An Alaskan glacier is under study from a unique vantage point: space. A series of satellites that estimate precise positions on Earth from space, the Global Positioning System, are helping scientists study the the movement of ice and Earth at the surging Bering Glacier.
by Jeanne Sauber, Geodynamics Branch, Laboratory for Terrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Md.; George Plafker, Branch of Alaskan Geology, U.S. Geological Survey, Menlo Park, Calif.;and John Gipson, NVI, Greenbelt, Md.
Surging is periodic, rapid movement of large quantities of ice within a glacier, alternating with much longer periods of near stagnation or retreat. The Bering Glacier in southern Alaska has undergone at least six surges this century and it is currently surging again. When ice is removed from a glacier's reservoir during a surge, its surface lowers by tens or hundreds of meters, and ice is added to the receiving area, where ice thickens and advances. Glaciological studies have detailed the timing of the glacier's recent surge and the advancement of its lower margin, the terminus.
The surge originated well below the equilibrium line in the spring of 1993; in late August of 1993, a rapid transfer of ice from the Bagley Icefield and upper reaches of the Bering Glacier to the glacier's terminus advanced the glacier's lower margin. This advance has resulted in a total horizontal movement of about 5 km. Although recent surges at the Bering and Variegated Glaciers have been well documented, little is known about most surges, particularly about what happens in the upper reaches of the glaciers.
A set of Global Positioning System measurements collected adjacent to the Bagley Icefield (see figure 1) and along the Gulf of Alaska (see figure 2) are being used to study the Bering Glacier surge that began in the spring of 1993. The dramatic changes in a surging glacier's extent and thickness should result in elastic deformation of the solid Earth. This distortion of the Earth due to the changing glacier load occurs instantly and would disapear if the load were removed.
Repeated GPS measurements can be used to estimate the distortion. Using a representation of the ice thickness changes, we calculated the elastic deformation of the Earth at specific locations. It is expected that where ice was lost, the solid Earth would uplift; where additional ice was added, it would be depressed. The Earth is predicted to subside up to 17 cm and move horizontally up to 2.5 cm.
Fig. 1.
Clobal positioning satellite measurements were taken just north of Bagley Icefield, Alaska, in June 1993 within the Wrangell-St. Elias Park. In the foreground is a global positioning antenna mounted on a tripod facing the Bagley Icefield at right and the Jeffries Glacier at left. The Bagley Icefield occupies a long, narrow, east-west-trending basin in the eastern Chugach mountains of southern Alaska and flows primarily westward of Bering Glacier.
Fig. 2.
Map of the region affected by the Bering Glacier surge and the predicted horizontal deformation of the solid Earth caused by the surge. Triangles indicate the global positioning stations occupied in June 1993. Thin lines outline of the major glaciers. The thick line shows the Gulf of Alaska coastline.
In southern Alaska, two plates are slowly but continuously colliding as one is forced into the Earth's interior beneath a second, overriding plate. To understand this subduction process we must know the rate and orientation of the distortion in the overriding tectonic plate. We can measure this distortion, which is referred to as tectonic strain, by using repeated precise, space-based measurements of positions.
An estimate of the rate of deformation associated with subduction can be obtained with very-long-baseline interferometry (VLBI), a method similar to GPS. Instead of using satellites like GPS does, however, VLBI uses radio sources such as quasars to estimate the position of a receiver on the ground. VLBI measurements made at the Cape Yakataga site between 1984 and 1990 shows that relative to Fairbanks, deformation occurred at this station at a rate of about 38 mm per year at N29°W and was uplifted at a rate of about 48 mm/yr. Other geologic studies and earlier geodetic results from the Cape Yakataga region suggest north-south shortening of the crust associated with the east-west trending thrust faults in the region. A component of right-lateral, strike-slip motion may be associated with a fault that runs from east to west near the Bagley Icefield. Although the overall diplacement pattern in the figure 2 differs from that predicted for tectonic strain, the northwestward-directed displacement and uplift is similar to the tectonic strain at some of the sites north of the Bagley Icefield. This will make the interpretation of the displacement results more challenging.
The glaciers of southern Alaska have been receding since early this century. The predicted deformation of the region associated with an average glacial thinning rate of 1 to 6 m/yr in the region where ice and snow are lost from the glacier is generally less than 1 cm/yr of uplift and the horizontal movement is less than 2 mm/yr. For the sites given in the figure 2 the displacements associated with the recent surge will be larger than the displacements due to the retreat of other glaciers.
The GPS measurements collected in June will soon be used to estimate the amount of displacement between 1993 and 1995 at the stations on the opposite page. We could use these results to roughly infer the ice thickness changes in different parts of the glacier. Alternatively, by combining the glaciological information on the surge's extent in the reservoir and receiving area with a physical model of the surge that predicts thickness change along the longitudinal profile of the glacier, we could solve for the overall amount of ice transferred during the surge.
As is evident from the results, additional stations, especially near Bering Glacier's terminus, would have provided better constraints on the cumulative ice thickness changes in the surge region. The stations were chosen to improve tectonic models of what happens in the subduction zone. The GPS measurements, however, should allow us to roughly estimate the magnitude of ice change in regions affected by the surge and estimate how much ice is transferred from the reservoir to the receiving area during the surge. The results will give us a measure of how useful GPS data are to understanding glacier fluctuations.
Return to Science and Society
Return to Starting Point
