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A high-resolution record of Holocene drought variability and the diatom stratigraphy of Foy Lake, Montana

Jeffery Robert Stone, University of Nebraska - Lincoln


A combination of modeling, taxonomic, and stratigraphic approaches was used to examine fossil diatom assemblages of Foy Lake and reconstruct Holocene climate variability. First, a three-dimensional model of Foy Lake was generated to assess how ratios of planktic and benthic habitat area change as lake level is lowered or raised. Model results allowed semiquantitative lake-level depths to be reconstructed from 20th-century and late-Holocene sedimentary diatom records. Comparison of the model results with sedimentary records suggests that planktic:benthic ratios may be a better predictor of mean lake depth than maximum lake depth at this site. ^ Diatoms belonging to the Cyclotella bodanica/radiosa species complex are the dominant diatom from late-Holocene sediments of Foy Lake. Scanning electron microscope and light microscope techniques were used to examine the valve morphology and ultrastructure of fossil bodanicoid populations of Foy Lake and three other sites in the northern Rocky Mountain region. These analyses reveal that the Foy Lake populations share identical valve characters with the populations from one lake but differ from populations in the other two lakes examined. Three features are recognized that consistently separate the populations, which may represent undescribed species or varieties. ^ A 13,000-yr record of drought variability at sub-decadal resolution was inferred from fossil diatom assemblages recovered from Foy Lake, Montana. Spectral analyses of benthic diatom proportions, which are altered by drought-driven lake level change, indicate that multi-decadal drought periodicities characteristic of the PDO (particularly 50 to 70-yr) have undergone fundamental state changes throughout time. The strongest periodicity occurs during the mid-Holocene. ^ Fossil diatoms and δ18O values of authigenic carbonate from the late Holocene indicated a dry climate and low lake levels from 200 B.C. to A.D. 800. At A.D. 1200, lake level increased to approximately 6 m below present elevation and over the last 800 years, the lake reached maximum stage. Spectral analysis of the isotopic data indicates that droughts in the late Holocene occurred with a persistent multi-decadal (50 to 70-yr) frequency.* ^ *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Adobe Acrobat; Microsoft Office.^

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Recommended Citation

Stone, Jeffery Robert, "A high-resolution record of Holocene drought variability and the diatom stratigraphy of Foy Lake, Montana" (2005). ETD collection for University of Nebraska - Lincoln. AAI3180728.