Abstract. The late-Pleistocene history of the coastal Cordilleran Ice Sheet remains
relatively unstudied compared to chronologies of the Laurentide Ice Sheet.
Yet accurate reconstructions of Cordilleran Ice Sheet extent and the timing
of ice retreat along the Pacific Coast are essential for paleoclimate
modeling, assessing meltwater contribution to the North Pacific, and
determining the availability of ice-free land along the coastal Cordilleran
Ice Sheet margin for human migration from Beringia into the rest of the
Americas. To improve the chronology of Cordilleran Ice Sheet history in the
Alexander Archipelago, Alaska, we applied 10Be and 36Cl dating to
boulders and glacially sculpted bedrock in areas previously hypothesized to
have remained ice-free throughout the local Last Glacial Maximum (LLGM;
20–17 ka). Results indicate that these sites, and more generally the coastal northern Alexander Archipelago, became ice-free by 15.1 ± 0.9 ka (n = 12 boulders; 1 SD). We also provide further age constraints on deglaciation along the southern Alexander Archipelago and combine our new ages with data from two previous studies. We determine that ice retreated from the outer coast of the southern Alexander Archipelago at 16.3 ± 0.8 ka (n = 14 boulders; 1 SD). These results collectively indicate that
areas above modern sea level that were previously mapped as glacial refugia
were covered by ice during the LLGM until between ∼ 16.3 and
15.1 ka. As no evidence was found for ice-free land during the LLGM, our
results suggest that previous ice-sheet reconstructions underestimate the
regional maximum Cordilleran Ice Sheet extent, and that all ice likely
terminated on the continental shelf. Future work should investigate whether
presently submerged areas of the continental shelf were ice-free.
