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TREE-RING DATED 1000-YEAR ADVANCE OF COLUMBIA GLACIER

BECKWITH-LAUBE, MATTHEW J  The College of Wooster.
Wiles, Greg C  The College of Wooster.
Calkin, Parker E  INSTAAR.
Post, Austin  University of Washington (Retired).

An almost 14-kilometer catastrophic retreat of Columbia Glacier initiated in the early 1980s has exposed thousands of hemlock logs overrun by a previous 1000-year ice advance. Over 400 tree-ring dates from this forest suggest advance began before AD 1020 and reached its most recent maximum about AD 1810. An early expansion through the deepest reaches of the upper fjord averaged 20 m/yr attaining a mid-fiord position about AD 1450. At this position the ice margin laterally expanded into tributary valleys increasing the width of the calving margin and stalling the down fjord progress.

The next evidence of advance into forest is detected just south of the AD 1450 position with calendar dates in the mid- to late-1700s. Tree-ring dates here show a near-continuous ice expansion to the recent terminal moraine shoal at Heather Island. This latter 6-kilometer expansion attained advance rates of up to 150 m/yr.

Based on these data, fjord geometry and especially water depth appear to be the dominant controls on the advancing Columbia glacier. The increasing rates of advance documented by tree-ring dating along the 14-kilometer fiord reflect the overall decreasing water depth from the deep water of the upper fiord to the relatively shallows near the terminal moraine shoal. This reconstruction of ice advance is consistent with the model put forth for Columbia Glacier by Post (1975) and Meier and Post (1987) who suggested a relatively slow advance of the tidewater margin down its fiord for millennia and then after an extended phase, catastrophic retreat. Continued retreat and subsequent exposure of subfossil forests will reveal more of Columbia’s glacial history into the first millennium AD.

REFERENCES
Meier, M.F. and Post, A., 1987, Fast Tidewater Glaciers: Journal of Geophysical Research, v.92, no.9, p. 9051-9058.



Post, A., 1975, Preliminary hydrography and historic terminal changes of Columbia Glacier, Alaska: U.S. Geological Survey Hydrologic Investigations Atlas 559, 3 sheets.


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