AN UNPRECEDENTED, MULTI-PROXY, HIGH-ELEVATION RECORD OF THE SANGAMON INTERGLACIAL IN THE COLORADO ROCKIES

The study of terrestrial biotic and environmental dynamics of the penultimate interglacial [Sangamon Interglacial/Marine Isotope Stage (MIS) 5] provides insight into long-term effects of changing climates on Pleistocene ecosystems. In North America, there are hundreds of well-preserved fossil sites that date to MIS 5. These sites have formed the basis of our interpretation of ecosystems and ecosystem change through that time. However, our view of these ecosystems is heavily biased by the fact that there are no high-elevation fossil sites that contain a continuous record of multiple aspects of the biotic community and corresponding environment, leaving mountain ecosystems during MIS 5 virtually undocumented. Moreover, our understanding of ecosystem response is necessarily a composite view because single sites at which insects, invertebrates, micro- and macro-plants, and micro- and macro-vertebrate fossils are preserved and have been studied are exceptionally rare. A new site, discovered on Oct. 14, 2010 by a construction crew while enlarging a small reservoir near Snowmass Village, Colorado, elevation 2,720 m, revealed a series of stacked fossil ecosystems. Subsequent investigation recovered a diverse biota including more than 5,000 bones from late Pleistocene megafauna. Geomorphic evidence and multiple dating techniques indicate that a small, ridgetop basin was formed by a lateral lobe of a valley glacier that filled the Snowmass Creek drainage during Bull Lake time (MIS 6/Illinoisan). When the glacier receded, the basin became a lake and began to slowly fill with eolian sediment, with occasional input from slope failures of the impounding moraine. The resulting sedimentary record spans ~80,000 years between ~130 and 50 ka capturing all of MIS 5 and 4. Preservation of organic material at the site is exceptional; sedge and willow leaves are often still green, beetle parts are iridescent, and cones are often found still intact. MIS 5e/d are especially well represented, and include the most complete record to date of biotic communities during the Sangamon Interglacial at high-elevation in North America.