10BE DATING MIS 2 AND MIS 4 MORAINES IN THE REVELATION MOUNTAINS, ALASKA

Mountain glaciers record the timing and pace of climate change through moraine sequences preserved in mountain ranges across the globe. In western North America, moraines from the Last Glacial Maximum [LGM, marine isotope stage (MIS) 2] lie inboard of moraines dating to MIS 6. However, in Beringia, LGM moraines lie inboard of MIS 4 moraines, and together provide a moraine sequence that allows for a rare comparison of the timing of MIS 4 vs. MIS 2 glacial culmination if precisely dated. Here, we present 32 new ¹⁰Be exposure ages along with four previously published ¹⁰Be ages from a moraine sequence deposited during Wisconsinan glaciation in the Swift River valley of the Revelation Mountains, western Alaska Range. Exposure ages are derived from large granitic boulders deposited on left lateral moraines at and within the late Wisconsin limit, and on a well-preserved early Wisconsin lateral moraine slightly outboard of those lateral moraines. Exposure ages from the early Wisconsin moraine average 59.9 ± 2.8 ka (n = 9; not including one outlier), ages from the late Wisconsin limit average 20.4 ± 0.7 ka (n = 9; not including three outliers), and ages within the late Wisconsin limit date two recessional moraines to 19.6 ± 0.7 ka (n = 2; not including one outlier) and 17.7 ± 0.5 ka (n = 4; not including two outliers). Our ages support the notion of a glacial termination corresponding to the end of MIS 4, contrasting with sites investigated elsewhere in western North America. One hypothesis for the moraine pattern in Alaska is that low moisture availability stemming from the emergence of the Bering land bridge limited MIS 2 glaciation in Alaska, leading to the preservation of MIS 4 moraines. However, new evidence from Lake El’gygytgyn, eastern Beringia, reveals a larger temperature depression during MIS 4 than during MIS 2; thus, the temperature pattern in Beringia may have led to the pattern of glaciation. We also characterize the temporal pattern of MIS 2 and MIS 4 glaciation in Alaska with respect to other forcing mechanisms such as atmospheric CO₂ and insolation fluctuation. We find that in both cases, the culmination is preceded by increases in both insolation and CO₂ concentration. Our precise chronology provides the first opportunity to compare glaciation in Alaska to other sites in the Southern Hemisphere with preserved MIS 4 moraines.