COHERENT SHIFTS IN HOLOCENE CLIMATE AND THERMAL LAKE STRUCTURE RECONSTRUCTED FROM FOSSIL DIATOM ASSEMBLAGES

Changes in fossil diatom assemblages from the sediment archives of lakes in the U.S. Rocky Mountains region reveal two substantial unidirectional shifts in the dominant planktic species ~3400 and ~350 years before present. Evidence from modern in-lake ecological experiments along with published autecological information and calibration data suggest that shifts in the dominant plankton are likely responding to changes in the thermal structure (mixing depth) of the lakes. Trends in the fossil diatom assemblages, which span from the Beartooth Mountains of northwestern Wyoming to Glacier National Park in northwestern Montana, are similar despite differences in lake size, basement geology, nutrient regimes, and elevations. Here we present evidence of regionally-coherent changes in lake mixing depths from the sediment archives over the last 5,000 years and explore the potential for using planktic diatom indices and mixing depth calibration techniques as a metric for reconstructing past changes in average air temperatures and wind regimes.