TIMING AND PALEOCLIMATIC SIGNIFICANCE OF LATEST PLEISTOCENE AND HOLOCENE CIRQUE GLACIATION IN THE ENCHANTMENT LAKES BASIN, NORTH CASCADES, WA

The Enchantments Lakes Basin in the Alpine Lakes Wilderness, WA, preserves two sets of moraines that record distinct post-Wisconsin maximum advances of cirque glaciers in the eastern North Cascades. Cores collected from five lakes adjacent to the moraines preserve detailed records of the timing and extent of the advances. The cores show no evidence for early Holocene advances, in contrast to some other studies in the North Cascades. (e.g., Heine,1998; Thomas, 1997).

Upstream glacier activity, as indicated by rock-flour production, is recorded in the lake sediments as fluctuations in magnetic susceptibility, organic content, and sediment particle size. Tephra identification, AMS ¹⁴C dating and secular variation recorded by the sediments provide detailed age constraints for the lake cores. The presence of the 475-year-old Mt. St. Helens W_n tephra within outwash associated with the inner (Brynhild) moraines indicates that they are Little Ice Age (LIA) equivalent, and that the advance began between ~1100-700 cal yr B.P. The age of the outer (Brisingamen) moraines, previously reported as early Holocene (Waitt et al.,1982), are instead latest Pleistocene; basal ¹⁴C dates demonstrate that this advance ended shortly before ~11,300 cal yr BP, suggesting temporal equivalence with the North Atlantic Younger Dryas climatic reversal (12940±260 – 11640±250 cal yr BP; Alley et al., 1993). An early Neoglacial advance, which was less extensive than the subsequent LIA advance, is recorded in the sediment cores and lasted roughly 400 years between ~3300 and ~2900 cal yr BP. Steady-state equilibrium-line altitudes (ELAs) for Brynhild and Brisingamen advances estimated with accumulation-area ratio and balance-ratio methods are distinct but nearly indistinguishable at ~2365 m. Following Leonard (1989), conditions required to form and sustain the Brisingamen and Brynhild paleoglaciers include a summer temperature depression of ~3° C, an increase of ~90 cm water-equivalent in winter precipitation, or, more likely, some intermediate combination of the two. These constraints imply a regional climate that could support only minor advances in the latest Pleistocene, and none in the early Holocene.