A HIGH-ELEVATION, MULTI-PROXY BIOTIC AND ENVIRONMENTAL RECORD OF MIS 6-4 FROM THE ZIEGLER RESERVOIR FOSSIL SITE, SNOWMASS VILLAGE, COLORADO, USA

In North America, terrestrial records of biodiversity and climate change that span Marine Oxygen Isotope Stage (MIS) 5 are rare. Where found, they provide insight into how the coupling of the ocean-atmosphere system is manifested in biotic and environmental records and how the terrestrial biosphere responds to climate change. In 2010–2011, construction at Ziegler Reservoir near Snowmass Village, Colorado (USA) revealed a nearly continuous, lacustrine/wetland sedimentary sequence that preserved evidence of past plant communities between ~140 and 55 ka, including all of MIS 5. At an elevation of 2705 m, the Ziegler Reservoir fossil site (ZRFS) also contained thousands of well-preserved bones of late Pleistocene megafauna, including mastodons, mammoths, ground sloths, horses, camels, deer, bison, black bear, coyotes, and bighorn sheep. In addition, the site contained more than 26,000 bones from at least 30 taxa of small animals, including salamanders, otters, muskrats, minks, rabbits, beavers, frogs, lizards, snakes, fish, and birds. The combination of macro- and micro-vertebrates, invertebrates, terrestrial and aquatic plant macrofossils, a detailed pollen record, and a robust, directly dated stratigraphic framework, shows that high-elevation ecosystems in the Rocky Mountains of Colorado are climatically sensitive and varied dramatically throughout MIS 5. Overall, the proxy data show that the local ecosystem responded to global climate in a way that was similar to near-shore marine and lacustrine sites elsewhere in North America with two key exceptions. First, the response at the ZRFS during MIS 5d was significantly muted compared to other North American sites and remained warm (similar to MIS 5e and 5c), despite a decrease in global temperatures and possible continental ice growth at this time. Second, the response during MIS 5b was amplified and significantly colder than expected, culminating in conditions that were similar to late MIS 6. These results demonstrate the importance of hemisphere-scale teleconnections while underpinning the fact that regions, and even sub-regions at high elevations, may respond to changing climate conditions in ways that are similar or different from hemsipheric or global averages depending on whether certain environmental thresholds are met.