EMERGING EVIDENCE FOR A 50-60 KA PENULTIMATE GLACIATION IN BERINGIA

Mountain glaciers are a key terrestrial archive of past climate change, and where well dated, they record the synchroneity, or asynchroneity, of climate change across the globe. The most extensive glacier advance of the last glaciation (120-10 ka) in Beringia occurred prior to the last global glacial maximum (25-15 ka). Two moraines deposited by this ‘penultimate' advance in the Alaska Range have 10Be ages suggesting it culminated around 60-55 ka. 36Cl ages from erratic boulders deposited during the penultimate advance in the Ahklun Mountains, southwestern Alaska, are 56-54 ka; this age agrees with a maximum thermoluminescence age of 70±10 ka and with amino acid and optically stimulated luminescence ages between 90 and 55 ka. 10Be ages from erratics in penultimate drift from the Yukon Territory are also 60-50 ka. This marine-isotope-stage (MIS) 4/3 age for the culmination of the penultimate glaciation at many sites, but not all, in Beringia contrasts with a MIS 6 (~140 ka) age for the penultimate glaciation at many locations elsewhere in the world, and in the western US in particular. Although Parker Calkin's work on the Holocene glaciation of Alaska is most widely known, he also made significant contributions to understanding the Pleistocene glacial geology of the state. The relative weathering studies on Seward Peninsula by Parker and his students demonstrated that the penultimate moraines post-dated the last interglaciation, a conclusion supported by these more recent absolute dating results.