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Browsing by Subject "Anthropogenic impacts"

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    Anthropogenic climate change has driven Lake Superior productivity beyond the range of holocene variability: an organic and stable isotopic study of Human impacts on a pristine biogeochemical system
    (2013-07) O'Beirne, Molly D.
    Recent studies have noted that changes in Lake Superior's physical, chemical and biological processes are apparent - including a warming of the surface waters at a rate twice as great as the surrounding airshed in the last 20 years. These changes are often difficult to perceive as cause for concern when not placed within a historical context. In this study, bulk C and N elemental abundance and stable isotope analysis of sediments from three piston and corresponding gravity cores, representing a record of lake-wide paleoproductivity trends spanning the Holocene, allows for the historical comparison with recent (1800 A.D. to present) productivity trends. Overall, Lake Superior experiences a slow, steady increase in productivity consistent with the concept of `natural' eutrophication, which is characterized by gradual increases in TOC and TON, as well as the steady 13C-enrichment of bulk sedimentary organic carbon and 15N-enrichment of bulk sedimentary organic nitrogen compositions. Over the last 200 years bulk concentrations and stable isotope compositions of carbon and nitrogen from eight multicores sampled at high resolution indicate that the Lake Superior basin has undergone productivity changes in the last two centuries (1800 to present) which are unique in the context of the Holocene. Overall, lake-wide sedimentary bulk organic proxy data show increasing primary production between 1900 and present, as indicated by an ~2 ‰ increase in δ13Corg. The most recent increases in productivity are likely a response to increasing water temperatures and longer stratified periods reported in Lake Superior. Down-core variations in the δ13C composition of algal-derived short-chain n-alkanes do not exhibit the same trend as that observed for bulk sedimentary organic matter (δ13Corg). The δ13C of bulk sedimentary organic matter shows systematic 13C-enrichment over the last ~9000 years, while the δ13C values of aquatic derived n-alkanes exhibit a systematic 13C-depletion to present-day. Down-core variation in δ13C values of n-alkanes likely reflect multiple isotope effects associated with carbon partitioning and fractionation associated with the biosynthesis of n-alkanes.
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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.