1,721,284 research outputs found
The origins and geochemical behavior of arsenic in a fractured bedrock aquifer, New Hampshire.
No full textElevated arsenic concentrations in a fractured crystalline bedrock aquifer in Bow, New Hampshire are the result of weathering of arsenopyrite (FeAsS) in granitic pegmatites and are controlled by pH dependant adsorption/desorption reactions on iron oxyhydroxide surfaces. Arsenic concentrations were measured in 1,119 water samples using online hydride generation ICP-MS. In randomly selected households, concentrations ranged from 4 · 2H2O) was observed in aquifer materials. Groundwaters in contact with these lithologic units have arsenic concentrations that range from 0.020 mug/L to 400 mug/L. Sulfate is observed in excess of iron in the groundwaters and probably indicates the loss of iron as an oxyhydroxide precipitate, which then affects arsenic mobility via adsorption/desorption reactions. At pH values between 5 and 7, iron oxyhydroxides adsorb arsenic oxyanions and limit arsenic concentrations in solution. At pH values greater than 7, iron oxyhydroxides have a positive surface charge, do not adsorb arsenic, and therefore results in higher concentrations of dissolved arsenic. The distribution of arsenic species (As(III) or As(V)) in water samples dominated by only one of these species is in equilibrium with measured Eh-pH conditions, while samples with >20% of both As(IH) and As(V) are out of equilibrium. This disequilibriurn may be the result of recent changes in the oxidation/reduction status of the water caused by mixing of waters or movement of groundwater into a different redox zone.PhDEarth SciencesEnvironmental scienceGeochemistryGeologyHealth and Environmental SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124413/2/3002548.pd
Silicate versus carbonate weathering in the Himalaya Mountains and the New Zealand Southern Alps.
No full textThe research presented in this dissertation examines how the relative dominance of silicate versus carbonate weathering varies both spatially and temporally in two rapidly uplifting, young orogenic belts---the Himalaya Mountains and the New Zealand Southern Alps. Silicate rocks in the Pakistan Himalaya contain small amounts of disseminated calcite precipitated by hydrothermal fluids during recent uplift of the region. Disseminated calcite has a wide range of Ca/Sr and 87Sr/ 86Sr ratios, with the latter similar to values measured within the silicate host rock. The rapid weathering of disseminated calcite with high Ca/Sr and 86Sr/86Sr ratios strongly influences stream water chemistry and exceeds contributions from silicate dissolution. Analyses of a glacial soil chronosequence show that carbonate weathering continues to dominate the dissolved flux of Ca2+ and Sr2+ from stable landforms for at least ∼55 kyr after the exposure of rock surfaces, even though carbonate represents only a minor proportion of fresh glacial till. Findings from northern Pakistan were used as a model to evaluate silicate vs. carbonate weathering throughout the Himalaya. Himalayan streams are predominantly Ca2+-Mg2+-HCO3- waters derived from carbonate dissolution. The dominance of carbonate weathering is less apparent when Ca/Sr and 86Sr/86Sr are considered. However, Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, suggesting preferential removal of Ca 2+ by calcite precipitation. By accounting for the nonconservative behavior of Ca2+ mass-balance calculations demonstrate that carbonate weathering dominates both the solute and Sr isotope geochemistry of Himalayan streams. The New Zealand Southern Alps provided an opportunity to examine the effects of climate, tectonics, and bedrock mineralogy on chemical weathering. Similar to the Pakistan Himalaya, hydrothermal calcite weathering in the Southern Alps dominates the solute and Sr isotope geochemistry of stream waters. Streams underlain by highly metamorphosed bedrock in regions of active uplift have the highest abundance of carbonate-derived solutes. The results presented in this dissertation suggest that the Sr isotope equilibration between silicate and carbonate minerals during uplift and the relatively rapid rate of carbonate versus silicate weathering may be an important process linking orogenesis to patterns of river water chemistry.PhDBiogeochemistryEarth SciencesGeochemistryGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/127800/2/3029354.pd
The isotope geochemistry of calcium and strontium in tree tissues as a monitor of calcium sources and cycling in a base-poor forested ecosystem, New Hampshire, United States.
No full textCalcium is leached from soil exchange pools by acid deposition, potentially impacting forest productivity, soil fertility, and water quality. This dissertation examines the biogeochemistry and cycling of Ca in terrestrial ecosystems and includes studies conducted in base-poor northeastern forests. In the Hubbard Brook Experimental Forest, a whole-watershed Ca addition with Ca/Sr and 87Sr/86Sr distinct from natural sources within the watershed, permits tracking of Sr, and by proxy Ca, as it cycles through the ecosystem. Incorporation of Ca and Sr into plant tissue was examined, and it was determined that relative to the exchange pool of plant-available nutrients, Ca is preferentially allocated to foliage formation, whereas Sr is sequestered in wood. A discrimination factor (DF) was defined, with values greater than unity indicating preferential uptake of Ca over Sr. Foliar DFs estimated for dominant species include 1.14 for sugar maple, 1.31 for yellow birch, 1.33 for white birch, 1.43 for pin cherry, 1.78 for American beech, and 1.93 for red maple. Understanding the degree to which Ca is fractionated from Sr in different species allows for refinement in the use of 87Sr/ 86Sr and Ca/Sr ratios to trace Ca sources to foliage. The fidelity of tree ring chemistry in sugar maple and red spruce was investigated as a potential biomonitor of past soil chemistry using Sr isotopes. It was found that Sr, and by proxy Ca, are laterally re-equilibrated in a >30 year window in both sugar maple and red spruce, demonstrating that tree rings cannot be used as a precise chronometer of soil chemistry. A watershed-scale investigation of Ca isotopic compositions was carried out and delta44/42Ca values were found to vary by >2‰ in ecosystem pools. American beech and sugar maple bark have a delta 44/42Ca of -1.62 (+/-0.08)‰, which is significantly different from wood, which has a value of delta44/42Ca (1.14 +/-0.14‰). delta 44/42 Ca values of lysimeter water (-0.66‰) and stream water (0.32‰) are significantly different from each other and from bark and wood. No change in Ca isotopic composition was observed with increasing height in trees in either wood or bark.PhDBiogeochemistryEarth SciencesGeochemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126132/2/3237940.pd
Source apportionment of mercury to the atmosphere from wildfires and other sources in the western USA.
No full textMercury (Hg) is a toxic trace metal with globally distributed emissions, and the potential to bioaccumulate in terrestrial and aquatic animals. This dissertation characterizes source apportionment of Hg to the atmosphere with the goal of better understanding the global Hg cycle in order to mitigate the bioaccumulation of methylmercury in terrestrial and aquatic ecosystems. Mercury emissions from wildfires in northwestern Wyoming and central Washington were investigated. Mercury release during wildfires of low to high fire severity were quantified based on comparisons of total soil Hg and foliage at unburned and recently burned forests. At the Rex Creek Fire (central Washington) we compare the results of this method with previous estimates of Hg release based on atmospheric sampling of Hg and CO in smoke. The discrepancy between these estimates suggest that they are measuring different components of Hg release during wildfires. We use the difference between these estimates to suggest that ∼1/2 of the Hg emitted was rapidly deposited locally downwind by scavenging of Hg by coarse particulates and impaction with rugged topography. Source apportionment of Hg to Camp Davis, Wyoming was investigated. Total gaseous Hg (TGM), particulate Hg, particulate major and trace metals in ambient atmospheric particles, and wet Hg deposition were collected during two intensive summer sampling seasons in 2002 and 2003. Principal components analysis (PCA) of particulate data and back-trajectories of air masses indicate that easily eroded soils and exposed rock formations contribute in addition to a combination of production, wildfires, cement production together account for ∼1/4 of the pHg and TGM concentrations observed. The results of the PCA indicate the presence of a regional Hg source contributed ∼75% of the pHg and TGM in ambient air at Camp Davis. We present the first high precision stable isotopic measurements of mercury isotopes (by MC-ICP-MS) in a series of soils, peats, and coals. The variety of sample types analyzed during this study have permitted a preliminary Hg mass balance, based on Hg isotopic compositions, and indicate that approach will be useful in identifying the pathways by which Hg is distributed on the Earth's surface.PhDApplied SciencesBiogeochemistryBiological SciencesEarth SciencesEnvironmental engineeringForestryGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126361/2/3253217.pd
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
Mineral weathering in soil as a function of landscape position, vegetation and fungi at the Hubbard Brook Experimental Forest, New Hampshire.
No full textMineral weathering provides nutrients to vegetation, neutralizes acid deposition, and controls soil water composition. In the northeastern USA, acid deposition has leached exchangeable calcium, an essential plant nutrient, from soil. This dissertation investigates factors that affect the replenishment of exchangeable calcium by mineral weathering, especially landscape position, vegetation type and fungi, at the Hubbard Brook Experimental Forest (HBEF), New Hampshire. Long-term mineral weathering rates, calculated from changes in soil chemical composition compared to parent material, are twice as rapid at higher elevations where conifers are abundant than at lower elevations where deciduous trees are dominant. The average weathering rate is 35 meq (Ca+Na+Mg+K) m -2 yr-1 - one-third the current rate of base cation loss from the watershed via stream flow. This confirms that the pool of exchangeable base cations in HBEF soil is being diminished. The relative availability of different Ca-bearing minerals to the plant-available pool was investigated. A sequential extraction method was developed to identify the following soil pools of Ca: exchangeable pool, easily-weathered minerals and weathering-resistant minerals. The weathering of apatite [Ca5(PO 4)3(F,Cl,OH)], a trace mineral in HBEF soil, was found to account for ∼20% of Ca loss from HBEF during the last 14,000 years. To explore the distribution of apatite in soils across the northeastern USA, the sequential extraction procedure was performed on several types of soil parent material collected from 18 locations. Apatite, identified by a Ca:P ratio of 5:3 in a 1M HNO3 extract, is present in soils developed on crystalline-silicate rocks. Soils developed on sedimentary rocks do not contain appreciable amounts of apatite. Fungal response to Ca- and P-bearing minerals in soil was also examined. Mesh bags containing apatite or wollastonite (CaSiO3) were buried in HBEF soils for two growing seasons. Fungal biomass, quantified as phospholipid fatty acid content, increased in bags containing apatite beneath beech stands. Fungi in a mixed spruce-fir stand showed no response to either Ca-bearing mineral. Fungal biomass in sugar maple decreased in wollastonite-amended bags relative to a control.PhDBiogeochemistryEarth SciencesGeochemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126245/2/3238042.pd
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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