1,720,980 research outputs found
Plant fingerprints in the deep Earth
Full text linkThe colonization of Earth landmasses by vascular plants around 430 million years ago substantially impacted erosion and sediment transport mechanisms. This left behind fingerprints in magmatic rocks, linking the evolution of Earth’s biosphere with its internal processes
Chemical evolution of the continental crust from a data-driven inversion of terrigenous sediment compositions
Full text linkThe nature of emerged continents through time is highly debated. Several studies relying on trace element data concluded that the Archaean crust was predominantly mafic, while Ti isotope systematics point to an Archaean crust that was predominantly felsic. Here, we resolve the inconsistency between these two approaches by applying a novel statistical method to a compilation of published elemental concentrations in terrigenous sediments (the OrTeS database). We use a filter based on the Local Outlier Factor to reject sediment samples that have been affected by alteration processes or mineral fractionation during transport. The nature of the emerged continents is calculated using an inverse mixing model based on a Markov Chain Monte Carlo algorithm. A procedure is presented to automatically select elemental ratios that are best suited for constraining the sediment provenance. We find that for all systems that accurately reconstruct the modern-day composition of the continents, a continuous >50% felsic contribution is required to explain the composition of fine-grained terrigenous sediments starting from 3.5 billion years ago. This finding is consistent with an early onset of plate tectonics. We estimate the geothermal gradient in the Archaean upper continental crust by tracking the reconstructed concentrations of the radiogenic heat-producing elements K, U, and Th through time. Radioactive heat production in the bulk continental crust was 50% higher in the Archaean compared to the present, resulting in a continental geothermal gradient that was about 40% higher
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
Titanium isotopic compositions of bulk rocks and mineral separates from the Kos magmatic suite: Insights into fractional crystallization and magma mixing processes
Full text linkTerrestrial and extraterrestrial rocks exhibit significant variations in their mass-dependent Ti isotopic compositions, with basalts being isotopically lighter than evolved lithologies. The observed trend from light to heavy Ti isotopic compositions from more primitive to more differentiated rocks agrees with theoretical predictions that light Ti isotopes are sequestered in Fe–Ti oxides. However, there are lingering questions about the exact extent of this fractionation and whether it is influenced by the nature of oxides and silicate melt. To improve on this matter, we measured the Ti isotopic compositions of mineral separates and bulk rocks from the calc-alkaline Kos volcano-plutonic system, Aegean arc, Greece. Bulk rock Ti isotopic compositions (δ49Ti) increase with differentiation of the magmatic system, from δ49Ti of +0.042 ± 0.033‰ in basalt to +0.654 ± 0.034‰ in rhyolite. We document two different Ti isotope trends produced by (i) fractional crystallization, and (ii) mixing between a basaltic melt and an evolved (rhyolitic) magma. Trend (i) can be explained by a melt-cumulate Ti isotopic fraction factor α of 0.9998 (i.e., the bulk cumulate is on average 0.20‰ lighter than the melt). The mineral separates reveal variable δ49Ti values, with magnetite having the lightest 49Ti/47Ti isotopic composition, biotite being intermediate and neso- and tectosilicates (i.e., olivine, plagioclase and quartz) heaviest. Comparing the TiO2 concentrations of the low-Ti minerals olivine, plagioclase and quartz determined with LA-ICP-MS and isotope dilution shows that the δ49Ti values measured in these minerals reflect their isotopic compositions, and contamination by inclusions is minimal. The difference in δ49Ti between different minerals is smallest in a basalt (Δ49Tiolivine-magnetite = +0.426) and largest in two rhyolites (Δ49Tiquartz-magnetite = +1.083; both ± 0.046‰). Our data agree with theoretical predictions that Fe–Ti oxides have a light δ49Ti signature, and neso/tectosilicate minerals are heavy. Furthermore, the measured difference in δ49Ti between magnetite-olivine, magnetite-plagioclase and magnetite-quartz agree to first order with theoretically predicted inter-mineral Ti isotopic fractionation factors, thus suggesting that the measured inter-mineral Ti isotopic variations are equilibrium in nature
Constraining the mass-dependent Ti isotope composition of the chondritic reservoir – An inter-laboratory comparison study
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