1,720,992 research outputs found
Mercury Isotope Fractionation during Aqueous Photo-reduction of Methylmercury in Presence of Different Types and Amounts of Dissolved Organic Matter
No full textThe effects of different types and amounts of dissolved organic matter (DOM) on the isotope fractionation of Hg isotopes during aqueous photo-reduction of monomethylmercury (MMHg) were investigated to assess whether mass-independent fractionation (MIF) signatures can be used to track photo-degradation of MMHg in natural waters. From experiments conducted with different amounts of reduced organic sulfur (Sred-DOM), it appears that MIF during photo-reduction may be dependent on whether MMHg is dominantly bound to Sred-DOM. Similar fractionation factors were observed for experiments where Sred-DOM was in far excess of MMHg, while significantly lower fractionation factors were observed with lower Sred-DOM. We also characterized the signature of MIF (i.e. Δ199Hg/Δ201Hg) during MMHg photo-degradation to assess if it was similar in different matrices. The experimental Δ199Hg/Δ201Hg was very similar for different matrices. However, the experimental slope is slightly but statistically different than the slope observed in freshwater fish, which preserve MMHg in nature.MAS
Understanding Sources and Cycling of Mercury in Terrestrial and Aquatic Ecosystems Using Mercury Stable Isotopes
Get PDFMercury (Hg) is a trace metal pollutant that is a neurotoxin, found in both inorganic (IHg: Hg0, Hg2+) and the more toxic organo-metallic (e.g., monomethylmercury, MMHg) forms. Both historically and currently, IHg was/is released into aquatic ecosystems through industrial and mining activities, where it can be transformed into MMHg, that bioaccumulates in food webs and poses risks to both humans and wildlife. Legacy Hg pollution can still pose a threat to modern ecosystems if it becomes methylated. Hence, it is crucial to locate legacy IHg sources, understand the processes and environments that convert IHg to MMHg, and determine whether this MMHg is still entering food webs. In chapter 2, Hg isotopic compositions are used to link Hg discharges from a former chlor-alkali plant to Hg in contaminated sediments and lower trophic biota in the St. Clair River. This project is a good example of a setting where Hg isotopes successfully delineated Hg sources and demonstrated that a legacy Hg source (decades old) is still entering the food web. Chapter 3 presents a more complex system where multiple Hg sources converge and mix. Because the isotopic compositions of Hg-contaminated sediments from the historically industrialized St. Lawrence River fell within the range of documented industrial Hg isotopic signatures, they were not able to distinguish between various industrial sources, such as chlor-alkali and paper and pulp industries, due to the mixing and similarity of the isotopic compositions of the sources. However, Hg isotopes support how pervasive industrial Hg contamination is in the region. Chapter 5 delves into the atmosphere-terrestrial cycling of Hg, particularly the isotopic fractionation as plants uptake and adsorb Hg from the atmosphere. Because Hg isotopes are now an important tool to delineate and quantify sources and deposition pathways to forest soils, which are one of the largest sinks of Hg, I investigated the consistency of the atmosphere-vegetation Hg isotopic offsets between and within different plant species, as this is a crucial variable in isotopic modelling. In a Canadian boreal-deciduous mixed forest (Limberlost Forest and Wildlife Reserve), offsets were reasonably consistent between and within species. This dissertation demonstrates the utility of Hg isotopes in source characterization and quantification, but also highlights some of the limitations of using Hg isotopes.Ph.D
Mercury Isotope Fractionation during Aqueous Photo-reduction of Methylmercury in Presence of Different Types and Amounts of Dissolved Organic Matter
Get PDFThe effects of different types and amounts of dissolved organic matter (DOM) on the isotope fractionation of Hg isotopes during aqueous photo-reduction of monomethylmercury (MMHg) were investigated to assess whether mass-independent fractionation (MIF) signatures can be used to track photo-degradation of MMHg in natural waters. From experiments conducted with different amounts of reduced organic sulfur (Sred-DOM), it appears that MIF during photo-reduction may be dependent on whether MMHg is dominantly bound to Sred-DOM. Similar fractionation factors were observed for experiments where Sred-DOM was in far excess of MMHg, while significantly lower fractionation factors were observed with lower Sred-DOM. We also characterized the signature of MIF (i.e. Δ199Hg/Δ201Hg) during MMHg photo-degradation to assess if it was similar in different matrices. The experimental Δ199Hg/Δ201Hg was very similar for different matrices. However, the experimental slope is slightly but statistically different than the slope observed in freshwater fish, which preserve MMHg in nature.MAS
Tracing the Distribution of Pb and Trace Elements in the Canadian Arctic from the Atmosphere to the Ocean
Get PDFTrace elements (TEs) play important roles in biogeochemical processes in the Arctic and understanding their cycling along with their isotopes will help us understand better the warming induced changes in the Arctic. This dissertation measured Pb, its isotopes and other TEs in aerosols and seawater samples collected in the Canadian Arctic. Lead and its isotopes are useful particulate source tracers and can be used to understand particulate contaminant inputs and oceanic circulation. Iron is also evaluated because it is an important micronutrient for primary production in the ocean. This study shows that anthropogenic aerosols from Europe and Russia (Eurasia) still dominate during the Arctic Haze period (winter–spring) and particulate pollution levels are relatively stable in the Arctic since the 2000s. This study also traced the anthropogenic Pb from aerosols to the Canadian Arctic seawater using Pb isotope measurements. Historic Eurasian Pb with distinctively low 206Pb/207Pb (Ph.D
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Lead Isotopes as Particulate Contaminant Tracers and Chronostratigraphic Markers in Lake Sediments in Northeastern North America
Get PDFThe utility of lead isotopes as particulate contaminant tracers and chronostratigraphic markers was assessed in six lakes from the Great Lakes region. The geographic range of the 19th century Upper Mississippi Valley ore smelting marker was expanded into southern and central Ontario and its northwestern extent was reached within Lake Superior. This marker is useful for dating sediments deposited 100-200 years ago when other dating techniques are problematic. Leaded gasoline was dominant in one lake in the 20th century; its signal was absent elsewhere because the other lakes were affected by regional industrial output or were too remote to receive the leaded gasoline signal. Lead isotopes and trace metals identified a coal combustion marker in the 20th century in southern Ontario, where coal combustion was important enough to outweigh leaded gasoline input. This thesis demonstrates the usefulness of lead isotopes for identifying contamination sources where several pollution sources exist.M.A.S
Lead Isotopes as Particulate Contaminant Tracers and Chronostratigraphic Markers in Lake Sediments in Northeastern North America
No full textThe utility of lead isotopes as particulate contaminant tracers and chronostratigraphic markers was assessed in six lakes from the Great Lakes region. The geographic range of the 19th century Upper Mississippi Valley ore smelting marker was expanded into southern and central Ontario and its northwestern extent was reached within Lake Superior. This marker is useful for dating sediments deposited 100-200 years ago when other dating techniques are problematic. Leaded gasoline was dominant in one lake in the 20th century; its signal was absent elsewhere because the other lakes were affected by regional industrial output or were too remote to receive the leaded gasoline signal. Lead isotopes and trace metals identified a coal combustion marker in the 20th century in southern Ontario, where coal combustion was important enough to outweigh leaded gasoline input. This thesis demonstrates the usefulness of lead isotopes for identifying contamination sources where several pollution sources exist.M.A.S
Evaluating the Application of Photochemical Mass-Independent Fractionation of Mercury (Hg) in Natural Systems
Get PDFMercury (Hg) is a toxic and globally distributed contaminant that poses a serious risk to humans, wildlife, and the environment due to its ability to bioaccumulate along the food webs in aquatic ecosystems. The overarching goals for this dissertation are to: (1) evaluate the potential of mass-independent fractionation (MIF) of Hg during photochemical reactions to understand biogeochemical Hg cycling in natural systems; and (2) utilize Hg mass-dependent (MDF) and mass-independent fractionation (MIF) to characterize and trace Hg sources in the environment. In Chapter 2, Hg isotopes link environmental factors to Hg MIF preserved in freshwater lower trophic organisms to quantify MMHg photodegradation in freshwaters and understand how Hg MIF is preserved in natural systems. In Chapter 3, Hg isotopes in pelagic ocean higher trophic biota assess the relative contribution of Hg sources, transformation processes (methylation/photodemethylation), and other factors that may control the Hg isotopic signature of open-ocean fish in different regions and depths of the pelagic ocean. Chapter 4 assess Hg isotopes in atmospheric gaseous Hg(0), particulate Hg (PHg), and snow to understand Hg photochemical processes and sources in the Arctic. This dissertation demonstrates source characterization of atmospheric Hg species from multiple emission sources and better understanding of transformation processes that alter Hg source signatures, and highlights the potential of stable Hg isotopes to contribute to the increasing knowledge of Hg sources and transformation processes in the atmosphere, aqueous ecosystems and cryosphere.Ph.D
Tracing Atmospheric Sources of Mercury through Passive Air Sampling and Isotope Characterization
Get PDFGaseous elemental mercury (GEM) is the dominant form of mercury (Hg) in the atmosphere and is the main species responsible for Hg being globally distributed far from point sources. Tracing Hg sources in the atmosphere remains challenging, but a potential new tool to aid in this is the measurement of stable Hg isotope ratios in atmospheric samples. Mercury isotopes undergo both mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) making them useful for identifying and quantifying sources and transformations of Hg. Current collection methods for isotope characterization of atmospheric GEM rely on power and instrumentation requiring technical expertise, making sampling on large spatial scales and in remote locations difficult. Here, we assess the ability of a passive air sampler (PAS), which samples GEM, to collect and preserve Hg for isotope analysis. Based on both field and laboratory experiments, this thesis demonstrates that no MIF occurs during sampling of GEM by the PAS and that there is consistent MDF offset of ≈-1.2 ‰ in δ202Hg. The PAS was then used in two reconnaissance studies on the spatial distribution of GEM concentrations and isotopes across large regions (Ontario, Canada and Madre de Dios, Peru). In both Ontario and Peru, the Hg isotopic composition of GEM collected with the PAS allowed regional and local sources to be identified and traced. In Ontario, southern regions with greater local urban/industrial emission sources and/or trans-regional air masses containing industrial sources had GEM that was isotopically distinct from northern regions, which have fewer emission point sources. In Madre de Dios, artisanal and small-scale gold mining (ASGM) activity is a main Hg source to the region and is isotopically distinct, allowing the contribution of ASGM derived GEM to be estimated. In addition to source tracing, Hg isotopes in GEM were also affected by vegetation uptake of GEM in both Ontario and Peru, which is a major removal process of GEM from the atmosphere and a major source of Hg to terrestrial ecosystems and soils. In Peru, isotope compositions in foliage and soils in Amazonian forests near mining indicated uptake of ASGM-GEM. These results demonstrate that Hg isotopes can be used to improve our understanding of sources to the atmosphere and soils (local/regional/global) and also assess major removal processes from the atmosphere.Ph.D
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