Timing and Paleoclimatic Significance of Latest Pleistocene and Holocene Cirque Glaciation in the Enchantment Lakes Basin, North Cascades, WA

The Enchantment Lakes Basin in the Alpine Lakes Wilderness, Washington, 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 indicate that there were two Neoglacial advances, culminating with the Little Ice Age, and one slightly larger advance that ended coincident with the termination of the North Atlantic Younger Dryas event. The cores show no evidence for an early Holocene advance, in contrast to some other studies in the North Cascades, (e.g., Heine, 1998; Thomas, 1997; Thomas et al., 2000). 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 paleomagnetic secular variation of the sediments provide detailed age constraints for the lake cores. The presence of the 475 cal yr B.P. Mount St. Helens Wn tephra within outwash associated with the inner (Brynhild) moraines indicates that they are Little Ice Age (LIA) equivalent. The age constraints on the lake sediments show that this advance began between ~ 1000-800 cal yr B.P. and culminated after the Wn tephra was deposited. The age of the outer (Brisingamen) moraines, previously reported as early Holocene (Waitt et al.,1982), are instead latest Pleistocene; close limiting 14C dates demonstrate that this advance ended shortly before ~11,300 cal yr B.P., suggesting temporal equivalence with the North Atlantic Younger Dryas climatic reversal (12,940 ± 260 - 11,640 ± 250 cal yr B.P; Alley et al., 1993). A ~500-yr interval of high rock-flour flux in the cores records an early Neoglacial advance between ~3300 and ~2800 cal yr B.P. that was less extensive than the subsequent LIA advance. 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 ~2355 m, roughly 200 m below the modern ELA. 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 lesser combination of the two. These constraints imply a local climate that could support only small-scale advances in both the latest Pleistocene and late Holocene, and warmer as well as drier conditions throughout the early Holocene