A 10Be Chronology of Cordilleran Ice Sheet Configuration During the Last Glacial Maximum in the Northern Alexander Archipelago, Alaska

Abstract

The late-Pleistocene history of the coastal Cordilleran Ice Sheet (CIS) remains relativelyunstudied compared to chronologies of the Laurentide (LIS) Fennoscandian, and Greenland icesheets. Yet accurate reconstructions of CIS extent and timing of ice retreat along the PacificCoast are essential for a variety of reasons including ice sheet modeling, assessing meltwatercontribution to the North Pacific, and determining the availability of ice-free land along thecoastal CIS margin for human migration from Beringia into the Americas. To improve thechronology of CIS history in the Alexander Archipelago, Alaska, I applied 10Be dating in areaspreviously hypothesized to have remained ice-free throughout the local Last Glacial Maximum(lLGM; 20-17 ka). My field reconnaissance at these locations revealed freshly ice-sculptedbedrock outcrops draped with glacial boulders, implying recent ice cover, in contrast to previousstudies that mapped these areas as ice-free. My samples for 10Be dating include both bouldersand glacially sculpted bedrock. Results indicate that these sites, and more generally the coastalnorthern Alexander Archipelago, became ice-free by 15.1 ± 0.9 ka (n = 12 boulders; 1 SD).Hence, at least these several areas mapped as ice-free were in fact covered by ice during thelLGM. I combine these results with data from two previous studies to determine that ice retreatedfrom the outer coast throughout the Alexander Archipelago at 15.7 ± 1.0 ka (n = 25 boulders; 1SD). Combined, these results indicate that all recently investigated areas above modern sea levelthat were previously mapped as glacial refugia were covered by ice during the lLGM until ~15.7ka; we found no evidence of ice-free land during the lLGM. Previous ice-sheet reconstructionsunderestimate the regional maximum CIS extent, as all ice likely terminated on the continentalshelf. These data support the hypothesis that no areas currently above sea level were ice-freethroughout the lLGM. Future work should investigate whether presently submerged areas of thecontinental shelf were ice-free.