. The values for c and γ we found are in good agreement with earlier published values. Using the scaling relation we complete the volume estimate for the Swiss Alps. The total ice volume present in the Swiss Alps by the year 1999 (about 1500 glaciers) is estimated to be 75 ± 10 km3. The analysis show the importance of large glaciers as ice reservoirs: more than 80 % of the total ice volume is stored in the 50 largest glaciers, the glacier complex of Grosser Aletschgletscher storing alone about 20 % of the total volume.
C22A-05
Is Glacier wastage continuing to accelerate in NW North America?
Laser altimetry elevation profiles of glaciers have been collected in Alaska, Yukon, and NW British Columbia
(herein NW North America) by the University of Alaska Geophysical Institute beginning in 1993. Since that
time, over 27,000 km of glacier profiles have been acquired. Nearly 200 glaciers throughout NW North
America have been measured, many of them multiple times. All of the largest glaciers in NW North America
have been profiled, including at least some representative glaciers from every major icefield in NW North
America. This dataset includes a broad range of glacier sizes distributed across the region. Several glacier
and icefield regions within Alaska and adjoining Canada have been profiled multiple times at this stage, and
data from these regions are being analyzed for changes in the rates of thinning. Specific regions include
Harding Icefield, the central Alaska Range, Yakutat Icefield, Glacier Bay, and Juneau Icefield. Hypsometry
appears to be a significant factor, with those areas that have relatively low elevation accumulation areas
showing signs of accelerated thinning, particularly the Harding and Yakutat Icefields. Other areas that have
relatively high elevation accumulation areas appear to have steady rates of thinning, such as within the St.
Elias Mountains. The profile ground tracks have all been converted into files that can be displayed in Google
Earth, and are available for easy download from our webpage (http://fairweather.gps.alaska.edu/chris). In
order to allow for open distribution to the community, all of the profile elevation data are being reprocessed
into a common reference frame (ITRF), which not only allows for accurate inter-comparison of repeated laser
altimetry data but also will allow for comparison with other elevation datasets, such as digital elevation models
derived from satellite remote sensing.
http://fairweather.gps.alaska.edu/chris
C22A-06
Projections of 21st Century Sea Level Rise From the Melt of Mountain Glaciers and Ice Caps
An ensemble of 21st century volume projections for all mountain glaciers and ice caps from the World Glacier Inventory is derived by modeling the surface mass balance coupled with volume--area--length scaling and forced with temperature and precipitation scenarios with A1B emission scenario from ensemble of GCMs. By upscaling the volume projections through a regionally differentiated approach to all mountain glaciers and ice caps outside Greenland and Antarctica (514,380~km2) we estimated total volume change to range from -0.039~m to -0.150~m of sea level equivalent for the time period 2001--2100. A major source of uncertainty in the methodology is the temperature forcing in the mass balance model which depends on bias correction of ERA-40 temperatures in order to simulate the local temperatures on a mountain glacier or ice cap. Other major sources of uncertainties are the volume-area scaling in deriving initial glacier volume and upscaling the volume changes with assumptions on glacier-size distributions in each glacierized region. Our projected 21st century volume loss is probably a lower bound since no calving is modeled and no mountain glaciers and ice caps surrounding Antarctica and Greenland are included due to a lack of glacier inventory data. Nevertheless, the large range of our projections depends on the choice of GCM emphasizing the importance of ensemble projections.
C22A-07
Testing a numerical model for thermokarst lake expansion using morphologic measurements, N. Seward Peninsula, Alaska
The initiation and growth of thermokarst lakes are major factors in the dynamics of ice-rich permafrost lowlands in N. America and Siberia. These landscapes may be a globally-significant influence to past and present climates, because they store Pleistocene and Holocene C and may release it as CH4 to the atmosphere over relatively rapid time-scales. A numerical model that combines thermal processes (heat flow in lake water and permafrost) and geomorphic processes (thaw subsidence and mass movement) is a new and promising tool to investigate the influence of substrate, ground ice content, and climate on the expansion of thermokarst lakes. Models also might be used to predict dynamics of lakes and related biogeochemical processes in coming decades, given anticipated continued warming in high latitude regions. The geomorphic processes that shape natural thaw lake margins include a complicated range of diffusive and advective processes spanning simple creep to more complicated mechanisms including ice wedge melting, thaw slumping, peat-block toppling, turbidity flows, and possibly animal disturbance. The model treats the combined and time-averaged effect of these processes using new and relatively simple algorithms for slope failure and transport distance, which together produce both diffusive and advective behavior. Initial comparisons of the model with measured lake bluffs and bathymetry indicated a good match, but more measurements are needed to fully test and calibrate the model. We measured the bluff morphology and bathymetry of selected lakes of various sizes and age in the Kitluk River, Seward Peninsula, NW Alaska using sonar and DGPS. The region is characterized by continuous permafrost, a highly dissected and dynamic thermokarst landscape, uplands of Late Pleistocene permafrost deposits with high excess ice contents, and a large total volume of permafrost-stored carbon. We drive the model using approximate ground ice conditions for each lake, and compare modeled morphology to the measured morphology of our study lakes. Initial results are used to develop new model subroutines/algorithms which can be used to treat particular types of lake margins, such as thermally-undercut and floating peat mats, extending the model's applicabability to a wider range of thermokarst lakes.
C22A-08
Dynamics of Polygonal Terrain in the Dry Valleys, Antarctica
The polar regions of both the Earth and Mars host a surprising variety of surface patterns. These arise from complex interactions between soil, ice, and in some cases, liquid water, and can dominate the dynamics of near-surface soils in these regions. However, the rates at which these processes work are still poorly constrained. We present observations of polygonal terrain, which occurs in areas of icy permafrost that see large temperature variations over the year. The winter cooling creates a large stress in the ice component of these soils, causing fracture. While the cracks are open, snow and detritus can fall into them. Thus, in the summer, when the cracks relax, there is a net addition of material. Over many such cycles, the crack networks grow and develop into beautiful regular polygons. We have recently made measurements of the surface topography and trough dynamics of sand-wedge polygons in Beacon, Taylor, and Victoria valley, of Antarctica. In Beacon and Taylor Valleys we report new measurements of the inclination, rod height, and inter-rod distance of a series of rods which were originally introduced into the polygonal landscape in the 1960s, and which serve as a long-baseline study of soil motion. In all three valleys we have surveyed the surface topography of several polygons, by theodolite. We focussed on characterizing the size, and shape, of the mounds which surround the crack troughs, and measured the height profiles of the ice-cemented soil horizons around the troughs. Assuming steady-state conditions, these measurements allow an independent estimate of surface soil motion rates. Our observations will be used to refine models of polygonal terrain, and to tie into efforts to understand the history and evolution of surface, and near-surface, features in the Dry Valleys. With the Phoenix mission's recent observations of subsurface ice on Mars, it is also hoped that a deeper understanding of terrestrial features will also aid in our understanding of how polygonal terrain develops there.