Hunting for Ice Age Refugia: A 10Be-Based Deglacial Chronology from the Northern Alexander Archipelago, Alaska

Abstract:

Recent studies from the North American interior necessitate an alternative to the “ice-free corridor” human migration route from Beringia into the Americas. The alternative – a hypothesized “coastal route” - relies on the availability of refugia (areas that were ice-free throughout the Last Glacial Maximum; LGM) or coastal land that deglaciated quickly after the LGM along the coasts of SE Alaska (SEAK) and British Columbia where humans could have lived during migration. We present new data from SEAK constraining rates of deglaciation along a potential coastal human migration route from Beringia into the Americas following the LGM. Specifically, we targeted areas of SEAK that were mapped to be ice-free in previous studies for ¹⁰Be surface exposure dating. Our new ¹⁰Be ages from glacial erratics indicate that western Baranof Island were covered by ice until 15.1 ± 0.1 ka, and Biorka Island - a small island offshore Baranof near Sitka, AK - until 14.8 ± 0.7 ka. We also sampled glacially-sculpted bedrock; ages are ~5 kyr older than erratic ages at several locations on Baranof, likely caused by inheritance due to insufficient glacial erosion. These new deglaciation ages of Baranof Island are ~2 kyr younger than those farther south in SEAK, suggesting asynchronous retreat of the Cordilleran Ice Sheet or persistence of local ice on Biorka and Baranof Islands. Our preliminary data indicate that coastal Baranof Island and Biroka Island did not host LGM refugia (despite being previously postulated as such) and were thus not available for humans during the hypothesized time of migration (~17 ka). While we are currently exploring other areas of SEAK mapped as ice-free, it is also plausible that continental shelf areas presently submerged – but above sea-level during the LGM – were ice-free, and further studies should look offshore to investigate these possibilities.