1,721,365 research outputs found
Geochemical differences between along-arc and across-arc volcanics in west-central Nicaragua
No full textThe La Paz Centro - Malpaisillo Lineament (LPML) in west-central Nicaragua is a north-south striking, 20 km long chain of maars and cinder cones, which intersects the northwest-southeast striking main volcanic front. A tectonic control of LPML volcanism is likely but only evident for the Malpaisillo fissure at the northern end of the LPML. Previous work demonstrated geochemical variations implying changes in mantle-source composition (i.e., added slab components) along the Central American Volcanic Arc at spatial scales of some 10’s of kilometers. Our study of the LPML shows that minor but systematic changes also occur across the arc within 20 km distance. Variations in trace element ratios such as Zr/Nb, Ba/Th, Ba/La, Th/Zr, U/La and La/Yb along the LPML, i.e. across the volcanic front indicate little change in the degree of partial melting but an increase particularly in the hemipelagic sediment component in the mantle source from the fore arc towards the arc front, followed by a decrease behind the arc. Interestingly, the slab component is most prominent just in front of the arc.
About 60 km southeast of the LPML, the Nejapa-Miraflores volcanic and tectonic lineament, which marks a 20 km north-south offset in the arc, differs substantially from the LPML. There is a wide scatter in incompatible trace element ratios indicating a heterogeneous mantle source at small spatial scales (c. 1 km). This mantle heterogeneity may represent vertical rather than across-arc variations and is probably related to the arc offset, because in the absence of such offset at the LPML mantle source conditions vary much less but more systematically
Constraining input and output fluxes of the southern Central Chile Subduction Zone: water, chlorine, sulfur
No full textIn this paper, we constrain the input and output fluxes of H2O, Cl and S into the southern-central Chilean subduction zone (31°S–46°S). We determine the input flux by calculating the amounts of water, chlorine and sulfur that are carried into the subduction zone in subducted sediments, igneous crust and hydrated lithospheric mantle. The applied models take into account that latitudinal variations in the subducting Nazca plate impact the crustal porosity and the degree of upper mantle serpentinization and thus water storage in the crust and mantle. In another step, we constrain the output fluxes of the subduction zone both to the subcontinental lithospheric mantle and to the atmosphere–geosphere–ocean by the combined use of gas flux determinations at the volcanic arc, volume calculations of volcanic rocks and the combination of mineralogical and geothermal models of the subduction zone. The calculations indicate that about 68 Tg/m/Ma of water enters the subduction zone, as averaged over its total length of 1,480 km. The volcanic output on the other hand accounts for 2 Tg/m/Ma or 3 % of that input. We presume that a large fraction of the volatiles that are captured within the subducting sediments (which accounts for roughly one-third of the input) are cycled back into the ocean through the forearc. This assumption is however questioned by the present lack of evidence for major venting systems of the submarine forearc. The largest part of the water that is carried into the subduction zone in the crust and hydrated mantle (accounting for two-thirds of the input) appears to be transported beyond the volcanic arc
From marine sediments to explosive eruption record: A case study from New Zealand and a revised methodical approach
Full text linkThe investigation of marine tephra records allows the establishment of most complete long-term eruptive sequences of volcanoes, volcanic centres, and arc systems. These eruptive sequences enable further research of systematic changes, including potential links to external forcing, over entire life cycles of volcanic systems which is a major goal of this thesis. Five deep ocean drilling campaigns targeted the marine sediments offshore New Zealand [...] covering the marine environment towards the East and Southeast of New Zealand [...]. The recovered sediments comprised several intercalated volcanic deposits (i.e. tephra) and therefore enabled the investigation of the marine tephra record from New Zealand’s volcanic arc systems. The result of this investigation is a most complete marine tephrochronostratigraphy of New Zealand’s volcanism reaching back to the Neogene. The marine tephra record is continuous until c. 12 Ma and therefore complements and extends the volcanic onshore record of New Zealand’s volcanoes, which is considered to be well-established until ≤61 ka. [...] This cumulative thesis is based on four independent manuscripts. The first three contributions present the marine tephrochronostratigraphy of New Zealand’s volcanism [...]. The fourth contribution presents a methodical approach designed for marine tephra studies and optimised for large sample quantities and can be used as a “core to correlation” - recipe. [...] The major findings of this thesis are: 1) a continuous marine tephrochronostratigraphy for New Zealand’s volcanic arcs for the last 12 Myr [...], 2) temporal geochemical variations and trendswithin and between the two arc systems, 3) insights into the dispersal, depositional processes and overall preservation of tephra in the marine environment, 4) first estimates of tephra volumes and equivalents [...], and 5) eruption frequencies and repose times [...]
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
Data report: core-seismic integration and time-depth relationships at IODP Expedition 398 Hellenic Arc Volcanic Field sites
No full textWe present seismic two-way traveltime depth relationships for all sites drilled by the International Ocean Discovery Program Expedition 398, Hellenic Arc Volcanic Field, using high-resolution multichannel seismic and core data. First, we filter and interpolate P-wave velocity and density data taken from (1) whole-round cores and (2) discrete measurements on half-round cores. We establish the reliability of shipboard density measurements by comparing them with in situ logging data. Using these validated measurements, we estimate acoustic impedance and synthetic seismograms. By correlating synthetic seismograms with those extracted from multichannel seismic profiles at each site, we establish time-depth relationships. We assess the quality of these relationships by examining the alignment of major lithologic boundaries with prominent unconformities or correlated conformities in the reflection seismic data. The results of this report facilitate the mapping of core data onto the multichannel seismic profiles at each site, allowing for spatial tracing of core data across the Christiana-Santorini-Kolumbo volcanic field
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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