Post Little Ice Age Collapse of the Glacier Bay Icefield, Alaska

Glacier Bay provides an excellent example of the tidewater glacier cycle proposed by Austin Post. It has a complete record of an advancing phase, stability, rapid calving and drawdown, lengthy retreat, and then readvance behind protective sediments. Glacier Bay currently consists of numerous discrete glaciers and small isolated icefields, but it recently contained a huge continuous icefield up to 2 km thick that covered more than 6000 km2 at the peak of the Little Ice Age (LIA) (1750 AD). Rapid calving and associated upstream drawdown lead to its collapse. In less than 160 yrs, the main trunk of the icefield retreated 120 km in fjords as deep as 500 m. We evaluated the LIA volume and topography of the Glacier Bay Icefield based on mapping of trimlines, lateral moraines and terminal moraines. We used light aircraft to identify these geomorphic markers as well as analysis of vertical airphotos, hydrographs, seismic profiles, and the SRTM digital elevation model. Our reconstruction indicates an ice volume loss of over 3000 km3 during the post-LIA collapse. This localized ice wastage represents the largest post-LIA deglaciation known to us, and is greater than the volume lost from all Alaskan and neighboring Canadian Glaciers from 1955-2002, greater in volume than the Larsen B 2002 ice shelf collapse, comparable in volume to Lake Huron, and equivalent to a global rise in sea level (SLE) of 8 mm. The collapse of the Glacier Bay icefield stranded many tributary glaciers. Some were entirely isolated from any source of accumulation and are now simply wasting away (e.g., Burroughs Glacier). Other glaciers in the region have had their accumulations areas severely reduced as the icefields feeding the LIA tidewater glaciers disappeared (e.g., Casement and Brady Glaciers). The vast loss of ice has lead to some of the highest rates of glacier rebound presently occurring in the world (32 mm/yr) with total uplift since the 18th century of as much as 5.8 m. Facilitated by infill of fjords from erosion and remobilization of subglacial sediments, several glaciers are now in the advancing phase of the tidewater glacier cycle despite the regional trend of glacier wastage. Post-LIA Glacier Bay could provide an analogue to collapse of other tidewater glacier systems and outlet glaciers from polar ice sheets.