RECONSTRUCTING ICE DYNAMICS FROM FORESTS PRESERVED IN THE WAKE OF THE CATASTROPHIC RETREATING COLUMBIA GLACIER, ALASKA
Tree-rings dates on wood collections from 2014 and 2015 provide new advance dates for the East Branch of the Columbia Glacier that show ice expansion as early as 1008 CE. These are dates on logs only a few kilometers away from the 2014 retreating calving margin. Most of these sub fossil logs remain in-situ or are near their original living sites associated with soils and forest debris. A spatial array of these new dates provide a preliminary average advance rate for the East Branch maximum of 38 meters per year and an ice thickening of 6 meters per year. Additional tree-ring dates on overrun forests show ice continued to advance to a lower-fjord position by ~1400 CE where it remained for about 250 years, prior to its push to an outer moraine by 1810 CE at Heather Island.
Past advance rates of the distributary Land Lobe Glacier are estimated at an average of 5 meters per year based on tree-ring dates. The land-terminating branch of the Columbia thus records a 5-fold slower advance rate than the larger, iceberg calving glacier. Although these glaciers are controlled by different dynamics, Columbia Glacier a tidewater glacier and the Land Lobe Glacier, both share the same ice fields and accumulation areas from the Chugach Mountains. Ongoing research strives to investigate the relationship between the Columbia Glacier and the Land Lobe Glacier to better explain the climatic and non-climatic controls on the different rates of ice expansion.