1,721,199 research outputs found
Reconstructing past seawater Mg/Ca and Sr/Ca from mid-ocean ridge flank calcium carbonate veins
No full textProxies for past seawater chemistry such as Mg/Ca and Sr/Ca ratios provide a record of the dynamic exchanges of elements between the solid Earth, atmosphere and hydrosphere, and the evolving influence of life. Here, we estimate past oceanic Mg/Ca and Sr/Ca ratios from suites of 1.6 to 170-million-year-old calcium carbonate veins precipitated from seawater-derived fluids in ocean ridge flank basalts. Our data indicate that prior to the Neogene, oceanic Mg/Ca and Sr/Ca were lower than in the modern ocean. Decreased ocean spreading since the Cretaceous and the resulting slow reduction in ocean crustal hydrothermal exchange throughout the early Tertiary may explain the recent rise in these ratios
Controls on thallium uptake during hydrothermal alteration of the upper ocean crust
No full textHydrothermal circulation is a fundamental component of global biogeochemical cycles. However, the magnitude of the high temperature axial hydrothermal fluid flux remains disputed, and the lower temperature ridge flank fluid flux is difficult to quantify. Thallium (Tl) isotopes behave differently in axial compared to ridge flank systems, with Tl near-quantitatively stripped from the intrusive crust by high temperature hydrothermal reactions, but added to the lavas during low temperature reaction with seawater. This contrasting behavior provides a unique approach to determine the fluid fluxes associated with axial and ridge flank environments. Unfortunately, our understanding of the Tl isotopic mass balance is hindered by poor knowledge of the mineralogical, physical and chemical controls on Tl-uptake by the ocean crust.Here we use analyses of basaltic volcanic upper crust from Integrated Ocean Drilling Program Hole U1301B on the Juan de Fuca Ridge flank, combined with published analyses of dredged seafloor basalts and upper crustal basalts from Holes 504B and 896A, to investigate the controls on Tl-uptake by mid-ocean ridge basalts and evaluate when in the evolution of the ridge flank hydrothermal system Tl-uptake occurs.Seafloor basalts indicate an association between basaltic uptake of Tl from cold seawater and uptake of Cs and Rb, which are known to partition into K-rich phases. Although there is no clear relationship between Tl and K contents of seafloor basalts, the data do not rule out the incorporation of at least some Tl into the same minerals as the alkali elements. In contrast, we find no relationship between the Tl content and either the abundance of secondary phyllosilicate minerals, or the K, Cs or Rb contents in upper crustal basalts. We conclude that the uptake of Tl and alkali elements during hydrothermal alteration of the upper crust involves different processes and/or mineral phases compared to those that govern seafloor weathering. Furthermore, a correlation between the Tl and S concentrations of upper crustal basalts from Holes U1301B, 504B and 896A indicates that Tl is primarily incorporated into secondary sulfides. Given that some of these secondary sulfides formed as a result of microbial sulfate reduction, microbial action is at least indirectly responsible for Tl-uptake.Thallium-enrichment of ridge flank basalts requires a Tl-bearing fluid and physical, chemical and microbial conditions that favor secondary sulfide formation. Uptake of Tl occurs in reducing environments in the background rocks away from fluid flow pathways during early ‘open’ circulation of oxidizing seawater but more pervasively throughout the system during later ‘restricted’ circulation of reducing fluids. The Tl-isotope system is therefore a useful tracer of the fluid flux through both the ‘open’ and ‘restricted’ ridge flank hydrothermal regimes
Determination of the volcanostratigraphy of oceanic crust formed at superfast spreading ridge: electrofacies analyses of ODP/IODP Hole 1256D
Full text linkThe objective of this study is to construct a representative volcanostratigraphy of Ocean Drilling Program Hole 1256D, the first complete penetration of intact upper oceanic crust formed at a superfast spreading rate. An accurate knowledge of the volcanostratigraphy is vital to understand processes of crustal construction and submarine magmatism and to estimate chemical exchange with seawater, but this is rarely achieved due to very low recovery rates in most basement holes. We used two approaches to determine the rock types that form the wall rocks in the basement sections of Hole 1256D: (1) user guided interpretations of electrofacies acquired by imaging tools combined with other wireline tools; and (2) the use of an artificial neutral network to objectively classify the responses of all available logging information. Great availability of formation microscanner (FMS) images provided superior coverage of the borehole wall compared to previous attempts at core-log integration. This has resulted in more confident and detailed lithologic classifications, such as with the distinction between pillows and different styles of breciation. Ten lithology types are suggested for a volcanostratigraphy model: massive flows, ponded lava, fractured massive flows, fragmented flows, thin flows or thick pillows, pillows, breccias, dikes in dike complex, isolated dikes, and gabbros. Three major lithology types in the extrusive section are massive flows (both massive and fragmented massive flow, 22%), fragmented flows (32%), and breccias (19%). Pillow lavas make up only 1.9% of the volcanic section and are confined to a 100 m interval. Below the extrusive section, subvertical contacts interpreted to be dike margins are typically observed every 1 to 2 m with brecciated zones along the contacts. The dikes dip steeply to the northeast indicating slight rotation away from the ridge axis. We used an artificial neural network (ANN) approach to determine a quantitative lithostratigraphy. The ANN is most strongly influenced by porosity and alteration degrees and the resulting stratigraphy most closely resembles the above classifications when clustered by FMS texture as opposed to lithologic interpretation. The ANN thus provides a porosity-based stratigraphy of the basement rather than the traditional lithology-based stratigraphy
(Table 2) Geochemistry of whole rock samples of ODP Hole 136-843B
No full text(Table 2) Geochemistry of whole rock samples of ODP Hole 136-843
(Table 3) Stable isotopic analyses of vein carbonates of ODP Hole 136-843B
No full textSediment depth is given in mbsf. Equilibration temperatures calculated assuming formation in equilibrium with water having d18O of normal seawater (O per mil) and d18O-depleted pore fluids (-2 per mil) using fractionation factors of O'Neil et al. (1969)
(Table 1) Geochemistry of phyllosilicates and calculated mineral formulas for ODP Hole 136-843B samples
No full textCalculated formula atoms/11 oxygen
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
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