1,721,023 research outputs found
Provenance and Evolution of the Yangtze River constrained by Detrital Minerals
Full text linkWijbrans, J.R. [Promotor]Kuiper, K.F. [Copromotor
Improving the precision of single grain mica 40Ar/39Ar-dating on smaller and younger muscovite grains: Application to provenance studies
Full text linkCurrent generation multi-collector mass spectrometers allow for increasingly more precise measurements of small ion beams. The improvement of instrument sensitivity and resolution compared with older generation mass spectrometers has important implications for 40 Ar/ 39 Ar dating and allows the expanding of its range of applicability. Thermochronological analysis of detrital modern river sands is a powerful tool for unraveling provenance and exhumation histories of eroding hinterlands. Better instrument sensitivity allows refining the precision of dates for young and small grains, which in turn acknowledge an interpretation of the detrital signals from a wider range of micas. Previous studies have used the 40 Ar/ 39 Ar method to assess how the detrital mineral age signals can evolve downstream in the river trunk of an active mountain range. So far, however, there has not been a robust assessment of how grain-size variability can introduce biases in the analysis of age distributions. For example, the white mica signal from the Namche Barwa syntaxis in the eastern Himalaya is interpreted to be diluted downstream from its source due to the admixture of micas from downstream sources to the total population, but grain-size variability biases were not evaluated. Here we use the latest developments in multi-collector noble gas mass spectrometry to (1) test if the precision in the analysis of young and small muscovite samples can be improved by use of new faraday collector amplifier technology and (2) to apply this approach to test the variability of the age distribution as a function of the grain size from five modern rivers samples draining the Eastern Himalaya. The Helix MC plus at VUA is equipped with 10 13 Ohm amplifiers on the H2-H1 Faraday cups. We compare the functioning of the 10 13 Ohm amplifiers with the 10 12 amplifiers on the in-house Drachenfels (DRA) standard. The use of the new 10 13 Ohm amplifiers to measure the 40 Ar and 39 Ar ion beams improved the precision when measuring standards by a factor of two. We show that for larger catchment areas multi grain-size analyses lead to a more complete assessment of the full spectrum of ages obtained from different sources. The analyses of smaller grain sizes ( 250 μm) of the analyzed samples. This outcome potentially has important implications for future provenance studies
Impact of hydraulic sorting and weathering on mica provenance studies:An example from the Yangtze River
Full text linkDetrital muscovite and biotite 40Ar/39Ar analyses are useful tools for studying regional tectonic histories, sediment provenances and paleo-drainage reconstructions. During transport and recycling of detrital micas physical and chemical weathering occurs. This process effects the grain size and age populations ultimately found in river sediments, but is often ignored in provenance studies. Here, we present detrital muscovite and biotite 40Ar/39Ar results of 15 modern sediments from the Yangtze River to address the impact of grainsize on provenance age populations. The beam intensities of 39Ar, formed from 39K by neutron capture reaction during sample irradiation, have been used as an index for grain size. We found that relatively older detrital mica ages of the Yangtze River are often characterized by small 39Ar signals (i.e., grain sizes), and large grain sizes correspond to younger grains. This observation is also revealed by reanalysis of previously reported detrital mica studies in other major river systems (Red and Brahmaputra rivers) and sediments (Scotian Basin, Canada and Antarctic) and probably results from physical and chemical weathering during transport and recycling. Our Yangtze results indicate that detrital muscovite and biotite ages of grainsize ranging from 100 to 1000 μm cover all age components as identified in all dated grains (with a size of >100 μm), and thus indicate that detrital mica 40Ar/39Ar analyses should include also small grains from >100 μm to reduce the effects of hydraulic sorting and weathering. Grainsizes smaller than 100 μm have not been tested in this study, but will be more difficult to date due to both smaller beam intensities and possible recoil effects.</p
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
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
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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