1,720,963 research outputs found
Effect of pressure on silica solubility of diatom frustules in the oceans: Results from long-term laboratory and field incubations
No full textThe oceanic cycle of silicon (Si) has been studied extensively due to its close coupling to the oceanic carbon cycle and the biological CO2 pump. The oceanic Si cycle is dominated by the uptake of dissolved silicate (dSi) by planktonic organisms, predominantly diatoms, which use it to synthesize siliceous frustules. As oceanic waters are undersaturated with respect to biogenic silica (bSiO2) the frustules dissolve after death of the organisms, thereby regenerating dSi. Because the dissolution rate of bSiO2 depends on the degree of undersaturation, the thermodynamic solubility of bSiO2 is a key parameter controlling the recycling efficiency of nutrient Si in the water column and sediments. While an extensive body of data exists describing the dependence of bSiO2 solubility on temperature, the effect of pressure on the solubility of natural diatom frustules has never been measured directly. In this study, we conducted long-term (up to 22 months) laboratory and field equilibration experiments to determine the solubility of cleaned frustules of a cultured marine diatom (Thalassiosira punctigera) in seawater, for pressures between 1 and 700 bar, and temperatures between 2 and 21 °C. According to our results, the solubility of the frustules decreases by about 10% when pressure increases from 1 to ~ 200 bar. From 200 bar on, the pressure dependence reverses, and at 700 bar the solubility is about 15% higher than at atmospheric pressure. Integrated over an average oceanic water depth of 4000 m, a drop in temperature of 15–20 °C has a far more significant effect on the solubility of bSiO2 than a corresponding 400 bar increase in pressure
Silicate in porewater of sediment core NIOP-C2_905
No full textSilicate in porewater of sediment core NIOP-C2_90
Silicate in porewater of sediment core NIOP-C2_915
No full textSilicate in porewater of sediment core NIOP-C2_91
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
Seawater-mediated interactions between diatomaceous silica and terrigenous sediments: Results from long-term incubation experiments
No full textReactors containing frustules of the cultured diatom Thalassiosira punctigera suspended in seawater were
incubated with or without added sediment from the Mississippi River Delta or the Congo River Fan. The
diatom frustules were separated from the terrigenous sediments by a dialysis membrane, thereby only
allowing the exchange of dissolved species. One series of incubations was carried out in the laboratory, at
room temperature (21 °C) and for a period of 10 months. Another series of reactors was deployed along a
mooring in the Mozambique Channel at three water depths (500, 1250, and 2000 m), for a period of
22 months. Chemical analyses after total destruction of frustules collected at the end of the incubations
showed elemental transfer from seawater (Mg and K) and the sediments (Al, Fe, Mn, P and Ca) to the
frustules. In the presence of the terrigenous sediments, the dissolved silicate concentrations at the end of the
incubations were systematically lower that those measured in the incubations without the sediments. In
addition, electron microscopy revealed the formation of new mineral precipitates. These included
amorphous deposits on the frustules containing Si, Fe, Al, Mg, K and P, as well as euhedral clay crystallites.
Differences were observed between the incubations performed in the laboratory and those deployed at sea,
likely as a result of differences in redox conditions, temperature and reaction time. Overall, the interactions
between biogenic silica, seawater and lithogenic minerals reduce the regeneration of nutrient silicon fixed by
siliceous organisms. These interactions take place on relatively short time scales (months to years), and
affect not only the marine cycle of silicon, but also those of other major and minor elements, such as Al, Fe,
Mn, K, and Mg
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