Dendrochronological Dating of Glacier-Dammed Lakes: An Example from Yukon Territory, Canada

Abstract

The chronology of prehistoric, glacier-dammed lakes can be established, in many cases, through the use of dendrochronological techniques. Driftwood deposited at the margins of a glacier-dammed lake can be precisely dated, and the history of the lake thereby deduced, by matching the annual ring patterns in the driftwood with those of nearby old living trees. Dating is facilitated by X-ray densitometric analysis, whereby digital data on ring width and intraring density variations are obtained from X-ray images of wood.

Dendrochronology is not subject to the severe limitations on precision inherent in radiocarbon and other absolute dating techniques. It is the preferred method for dating recent geologic and other events where exact ages are required and where appropriate fossil material (i.e., reasonably well preserved wood with sensitive annual ring patterns) is available.

Dendrochronological techniques were applied to Neoglacial Lake Alsek, a former glacier-dammed lake in southwestern Yukon Territory with a complex history of filling and emptying. Lake Alsek forms whenever Lowell Glacier, a large surging glacier in the St. Elias Mountains, advances across Alsek Valley and blocks Alsek River. Dendro-dating of driftwood, combined with sparse historical and other data, indicate that Lake Alsek last extended beyond the front of the St. Elias Mountains to the vicinity of Haines Junction between A.D. 1848 and 1891, probably in the early 1850s.