Browsing by Subject "Dissolved organic carbon"
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Item The behavior of dissolved organic carbon (DOC) at geological sequestration sites(2015-05) Patson, Michael Edwin; Breecker, Dan O.; Larson, Toti Erik; Bennett, Philip CGeologic carbon sequestration has been proposed as a means of mitigating anthropogenic greenhouse gas emissions. At depth, supercritical CO₂ may rise above the surrounding fluid. Detecting leaks from CO₂ storage reservoirs is important to evaluate the effectiveness of carbon sequestration and address public concern for negative environmental impacts. Other attempts have been made to detect leaks, such as changes in pH, pressure and direct observation of CO₂ in the AZMI (Above Zone Monitoring Interval). Each has limitations and here we investigate dissolved organic carbon (DOC) as a potential indicator for fugitive CO₂. This study uses a series of batch experiments to evaluate the interaction between dissolved CO₂ and DOC. The batches consist of homogenized and sieved 250 micron to 425 micron matrix samples of varying mass and type, 2mL of DI water and a headspace of pure carbon dioxide or air. The three different rock samples analyzed are Buffalo River Sediment, illite and Barnett Shale. A pure CO₂ headspace results in lower amount of DOC in solution than an air headspace. All matrix samples demonstrated this effect. The proposed mechanism to describe the observed results is that a lowered pH shifts speciation of weak organic acids and protonated humic substances causing decreased solubility and increasing the adsorption of these compounds. These results suggest that a decrease in DOC concentrations could be used to detect CO₂ leakage and that CO₂ leakage would not deteriorate water quality by releasing DOC.Item Factors Affecting Carbohydrate Production and the Formation of Transparent Exopolymer Particles (TEP) by Diatoms(2014-03-25) Chen, JieDiatoms exude large amounts of exopolymers (EPS), which are predominantly composed of carbohydrates. EPS may coagulate into transparent exopolymer particles (TEP). Sticky TEP affects the formation of aggregates and marine snow, and consequently, the efficiency of the biological carbon pump. The objective of this research was to determine how different factors affect carbohydrate production and the formation of TEP by diatoms, and their role in aggregation. Diatoms were grown in laboratory cultures to test the hypothesis that stress increases the cell membrane permeability and subsequently enhances TEP formation. In addition, an experiment was conducted to compare the effect of oxidative stress on both a diatom (Thalassiosira weissflogii) and a cyanobacterium (Synechococcus elongates). For some diatoms (Thalassiosira weissflogii and Skeletonema marinoi) and the cyanobacterium Synechococcus elongatus, TEP formation was associated with permeable cells. Greater TEP production was observed in cultures under stress conditions (higher temperature, nutrient limitation, and oxidative stress), and more dissolved extracellular carbohydrate was released by dying cells. In the contrast, TEP formation by Cylindrotheca closterium was associated with healthy cells. More dissolved extracellular carbohydrate produced by healthy cells, rather than permeable cells. Therefore, my results indicate that carbohydrate production is important for TEP formation. Stress causes cell leakage, but TEP formation is a complex process. Cell leakage does not always result in the release of dissolved extracellular carbohydrate and enhanced TEP production. In addition, this study investigated the relationship between TEP and aggregate formation. Higher temperature increased TEP production, which was associated with greater aggregation in cultures of S. marinoi, but not in T. weissflogii. Therefore, enhanced TEP production by diatoms does not always affect aggregate formation. This research indicates that environmental factors affect carbohydrate and TEP production by diatoms, and consequently influences aggregate formation. These influences have a profound impact on biogeochemical cycling of carbon.Item Seasonal dynamics of organic matter and inorganic nitrogen in surface waters of Alaskan Arctic streams and rivers(2015-12) Khosh, Matthew Solomon; McClelland, James W.; Dunton, Kenneth H; Liu, Zhanfei; Shank, Gerald C; Townsend-Small, AmyClimate-linked changes in hydrology and biogeochemical processes within Arctic watersheds are likely already affecting fluvial export of waterborne materials, including organic matter (OM) and dissolved inorganic nitrogen (DIN). Our understanding of Arctic watershed OM and DIN export response to climate change is hampered by a lack of contemporary baselines, as well as a dearth of seasonally comprehensive studies. This work focuses on characterizing OM and DIN concentrations and sources in six streams/rivers on the North Slope of Alaska during the entirety of the hydrologic year (May through October) in 2009 and 2010. The highest OM concentrations occurred during spring snowmelt, with results indicating that terrestrial vegetation leachates are the major source of dissolved OM, while particulate OM originates from a degraded soil source. Over the hydrologic year, soils became a progressively increasing source of dissolved OM, while autochthonous production made up a sizeable proportion of particulate OM during base flow conditions. DIN concentrations were low throughout the spring and summer and increased markedly during the late summer and fall. Our findings suggest that penetration of water into thawed mineral soils, and a reduction in nitrogen assimilation relative to remineralization, may increase DIN export from Arctic watersheds during the late summer and fall. Although recent studies of Arctic rivers have emphasized the importance of the spring thaw period on OM export, our understanding of the mechanisms that control water chemistry observations during this time are still lacking. Experimental leaching results, from experiments conducted in 2014, suggest that aboveground plant biomass is a major source of dissolved OM in Arctic catchments during the spring, and that the timing of freezing and drying conditions during the fall may impact dissolved OM leaching dynamics on that same material the following snowmelt. Improved knowledge of OM and DIN temporal trends and the mechanisms that control seasonal concentrations is essential for understanding export dynamics of these water constituents in Arctic river systems. Perhaps more importantly, increased understanding of the seasonal controls on OM and DIN export in Arctic rivers is critical for predicting how these systems will respond under future climate change scenarios.