DISARTICULATION AND RECENT RAPID RETREATS OF THE BERING GLACIER, ALASKA

The surging Bering Glacier is the largest glacier in continental North America. During its two most-recent, two-part surge events (the 1957-1960 & 1965-1967 surge and the 1993-1994 & 1995 surge), rapid ice displacement resulted in part of Bering's piedmont lobe advancing > 10 km. Following each surge event, terminus retreat was also rapid. Between 1967 and 1993, Bering’s terminus retreated as much as 10.7 km. The average rate of retreat at four locations was 0.43 km/yr. Annualized rates ranged from 0.04 km/yr to 2.8 km/yr. Extreme rapid retreat events were observed in 1977 and 1984, in which parts of Bering's terminus retreated ~ 1.0 km in just a few hours.

Since 1996, rapid retreat is again underway. Between 1996 and 2002, part of the terminus retreated ~ 6 km (~ 1 km/yr). By 2001, continuing retreat resulted in much of the terminus returning to near its 1993 pre-surge position. In 2000, 2001, and again in 2002, parts of the terminus were observed retreating as much as 700 m in less than 24 hours, as successions of large icebergs disarticulated from the margin and drifted into ice-marginal Vitus Lake. In June 2002, a 1.2-km-long iceberg, the largest yet seen, separated from the terminus.

Bering Glacier’s rapid retreats are not the exclusive result of melting and calving. The greatest loss of ice results from disarticulation, a previously unreported process that not only affects Bering Glacier, but also many other large, rapidly-thinning glaciers that terminate in deepwater lakes and fiords. As the glaciers thin, buoyancy lifts their termini from their beds. Once a terminus loses contact with the bottom, disarticulation begins along pre-existing planes of weakness, such as surge-fracture scars or old crevasses. Large irregularly-shaped, tabular icebergs result, many greater than a kilometer in maximum dimension. As with ice shelves, the presence of meltwater may facilitate disarticulation.

Calving events are gravity-driven, occur at the ice edge, and are high energy, often producing high-intensity noise and a wave. Disarticulation events are buoyancy-driven, may simultaneously occur in a zone that extends as much as 1.5 km inward from the terminus, and are generally quiet and wave-free.

Aerial photographs of Bering Glacier made in 1936 and 1948 capture on-going disarticulation events. These photographs document rapid retreat following surges that occurred about 1920 and from 1938-1940.