1,720,967 research outputs found
In situ strontium isotope analysis on biogenic apatite: the use of Laser Ablation and Multi Collector Inductively Coupled Plasma Mass Spectrometer (LA–MC–ICPMS) in anthropological research
No full textThe use of LA–MC–ICPMS in anthropological research is an innovative approach for measuring strontium isotopic ratios of human enamel. This technique, due to its micro-destructivity, allows to exam 87Sr/86Sr on precious human remains, without the drawbacks of the dissolution method. Despite this advantage, the laser ablation technique (LA) is not without flaws, principally related to unavoidable analytical interferences. For these reasons, data obtained with the laser are usually less precise and accurate than data obtained with the classical dissolution method. In particular, problematic interferences are represented by 86Kr formed in the gas, by double-charged REE (Rare Earth Element), by 87Rb, and by other polyatomic isobaric interferences (i.e. 44Ca40Ar, 40Ca44Ca, 40Ca13P16O). In this work, we measured the Sr isotopic ratio of a shark tooth with both dissolution and LA method, to test the precision of the LA method. A shark tooth is a bio-apatite formed in a marine environment, thus its 87Sr/86Sr ratio reflects the modern marine ratio of ~0.7092. Our preliminary laser analyses show that we are able to reproduce the isotopic ratio of our shark tooth obtained by Sr chromatographic separation and HR–MC–ICPMS with a precision to the fourth decimal place and that this ratio reflects the modern sea water value. If confirmed by further studies, our preliminary results suggest that the LA technique is a reliable method to explore hominin movement and migrations
Sr Curve for the Ain El Beida Section (Morocco): An Atlantic Perspective of the Mediterranean Salinity Crisis
No full textWe present a new 87Sr/86Sr curve for the time interval between 6.5 and 5.5 Ma of the Ain El Beida section (AEB, Atlantic side of Morocco). The location and the recessional-scale resolution of our curve allow to investigate the possible relationships between global paleoclimatic and palaeoceanographic trends and the high-amplitude hydrological changes of the Mediterranean during the Messinian salinity crisis (MSC). The values, obtained from planktic foraminifers, plot in the upper portion of the global ocean curve and show long- (at eccentricity scale) and short-term (precessional scale) oscillations; the latter are more evident during the maximum of eccentricity for both the 100 and 400 ka components, between 6.00 and 5.85 Ma. Negative 87Sr/86Sr anomalies are observed at insolation minima in phase with δ18O maxima, marking the glacial stages from TG34 to TG26. It is unclear whether these oscillations originated from local (river runoff) or global (glacial/interglacial) climatic forcing. Conversely, the evaporites of the coeval Primary Lower Gypsum (5.97–5.62 Ma) accumulated in the Mediterranean at insolation minima show positive anomalies at glacial stages TG32, 30, 28, and 26. Such an opposite trend is possibly related to the greater sensitivity of the Mediterranean Sea to river runoff during insolation maxima compared to the
global ocean, as recorded at the Moroccan Atlantic margin. These observations and the deviation toward lower values of the Mediterranean Sr isotope curve starting at around 6.5 Ma, support the hypothesis of a progressive restriction of the Atlantic connections since the early Messinian, leading to the MSC
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
Seasonal Pattern In the High-Elevation Fluvial Travertine From the Jiuzhaigou National Nature Reserve, Sichuan, Southwestern China
Full text linkThe Jiuzhaigou National Nature Reserve on the Tibetan Plateau (Sichuan, southwestern China) is
characterized by the deposition of fluvial travertine in a spectacular array of shoals, waterfalls, pool
dams, and multicolored lakes. This is possibly the highest vegetated travertine setting of the world,
from 2200 to 2900 m above sea level, an environment sensitive to minimal changes in temperature
and precipitation regime. The evolution of the system is driven by two seasonal monsoon climate
patterns with a wet spring-summer travertine deposition and then a dry fall-winter characterized by
no precipitation or erosion. Spring and phreatic-vadose deposits transition from laminated columnar
calcite to clotted micrite encrustation, possibly correlated with a mid-Holocene peak in precipitation
and high lake levels in the Northeastern Tibetan area connected to glacial advance. The most
peculiar features are the fluvial shoals, a rather uncommon travertine surface consisting of two main
superimposed facies of alternating weak- and strong-turbulence water flow. The low-turbulence
facies consists of clotted micrite encrustation of mosses and cyanobacteria filaments, and platy
calcite crystals covering algal filaments. The high-water-turbulence facies show seasonal alternation
of diatom-rich bundles of Phormidium sp. (late spring) with algal Oocardium stratum levels (summerfall).
Upslope and downslope of the shoals, the precipitation of calcium carbonate results in the
formation of prograding waterfalls and dam-pool systems that encrust macrophytes with microspar
and clotted micrite. Encrusted chironomids larvae tubes are present in the waterfall walls up to an
elevation of 2860 m, possibly the highest ever recorded. Early diagenetic processes such as
dissolution of diatom frustules take place over the span of several years, a slower phenomenon
compared to other travertine occurrences. In this high-elevation extreme setting, travertine sand and
gravel bar deposits are produced by the exposure to severe weathering of a section of the valley
that is bypassed through an underground karst system during the winter dry season.
High-elevation travertine probably has a low preservation potential, but it appears to be more
sensitive to climate changes than other depositional settings, especially in the Himalayan-Tibetan
area, where the extent of the Quaternary glacial advances and retreats is still a matter of debate
STRONTIUM ISOTOPE STRATIGRAPHY AS A CONTRIBUTION FOR DATING MIOCENE SHELF CARBONATES (S. MARINO FM., NORTHERN APENNINES)
Full text linkThis paper provides new data on strontium isotope stratigraphy applied to the Miocene heterozoan
shelfal carbonates of the S. Marino Fm. (Marecchia Valley, northern Apennines). Sr isotopic analyses were
carried out on oyster shells, bryozoans and bulk-rocks from the lower-middle carbonate portion of the section.
In the upper part of the succession that shows evidence of detrital influx, 87Sr/86Sr analyses were performed on
foraminifera tests, separating planktonic and benthic forms. Results were compared with calcareous nannofossil
biostratigraphic data from the same levels, in order to test the reliability of Sr dating in mixed carbonate-siliciclastic
sediments. Mean ages obtained from oysters range between 16.9 Ma and 16.3 Ma. Very similar results are obtained
using bryozoans (16.5 Ma to 16.1 Ma) and bulk-rocks (16.8 Ma to 16.2 Ma). These results allow to better constrain
the age of the massive carbonate shelf, referable to the upper Burdigalian. In the upper carbonate-siliciclastic portion
of the shelf, numerical ages obtained from planktonic and benthic foraminifera are in good agreement with
nannofossil biozones (mean ages respectively around 15.3 Ma and 14.5 Ma) although they display wide confidence
intervals. These wide age uncertainties depend on the slow rate of change of marine 87Sr/86Sr through time that
characterizes the interval between ~15 and ~13.5 Ma
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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