HR: 10:50h
AN: C42A-03    [Abstracts]
TI: Repeated Rapid Retreats of Bering Glacier by Disarticulation - The Cyclic Dynamic Response of an Alaskan Glacier System
AU: * Molnia, B F
EM: bmolnia@usgs.gov
AF: U.S. Geological Survey, 12201 Sunrise Valley Drive MS 926A, Reston, VA 20192 United States
AB: Bering Glacier is the largest glacier in continental North America, with an area greater than 5,000 square kilometers. Alone, it accounts for more than 6% of the glacier-covered area of Alaska and perhaps 15% of Alaska's glacier ice. In places, its bed is more than 250 m below sea level. It is also the largest surging temperate glacier on Earth. Surges, some with maximum ice displacements exceeding 13 km, occurred at least five times during the 20th century. Analysis of aerial photography, dating from 1936, and satellite imagery, dating from 1972, documents that following each of the last four surges, Bering Glacier experienced post-surge, decadal-scale, cyclic episodes of rapid retreat. In each instance, the primary mechanism responsible for the rapid retreat was a buoyancy-driven process, here named `disarticulation.' Abundant imagery exists to carefully document the post-surge retreat pattern during the two most recent cycles, 1967-1992 and 1996-present. In each cycle retreat began by calving. However, within a few years, as the piedmont lobe rapidly thinned, often by more than 20 m/yr, the dominant process transitioned to disarticulation, and the rate of retreat greatly increased. Disarticulation events occur when the thinning, low-gradient, distal end of the glacier's piedmont lobe reaches a state of buoyancy and separates from its bed. As it begins to float, large tabular pieces of ice up to a kilometer in maximum dimension passively separate from the terminus. Separation usually occurs along old crevasse and fracture planes and may begin at distances of more than 2 km behind the terminus. Often, hundreds of large icebergs simultaneously separate. Disarticulation events were also identified as being underway on photographs from 1936 and 1948. These followed surge that ended in the 1920s and 1940. In both 1936 and 1948, disarticulation was occurring at the same location. This location was also a focal point for disarticulation during the last two post-surge cycles. The post-1967 surge cycle spanned 25 years and resulted in a maximum of 10.7 km of terminus recession. Maximum annual recession exceeded 2.5 km. In the post-1996 cycle, maximum retreat is more than 6 km. Disarticulation is not unique to Bering Glacier, and probably not unique to Alaska. A 2005 aerial survey of Alaskan coastal glaciers identified more than a dozen glaciers, many former tidewater and calving glaciers, including Grand Plateau, Alsek, Bear, and Excelsior Glaciers that were rapidly retreating through disarticulation.
DE: 0700 CRYOSPHERE (4540)
DE: 0720 Glaciers
DE: 0726 Ice sheets
DE: 0738 Ice (1863)
DE: 0776 Glaciology (1621, 1827, 1863)
SC: Cryosphere [C]
MN: Fall Meeting 2005