Dust and Pleistocene ice ages: Eolian sediments and climate change at Ziegler Reservoir, Snowmass Village, Colorado

The discovery of the Ziegler Reservoir fossil site near Snowmass Village, Colorado presents an opportunity to examine subalpine ecosystem response, during a relatively unknown period, in which climate conditions were similar to present. Fine-grained sediments at Ziegler Reservoir represent continuous deposition between ~140--55 ka (thousand years before present), spanning the close of the Bull Lake glacial period [marine isotope stage (MIS) 6], the Sangamon interglacial (MIS 5) and the early stages of the Pinedale (early Wisconsin) glacial period (MIS 4). Ziegler Reservoir is positioned on top of a ridge, at an elevation of 2705 m, and has a small watershed area (~14 ha), with little evidence of fluvial sediment transport. Particle size distributions, mineralogy, and geochemistry indicate eolian processes were the likely mechanism for deposition of the local, silt-rich and clay-rich sediments that ultimately filled the basin. The presence of clay-rich units (up to three-times more clay than background), is interpreted to represent glacial conditions at two distinct intervals: 1) ~140--134 ka (MIS 6) and 2) ~71--55 ka (MIS 4/3), which correspond to late Bull Lake and early Pinedale glacial periods, respectively. In addition, the absence of clay-rich sediment in the intervening sediments suggests a lack of glacial activity during sub-stages MIS 5d and 5b. In all, the Ziegler Reservoir sedimentary record provides critical information regarding environmental response of high-alpine ecosystems to climate change in the Rocky Mountains during the late Pleistocene.