Response of the Alpine Timberline Ecotone to the Younger Dryas Climate Oscillation in the Colorado Rocky Mountains: Evidence From two Well-Dated, High-Resolution Pollen Records

Reasoner, Mel A. (Institute of Arctic and Alpine Research, Campus

Box 450, University of Colorado, Boulder, Colorado 80309-0450; ph.

303-492- 4703; Internet: reasoner@spot.colorado.edu); Jodry, Margaret A. (Smithsonian Institution, National Museum of Natural History 304, Paleoindian/Paleoecology Program, MRC 112, Washington, District of

Columbia 20560; ph. 202-357-2672; Internet: jodrym@nmnh.si.edu) (AGU Sponsor: Dr. Eric J. Steig)

Preliminary pollen and macrofossil results from two high-altitude lacustrine records in Colorado register a significant vegetation response to the Younger Dryas (YD) oscillation. Sky Pond (3320 m) and Black Mountain Lake (3415 m) are situated directly above and below alpine timberline respectively. Both records show a substantial increase in arboreal pollen taxa between ca.13,600 and 13,000 (11,600 and 11,100 14C) yr BP. Arboreal taxa reach 40% in the Sky Pond record and 45% in the Black Mountain Lake record during this interval. Both records also show subsequent declines in arboreal taxa to 25% and 35% during the YD Chron followed by increases to Holocene levels after ca. 11,500 (10,100 14C) yr BP. Pollen accumulation rate data indicate that these fluctuations in relative abundance are primarily driven by changes in Pinus, Picea, and Abies pollen. Comparisons with modern pollen transect data suggest that alpine timberline moved up-slope between ca. 13,600 and 13,000 (11,600 and 11,100 14C) yr BP but may not have reached the elevation of the sites. Subsequent declines in arboreal pollen percentages during the YD Chron reflect a lowering of alpine timberline elevation that was concomitant with an advance of cirque glaciers in the Rocky Mountains. The predominance of xerophytic taxa in the early portions of both the Sky Pond and Black Mountain Lake records suggest that the relatively small YD-age glacial advances in the Colorado Rocky Mountains were primarily influenced by cooler temperatures rather than by increases in precipitation. Alpine timberline advanced up-slope to elevations at or above both sites shortly after ca. 11,600 yr BP.