Browsing by Subject "Radiocarbon"

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    Sources, biogeochemical cycling, and fate of organic matter in Lake Superior: an investigation using natural abundance radiocarbon and stable isotopes
    (2012-01) Zigah, Prosper Kojo
    The natural abundance radiocarbon and stable isotopic distributions of bulk dissolved organic carbon (DOC), particulate organic carbon (POC), dissolved inorganic carbon (DIC), zooplankton, size-fractionated organic matter, and biochemical compound classes were used to investigate the sources, biogeochemical cycling, and fate of organic matter in the water column of Lake Superior. DIC pool appears to reset rapidly, showing radiocarbon values similar to atmospheric values from approximately 3 years previous to sampling. DIC concentrations and isotopic compositions were mostly homogeneous across the entire lake. POC was generally more depleted in stable carbon isotopic values than concurrent DOC. POC was also consistently depleted in radiocarbon (thus, older) relative to DOC and DIC. Radiocarbon ages of POC was spatially heterogeneous (range, modern to 2,840 year BP), and appear to be related to total water depth, exhibiting more older and more variable ages in the deepest basins of the lake. The ages and reactivity of bulk DOC did not change radically across the lake. DOC pool appears to be semi-reactive, recycling over up to 60 years in the entire water column. The radiocarbon signatures of the various DOC size fractions show that they recycle on similar time scales, with consistently modern (post 1950) radiocarbon values. Radiocarbon and Nuclear Magnetic Resonance (NMR) data show most of the high molecular weight dissolved organic matter (HMW DOM) originates from contemporary origin and was dominated by carbohydrates, aliphatic compounds, and acetate, with little aromatic compounds. Total hydrolyzable free carbohydrates and amino acids within HMW DOM exhibited modern radiocarbon signatures and recycled rapidly in the lake. In contrast, extractable lipid was pre-aged (20 to 2,320 years BP) due to older sources and/or general long term persistence in the lake. Coupled radiocarbon and stable carbon isotopic values indicate multiple sources, and variable formation pathways for the acid insoluble organic fraction within HMW DOM in the lake. Radiocarbon and stable isotopic values show zooplankton in Lake Superior selectively feed on within-lake produced organic matter even though other organic carbon sources represented a considerable portion of the available food resource.
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    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 Ruth
    Organic 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.