Rapid Retreat of Alaska Glaciers by Floatation and Passive Calving

Floatation and rapid passive calving is the last phase of a complex sequence of retreat-related events that has been observed at more than 50 low elevation, low gradient, Alaskan valley glaciers. Floatation and passive calving is the final component of a five-part process that characterizes the retreat behavior of many valley glaciers under warming climate conditions. Components of this sequence are: 1) Initiation of slow, melting-dominated terminus retreat from a position of maximum advance, typically the Little Ice Age maxima. Typically, retreat rates are meters to tens of meters per year; 2) Continued terminus thinning and retreat up valley, often culminating in retreat into an over deepened part of the glacier’s bed, usually at the head of the valley and adjacent to the foot of the adjacent mountains. This often results in the development of an ice-marginal lake. Often, this area of deeper bed erosion is the result of multiple cycles of advance and retreat and has previously been eroded below sea level; 3) With the development of water adjacent to the terminus, the rate of terminus retreat increases by an order of magnitude or more, dominated by active calving rather than melting; 4) Continued terminus retreat, accompanied by an acceleration in the rate of glacier thinning, results in the terminus transitioning from grounded to floating; and 5) With the continuation of terminus thinning and retreat, the terminus becomes afloat resulting in the cessation of active calving and the initiation of passive calving. Intensive passive calving of the floating terminus produces tabular icebergs, and generates retreat rates that may be an order of magnitude greater than active calving rates. Typically, tabular iceberg production accelerates the rate of glacier retreat from the valley floor, resulting in the terminus ascending the adjacent mountain slope and returning the glacier to a melting-dominated retreat mode.