1,720,975 research outputs found
Late quaternary tephra layers around Raoul and Macauley islands, Kermadec arc: implications for volcanic sources, explosive volcanism and tephrochronology
No full textA suite of deep-sea cores were collected along transects up to 100?km across the fore-arc and back-arc regions of the predominantly submarine Kermadec arc near Raoul and Macauley islands, southwest Pacific. The cores reveal a macroscopic tephra record extending back >50?ka. This is a significant addition to the dated record of volcanism, previously restricted to fragmented late Holocene records exposed on the two islands. The 27 macroscopic tephra layers display a wide compositional diversity in glass (?50–78?wt% SiO2). Many tephra layers comprise silicic shards with a subordinate mafic shard population. This could arise from magma mingling and may reflect mafic triggering of the silicic eruptions. Broadly, the glass compositions can be distinguished on diverging high-K and low-K trends, most likely arising from different source volcanoes. This distinction is also reflected in the tephra records exposed on Raoul (low-K) and Macauley (high-K) islands, the likely source areas. Heterogeneous tephra comprising shards of both high- and low-K affinity, silicic and mafic compositions, and more homogeneous tephra with subordinate outlier shard compositions, are best explained by post-depositional mixing of separate eruption deposits or contemporaneous eruptions. Evidently, the slow sedimentation rates of the calcareous oozes (?101–102?mm?ka?1) were insufficient to adequately separate and preserve closely spaced eruption deposits. This exemplifies the difficulty in assessing eruption frequencies and magmatic trends, and erecting a tephrostratigraphy, using geochemical fingerprinting in such environments. Despite these difficulties, the ca. 5.7?ka Sandy Bay Tephra erupted from Macauley Island can be correlated over a distance of >100?km, extending east and west of the island, showing that the mostly submerged volcanoes are capable of wide tephra dispersal. Hence there is potential for developing chronostratigraphies for the southwest Pacific beyond the region covered by the extensive rhyolite marker beds from the Taupo Volcanic Zone
Submarine silicic volcanism of the Healy caldera, southern Kermadec arc (SW Pacific): I - volcanology and eruption mechanisms
No full textThe submarine Healy volcano (southern Kermadec arc), with a 2-2.5 km wide caldera, is pervasively mantled with highly vesicular silicic pumice within a water depth of 1,150-1,800 m. Pumices comprise type 1 white-light grey pumice with h30 mm vesicles and weak-moderate foliation, type 2 grey pumice with millimetre-scale laminae, flow banded foliation, including stretched vesicles h55 mm in length, and a minor finely vesicular type 3 pumice. All types are sparsely porphyritic, with undevitrified glassy groundmass (68-70% SiO2), which is microlite and lithic free. Coexisting pyroxenes yield magma temperatures of ~950 °C. Pumice density is h0.5 g cm-3 and vesicularity is 78-83%. Vesicle size distributions for types 1 and 2 pumice, range from ~20 µm to >20 mm, with a strong power-law relation (with d=-2.5-0.4) for vesicles <1-2 mm. Larger vesicles have variable size modes. The vesicle size distribution and packing indicates rapid magma decompression and ascent. Consideration of the pressure dependent, solubility of H2O at a magma temperature of h950 °C and water content of h6 wt%, with pumice petrography and vesicle granulometry, strongly suggests a pyroclastic eruption. Reconstructions of the submarine edifice between water depths of 1,000 and 550 m constrain the ambient hydrostatic pressure to ~6-9 MPa. Pressures >~9 MPa will limit vesicularity to less than the observed 78-83%, whereas pressure <~6 MPa require a more shallower reconstruction of the edifice and larger-volume syn-eruptive collapse. Uniformly high vesicularity is interpreted as evidence of insulation within an eruption column comprising steam and hot pyroclasts. Most pyroclasts cool, condensing and ingesting water into steam-inflated vesicles, and then sink. Progression into pyroclastic mode would expand the eruption column, displace ambient water, reduce the hydrostatic load, and further promote vesiculation and fragmentation. Pyroclasts within the column would quench at these reduced pressures. We argue that Healy eruptions deeper than ~1,000 m cannot be pyroclastic. Volumes for the lower and upper bounds of edifice size are 2.36 and 3.58 km3, respectively, but do not account for intra-caldera pumice fill. These volumes are considered to be predominantly primary eruption output, as shown by a dearth of accessory lithics in all pumice, yielding (at an average 81% vesicularity) eruptive pumice volumes of between 10 and 15 km3. Some pyroclasts may have risen to the sea surface and be a correlative of the sea-rafted Loisels pumice; the latter occurs in some New Zealand Holocene beach sequences and has a estimated age of 590-80 calendar years.<br/
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
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
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
Isotopic and Geochemical Records of Magmatic Processes Captured in Crystals from Rhyolites at Okataina Volcano
No full textThe Okataina Volcanic Centre (OVC) is a dormant silicic caldera in the Taupo Volcanic Zone of North Island, New Zealand that has been active since ca. 0.6 Ma. Eruption episodes at the OVC occur on a millennial scale and range from rhyolitic caldera-forming and dome-building events to mafic fissures. To understand how mafic and crustal inputs into magma genesis have varied across the life of the volcano, isotopic fluctuations in silicic minerals from ten temporally, volumetrically, and spatially varied rhyolite units (two caldera-forming, one syn-caldera, four intra-caldera, and three extra-caldera) were investigated. This study examines textures and compositions of major mineral phases along with, for the first time at the OVC, their sub-crystal isotopic compositions. Sub-crystal isotopic investigations consist of ⁸⁷Sr/⁸⁶Sr ratios within plagioclase, a ubiquitous and early-crystallizing phase, and δ¹⁸O within quartz, an abundant later-crystallizing phase. Results suggest diverse origins and transport of plagioclase and quartz cores prior to assembly in the erupting magma. In contrast, the outermost rims of plagioclase and quartz suggest crystallization in more uniform melts. Despite geochemical variations, sub-crystal isotopic compositions are homogeneous in plagioclase (⁸⁷Sr/⁸⁶Sr = ~0.705-0.706) and quartz (δ¹⁸O = ~7.6‰) to within the resolution of this study (±0.0004 average 2σ and ± 0.12‰ average 2sd, respectively). Higher-precision whole crystal Sr and Pb isotopes in plagioclase and oxygen isotopes in quartz were undertaken as well. These analyses also demonstrate prevalent homogeneity across the magmas, although variations in Pb isotopes are noted in caldera-forming magmas. Isotopic mixing models using sub-crystal and whole crystal isotopic ratios of plagioclase and quartz imply likely origins from subduction-modified basaltic melts that experienced ≥20% contamination from regional Torlesse metasediments. However, variations in mineral and whole rock ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb suggest contributions from additional sources are likely (e.g., Waipapa metasediments, metaigneous lower-crust, and/or unknown lower crust). Findings from this study demonstrate fairly consistent radiogenic and stable isotopic homogeneity across the history of this volcanic system, indicating steady inputs from mantle and crustal sources over time. This contrasts with other silicic systems that often exhibit dramatic shifts in isotopic compositions in pre- or post- caldera-forming eruptions (e.g., Yellowstone, Toba)
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