A DEGLACIATION CHRONOLOGY FROM THE WESTERN ALASKA RANGE: GLACIER FLUCTUATIONS DRIVEN BY REGIONAL CLIMATE MECHANISMS (Invited Presentation)

Moraines deposited in high-northern-latitude mountain ranges have not yet been dated in as much detail as sites at lower latitudes. As such, there is uncertainty regarding which climatic factors were most important for driving the last deglaciation in polar mountains. The presence of detailed moraine sequences deposited in alpine valleys in Alaska provides opportunities to study how glaciers in these high latitude locations responded to rapid climate change through the last deglaciation.

We combine new and previously published cosmogenic ¹⁰Be ages from moraine sequences in the western Alaska Range through the last deglaciation. 50 new ¹⁰Be ages and 22 previously published ages from 15 distinct moraine crests dispersed from ~20 km to 1.5 km down-valley of modern glacier termini range from 28.4 – 11.6 ka. From outermost to innermost, moraines were deposited between 21.3 ± 0.8 ka (n = 3) and 12.8 ± 0.6 ka (n = 5) and generally conform stratigraphically, giving us high confidence in the chronology.

Steady glacier recession from ~21 ka – 16 ka was punctuated by faster retreat from ~16 ka – 12.8 ka. Because this pulse of deglaciation beginning around 16 ka occurred after the rise in global CO₂ yet prior to Bølling warming at ~14.7 ka, we hypothesize that the glacier was not responding to some climate signals commonly observed in the North Atlantic region. Furthermore, the innermost moraine dated (12.8 ± 0.6 ka) lies directly outboard of the late-Holocene glacier maximum, which suggests either the glacier had a minor re-advance at the culmination of the Antarctic Cold reversal (14.5 – 12.8 ka) or during the early Younger Dryas cold period (12.8 – 11.7 ka). The lateglacial advance was relatively minor, which implies that the magnitude of temperature depression below pre-industrial levels was minimal, especially compared to other regions across the globe that indicate a relatively large temperature depression at this time. We hypothesize that global and/or Beringia-specific regional climatic factors played a more important role in the deglaciation of Alaska than North Atlantic-specific regional forcings.