1,721,116 research outputs found
Early Holocene collapse of Volcan Parinacota, central Andes, Chile: Volcanological and paleohydrological consequences
No full textThe catastrophic gravitational collapse of the Old Cone of Volcan Parinacota produced a 6 km(3) debris avalanche that traveled similar to 22 km and covered more than 150 km(2). The Upper Lauca drainage, a broad high-altitude basin in the Chilean Altiplano, was permanently altered by the collapse. Although the eruptive history of Parinacota before and after-the debris avalanche is well known from petrologic and geochronologic studies, previous age limits on the debris avalanche (based on multiple chronometers) span ca. 8-20 ka. New cosmogenic surface-exposure ages from boulders atop the deposit are based on a regionally calibrated production rate of in situ Be-10 and indicate that the avalanche occurred at 8.8 +/- 0.5 ka. These data demonstrate that cosmogenic 10Be surface-exposure dating can be successfully applied to quartz-bearing, volcanic debris avalanche deposits, and that this method offers a distinct advantage over C-14 chronologies that provide only minimum or maximum age limits. The 8.8 ka exposure age for the debris avalanche (1) agrees with 14C age limits of paleosol material incorporated in the debris avalanche, (2) requires a voluminous initial phase of postcollapse volcanism with an eruptive rate exceeding that of recent cone-building episodes at most continental arc volcanoes, and (3) suggests that volcano collapse did not result in the formation of Lago Chungara, but instead led to a major expansion of a preexisting closed basin
Cosmogenic nuclides: Beryllium-10 and Chlorine-36 in the West Antarctic Ice Sheet Divide ice core and advances in accelerator
No full textPolar ice cores continue to provide a wealth of information about the history of Earth\u27s environment. Cosmogenic radionuclides, produced in the atmosphere by cosmic rays and deposited on ice sheets, carry clues to the nature and variability of solar activity and the geomagnetic field in the past. They can also be used to independently measure the snow accumulation rate and provide chronological markers to compare with other paleoarchives such as sea sediment cores. The present work details measurements of 10Be and 36Cl in the West Antarctic Ice Sheet Divide ice core. We have established a new chemical processing line at Purdue University for this work and demonstrated that it produces results identical to a sister lab at UC Berkeley. 10Be and 36Cl have been measured in the top 560 m of core at ∼12 and 24 year resolution. In addition we have measured 10Be at annual resolution from 1586–2006 AD. We find good agreement between our results and a 10Be record from Greenland and with the historic sunspot record. In addition, work is described on the development of several new upgrades to the Purdue Rare Isotope Measurement Lab accelerator mass spectrometry facility. Two new detectors for 14C and 10Be have been constructed as well as a new gas-filled magnet system. The new detectors have capabilities comparable to those of the previous detector and have been easier and faster to use. The gas-filled magnet system is not yet ready for routine use but preliminary tests have been performed and further development of the system is underway
Cosmogenic Ages from Alluvial Fans offset by Central Walker Lane Faults
No full textHere are a series of cosmogenic 10Be and 36Cl ages and metadata for samples related to a faulting study southeast of Reno, NV, USA. Both 10Be and 36Cl concentrations were measured at the PRIME lab at Purdue. All 10Be samples were processed in the Geochronology Laboratories at the University of Cincinnati. The 36Cl samples were processed and analyzed at the PRIME lab. Boulder sampling focused on the largest boulders (~50–150-cm-diameter) from alluvial fan surfaces. Approximately 500 g samples were taken from the upper 2–5 cm of each of these boulders. 10Be concentrations and laboratory data are listed in these tables. The 10Be boulder exposure ages were calculated using the Cosmic Ray Exposure Program (CREp). The calculator requires input describing the geographic coordinates and elevation of the samples, local shielding of the sample, density of the sample, and estimation of the boulder erosion rates resulting from processes such as boulder grussification and spalling. The age estimates are also dependent on the assumption of particular scaling models designed to estimate the long-term production rate of cosmogenic 10Be. The 10Be ages use a production rate of 4.05 ± 0.30 at/g SiO2/yr determined at Twin Lakes, which is located at a higher elevation than the fan surfaces here, but is within 100 km of all study sites, the ERA40 atmosphere model, the Lifton-VDM2016 geomagnetic database, and the LSD scaling scheme.The 36Cl boulder ages are calculated using the CRONUS calculator for 36Cl
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
Sample preparation and laboratory measurement values for coarse-grain meteoric Beryllium 10 concentrations before and after Hurricane Maria in Dominica and the percent difference between the two time periods
No full text- …
