EARLY POSTGLACIAL TERRESTRIAL AND LIMNOLOGIC DEVELOPMENT IN THE NORTHERN YELLOWSTONE AREA INFERRED FROM MULTIPLE PROXY RECORDS FROM DAILEY LAKE, MONTANA

A sediment core from Dailey Lake, a small semi-closed basin lake located in southwestern Montana, provides new insight into early postglacial terrestrial and limnologic development in the Greater Yellowstone region from ~16,000 to 7000 cal yr BP based on pollen, charcoal, diatom, geochemical, and lithologic data. Immediately following deglaciation, low pollen influx in combination with high magnetic susceptibility indicates a sparsely vegetated unstable landscape with high clastic mineral input into the lake. As summers warmed and slopes stabilized, the pollen record indicates the expansion of Picea populations at 13,400 cal yr BP, followed by the development of closed Picea-Abies-Pinus forest at 12,200 cal yr BP and open Pinus-Pseudotsuga forest at 10,200 cal yr BP. Within the lake system, establishment of diatoms at 13,350 cal yr BP marks the onset of productive lake conditions and coincides closely with slope stabilization and Picea population expansion. With warmer summers, Dailey Lake became thermally stratified beginning at 13,100 cal yr BP as indicated by increased planktic diatoms, and a switch to high tychoplanktic diatom abundance between 12,300 and 11,800 cal yr BP indicates a moderately shallow unstratified lake during the summer months. After 11,800 cal yr BP, a resurgence of planktic diatoms implies that the lake became deeper and thermally stratified again, consistent with warm summer conditions and increased water balance due to enhanced winter precipitation and early spring warming. Lake level at Dailey Lake decreased at 11,100 cal yr BP, as indicated by increased benthic diatom percentages. In combination with an open forest and high fire activity, it appears that warm dry summers persisted near Dailey Lake until 7500 cal yr BP. This is in contrast to nearby records in northern Yellowstone National Park, which indicate prolonged summer-wet conditions until ~8000 cal yr BP. Because of its low elevation, Dailey Lake was apparently sensitive to the direct effects of increased summer insolation on temperature and effective moisture, registering dry summers. In contrast, higher elevations in northern Yellowstone responded to the indirect effects of an amplified seasonal insolation cycle on atmospheric circulation, including increased winter snowpack and/or enhanced monsoonal circulation.