ADVANCE OF TIDEWATER GLACIERS AS RECORDED BY RADIOCARBON DATING OF OVERRIDDEN TREES IN GLACIER BAY NATIONAL PARK, ALASKA

Current understanding of the advance and retreat rates of tidewater glaciers suggests that while ice margins can retreat quite rapidly (km/yr), their advance is rather slow (m/yr). Our studies of the glacial history and interglacial climate of Glacier Bay in southeast Alaska indicate that past rates of ice margin advance were highly variable, including periods when ice advanced at rates equivalent to those during a rapid recession. We infer rates of advance during the Holocene from radiocarbon dates of tree stumps that were overridden as ice moved down-fjord. We have sampled over 100 stumps rooted in growth position, and dated their outer rings, which mark the death of the tree, using the AMS technique. In addition, the ages of about 100 logs in glacial deposits of moraines adjacent to the fjords also record approximate ice marginal positions. Our data cover the advance of ice from accumulation areas in the western Fairweather Range and eastern Takhinsha Range into lower Glacier Bay during four apparently distinct periods: ~ 9.5 to 6.0 K radiocarbon years BP, ~ 4.4 to 3.0 Kyr BP, ~2.4 to 2.0 Kyr BP, and ~1.6 to 1.2 Kyr BP. Asynchronous ice advance and retreat in the eastern and western source areas as well as advance during periods characterized globally as warming were identified. A late stage, possibly rapid ice advance near the entrance to Glacier Bay may have taken place about 600 yrs BP. Although our data are preliminary and the reasons are unknown, there is a clear indication that a tidewater ice margin can advance rapidly into fjords and that in Glacier Bay, climatically asynchronous advances were characteristic of the Holocene.