The Last Deglaciation of the Revelation Mountains, Alaska
Tulenko, Joseph P.
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Global mean sea level is expected to rise by as much as 45 cm by the end of the 21 st century. While many factors contribute to sea level rise, meltwater from alpine glaciers and ice caps is predicted to contribute the most in this century since both reservoirs respond more rapidly to climate change than ice sheets. Studying past alpine glacier behavior provides valuable insight into the fate of current and future alpine glaciers as climate continues to change. Mounting improvements to the 10 Be dating technique make it a precise and accurate dating tool for dating past glacier change, especially in regions where the use of other dating methods is limited. Motivation for this work is a recent synthesis of alpine glacier chronologies that pointed to globally synchronous glacier retreat forced by rising atmospheric CO 2 at ~18,000 yr ago following the Last Glacial Maximum (26-19 ka, Clark et al., 2009; Shakun et al., 2015). However, Shakun et al., (2015) found that greenhouse gas forcing does not fully explain why some glaciers began to retreat earlier than ca. 18 ka. Moreover, the study uses glacier records exclusively from the mid-low latitudes because suitable records from the high latitudes are not available. In order to address whether this is truly a global pattern, we wonder if alpine glaciers in the Arctic behaved similarly. Alaska provides a rare opportunity to study alpine-style glaciation in the Arctic during and following the LGM since ice sheets covered only a small portion of the state at that time. However, there are no published high-resolution records of glacier recession from a single valley in Alaska comparable to those compiled by Shakun et al. (2015) (Kaufman et al., 2011). For this project, we re-visit an area in Alaska surveyed by Briner et al. (2005) – they noted an exceptional moraine sequence with large granite boulders. Briner et al. (2005) dated 4 boulders on the outermost LGM moraine in the Swift River valley, which average 20.4 ± 0.7 ka (re-calculated here with updated production rates). Here, we present 22 new 10 Be exposure ages used to constrain glacier retreat following the LGM in the Swift River valley, which drains the Revelation Mountains in the western Alaska Range. The ages are from large granitic moraine boulders deposited on left-lateral LGM and recessional moraines. We dated boulders on moraine crests at and within the outermost LGM limit, and on two prominent recessional moraines in the sequence. Ages from the outermost LGM limit average 21.3 ± 0.1 ka (n = 3; 2 outliers excluded). Ages inboard of the outermost LGM limit exhibit some minor evidence of post-depositional degradation, but overlap, averaging 20.4 ± 1.0 ka (n = 5; 3 outliers excluded). Ages from the first recessional moraine in the valley average 19.6 ± 0.7 ka (n = 2; 1 outlier excluded). Finally, ages on the innermost recessional moraine that we sampled average 17.7 ± 0.5 ka (n=4; 2 outliers excluded). While there are occurrences of statistical outliers, such as older ages that suggest significant inheritance and younger ages that suggest post-depositional degradation, a majority of the ages cluster well. Moraine mapping of the site shows that the glacier had retreated in length by approximately 25% of its LGM extent when it deposited the ca. 17.7 ka recessional moraine. Thus, the chronology indicates significant retreat prior to global CO 2 rise. In comparison to glaciers from the mid to low latitudes, the Swift River valley glacier initiated retreat on pace with many of those glaciers. We suggest that glacier recession was initiated by orbital forcing, which was perhaps modulated by polar amplification.