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The chemistry of fine-grained terrigenous sediments reveals a chemically evolved Paleoarchean emerged crust
Full text linkThe nature of the rocks exposed to weathering and erosion on continents exerts an important control on weathering feedbacks and the supply of nutrients to the oceans. It also reflects the prevailing tectonic regime responsible for the formation of continents. How the chemical and lithological compositions of the continents evolved through time is, however, still a matter of debate. We use an extensive compilation of terrigenous sediment compositions to better constrain the nature of rocks at the surface of continents at 3.25 Gyr and 250 Myr ago. Specifically, we use geochemical ratios that are sensitive indicators of komatiite, mafic, and felsic rocks in the provenance of the sediments. Our results show that the average Al2O3/TiO2 ratio of fine-grained terrigenous sediments decreased slightly over time from 26.2 ± 1.3 in the Archean to 22.1 ± 1.1 (2SE) in the Phanerozoic. In contrast, in the same time interval, the average Zr/TiO2 ratio stayed nearly constant at ∼245. Considering the distinct behaviors of Al, Ti and Zr during sedimentary processes, we find that hydrodynamic mineral sorting had a minor effect on the chemical composition of Archean fine-grained sediments, but could have been more effective during periods of supercontinents. We show that the compositions of Phanerozoic sediments (Al2O3/TiO2, Zr/TiO2, La/Sc, Th/Sc, Ni/Co, Cr/Sc) are best explained with igneous rocks at the surface of continents consisting of 76 ± 8 wt% felsic, 14 ± 6 wt% Arc-basalts and 10 ± 2 wt% within-plate basalts, most likely in the form of continental flood basalts. Applying the same mass-balance calculations to the Paleoarchean suggests continental landmasses with 65 ± 7 wt% felsic, 25 ± 6 wt% mafic and 11 ± 3 wt% ultramafic rocks (all 2SE), likely in the form of komatiites. The presence of volumetrically abundant felsic rocks at the surface of continents (as evident from the sediment record) as well as at mid-crustal levels (as evident from presently exposed igneous rock record) in Paleoarchean cratons is currently best explained with the onset of subduction magmatism before 3.25 Gyr
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
Minéraux du gisement de molybdénite de l'Alpjuhorn dans le Baltschiedertal (VS)
No full textIm Zuge einer Masterarbeit an der Universität Bern und am Naturhistorischen Museum der Burger- gemeinde Bern wurde das Molybdänitvorkommen am Alpjuhorn im Baltschiedertal untersucht. Neben Molybdänit wurden auch Scheelit, Ilmenit, Galenit, Ferberit und die Sekundärmineralien Ferrimolybdit und Powellit gefunden. Speziell am Galenit des Alpjuhorns ist, dass er eine oktaedrische Spaltbar- keit aufweist. Das Auftreten von Ferberit ist insofern erwähnenswert, da aus der Schweiz nur wenige Funde von Mineralien der Wolframit-Reihe (Ferberit- Hübnerit) bekannt sind.Le gisement de molybdénite de l’Alpjuhorn dans le Baltschiedertal a été étudié dans le cadre d’un travail de master entrepris à l’Université de Berne et au Musée d’Histoire Naturelle de la Bourgeoisie de Berne. Outre la molybdénite, d’autres minérali- sations y ont également été trouvées: scheelite, ilménite, galène, ferbérite et les minéraux secondaires ferrimolybdite et powellite. La galène de l’Alpjuhorn est particulière par le fait qu’elle montre un clivage octaédrique. La présence de ferbérite est remarquable dans la mesure où très peu de découvertes de minéraux de la série de la wolframite (ferbérite-hübnérite) sont connues en Suisse
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
Magmatic–hydrothermal molybdenum isotope fractionation and its relevance to the igneous crustal signature
Full text linkWe analysed the Mo isotope composition of a comprehensive series of molybdenite samples from the porphyry- type Questa deposit (NM, USA), as well as one rhyolite and one granite sample, directly associated with the Mo mineralization. The δ98Mo of the molybdenites ranges between −0.48‰ and +0.40‰, with a median at −0.05‰. The median Mo isotope composition increases from early magmatic (−0.29‰) to hydrothermal (−0.05‰) breccia mineralization (median bulk breccia = −0.17‰) to late stockwork veining (+0.22‰). Moreover, variations of up to 0.34‰ are found between different molybdenite crystals within an individual hand specimen. The rhyolite sample with 0.12 μg g−1 Mo has δ98Mo = −0.57‰ and is lighter than all molybde- nites from the Questa deposit, interpreted to represent the igneous leftover after aqueous ore fluid exsolution. We recognize three Mo isotope fractionation processes that occur between about 700 and 350 °C, affecting the Mo iso- tope composition of magmatic–hydrothermal molybdenites. Δ1Mo: Minerals preferentially incorporate light Mo isotopes during progressive fractional crystallization in subvolcanic magma reservoirs, leaving behind a melt enriched in heavy Mo isotopes. Δ2Mo: Magmatic–hydrothermal fluids preferentially incorporate heavy Mo iso- topes upon fluid exsolution. Δ3Mo: Light Mo isotopes get preferentially incorporated in molybdenite during crys- tallization from an aqueous fluid, leaving behind a hydrothermal fluid that gets heavier with progressive molybdenite crystallization. The sum of all three fractionation processes produces molybdenites that record heavier δ98Mo compositions than their source magmas. This implies that the mean δ98Mo of molybdenites published so far (~0.4‰) likely represents a maximum value for the Mo isotope composition of Phanerozoic igneous upper crust
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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