1,720,985 research outputs found
Traces of the oxygen isotope composition of ancient air in fossilized cosmic dust
Full text linkAs a sub-type of micrometeorites, I-type cosmic spherules form by complete melting and oxidation of extraterrestrial Fe, Ni metal particles during their atmospheric entry. All oxygen in the resulting Fe, Ni oxides sources from the Earth’s atmosphere and hence makes them probes for the composition of atmospheric oxygen. When recovered from sedimentary rocks, they allow the reconstruction of the triple oxygen isotope composition of past atmospheric O2, providing quantitative constraints on past CO2 levels or global primary production. Here we establish using fossil I-type cosmic spherules as an archive of Earth’s atmospheric composition with the potential for a unique record of paleo-atmospheric conditions dating back billions of years. We present combined triple oxygen and iron isotope compositions of a collection of fossil I-type cosmic spherules recovered from Phanerozoic sediments. We reconstruct the triple oxygen isotope anomalies of past atmospheric O2 and quantify moderate ancient CO2 levels during the Miocene (~8.5 million years) and late Cretaceous (~87 million years). We also demonstrate this method’s competitive precision for paleo-CO2 determination, despite challenges in finding micrometer-sized unaltered fossil I-type cosmic spherules. Our work indicates that morphologically intact spherules can be isotopically altered by terrestrial processes, underscoring the need for rigorous sample screening
Multi-collector 40Ar/39Ar dating of microtektites from Transantarctic Mountains (Antarctica): A definitive link with the Australasian tektite/microtektite strewn field
No full textMicrotektites represent high-velocity/distal meteorite impact ejecta. Demonstrating that microtektites found at several locations throughout East-Antarctica consist of a homogeneous class of geological objects belonging to the Australasian tek- tite/microtektite strewn field is fundamental to define the actual extent of the largest and youngest known tektite field on Earth produced by an asteroidal impact 4 times a previous fission track date, thus providing conclusive evidence that microtektites found throughout the Transantarctic Mountains of Antarc- tica belong to a single source – the Australasian field. This study strengthens the southward extension of the Australasian field ($4,000 km southward with respect to Australasian microtektites recovered at lower latitudes from deep sea sediments), thus implying a launch distance of nearly 12,000 km from the putative impact location in Indochina. From a broad perspective, results also reveal a contrasting behavior between microtektites from the Transantarctic Mountains, highly contaminated by extraneous Ar, and Australasian macroscopic tektites, weakly or negligibly contaminated. Although future dedicated exper- imental work, aimed at the definition of physical homogeneity of microtektites at the submicroscale and at the understanding of the true intra-particle spatial distribution of Ar isotopes are necessary, we speculatively hypothesize that the contrasting behavior between tektites and microtektites may reflect displacement in different environments
Exploring the Variability of Primitive Materials and Processes in the Early Solar System: Characterisation of Four Chondritic Meteorites
No full textPrimitive asteroids provide unique clues about the astrophysical and geological processes that occurred at the birth of the solar system ~4.56 billion years ago. Primitive asteroids are thus targets of several national and international space missions (Dawn, OSIRIS REx, Hayabusa II) and sample returns from these bodies are under intense cosmochemical investigation in a major international complementary effort. Within this framework, I have petrographically (and spectroscopically) characterized four new primitive chondrites from Antarctica with two specific objectives: i) to complete and publish their classification in the Meteoritical Bulletin Database of the Meteoritical Society; ii) to highlight unusual features for further in-depth investigations that will expand our knowledge on the aqueous alteration processes in primitive asteroids and the geological context for the formation of extraterrestrial organics.
The four meteorites studied in this work are Reckling Peak (RKP) 17043, 17075, 17085 and Elephant Moraine (EET) 14079. They were recently collected (in 2017 and 2014) by the Italian Programma Nazionale delle Ricerche in Antartide (PNRA). Polished thin sections were investigated by means of optical microscopy (transmitted and reflected light), Raman Spectroscopy and microanalytical Field Emission Gun Scanning Electron Microscopy (FEG-SEM-EDS) at the Dipartimento di Scienze della Terra and Centro per la Integrazione della Strumentazione dell’Università di Pisa (CISUP), Italy. In addition to my analytical work, I conducted X-ray mapping tests to generate high-quality compositional maps in relatively short acquisition times in order to maximize the analytical potential of the CISUP FEG-SEM-EDS laboratory (see Appendix).
Based on petrographic analyses RKP 17043, 17075, 17085 and EET 14079 are classified as an L3 ordinary chondrite, a CM1 and a CM2 Mighei-type carbonaceous chondrites and a CV3 Vigarano-type carbonaceous chondrite, respectively. The diversity of the chondrites studied allowed to highlight similarities and differences between chondrites belonging to different classes (ordinary versus carbonaceous chondrites), groups (CM and CV carbonaceous chondrites) and petrologic types, as reflection of different geologic processes on different parent asteroids. The following are examples of remarkable features that would require further detailed studies: i) aqueous alteration fronts as key features for investigating pathways for fluid/element mobilization on CM hydrous/icy parent asteroids (RKP 17085), (ii) unusual mineral assemblages in chondrule pseudomorphs dominated by sulfide-carbonate-rich phases as ground for expanding our knowledge on aqueous alteration reactions and conditions (RKP 17075), (iii) melt inclusions held within olivine crystals in the CM2 chondrite RKP 17085 that could provide insights into chondrule formation processes.
Finally, my Master thesis work comprised the on-line publication of the Atlas of Meteorites in Thin Sections, an educational tool for students interested in the petrography and petrology of planetary materials. The atlas is a collection of 123 microphotographs of a wide spectrum of thin stony meteorites from the collection of the Museo Nazionale dell'Antartide in Siena, Italy
X‐ray computed tomography: Morphological and porosity characterization of giant Antarctic micrometeorites
Full text linkGiant micrometeorites (MMs; 400–2000 μm) are exceedingly rare and scientifically
valuable. Three-dimensional nondestructive characterization by X-ray computed
tomography (X-CT) provides information on the petrography and thus petrogenesis of
MMs and serves as a guide to maximize subsequent multi-analytical studies on such
precious planetary materials. Here, we discuss the results obtained by X-CT on 22 giant
MMs and the classification based on their 3-D density contrast images. Scoriaceous and
unmelted MMs have distinct porosity ranges (10–40 vol% versus 0–25 vol%, respectively).
We observe a porosity variation inside scoriaceous MMs, which allows their atmospheric
entry flight history to be resolved. For the first time, spinning entry is explicitly
demonstrated for four partially melted MMs. Furthermore, we are able to resolve the
thermal gradient in a single particle, based on porosity variation (seen as a progressive
increase in pore abundance and size with higher peak temperatures). Moreover, we explore
parent body alteration through the 3-D analysis of pores distribution, showing that shock
fabrics are either absent or weakly developed in our data set. Finally, owing to the detection
of pseudomorphic chondrules, we estimate that the intensively aqueously altered C1 or CIlike
material could represent 18% of the MM flux at this size fraction (400–1000 μm)
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
Microtektites and glassy cosmic spherules from new sites in the Transantarctic Mountains, Antarctica
No full textWe report on the geochemical analyses of glassy spherules from sediments at three Transantarctic Mountain locations and the discovery of Australasian microtektites at two of these sites. Australasian microtektites are present at Mt. Raymond (RY) in the Grosvenor Mountains and Meteorite Moraine (MM) at Walcott Neve, in the Beardmore Glacier region of Antarctica. The microtektites were identified based on their pale yellow appearance, the high concentrations of silica (SiO2 = 60 7 wt%) and alumina (Al2O3 = 23 4 wt%), and a K2O/Na2O > 1, which are all characteristics of microtektites and distinct from spherules of meteoritic origin. Additionally, trace element patterns for these microtektites match the upper continental crust compositions with enrichments in refractory elements and depletions in volatile elements, most likely as a result of melting and vaporization of source material. The presence of Australasian microtektites in RY sediment confirms the recent Australasian strewn field extension to Antarctica and the presence of highly volatile depleted microtektites. In addition to microtektites, thousands of chondritic spherules and a few unique differentiated cosmic spherules were identified in RY, MM, and Jacobs Nunatak sediments. Two unique spherules were calculated to have Fe/Mn ratios similar to micrometeorites assumed to be derived from Vesta (Fe/Mn 33.2 0.5 atom%) and two other unique spherules are extremely rich in refractory components (Al2O3 ~ 30% and TiO2 = ~2%). The three sites examined are evidently successful cosmic dust and impact debris collectors, and thus are useful traps for recording and examining the nature of influx events
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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