LAUSR

Abstract We present 32 new cosmogenic 10Be exposure ages from a moraine sequence deposited during the Wisconsinan glaciation in the Swift River valley, Revelation Mountains, western Alaska Range. 10Be ages from an early Wisconsinan [Marine Isotope Stage 4] moraine average 59.7 ± 3.6 ka (n = 9; excluding one outlier), and 10Be ages from a late Wisconsinan [Marine Isotope Stage 2] terminal moraine, inboard moraines, and two recessional end moraines average 21.3 ± 0.8 ka (n = 3; excluding two outliers), 20.2 ± 1.0 ka (n = 7; excluding three young outliers), 19.6 ± 1.0 ka (n = 2; excluding one outlier) and 17.7 ± 0.8 ka (n = 4; excluding two outliers), respectively. The early Wisconsinan moraine age coincides with the close of MIS 4, consistent with previous chronologies from Beringia. Whereas Southern Hemisphere glaciers retreated prior to the onset of Heinrich Stadial 6 (64–60 ka), the MIS 4 glacier advance in the Swift River valley terminated at the close of Heinrich Stadial 6, possibly in response to abrupt North Atlantic warming. Our late Wisconsinan ages indicate significant initial glacier retreat between ca. 21.3–17.7 ka (prior to any significant increase in CO2) likely in response to amplified warming from rising insolation. Preservation of MIS 4 moraines in Beringia has traditionally been ascribed to limited MIS 2 glacier advance modulated by low regional moisture availability, stemming from the exposed Bering Land Bridge. However, we hypothesize that atmospheric re-organization forced by the Laurentide Ice Sheet resulted in relatively higher temperatures and drier conditions in Beringia during MIS 2 than MIS 4, allowing for the preservation of early Wisconsinan moraines.