COMPLEX TIMING AND PATTERNS OF GLACIATION IN THE AMERICAN CORDILLERA DURING TERMINATION 1

Evidence from alpine glacial deposits in the American Cordillera suggest that glacier fluctuations, and therefore the climatic conditions that caused them, during the late-glacial were spatially and temporally complex.

In the Sierra Nevada, LGM glaciers retreated gradually between 17,000 and ~15,000 ¹⁴C yr BP (~20,000-18,000 cal yr B.P.) and subsequently stagnated between ~14,000 and 15,000 ¹⁴C yr B.P. (~16,800-18,000 cal yr BP). By ~13,100 ¹⁴C yr BP (~15,500 cal yr. BP), ice had largely or entirely disappeared from even the highest cirques. Cirques remained essentially ice-free between 13,100 and 12,200 ¹⁴C yr BP (15,500-14,200 cal yr. BP), after which glaciers reformed and readvanced a short distance during the Recess Peak event. The Recess Peak glaciers lasted about 1000 yrs, disappearing by ll,200 ¹⁴C yr BP (~13,100 cal yr. BP). The Sierra Nevada remained largely or entirely free of glacier ice for the next ~9000 cal yr, including during the Younger Dryas (YD) chronozone.

Glacier deposits indicate a mixed record in the North Cascades; alpine deglaciation began by ~17,000 ³⁶Cl yr BP, with two late-glacial readvances (~14,000 and 13,000 ³⁶Cl yr BP; Swanson and Porter, 1997) in the eastern Cascades, but only one (ending by ~13,200 cal yr BP) in the western Cascades near Mt. Rainier (Heine, 1998). During the YD, glaciers appear to have retreated at Mt. Rainier while simultaneously advancing further north in the Enchantment Lakes Basin. Proposed YD alpine glacier advances to near sea-level in the northernmost Cascades (Kovanen and Easterbrook, 2002) remain controversial, and may instead have been substantially smaller (Burrows, 2000).

In southern Idaho, glaciers retreated to the cirques by 13,000 ¹⁴C yr BP (~15,400 cal yr BP), but experienced a minor readvance between ~12,800 – 11,000 ¹⁴C yr BP (~15,100-13,000 cal yr BP); no YD advance is apparent. In the Rockies, small alpine moraines record one or two late-glacial advances, an earlier one that predates YD and a later that is synchronous with it (e.g., Menounos and Reasoner, 1997). These inconsistent results suggest either that current age constraints are in error, or more likely, that the American Cordillera experienced rapid yet localized variations in temperature and moisture delivery, possibly related to an unstable Pacific storm track.