HOLOCENE CLIMATIC CHANGES INFERRED FROM PLANT MACROFOSSILS FROM BLUE LAKE, EAGLE COUNTY, COLORADO

Blue Lake (2539m) is located in a small basin formed by a landslide ~8 km north of the Eagle River near Wolcott, Colorado. The lake has very limited surface water inflow, is primarily fed by groundwater, and lies within a relatively dry area of rolling topography and small peaks that is bounded by the more massive Gore Range to the east and the Flattops to the west. The present-day vegetation consists of stands of conifers and aspen (Picea pungens, Pseudotsuga menziesii, Populus tremuloides) amongst grassy open parklands with sagebrush and juniper (Artemisia tridentata, Juniperus scopulorum).

Three marl-dominated sediment cores from the lake were analyzed for plant macrofossils to produce a record of vegetation change over the past ~9700 years. The cores contain evidence of what appears to be the eruption of the Dotsero marr volcano during the middle Holocene, which, along with radiocarbon dates on plant macrofossils, establishes the chronology of the sediment record. At the base of all three sediment cores there is a layer of woody material that was deposited before the establishment of the lake at ~9.7 cal ka, suggesting that either the landslide occurred at that time or that conditions were too dry to support a lake prior to that time. The plant macrofossil record from sediments above the woody layer includes needles of ponderosa pine (Pinus ponderosa; dated at 8.0 and 9.6 cal ka), a species that is not present near Blue Lake today.  It does occur at sites near the Eagle River at elevations that are ~300m lower than the lake, suggesting that climatic conditions were warmer and drier than present during the early and middle Holocene. The presence of Picea and Pseudotsuga macrofossils in the upper core sediments, combined with the absence of ponderosa pine remains, suggests that the climate was cooler after ~4.5 cal ka during the late Holocene.