HIGH-ELEVATION PALEOENVIRONMENTAL CHANGE DURING MIS 4 - 6 IN COLORADO'S CENTRAL ROCKIES, AS DETERMINED FROM POLLEN AND CHARCOAL ANALYSES

Our ability to determine the long-term record of vegetation and climate change in western North America has been enhanced by analysis of long pollen records from long-lived basins. However, in the high elevations of the central Rockies, paleoecological sites extending backward in time beyond the Last Glacial Maximum are extremely rare, due largely to widespread ice that covered the mountains during the Pinedale Glaciation. One such site where pre-Pinedale conditions are preserved is the Ziegler Reservoir fossil site, discovered in October, 2010, as part of excavations in Snowmass Village, Colorado. Sediments from the ~ 5-ha site at 2,720 m elevation are being studied using a number of proxies to reconstruct environments in the area when the small lake was in existence. Our high-resolution pollen and charcoal analysis of the nearly 10-m section exposed during the excavations is unique in that it covers a time range of MIS 6 through MIS 4 in the mountainous terrain of central Colorado. Today the site occurs within the conifer – aspen forest. The transition out of Artemisia-dominated MIS 6 into the MIS 5 interglacial is documented by a rapid increase in tree species, including first Picea then Pinus, which is analogous to the late-glacial – early Holocene transition at other high elevation lakes in Colorado. Quercus is most abundant at Ziegler during the early interglacial, peaking in the middle of 5e. This suggests the importance of oak in the local vegetation around the Ziegler Reservoir site during the last interglacial, a situation that may have no analog for this elevation today, and supports the contention that MIS 5e was warmer than the present interglacial. The transition to MIS 5d is suggested by subtle changes in the pollen record, with increasing non-arboreal pollen types. Pollen spectra from 5c show an increasingly dense Picea, then Abies forest. This is followed by dominance again of Artemisia, suggesting very cool and/or dry conditions prevailed in 5b. The pollen assemblages are consistent with either spruce krummholz or alpine conditions. In 5a, a closed spruce forest once again prevailed around the site, but pollen during MIS 4 suggests cooler and drier conditions accompanied the infilling of the lake basin. In general, fire events were prominent during forested periods, and were rare during non-forested periods.