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TREE-RING AND HISTORICAL EVIDENCE FOR A LARGE AND RAPID ADVANCE OF AN ALASKAN TIDEWATER GLACIER

BARCLAY, DAVID J  SUNY Cortland.
Barclay, Julie L  SUNY Cortland.
Calkin, Parker E  INSTAAR.
Wiles, Gregory C  College of Wooster.

Icy Bay in southern Alaska extends 40 km inland from the Gulf of Alaska (Figure 1). Surficial stratigraphy and landforms show that the tidewater glacier termini of this system have coalesced and advanced from the deep-water inner fjords to the shallow outer bay several times during the late Holocene. We use tree-ring cross-dates of glacially killed trees and historical data to reconstruct the most recent of these expansions.

The coalesced margin of Yahtse, Guyot and Tsaa glaciers advanced southeastwards through the inner bay in the 1640s (Figure 1). Maps and descriptions by European explorers suggest that in the 1790s the coalesced ice margin had reached the turn at mid-bay and that the southeastern side of the outer bay was significantly farther east than today. Trees killed at three widely spaced locales record rapid expansion during the 1810s and 1820s; these tree-ring data are corroborated by E. Belcher who observed in 1837 that Icy Bay had largely disappeared beneath the advancing glacier. Climbing parties in the 1880s described the Icy Bay glacier as extending out into the Gulf of Alaska and outwash prograding rapidly into the area between Old Point Riou and the eastern Icy Bay glacier margin. Ice retreat began in the 1900s and left the whole bay deglaciated by the 1980s.

Advance through the deep-water inner bay between the 1640s and 1790s was at an average rate of 25 m/yr (Figure 2), a rate that is typical for tidewater termini in similar settings today. However, the 1790s to 1880s expansion of ~380 km2, an along-flowline distance of ~26 km at an average of 280 m/yr, is far larger and faster than any previously documented advance of an Alaskan tidewater glacier. The rapid advance appears to have begun as the ice margin entered shallow water in outer Icy Bay (Figure 2), and so we suggest that a decrease in iceberg calving was the primary cause of this rapid advance. This interpretation is consistent with observations of dense floating ice and large icebergs in Icy Bay the 1780s and 1790s, in contrast to 1837 when there was relatively little floating ice and a muddy beach at the ice margin.



Figure 1. Icy Bay, southern Alaska, with reconstructed ice margin positions.


Figure 2. Bathymetric profile of Icy Bay with dated ice margin positions.


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