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Iron isotopes in seawater:method development and results from the Atlantic ocean
No full textThe analysis of the iron (Fe) isotopic composition of seawater can provide unique information about Fe sources to seawater, and Fe cycling within the oceans, which are important for understanding global climate because of the links between the marine carbon and iron cycles. The low dissolved Fe (dFe) concentrations found in seawater mean that analyses of the iron isotopic composition of seawater is an analytical challenge. This thesis describes the development methods for accurate and precise analysis of Fe isotopes in seawater with concentrations as low as ~0.4 nM Fe, and the results of iron isotope analysis of seawater samples from within the oxygen minimum zone (OMZ) of the tropical Atlantic Ocean, and the dissolved phase of hydrothermal plumes in the Southern Ocean.Briefly, Fe is preconcentrated from seawater using NTA resin and is then purified by anion exchange chromatography. Iron isotope ratios are determined by high resolution multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS, Thermo Fisher Neptune) and mass bias effects are corrected using a double-spike technique. The Fe isotope spike consists of 47 % of 57Fe, 53 % of 58Fe, and a small amount (< 0.5 %) of 54Fe, which allows precise measurement of a wide range of sample to spike mixing ratios. Isotope ratios are expressed in delta notation (?56Fe), relative to 54Fe and relative to IRMM-14. The precision of the MC-ICPMS measurements is δ56Fe ~ 0.06 ‰ (2 SD), based on replicate analyses of two different iron isotope standards.The iron isotopic composition of dissolved Fe (δ56FedFe) was measured in seawater samples collected along an E-W transect along ~12 °N in the tropical Atlantic Ocean (GEOTRACES section GA06), extending from the Senegalese shelf towards the open ocean in order to assess the behaviour of dFe in the OMZ. Bottom waters within the OMZ have elevated dFe concentrations and low δ56Fe signatures, as low as ~ -0.3 ‰, which suggests that dFe is principally derived from sediment pore waters that have undergone dissimilatory iron reduction. Towards the open ocean, δ56FedFe values within the OMZ increase, due to formation of iron ligands and/or mixing with adjacent water masses. Dissolved aluminium concentrations in surface waters at the open ocean stations are very high (up to 27 nM), indicating significant supply from atmospheric dust, and the surface waters have high δ56Fe values (up to +0.5 ‰), much higher than bulk silicate Earth, indicating that Fe isotopes are fractionated during dust dissolution.Analyses of the Fe isotopic composition of dFe in hydrothermal plumes from 3 sites in the East Scotia Sea reveal that during the early stages of mixing between hydrothermal fluids and seawater, Fe isotopes are significantly fractionated from the vent fluid; the δ56FedFe value in the plume is as low as -1.2 ‰, compared to vent fluid values of -0.64 to +0.28 ‰. As the plume continues to mix with seawater, δ56FedFe values increase, converging to values of between -0.6 to -0.3 ‰. This strongly suggests that dFe is stabilised by the formation of colloidal Fe-(oxy)hydroxides and Fe-L in the distal parts of the plume. This stabilised dFe is likely to be transported long distances away from its source, contributing to the deep ocean dFe budget.<br/
Behaviour of chromium isotopes in the eastern sub-tropical Atlantic Oxygen Minimum Zone
No full textConstraints on the variability of chromium (Cr) isotopic compositions in the modern ocean are required to validate the use of Cr isotopic signatures in ancient authigenic marine sediments for reconstructing past levels of atmospheric and ocean oxygenation. This study presents dissolved Cr concentrations (CrT, where CrT = Cr(VI) + Cr(III)) and Cr isotope data (δ53Cr) for shelf, slope and open ocean waters within the oxygen minimum zone (OMZ) of the eastern sub-tropical Atlantic Ocean. Although dissolved oxygen concentrations were as low as 44-90 μmol kg-1 in the core of the OMZ, there was no evidence for removal of Cr(VI). Nonetheless, there was significant variability in seawater δ53Cr, with values ranging from 1.08 to 1.72‰. Shelf CrT concentrations were slightly lower (2.21 ± 0.07 nmol kg-1) than in open ocean waters at the same water depth (between 0 and 160m, 2.48 ± 0.07 nmol kg-1). The shelf waters also had higher δ53Cr values (1.41 ± 0.14‰ compared to 1.18 ± 0.05‰ for open ocean waters shallower than 160m). This is consistent with partial reduction of Cr(VI) to Cr(III), with subsequent removal of isotopically light Cr(III) onto biogenic particles. We also provide evidence for input of relatively isotopically heavy Cr from sediments on the shelf. Intermediate and deep water masses (AAIW and NADW) show a rather limited range of δ53Cr values (1.19 ± 0.09‰) and inputs of Cr from remineralisation of organic material or re-oxidation of Cr(III) appear to be minimal. Authigenic marine precipitates deposited in deep water in the open ocean therefore have the potential to faithfully record seawater δ53Cr, whereas archives of seawater δ53Cr derived from shelf sediments must be interpreted with caution
Sources of dissolved iron to oxygen minimum zone waters on the Senegalese continental margin in the tropical North Atlantic Ocean: Insights from iron isotopes
No full textOxygen minimum zones (OMZs) cover extensive areas of eastern boundary ocean regions and play an important role in the cycling of the essential micronutrient iron (Fe). The isotopic composition of dissolved Fe (dFe) in shelf and slope waters on the Senegalese margin was determined to investigate the processes leading to enhanced dFe concentrations (up to 2 nM) in this tropical North Atlantic OMZ. On the shelf, the δ56Fe value of dFe (relative to the reference material IRMM-014) was as low as -0.33 ‰, which can be attributed to input of dFe from both reductive and nonreductive dissolution of sediments. Benthic inputs of dFe are subsequently upwelled to surface waters and recycled in the water column by biological uptake and remineralisation processes. Remineralised dFe is characterised by relatively high δ56Fe values (up to +0.41 ‰), and the contribution of remineralised Fe to the total dFe pool increases with distance from the shelf. Remineralisation plays an important role in the redistribution of dFe that is mainly supplied by benthic and atmospheric inputs, although dust inputs, estimated from dissolved aluminium concentrations, were low at the time of our study (2 to 9 nmol dFe m-2 d-1). As OMZs are expected to expand as climate warms, our data provide important insights into Fe sources and Fe cycling in the tropical North Atlantic Ocean
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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