1,721,030 research outputs found
Variations of the CO2 concentration of occluded air and of anions and dust in polar ice cores
No full textAnalysis of impurities entrapped in natural ice is the most promising method for reconstructing the history of atmospheric composition before the period of direct measurement and offers the possibility of extending the record to at least 100,000 years B.P. We report here the present state of work in this field, with special emphasis on atmospheric CO2 concentration. After discussing the mechanism by which atmospheric gases are entrapped in ice, we report CO2 concentrations in ice core samples, up to 100,000 years old, from deep drilling projects in Greenland and the Antarctic. Results from ice deposited during the last 2000 years allow us to estimate the preindustrial atmospheric CO2 level, an important boundary condition for modelling the anthropogenic CO2 increase. Using older samples from a deep ice core drilled at Dye 3, Greenland, we show that the CO2 concentration was 180 to 200 ppmv at the end of the Wisconsin and increased during the transition to the Holocene to values in the 260 to 300 ppmv range. Detailed CO2 measurements on sections of the Wisconsin part of the Dye 3 core which, based on δ18O, were deposited during times of significant climatic variation, show that the δ18O variations were accompanied by simultaneous correlated rapid CO2 variations. Other parameters, including micro-particle concentration and Cl−, NO− 3 and SO2− 4 concentrations also showed significant variations which correlate with the measured δ18O shifts
Accelerator radiocarbon ages on foraminifera separated form deep-sea sediments
No full textA first set of accelerator radiocarbon dates for foraminifera shells separated from a deep-sea core from the western equatorial Pacific is reported. While the ultimate objective of this work is to obtain evidence for changes in the rate of deep-sea ventilation over the last 20,000 years, this preliminary study concentrates on illuminating some of the possible biases which will surely complicate such studies. The results reveal that while whole shells and shell fragments of a single species give ages which agree within experimental error, there are significant differences among the ages for coexisting whole shells of different planktonic species. It is not possible as yet to pin down the source of these differences. Because of this, the finding that the benthic-planktonic age difference was greater 6000 to 12,000 years ago than over the last 5000 years does not necessarily mean that the ventilation rate for the deep sea was significantly slower during late glacial and early Holocene times than it is today. Other equally plausible explanations are possible. Much has yet to be learned about the origin and seafloor history of the material in deep-sea cores before any firm answers regarding paleocirculation rates can be obtained by this approach. Such studies should initially be concentrated on cores from areas of the seafloor characterized by a higher ratio of sedimentation rate to bioturbation depth than is found for typical open sea sediments
(Table 1-2 and 3, page 797-798 and 799) 10Be, U, Th and Pa analyses for layers inside ferromanganese crusts of the Pacific Ocean (ARIES-13 and Mn139)
No full textUranium, Thorium and Palladium measurements were done using an alpha-spectrometry technique described in : Ku, T.L., Broecker, W.S., 1969. Radiochemical studies on manganese nodules of deep-sea origin. Deep Sea Research and Oceanographic Abstracts 16, 625-63
(Table 4, page 800) U, Th and Pa analyses for the top layers of a ferromanganese crust of the Pacific Ocean (Mn139)
No full textUranium, Thorium and Palladium measurements were done using an alpha-spectrometry technique described in : Ku, T.L., Broecker, W.S., 1969. Radiochemical studies on manganese nodules of deep-sea origin. Deep Sea Research and Oceanographic Abstracts 16, 625-63
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
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