POST-SURGE RETREAT OF BERING GLACIER FOLLOWING THE 1965-1967 AND 1993-1995 SURGES

During the 20th century, Bering Glacier surged at least five times. Following each surge, the glacier retreated significantly. For example, during the 27 year period following the 1965-1967 surge, part of Bering's piedmont lobe retreated >10.7 km. The pattern of retreat following the two most recent surges (1967-1992 and 1996-present) has been carefully documented. In each post-surge retreat cycle, initial retreat was by calving. Within three years, Bering Glacier's piedmont lobe rapidly thinned, often by >20 m/yr, and the intensity and the frequency of calving decreased. The dominant process responsible for terminus retreat became disarticulation.

Disarticulation is the passive separation of large tabular pieces of ice from the glacier's terminus. Typically, disarticulation events occur when the thinning, low-gradient, distal end of a retreating glacier reaches a state of buoyancy and separates from its bed. As the terminus region of a disarticulating glacier begins to float, tabular pieces of ice, some >1.5 km in maximum dimension, separate from the terminus or its lateral margins. Separation usually occurs along old crevasse and fracture planes and may begin at distances of >2 km behind the terminus. Often, hundreds of large icebergs simultaneously separate.

Following each of Bering Glacier's last two surges, the transition from calving to disarticulation resulted in an increase in the rate of terminus retreat. In western Vitus Lake, >2.5 km of retreat occurred between 1977 and 1978. Similarly, an increase in Bering Glacier's retreat rate occurred following the 1993-1995 surge. It is estimated that since 1998, >50 km² of Bering Glacier's piedmont lobe has been lost by disarticulation. Since 1996, part of the terminus has retreated >6 km.