Browsing by Subject "Sediment core analysis"

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    Completion Report Mille Lacs Lake Paleolimnology Project
    (University of Minnesota Duluth, 2003) Kingston, John
    Mille Lacs Lake is a high-priority lake in terms of its size and premier walleye fishery, and it has a relatively small watershed compared to the size of the lake surface. Expected future increases in development of the lakeshore have prompted a desire to know about water quality baselines for this important lake. This paleolimnological study examines a sediment core collected in late winter of 2002 using radioisotope dating, loss-on-ignition analysis of organic and inorganic sediment components, and diatom analysis as a proxy for nutrient loading. Land use changes in the watershed have caused accelerating soil erosion from 1960 until the present day. Diatom counts show a decline of benthos-dominated assemblages starting by the 1950s, with greater dominance of planktonic species for the last 60 years. This indicates nutrient loading increases and water transparency decreases. The core-top, representing conditions in 2001, shows the highest relative abundance of planktonic diatom species, indicating that nutrient loading is still increasing. Preliminary diatom- based reconstructions of past lakewater total phosphorus concentrations show that nutrient loading in Mille Lacs has increased approximately 30% during the past half century and remains at an historical high level.
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    Paleolimnological Reconstructions for the White Iron Chain of Lakes
    (University of Minnesota Duluth, 2013) Reavie, Euan D
    To quantify the environmental history of the White Iron Chain of Lakes (Lake and St. Louis Counties, Minnesota), five lakes were selected for retrospective analyses. Primary goals were to determine pre- European settlement conditions and track the timing and extent of anthropogenic impacts and remediation. Sediment cores were collected from each lake and sediment intervals were dated using isotopic analyses. Fossil remains, in concord with other stratigraphic indicators (organic and inorganic materials, sedimentation rates, other biological entities), were used to reconstruct the ~200-year history of each lake. Pollen analyses allowed for reconstruction of local and regional terrestrial conditions. Geochemical analyses provided data on historical flux of elemental trace metals to the sediments. Diatom assemblages were assessed from sediment intervals and inferred trophic conditions in the profiles were derived using a regional diatom-based model for Minnesota lakes. Eutrophication apparently occurred following settlement, particularly in White Iron Lake, but reconstructed phosphorus trends indicate more recent nutrient reductions. Pollen data track the decrease in pine abundance in the region and the rise of birch. Sedimentary metals largely reflect physical changes in the system, such as a change in sediment deposition regimes resulting from damming. Recent increases in metals are probably a result of increasing accumulation of soil and bedrock materials, a trend that is supported by increasing accumulation rates of overall organic and inorganic material. These recent increases in the last 30-40 years, which include increased algal deposition in Birch, Farm and Fall lakes, are not well explained at this time, but may be due to shifting water quality unrelated to phosphorus and possibly hydrological changes.