Radiocarbon age constraints on rates of advance and retreat of the Puget lobe of the Cordilleran ice sheet during the last glaciation

Calibrated radiocarbon dates of organic matter below and above till of the last (Fraser) glaciation provide limiting ages that constrain the chronolog y and duration of the last advance-retreat cycle of the Puget Lobe in the c entral and southeastern Puget Lowland. Seven dates for wood near the top of a thick proglacial delta have a weighted mean age of 17,420 +/- 90 cal yr B.P., which is the closest limiting age for arrival of the glacier near the latitude of Seattle. A time-distance curve constructed along a flowline ex tending south from southwestern British Columbia to the central Puget Lowla nd implies an average glacier advance rate of ca. 135 m/yr. The glacier ter minus reached its southernmost limit ca. 16,950 yr ago and likely remained there for ca. 100 yr. In the vicinity of Seattle, where the glacier reached a maximum thickness of 1000 m, ice covered the landscape for ca. 1020 yr. Postglacial dates constraining the timing of ice retreat in the central low land are as old as 16,420 cal yr B.P. and show that the terminus had retrea ted to the northern limit of the lowland within three to four centuries aft er the glacial maximum. The average rate of retreat was about twice the rat e of advance and was enhanced by rapid calving recession along flowline sec tors where the glacier front crossed deep proglacial lakes. (C) 1998 Univer sity of Washington.