ABRUPT LATE PLEISTOCENE CLIMATIC REVERSALS IN COLORADO AND WYOMING: EVIDENCE FROM LAKE SEDIMENTS AND MORAINES IN CIRQUES

Radiocarbon-dated lake sediment cores at four localities in Colorado and Wyoming suggest that alpine glaciers deposited moraines in cirques during the Younger Dryas (YD).

Zielinski and Davis (1987, Geogr phys et Quat) radiocarbon-dated basal sediments from lakes inside and outside the type Temple Lake (TL) moraines, which lie ~3 km beyond and ~100 m lower than the modern glacier margin in TL valley in the southern Wind River Range (WWR). Based on pollen analyses (Fall et al., 1995, Quat Res), these conventional 14C ages on bulk sediments of ~11,700 BP are believed to be a few hundred years too old, thus the type TL moraines are believed to be YD in age. Davis et al. (1998, GSA Abstr, Toronto) radiocarbon-dated sediment cores from two lakes in Titcomb Basin (TB), central WRR. The two lakes are separated by the type TB moraines, which lie ~3 km beyond and ~200 m lower than modern glacier margins and Gannett Peak (Little Ice Age) moraines in the basin. Over a dozen AMS 14C ages on macrofossils and bulk sediments at a transitional zone from inorganic, sticky gray silt below (< 1% LOI) to more organic, less sticky, light brown silt above (4-10% LOI) suggest that alpine ice receded from the moraines about 10,000 BP. This age accords with precise cosmogenic 10Be and 26Al data for boulders on the type TB moraines (Gosse et al., 1995, Geology). Davis (1988, Quat Sci Rev) radiocarbon dated basal lake sediments inside the type Triple Lakes moraines (TrpL), which lie ~1-2 km beyond and ~150 m lower than Arapaho glacier >10,200 BP. Menounos and Reasoner (1997, Quat Res) radiocarbon dated sediments in Sky Pond in Rocky Mountain National Park ~40 km north of Arapaho cirque that indicated the enclosing moraines are ~10,000-11,000 BP. Thus, the type TrpL moraines and those at Sky Pond are likely correlative with the TL and TB moraines, and probably reflect a global Younger Dryas (YD) cooling event.

LOI and pollen trends from all the lake sediment cores in the above studies, as well as a related site in western Colorado (Reasoner and Jodry, 2001, Geology), suggest a rapid warming immediately following glacier retreat from the YD cirque moraines.