A QUANTITATIVE MIDGE-BASED RECONSTRUCTION OF THERMAL CONDITIONS IN CENTRAL COLORADO DURING MARINE ISOTOPE STAGE 5

A sediment core recovered from the Ziegler Reservoir fossil site in Snowmass Village, Colorado was analyzed for subfossil chironomids. This newly developed midge stratigraphy spans an interval that includes the termination of the Bull Lake Glacial (~140 ka), the Sangamon, and the onset of the Wisconsin Glacial (~70 ka). Changes in midge community composition and the magnitude of faunal turnover provide evidence of notable environmental change during the transitions from MIS 5c to MIS 5b and from MIS 5b to MIS 5a. A midge-based inference model for mean July air temperature (MJAT) (2-component WA-PLS; r²_jack=0.61, RMSEP=0.97), based on a calibration set that incorporates lakes from the central Colorado Rockies, was applied to the Ziegler Reservoir midge stratigraphy and used to develop a quantitative reconstruction of MJAT for the site. Little variation in the midge-inferred MJAT was evident during the transition from MIS 5e to MIS 5d with MJAT decreasing from 10.6°C to 9.9°C. A gradual warming trend persisted through MIS 5, culminating in the warmest MJAT recorded (13.3°C) during MIS 5a at ~79.5 ka. The lowest inferred temperatures occur at the onset of MIS 5e and MIS 5a. Overall, the range in the reconstructed MJAT ranged from 8.5 to 13.3°C for the entire record. Qualitative changes within chironomid assemblages and individual taxa were also considered. The turnover in midge faunal composition that occurred midway through MIS 5a likely reflects a significant lowering of lake level at ~ 83.4 ka. The existence of extensive year round ice cover during MIS 5b (99 - 87 ka) is inferred to have occurred due to the complete absence of subfossil midge remains in sediment deposited during this interval. The chironomid stratigraphy corroborates the detailed pollen record produced for the site, which indicates that notable changes in local and regional environmental conditions occurred during the transition from MIS 6 to MIS 5e and from MIS 5b to MIS 5a.