1,721,006 research outputs found
The Bonarelli level (Gubbio, Italy) reveals a potential correlation between the occurrence of oceanic anoxic events and large-scale magmatic activity
No full textThe origin of nitrogen in Earth's mantle: constraints from basalts 15 N/ 14 N and N2/ 3 He ratios
No full textInternational audiencePlate tectonics is thought to be a major driver of volatile redistribution on Earth. The budget of nitrogen in Earth's mantle has been suggested to be almost entirely surfacederived. Recycling would contribute nitrogen with relatively heavy 15 N/ 14 N isotope ratios to Earth's mantle. This could explain why the Earth's mantle 15 N/ 14 N isotope ratio is substantially higher than both solar gases and chondritic parent bodies akin to enstatite chondrites. Here, published nitrogen isotope data of mid-ocean ridge and ocean island basalts are compiled and used to evaluate the nitrogen subduction hypothesis. Nitrogen isotope ratios are used in conjunction with published N2/ 3 He and K2O/TiO2 ratios on the same basalts. Assuming that 3 He is not recycled, N2/ 3 He ratios are argued to trace nitrogen addition to mantle sources via subduction. Various mantle source enrichments for basalts are tracked with K2O/TiO2 ratios: elevated K2O/TiO2 ratios are assumed to primarily reflect the contributions of recycled components in the basalts mantle sources. The main result of our data compilation is that for most basalts, d 15 N and N2/ 3 He remain constant across a vast range of K2O/TiO2 ratios. Mid-ocean ridge basalts have d 15 N signatures that are lower than air by ~ 4‰ and an average N2/ 3 He ratio of 3.7 (±1.2) x10 6 (95% confidence, n= 30). Published d 15 N and N2/ 3 He are invariant across K2O/TiO2 ratios that vary over a factor of ~ 20. Using estimates of slab K2O/TiO2 and [TiO2], the observed invariant d 15 N and N2/ 3 He ma
Methane thermometry in deep-sea hydrothermal systems: Evidence for re-ordering of doubly-substituted isotopologues during fluid cooling
No full textInternational audienceDeep-sea hydrothermal fluids are often enriched in carbon dioxide, methane, and hydrogen. Methane effuses from metal-rich black smokers such as the Rainbow hydrothermal field, at temperatures higher than 200 °C. At the Lost City field, CH4 emanates from alkaline fluids at < 100 °C. The abundance of the rare, mass-18 CH4 isotopologues, 13 CH3D and 12 CH2D2, can mitigate degeneracies in the conventional isotopic signatures of methane. We studied the isotopologue compositions of methane from the Rainbow, Lucky Strike, Von Damm, and Lost City hydrothermal fields. At Rainbow, where the vented fluids are at ~360°C, our coupled D 12 CH2D2-D 13 CH3D data establish that methane is in internal equilibrium at 343 #$% &'(°C. This may track the formation temperature of abiotic methane, or it may be the result of equilibration of methane isotopologues within the carrier fluid. Lucky Strike and Von Damm have fluid temperatures < 300°C and although D 13 CH3D values are indistinguishable from those at Rainbow, 12 CH2D2 abundances are marginally higher. At Lost City, D 13 CH3D data show a range of values, which at face value correspond to apparent temperatures of between 265 #,' &,-°C an
Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen
No full textInternational audienc
Origine, dynamique et multi-compositions isotopiques du soufre dans le manteau terrestre
No full textLa teneur et composition isotopique du soufre du manteau terrestre peut apporter des contraintes majeures sur la dynamique interne de cet élément ainsi que sur la nature des hétérogénéités mantelliques. La composition isotopique moyenne de ce réservoir en S peut également renseigner sur le potentiel équilibre entre manteau et noyau et donc sur l'origine du soufre sur Terre. Dans cette étude, nous rapportons les teneurs en S et les abondances relatives du 32S, 33S, 34S et 36S dans plus de 120 verres basaltiques provenant de 3 rides océaniques et d'une faille transformante. Nous proposons un nouveau protocole d'extraction du soufre de verres silicaté basé sur l'utilisation d'acide fluorhydrique. A cette occasion, nous mettons en évidence un biais analytique affectant les données précédemment publiées, lié à l'utilisation du réactif dit de "Kiba". Les 34S re-mesurés sur des échantillons précédemment analysés sont en moyenne 1 plus bas. Nous fournissons également les premièresdonnées de 33S et 36S de basaltes de dorsales. Ces données sont, dans l'erreur, indistinguables du standard international CDT, confirmant l'idée que le manteau a des 33S et 36S proches de 0.000 . Le 34S moyen des MORB est de -0,80 +- 0,38 , significativement inférieur à la valeur chondritique de 0,04 +- 0,31 , et montre une gamme de variation de près de 3 . Celui-ci présente des corrélations avec les isotopes radiogéniques, faisant du 34S un traceur de source. Les covariations entre 34S et isotopes du Plomb dans des basaltes de la ride Pacifique-Antarctique suggèrent le recyclage de croûte océanique portant un 34S positif au sein d'un manteau appauvri significativement négatif, que nous proposons à -1,40 +- 0,20 . Dans les basaltes de la ride Atlantique-Sud, le 34S est corrélé avec le rapport isotopique du Sr et les tendances isotopiques mettent en évidence le recyclage de sédiments dans le manteau source des basaltes étudiés. Dans l'ensemble, la variabilité du 34S associée à l'absence de variabilité du 33S et 36S montre que les composants recyclés sont d'âge post-archéens, en accord avec l'estimation de leur âge classiquement faite via les rapports isotopique du Pb. Enfin, le 34S négatif du manteau source des MORB peut être réconcilié avec un équilibre manteau-noyau, impliquant une origine interne du soufre par opposition à une origine tardive.Sulfur content and isotope composition of the terrestrial mantle can provide major constraints on the internal dynamic of this element and on nature of mantle heterogeneities. The mean S isotope composition of this reservoir can also characterize the potentiel mantle-core equilibrium and hence help to decipher the question of sulfur origin on Earth. In this study, we report S content and relative abundances of 32S, 33S, 34S and 36S in more than 120 glassy basalts coming from 3 ocean ridges and a transform fault. We propose a new sulfur extraction protocol for silicate glasses based on hydrofluoric acid. We evidenced an analytical bias affecting the previously published dataset, probably due to the use of the e"Kiba reagent". The redetermined 34S are approximately 1 lower than the previously published data. We also provide the first 33S and 36S data for ocean ridge basalts. These date are, within uncertainty, indistinguishable from our CDT estimate, confirming the idea that terrestrial mantle has a 33S and 36S close to 0.000 with respect to this international standard. The 34S variability encompasses a 3 range in ocean ridge basalts and correlations between O34S and radiogenic isotopes demonstrate that S isotopes are tracers of mantle heterogeneities. On the other hand, the mean 34S of MORB mantle is -0,80 +- 0,38 , significantly lower than the chondritic value of 0,04 +- 0,31 . The Pb-S isotope systematic in Pacific Antarctic basalts indicates that recycled oceanic crust bears a positive 34S. As a corrollary, this result implies that the depleted part of the mantle is even more negative thant the mean MORB value. We propose a 34S value of -1,40 +- 0,20 for this reservoir. In South Atlantic basalts, the 34S are correlated with Sr isotopes, trends being consistent with recycled sediment feeding the mantle source of these basalts. Taken together, the 34S variability associated to the 33S and 36S constancy indicates that the recycled components are a post-archean age, in agreement with independant estimation of their mean age using Pb isotopes. Finally, the negative 34S of MORB mantle can be reconcilied only with a mantle-core equilibrium, implying an internal origin of sulfur as opposed to a late origin.PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF
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
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