HR: 0800h
AN: C41A-0038    [Abstracts]
TI: Calving Theory and the Thinning, Retreat, and Disarticulation of Bear Glacier, Alaska
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: Bear Glacier, with an area of ~225 km², is a 25-km long valley glacier located in the southern Kenai Mountains. When first mapped in 1909, it had a piedmont lobe with an area of >30 km². By the mid- 1980s, the terminus had retreated from 1-3 km, thinned by >150 m, and was actively calving small icebergs into an ice-marginal lake. Through the end of the 20th century, the piedmont lobe continued to thin and narrow. Active calving decreased and then ceased as the thickness of the thinning glacier approached T_nb, the thickness of neutral buoyancy (the floatation thickness). As thinning continued, much of the terminus began to float. Passive calving, characterized by the release of large tabular icebergs from Bear's low gradient, low elevation terminus, became the dominant retreat process. Between 2000 and 2007, part of the terminus retreated about 3.6 km. Now, 0.5 km-long icebergs frequently separate from Bear's western terminus. An August 2006 bathymetric survey identified many locations with depths >75 m in Bear Glacier's ice-marginal lake. Calving theory can be used to explain much of Bear's observed behavior. Current calving theory suggests that active calving is initiated when a glacier terminus thins to a critical thickness, (i.e. T_nb plus an additional but limited thickness of ice such as 50 m [T_nb+ 50 m]). The thinning of Bear Glacier during much of the 20th century resulted in it approaching this thickness. In a freshwater lake with a depth of ~75 m, active calving would be initiated when T_{nb + 50 m} was about 120 m, continuing as the glacier thinned to <100 m. With further thinning, Bear Glacier would transition to a regime dominated by passive calving (disarticulation). This condition was initiated sometime after the mid-1990s, when Bear Glacier became significantly thinner than T_nb. Glacier ice floats because its density is at least 8% less than that of freshwater or saltwater. As Bear Glacier thinned to T_nb (approximately 85 m), active calving decreased and parts of the terminus began to float. Concurrently, lake water began to appear in crevasses and at low points on the glacier's surface. By 2002, most of Bear's terminus was afloat. With little or no piedmont lobe gradient and virtually no terminus margin height, active calving ceased and large icebergs began to be produced through passive calving (disarticulation), generally separating along former crevasse scars and fractures. These are the condition that resulted in the catastrophic retreat that characterizes Bear Glacier's 21st century behavior.
DE: 0720 Glaciers
DE: 0732 Icebergs
DE: 0746 Lakes (9345)
DE: 0758 Remote sensing
DE: 0776 Glaciology (1621, 1827, 1863)
SC: Cryosphere [C]
MN: 2007 Fall Meeting