1,721,139 research outputs found
The contribution of ice core studies to the understanding of environmental processes
No full textData obtained from the studies of polar ice cores supplement the records available from tree rings, peat bogs, lake and ocean sediments, and provide a relatively new data source to understand processes of the complex climatic and global cycles. The main sources of ice core data are stable and radioactive isotopes, soluble and particulate matter, and the composition of the gases occluded in the ice. Such information can be used to investigate the history and the variability of carbon dioxide and the climate system.
Temperature and other climatic data obtained from δ18O measurements of polar ice cores can be correlated with similar information obtained from carbonate lake sediments. Comparison of the δ18O profiles of the Dye 3 ice core and central European lake sediments show distinct similarities such as the identification of the Older Dryas-Bolling/Allerød-Younger Dryas-Preboreal sequence.
Measurements of the cosmic ray produced isotope 10Be on only 1 kg polar ice samples are possible by accelerator mass spectrometry. The resulting data reveals the 11-year solar modulation cycle and the Maunder Minimum of solar activity from 1645 to 1745 AD. The 10Be concentration values for the Maunder Minimum are a factor 1.6 higher than the average for the past 800 years. Using a carbon cycle model these 10Be variations can be compared to the 14C variations found in tree rings. The relatively good correlation suggests a common origin of the 10Be and 14C fluctuations and serves as a check of carbon cycle models. During the Wisconsin stage all of the Dye 3 ice core parameters measured to date (δ18O, CO2/air, SO4 −, NO3 −, Cl−, dust) show values fluctuating between two different boundary conditions. This suggests that the climate system existing at that time oscillated between a cold and a warm state, probably strongly influenced by different ocean circulations and ice cover. During the Wisconsin stage a cold system dominated; the transition to the Holocene is considered as the final transition to a warm state. Thereafter the boundary conditions did not allow the systems to switch back to a cold state
10Be variations in polar ice cores
No full text10Be is produced in the atmosphere by cosmic radiation and has a half life of 1.5 · 106 years. The produced 10Be atoms are attached to aerosols and washed out by precipitation. The mean residence time in the atmosphere is about one year. Annual snow layers with their 10Be contents are ideally preserved in polar ice sheets.
With the new accelerator based mass-spectrometry technique it is possible to measure the natural 10Be concentrations in ice samples of 1 kg each. 10Be, with its short residence time and long half life, is very well suited for the study of variations of the interaction of cosmic radiation with the atmosphere. We measured firn samples from Dye 3 from the surface to about 100 m depth, representing the precipitation of the last 140 years. One sample per 2 years has been measured. The results are compared with the sunspot-number record. A clear indication of the 11 year cycle modulation has been obtained.
In addition the 10Be activities of ice samples from 1600 m-1900 m depth have been measured. Samples from this depth interval represent precipitation from about 13,000 yrs B.P. to 6000 yrs BP. The results are compared with results from Antarctica, published by Raisbeck. By measuring the 10Be/36Cl ratio, it should be possible to date ice older than about 50,000 yrs
CO2 concentration in air extracted from Greenland ice samples
No full textThe principal aim of the analyses of the CO2 concentration in air extracted from ice samples is to reconstruct the CO2 concentration of the atmosphere during the last millenia. For this purpose ice from very cold regions is best suited. Ice samples from Dye 3, where the mean annual air temperature is −20°C and summer melting is frequent, are not very well suited from this point of view. The results of CO2 analyses give however very valuable information on a possible temperature effect on the CO2 concentration of air in the bubbles. The CO2 content show's seasonal variations with an annual maximum value in the summer melt layer. The annual minimum values correspond approximately to the estimated atmospheric CO2 concentrations. Based on this experience, in spite of the complications due to the melt features, we try to reconstruct the history of the CO2 concentrations of the atmosphere. of special interest are the fast climatic transitions in the course and especially at the end of the last glaciation which are represented in the ice core by changes of parameters like acidity, dust, and istopic ratios in short depth intervals.
In this respect we discuss the time lag between the climatic warming at the end of the last glaciation and the increase of the atmospheric CO2 concentration. Finally results of gas content and gas composition of two ice samples from the lowest, silty part of the ice core are discussed
Measurements of a kind of DC-conductivity on cores from Dye 3
No full textThe de-conductivity measured with two electrodes that move slowly over an ice sample shows generally a good correlation with the acidity of the melted ice sample. The method has been suggested and successfully applied by C. Hammer. In this paper the experimental setup and some physical aspects of the method are discussed; results obtained on the Dye 3 core and their climatological interpretation are discussed in another paper in this volume by C. Hammer. With the newly developed measuring device the current is measured as a function of depth and recorded by an XY-recorder as well as in digitized form on magnetic tape. The resolution of the two recording methods are compared.
Based on observations made during the measurements at Dye 3 the effect of ice-core temperature on electrical conductivity is discussed. This serves as a contribution to the understanding of the basic physical phenomena involved in the electrical conductivity of ice. Finally some possible explanations for the observed seasonal variations are suggested and discussed
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
- …
