Browsing by Subject "Isotopes"
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Item Exploring hydraulic residence in Minnesota's Sentinel Lakes: implications for management(2014-08) Engel, Lee CharlesLake systems present a challenge in determining how water associated solutes cycle with time. Lake hydraulic residence time is an important lake management variable dependent on several factors including: volume, watershed size, location within a watershed and climatic variability. The stable isotopes of hydrogen (Deuterium expressed as δD) and oxygen (δ18O) can provide some hydrologic insight to lake water quality management. Analyzing the stable isotopic composition of lake water δD and δ18O over time can aid in identifying source water input mixing and evaporative processes. Lake water δD and δ18O were compared to the isotopic composition of atmospheric water vapor which has a known isotopic concentration at specific latitudes and air temperatures (Burns and McDonnell, 1998; Dansgaard, 1964). Study lakes were sampled spring, summer, and fall over a three year period. Deviations in the amplitudes of fractionated lake water compared to water vapor was modeled to predict hydraulic residence time for twenty-four lakes throughout Minnesota. Results suggest hydraulic residence time was dynamic; variations occurred with annual source water contributions, watershed size and connectivity. Ranges of annual hydraulic residence time among individual lakes were as great as 18.8 years and as small as 0.4 years. δD and δ18O values were plotted in relation to the Meteoric Water Line (MWL) for all study lakes. A gradient of δD and δ18O values were found in relation to latitude. Lighter values ofδD and δ18O were found in northern Minnesota compared to southern Minnesota. Variations in seasonal δD and δ18O created annual amplitudes that provide insight into lake water budgets and residence times. The use of δD and δ18O offer water quality managers a tool to better understand, protect, and remediate lakes and their watersheds.Item An Investigation of Ni and Cu Isotopic Fractionation in Basal Duluth Complex Cu-Ni-PGE Mineralization, Northeastern Minnesota(2016-05) Asp, KristoferCu-Ni-PGE magmatic sulfide-style mineralization occurs along the western margin of the Duluth Complex in northeastern Minnesota. Previous studies have demonstrated a notable fractionation of 60Ni and 58Ni in terrestrial materials, including both primary and secondary phases, with a total range of up to 2.1 ‰. Other work has indicated a fractionation of 65Cu and 63Cu, with pronounced differences between primary copper sulfides and secondary copper phases in a variety of deposit types. Prior to this study, no δ60/58Ni or δ65/63Cu values have been measured in Duluth Complex rocks. The primary goal of this study is to measure Ni and Cu isotope values in a variety of Duluth Complex samples, and develop a possible model for the δ60/58Ni isotopic system in this geologic terrane. Based on the findings of previous studies, samples were collected to determine the isotopic differences between sulfide-bearing and sulfide-barren material. Samples were collected from a variety of locations in the basal Duluth Complex, including glacial till beds and surface outcrops in the vicinity of the I, II, Serpentine, Mesaba, and NorthMet deposits. Additional drill core material was obtained from the III, Wetlegs, and Wyman Creek deposits. A detailed characterization of till, weathered surface, and primary drill core samples revealed three main sources of nickel in Duluth Complex material: silicate, sulfide, and secondary oxide. The 24 δ60/58Ni values have an overall range from -0.97 to 0.22 ‰, but are correspondingly distinct in each type of material: silicate (-0.03 ‰ average), sulfide (-0.36 ‰ average), secondary oxide (-0.50 ‰ average). Further geochemical and microprobe work, along with the isotopic values, indicate two main stages of Ni fractionation in basal Duluth Complex rocks: a high temperature stage during crystallization, and a low temperature stage during surficial weathering. High-T fractionation is defined by a preferential incorporation of 58Ni into sulfide, while silicates, especially olivine, are reflective of the Bulk Silicate Earth value. Low-T fractionation results in a preferential incorporation of 58Ni into secondary oxide, while 60Ni possibly enters solution and leaves the system. The 22 measured Duluth Complex δ65/63Cu values have an overall range from -1.28 ‰ to 0.36 ‰, with an overall average of -0.35 ‰. Further work is necessary to better define and interpret the involved fractionation processes in the Duluth Complex.Item Sources, cycling, and fate of organic matter in large lakes: ingishts from stable isotope and radiocarbon analysis in Lakes Malawi and Superior(2014-08) Kruger, Brittany RuthOrganic matter (OM) in lake systems is sourced from in situ aquatic primary production (autochthonous), land based plant primary production or detrital material that ultimately originated from photosynthesis (allochthonous), or resuspension of organic rich sedimentary material that was ultimately sourced from a combination of all such sources. Studying the stable and radioisotopic signature of multiple chemical components of lacustrine OM can help elucidate which of the above is the dominant OM source to the lake, as well as how OM is incorporated into and cycles through lake systems. The high organic content and biodiversity in large lakes of the world make them excellent sites to investigate such questions, and this dissertation focuses on such questions in Lake Malawi (SE Africa), and Lake Superior (North America). In Lake Malawi, the organic carbon (OC) recently deposited (within the last 50 years) is largely dominated by aquatic input, and the influence of terrestrial riverine inputs dissipates as distance from shore and water depth increase. This confirms that parameters typically used to investigate historic lake levels (and thereby to infer past climates) can in fact function as robust indicators of distance from shore, and thereby lake level. This is supported by bulk and compound specific stable carbon isotopic and radiocarbon analysis of multiple sediment fractions. Most fractions exhibited isotopic signatures nearshore that were distinct from more offshore, open-lake locations. In Lake Superior, compound specific nitrogen isotope analysis (CSNIA) of specific amino acids from species occupying all levels of the food chain showed that Limnocalanus macrurus, a copepod, occupies a trophic level much higher than expected from known feeding habits, which may indicate the consumption of additional or unique food sources. Bulk radiocarbon analysis of the same suit of species from that lake showed Diporeia, a benthic amphipod, consumes an aged carbon source that does not appear to be significantly incorporated by other (more pelagic) organisms in this study, which rely primarily upon recently synthesized autochthonous organic carbon.