A HIGH-ELEVATION MIS 5 HYDROLOGIC RECORD USING MOLLUSKS AND OSTRACODES FROM SNOWMASS VILLAGE, COLORADO, USA

Sediments containing terrestrial and aquatic mollusks and ostracodes from Ziegler Reservoir fossil site (2705 masl) near Snowmass Village, Colorado (39.214° N 106.945° W), span ~130-87 ka (MIS 5e through 5b). Stable oxygen and carbon isotope values from ostracode calcite (δ¹⁸O_OST and δ¹³C_OST) are measured to provide additional insight into hydrologic conditions. Most taxa recovered have broad environmental tolerances, indicative of seasonal and/or annual physical and chemical variability. Mollusk and ostracode faunas indicate that the site was a shallow wetland during MIS 5e through 5c time, probably no more than 2 m deep, with significant evaporation, and a TDS value of ~200-2000 mg L^-1. The wetland may have been prone to episodic desiccation and cold springs or seeps were probably present. Hill slopes were shaded by shrubs and aspen trees producing ample forest litter. Preliminary δ¹⁸O_OST and δ¹³C_OST values are high (0.5‰ to 4‰ and 2‰ to 9‰ VPDB, respectively) and support the interpretation that the site was a shallow wetland with a long residence time, subject to photosynthetic enrichment of its DIC pool, and possibly the anaerobic production of methane. The mollusk and ostracode faunas did not show appreciable change across the MIS 5e/5d boundary. A distinct change in the ostracode fauna did occur across the MIS 5c/5b boundary, however, signaling increased surface runoff, deeper and potentially colder water, and probably a decrease in TDS. The reservoir may have become a deeper wetland or a shallow lake at this time. Even though water depth probably increased during MIS 5b, the δ¹⁸O_OST and δ¹³C_OST values remained high, indicating that the residence time remained long and buffered the isotopic response of the reservoir. The mollusk and ostracode faunal record from Ziegler Reservoir mirrors other MIS 5 records from the western U.S. in suggesting that the MIS 5b stadial resulted in a more pronounced terrestrial response than did the MIS 5d stadial.