1,721,360 research outputs found
Investigation of Thallium isotope fractionation during sorption to Mn oxides (abstract of paper presented at: 18th Annual V. M. Goldschmidt Conference Vancouver, Canada, July 2008)
No full textRetreat of the Laurentide ice sheet tracked by the isotopic composition of Pb in western North Atlantic seawater during termination 1
Full text linkDuring the Last Glacial Maximum much of North America was covered by the Laurentide ice sheet. Its melting during termination 1 led to systematic changes in proglacial lake formation, continental runoff, and possibly North Atlantic Meridional Overturning Circulation. The accompanying change in chemical weathering rates in the interior of North America throughout the deglaciation resulted in a pronounced change in seawater Pb isotope composition in the western North Atlantic Ocean. Here we present the first high-resolution records of seawater Pb isotope variations of North Atlantic Deep Water extracted from authigenic Fe–Mn oxyhydroxides in three sediment cores (51GGC, 1790 m depth; 31GGC, 3410 m depth; 12JPC, 4250 m depth) from the Blake Ridge off Florida. These data reveal a striking excursion from relatively unradiogenic 206Pb/204Pb as low as 18.93 towards highly radiogenic Pb isotope compositions that was initiated during the Bølling–Allerød interstadial and was most pronounced in both intermediate and deep waters during and after the Younger Dryas (206Pb/204Pb as high as 19.38 at 8.8 ka in 4250 m). This pattern is interpreted to be a direct function of increased inflow of continent-derived radiogenic Pb into the western North Atlantic, supplied through chemical weathering of North American rocks that had been eroded and freshly exposed during the preceding glacial cycle. These sediment-derived data are complemented by new laser ablation Pb isotope data from a ferromanganese crust from the Blake Plateau at 850 m water depth, which show only small glacial–interglacial Pb isotope variations of the Florida Current (206Pb/204Pb between 19.07 and 19.16). The lack of change in the Blake Plateau record at the same time as the radiogenic excursion in the deeper sediments supports a northern origin of the pulse of radiogenic Pb. After the Younger Dryas, the deep western North Atlantic has experienced a persistent highly radiogenic Pb supply that was most pronounced during the first half of the Holocene and still lasts until today
Controls on the incongruent release of hafnium during weathering of metamorphic and sedimentary catchments
No full textIt is well established that Hf weathers incongruently such that the isotopic compositions in seawater are offset from those of Nd relative to the correlation defined by bulk lithologies of the continental crust. Here we study this process in detail with new records of the seasonal variability of isotope compositions and concentrations of Hf and Nd in four Swiss rivers. The water has been filtered at a pore size of 0.45 mu m and therefore represents the truly dissolved and the colloidal pool of both elements. The studied rivers drain metamorphic (gneissic) or sedimentary (mixed carbonate/siliciclastic) lithologies. The dissolved isotope data are compared to the isotope compositions and concentrations of the suspended load and different fractions of the actual source rocks in the respective catchments, as well as to concomitant changes in the aqueous chemistry of the major elements.
Dissolved Nd concentrations span similar ranges for all rivers, whereas Hf concentrations are one order of magnitude lower in the rivers that drain gneissic catchments compared to those draining sedimentary rocks. This primarily results from the retention of most of the Hf in the gneissic zircons, as indicated by the Hf budget of the gneisses, whereas Hf in the sedimentary catchments is readily weathered from fine detrital silicates.
Large differences are found between the dissolved Hf isotope compositions of the rivers and those of the suspended load and the source rocks, consistent with the release of Hf from a radiogenic rock fraction during weathering. In the metamorphic catchments this primarily reflects that fact that zircons are barely accessible for weathering. The zircon-free portion of the rocks appears to weather congruently as the riverine Hf isotope compositions are similar to the zircon-free portion of the gneisses, rather than being distinctly more radiogenic. Leaching experiments performed to understand the riverine Hf budget in the sedimentary catchments reveal that the carbonate fraction of the sedimentary rocks is extremely radiogenic, yielding Hf isotope compositions up to epsilon(Hf) of + 208. However, the Hf concentrations in the carbonate fractions are too low to dominate the riverine Hf budget, which is instead controlled by the weathering of detrital silicate minerals.
Two of the catchments, a metamorphic and a sedimentary one, show relatively systematic changes towards more radiogenic dissolved Hf isotope compositions as discharge increases. This suggests that continental runoff conditions could be a relevant parameter for the control of the seawater Hf isotope composition, whereby more congruent weathering is achieved during low discharge when Hf is increasingly derived from weathering-resistant unradiogenic mineral
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
Reliable extraction of a deepwater trace metal isotope signal from Fe-Mn oxyhydroxide coatings of marine sediments
No full textThe extraction of a deepwater radiogenic isotope signal from marine sediments is a powerful, though under-exploited, tool for the characterisation of past climates and modes of ocean circulation. The radiogenic and radioactive isotope compositions (Nd, Pb, Th) of ambient deepwater are stored in authigenic Fe–Mn oxyhydroxide coatings in marine sediments, but the unambiguous separation of the isotopic signal in this phase from other sedimentary components is difficult and measures are needed to ensure its seawater origin. Here the extracted Fe–Mn oxyhydroxide phase is investigated geochemically and isotopically in order to constrain the potential and the limitations of the reconstruction of deepwater radiogenic isotope compositions from marine sediments.Our results show that the isotope compositions of elements such as Sr and Os obtained from the Fe–Mn oxyhydroxide fraction are easily disturbed by detrital contributions originating from the extraction process, whereas the seawater isotope compositions of Nd, Pb and Th can be reliably extracted from marine sediments in the North Atlantic. The main reason is that the Nd, Pb and Th concentrations in the detrital phase of pelagic sediments are much lower than in the Fe–Mn oxyhydroxide fractions. This is reflected in Al/Nd, Al/Pb and Al/Th ratios of the Fe–Mn oxyhydroxide fractions, which are as low as or even lower than those of hydrogenetic ferromanganese crusts. Mass balance calculations illustrate that the use of the 87Sr/86Sr isotope composition to confirm the seawater origin of the extracted Nd, Pb and Th isotope signals is misleading. Even though the 87Sr/86Sr in the Fe–Mn oxyhydroxide fractions is often higher than the seawater Sr isotope composition, the corresponding detrital contribution does not translate into altered seawater Nd, Pb and Th isotope compositions due to mass balance constraints. Overall the rare earth element patterns, elemental ratios, as well as the mass balance calculations presented here highlight the potential of using authigenic Fe–Mn oxyhydroxide coatings as paleoceanographic archives for the analysis of past seawater Nd, Pb and Th isotope compositions
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