1,721,118 research outputs found
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Influence of radiation damage on He diffusivity in apatite: implications and applications to low-temperature thermochronology
Full text linkThe radiogenic production and subsequent temperature-dependent diffusion of 4He in natural apatites provides means of constraining the thermal history of samples in the upper few kilometers of Earth’s crust. This technique, known as apatite (U–Th)/He thermochronology, has come into wide use in the past twenty years to quantify both rates and patterns of erosion in terrestrial settings. Critically important to the interpretation of these data, however, is the understanding of the diffusion of He and the effects of radiation damage over geologic time. Chapter 1 of this dissertation provides the background and basic concepts underlying the apatite (U–Th)/He thermochronology technique and describes the fundamental challenge of accounting for radiation damage when applying this technique. Chapter 2 describes one application of apatite (U–Th)/He thermochronology to a novel detrital study of glacially- transported cobbles in central Patagonia. This chapter provides evidence of a transient fast pulse of glacial incision with the onset of periodic glaciation in the region after ∼6 million years ago. As with all geochemical techniques used to study Earth science, the necessary assumptions made when interpreting data can have a consequential effect on the conclusions. Chapter 3 of this dissertation revisits a critical assumption—one regarding the role of radiation damage—that is made in the most often-used data interpretation models and then develops and proposes an alternate model (the alpha damage annealing model, or ADAM). In certain cases, but not all, the comparison between the ADAM and other radiation damage models shows that the radiation damage assumption can greatly affect the conclusions drawn from a data set. Additional experimental work aimed at improving the ADAM is described in Chapter 4, which finds that sample-dependent diffusion kinetics may be the key to interpreting challenging apatite (U–Th)/He thermochronologic data sets
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
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Reconstructing past Earth and planetary surface temperatures with cosmogenic noble gases
Full text linkCosmogenic nuclides, which are produced in the uppermost few meters of the Earth's crust by cosmic-ray particle interactions with atomic nuclei, are commonly used to quantify the rates and timing of surface processes. Some of the first terrestrial cosmogenic nuclide measurements revealed that the cosmogenic noble gases 3He and 21Ne are diffusively lost at Earth surface temperatures in common silicate minerals like quartz and feldspars. Viewed as a fatal limitation for geologic applications since then, the open-system behavior of cosmogenic noble gases can, in fact, be exploited to quantitatively reconstruct temperatures at the surfaces of Earth and other planetary bodies.In Chapter 1, I develop a theoretical framework for using cosmogenic noble gases as a paleothermometer based on the principles and mathematics underlying radiogenic noble gas thermochronometry. With this framework and published information on the diffusion kinetics of helium and neon in quartz and feldspars, I demonstrate that cosmogenic 3He–in–quartz measurements could be used to constrain past surface temperatures at high latitudes and elevations on Earth, while 21Ne–in–feldspar measurements could be used to constrain past surface temperatures at lower latitudes and elevations, and on other planetary bodies.I then explore the applicability of these published diffusion kinetics through a series of stepwise degassing experiments on quartz (Chapter 2) and feldspars (Chapter 3) containing initially uniform distributions of proton-induced 3He and 21Ne. These experiments reveal that 3He and 21Ne diffusion kinetics vary significantly across samples of different geologic origin, and that in many cases quartz and feldspars exhibit complex diffusion behavior mani- fest as nonlinearity in Arrhenius plots. The origin of this complex behavior is indeterminate, but I demonstrate that it is not caused by temperature-dependent structural transformations or anisotropy and that it is not an artifact of proton irradiation. Instead, complex diffusion behavior appears to be controlled by some intrinsic, sample-specific material property. I also demonstrate that we can mathematically model complex diffusion behavior, and use geologic examples with simple exposure and temperature histories to validate this mathematical model.Having laid out the theoretical and experimental backbone of cosmogenic noble gas paleothermometry, in Chapter 4 I present two applications of the technique to problems in paleoclimate and planetary science. In the first application, I use cosmogenic 3He and 10Be observations in quartz from a series of nested moraines in the Maritime Italian Alps to reconstruct temperatures since the Last Glacial Maximum (LGM). I demonstrate that temperatures reconstructed from the cosmogenic 3He observations are consistent with temperatures expected for this region since the LGM from a global circulation model (GCM) and other proxy data, but that additional constraints are necessary to fully interpret this dataset. In the second application, I use observations of cosmogenic neon isotopes in plagioclase feldspars from lunar sample 76535 to demonstrate that this sample only experienced solar heating during its 142 million year residency at the lunar surface. This constraint on the thermal history of 76535 agrees with existing argon measurements and confirms the fidelity of paleomagnetic measurements in the same sample, which have been used to demonstrate that the Moon had an early core dynamo
Variations on the Author
Full text link“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
Full text linkWe 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
Full text linkWe 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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Geochronologic Constraints on Earth Surface Processes and Their Response to Climate Change
Full text linkEarth’s topography is constantly evolving in response to surface processes that are modulated by changes in climate, tectonics, and sea level. Geochronology can used to identify the timing and rates of past Earth surface change and thereby advance our understanding of how these changes relate to different Earth processes that interplay over different geologic timescales. In this thesis, I contribute new geochronologic constraints on the extent to which Earth surface processes varied in the past and how that variation relates to past climate change. Chapter 1 describes the fundamentals of geochronology and provides an overview of the specific techniques used in this thesis. In Chapter 2, I present a quantitative framework for determining the timing and patterns of km-scale topographic change at the high-latitude Antarctic Peninsula using detrital apatite (U-Th)/He thermochronometry and 3D thermo-kinematic numerical modeling. While Plio-Pleistocene cooling accelerated glacial erosion rates at many mid-latitude glacial landscapes, my results reveal that Plio-Pleistocene cooling suppressed glacial erosion at the Antarctic Peninsula. These results highlight that landscapes at different latitudes had different responses to global cooling. In Chapter 3, I apply the approach developed in Chapter 2 to seven additional fjords along a latitudinal transect at the Antarctic Peninsula to investigate how the timing of km-scale topographic change relates to changes in climate and tectonics. At this site, the onset of km-scale topographic change occurs more than 15 million years after the initiation of glaciation and generally covaries with the arrival time of spreading ridge. These are among the first empirical results to show that a tectonic history, and its control on the regional topography of a landscape, has influenced glacial erosion rates over geologic timescales. In Chapter 4, I use the uranium-series comminution age technique to constrain sediment transport times to the Bengal Fan over the last 200 thousand years. I find pronounced variability in transport time that appears to be modulated by the climate and hydrological changes associated with the Late Pleistocene climate cycles. Because sediment transport times are more than 100 times smaller than the age of the Bengal Fan, Himalayan sediment transport keeps pace with erosion on million-year timescales. Each chapter demonstrates that the response of Earth surface processes to climate change depends on the geologic history of landscape. Thus, the past not only informs on the extent to which climate change can perturb Earth surface processes; the past influences how Earth surface processes will respond to future climate change
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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