Dendrochronological Estimation of Glacial Retreat and Glacioisostatic Uplift, Davidson Glacier, Southeast Alaska

Davidson Glacier (the Davidson), near Haines, Alaska is a prominent valley glacier with historical recognition by both John Muir and the Harriman Expedition (Davidson, 1904; Muir, 1915). These late 19th century visitors observed the Davidson with a substantial forest populating the landscape in front of the glacier. The question remained, however, as to whether that position reflected a terminal or recessional stage. That is, did the ice ever reach the sea as a tidewater glacier eliminating the potential of forest growth until recession? Since those early observations in the 1800’s, the Davidson has receded 3.5 km exposing a vast fan-shaped delta. Glacioisostatic rebound, uplift of the surface resulting from rapid deglaciation opposed to tectonic forcing, has also exposed huge tracts of land previously submerged by ocean waters. Larsen et al. (2005) reports glacioisostatic rebound in this region of Southeast Alaska between 22 and 24 mm per year making the rate of uplift some of the highest measured in the world today. Sitka spruce (Picea sitchensis) has recolonized the post-glacial and rebounded landscape. In a marine recessional environment spruce germination may average 30 to 40 years versus 5 to 8 years in post-glacial conditions (Motyaka, 2002). The presence of stranded marine terrace risers combined with stand age could also help delineate maximum glacial extent against LIA sea level. During the summer of 2017, using dendrochronologic techniques, I cored 90 Sitka spruce along transect lines from shore to the glacial terminus, and the resulting time series reflects progression of spruce establishment. Dating of the cores indicates ages of the trees increasing from the shore to approximately 250 meters landward before reversing that trend and decreasing in age to single digit years nearest the glacier face. Considering this trend from another perspective, the glacioisostatic rebound is also reflected in the tree ring record youthening from the oldest section of the forest towards the present day intertidal zone. These data suggest that the Davidson Glacier was a tidewater glacier before retreating in the 1760’s. Expanding the Little Ice Age history of northern Southeast Alaska contributes to datasets constructed in Glacier Bay National Park and refines timelines as to the end of the Little Ice Age in this region