Holocene glacier fluctuations, Waskey Lake, northeastern Ahklun Mountains, southwestern Alaska

Abstract

Lake sediments from Waskey Lake, Ahklun Mountains, southwestern Alaska were studied to decipher the history of upvalley glacier fluctuations during the Holocene. Several indicators of glacier activity were measured including: magnetic susceptibility, organic-matter content, grain-size distribution, bulk-sediment mineralogy and diatom assemblages. Seven radiocarbon ages on macrofossils, along with cross-checks by tephrochronology, provide the chronology of the cores. The results from core WL-1 indicate that glaciers lingered near Waskey Lake until 9100 cal. yr BP, perhaps under conditions of high winter accumulation. Peak organic-matter content occurs at 7400 cal. yr BP, when precipitation might have shifted to summer. The onset of Neoglaciation occurred 3100 cal. yr BP, and glaciers reached their maximum extenñ700 cal. yr BP. This chronology is consistent with the lichenometrically dated moraines from the glacier forefields. Although the ages are tentative, the youngest and most widespread group of moraines was deposited sometime between 650 and 200 cal. yr BP (during the‘Little Ice Age’). Since then, glaciers in the Waskey Lake area have shrunk bñ50% and equilibrium-line altitudes (ELA) have risen by 35± 22 m. This rise in ELA is much less than the 100 to 200 m rise observed elsewhere in Alaska and indicates considerable spatial variability in late-Holocene climatic change.