1,720,984 research outputs found
The thermal evolution of an Earth with strong subduction zones
No full textIt is commonly supposed that plate tectonic rates are controlled by the temperature-dependent viscosity of Earth's deep interior. If this were so, a small decrease in mantle temperature would lead to a large decrease in global heat transport. This negative feedback mechanism would prevent mantle temperatures from changing rapidly with time. We propose alternatively that convection is primarily resisted by the bending of oceanic lithosphere at subduction zones. Because lithospheric strength should not depend strongly on interior mantle temperature, this relationship decreases the sensitivity of heat flow to changes in interior mantle viscosity, and thus permits more rapid temperature changes there. The bending resistance is large enough to limit heat flow rates for effective viscosities of the lithosphere greater than about 1023 Pa s, and increases with the cube of plate thickness. As a result, processes that affect plate thickness, such as small-scale convection or subduction initiation, could profoundly influence Earth's thermal history.National Science Foundation (U.S.) (grant 9506427-EAR)National Science Foundation (U.S.) (Graduate Research Fellowship
Depth-dependent azimuthal seismic anisotropy governed by Couette/Poiseuille flow partitioning in the asthenosphere
No full textAzimuthal seismic anisotropy provides crucial knowledge on spatial patterns of past and present upper mantle deformation. Origins of this deformation were traditionally tied to relative shear between surface plates and mantle, and in turn a constant orientation of anisotropy azimuths with depth. However, observations of azimuthal seismic anisotropy based on surface-wave tomography often feature depth-dependent azimuths in the upper mantle. This is consistent with the existence of low-viscosity, thin asthenosphere that facilitates the channelization of both plate-driven Couette flow and pressure-driven Poiseuille flow. If the two flows are not aligned, their combination yields depth rotations of asthenospheric shear, giving rise to depth dependence of azimuthal seismic anisotropy. In this study, we utilize publicly available azimuthal seismic anisotropy together with predictions from a global mantle flow model that incorporates Couette/Poiseuille flow. We find that Poiseuille flow has significant influence on depth rotations of seismically inferred azimuthal anisotropy. Depth rotations are prominent under the Atlantic basin and the Nazca plate, where modeled asthenospheric flow regimes are dominated by Poiseuille flow. Significant Poiseuille flow may exist beneath the Indian basin, but its depth rotations are small, probably because subduction zones to the north align Couette and Poiseuille flows into the same direction. Our results indicate that interpretation of azimuthal seismic anisotropy cannot be simply associated with relative shearing between plates and mantle. Instead, the relative importance of Couette and Poiseuille flows must be considered to account its depth dependence.</p
Spatial variations in the rate of sea level rise caused by the present-day melting of glaciers and ice sheets
No full textThe redistribution of surface water mass associated with the melting of glacial ice causes uplift near areas of mass depletion, depression of the seafloors, and changes in the earth's gravitational field which perturb the ocean surface. As a result, local spatial variations exist in the rate of sea level rise. Tide gauges on continental coastlines measure a sea level rise 5% smaller than the global average. Tide gauges in the hemisphere opposite a source of continental mass depletion measure sea level rise 10 to 20% greater than the global average produced by that source while satellites make measurements 10% too low. Because most long duration tide gauges are in the northern hemisphere, if the sources of sea level rise are unbalanced between the two hemispheres, estimates of global sea level rise could be in error by 10 to 20%. Individual tide gauges could be more seriously unrepresentative if they are near regions of significant present-day mass depletion.United States. National Aeronautics and Space Administration (DOSE Grant NAG5-1911)National Science Foundation (U.S.). Graduate Research Fellowship Progra
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
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