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Interglacial ice core dust from Greenland
No full textAtmospheric dust is an active component of the climate system. Paleo dust records fromthe early Holocene and Eemian, which were both a few degrees warmer than today, are usedto constrain models of a future warmer climate. Ice core records provide a strong tie pointfor paleo dust models, as they have a high temporal resolution and are representative of alarge geographical area. However no insoluble dust record from Greenland of the Holoceneand Eemian has so far been published. This is partly due to the low dust concentrationin the ice, which demands high accuracy from the instruments.Long ice core dust concentration and size distribution records are often measuredby Abakus laser sensor. Abakus measurements deviate from measurements by the moreaccurate Coulter Counter, and the discrepancy can not be solved by a simple calibration.In this thesis it is shown that the discrepancy between the Coulter Counter and the Abakusis due to the non-spherical shape of the particles. The Abakus is strongly inuenced by Miescattering when measuring standard polystyrene spheres, while the Mie scattering eectscancel out for real dust particles due to their variable shape, as shown by AmsterdamDiscrete Dipole Approximation simulations. Furthermore, the Abakus assigns a largersize to the particles than the Coulter Counter, since the particles are elongated. A SingleParticle Extinction and Scattering Instrument (SPES) was used to measure an aspect ratioof 0:390:03 for local east Greenlandic dust and 0:330:03 for remote Asian dust in theeast Greenlandic RECAP ice core. The aspect ratio derived from the discrepancy betweenthe Coulter Counter and Abakus agrees with the SPES results, and local and remote dustcan be easily discerned. If the aspect ratio is known, the Abakus can be calibrated to theCoulter Counter, so it gives accurate concentrations and size distributions.The RECAP ice core was drilled in 2015 on the Renland ice cap by the East Greenlandcoast. Its dust record has been measured by Abakus calibrated using the known particleshape. It has a high concentration of large particles during the Holocene and Eemian,but low concentrations during the glacial. On the other hand, the glacial has a muchhigher concentration of small particles than the interglacials. This glacial record is almostidentical to the NGRIP dust record, which indicates that the RECAP glacial dust comesfrom Asia like the NGRIP dust. The 20 m mode and geochemical composition of theinterglacial dust shows that it has a local origin, coming from the Scoresby Sund area. Thelarge particle concentration fell by more than 90% from 116:6 0:7 to 111:1 0:5 ka b2k(before year 2000 CE) and rose again from 12:10:1 to 9:00:1 ka b2k. The decrease inlarge particle concentration at the onset of the glacial was because the Greenland ice sheetand glaciers grew and covered the dust sources. The large particle concentration increasedat the same time as the glaciers retreated. The non-zero large particle concentration showsthat some ice free areas persisted throughout the glacial.As opposed to RECAP, which is dominated by local sources, the NEEM Holocene dustrecord from north central Greenland has a remote source, and can therefore be used asa tie point for global dust models. It has an increasing ux from 10 to 15 mg/m2/yearthrough the Holocene, and the same concentration during the Eemian and early Holocene.The NEEM calcium ux, a proxy for dust, is around 1.5 mg/m2/year with no increasethrough the Holocene A comparison to the GRIP, NGRIP1 and GISP2 calcium recordsand the NGRIP2 dust record shows no consistent trend over the Holocene, and there isno geographical variation in ux. Modern global dust models predict up to 20 times moredust at NEEM than measured, and up to 10 times more dust at NEEM than in GRIPand GISP2. This inconsistency shows the need for further development of Greenlandatmospheric dust models
East Greenland ice core dust record reveals timing of Greenland ice sheet advance and retreat.
Get PDFAccurate estimates of the past extent of the Greenland ice sheet provide critical constraints for ice sheet models used to determine Greenland’s response to climate forcing and contribution to global sea level. Here we use a continuous ice core dust record from the Renland ice cap on the east coast of Greenland to constrain the timing of changes to the ice sheet margin and relative sea level over the last glacial cycle. During the Holocene and the previous interglacial period (Eemian) the dust record was dominated by coarse particles consistent with rock samples from central East Greenland. From the coarse particle concentration record we infer the East Greenland ice sheet margin advanced from 113.4 ± 0.4 to 111.0 ± 0.4 ka BP during the glacial onset and retreated from 12.1 ± 0.1 to 9.0 ± 0.1 ka BP during the last deglaciation. These findings constrain the possible response of the Greenland ice sheet to climate forcings
Particle shape accounts for instrumental discrepancy in ice core dust size distributions
Get PDFThe Klotz Abakus laser sensor and the Coulter counter are both used for
measuring the size distribution of insoluble mineral dust particles in ice
cores. While the Coulter counter measures particle volume accurately, the
equivalent Abakus instrument measurement deviates substantially from the
Coulter counter. We show that the difference between the Abakus and the
Coulter counter measurements is mainly caused by the irregular shape of dust
particles in ice core samples. The irregular shape means that a new
calibration routine based on standard spheres is necessary for obtaining
fully comparable data. This new calibration routine gives an increased
accuracy to Abakus measurements, which may improve future ice core record
intercomparisons. We derived an analytical model for extracting the aspect
ratio of dust particles from the difference between Abakus and Coulter
counter data. For verification, we measured the aspect ratio of the same
samples directly using a single-particle extinction and scattering
instrument. The results demonstrate that the model is accurate enough to
discern between samples of aspect ratio 0.3 and 0.4 using only the comparison
of Abakus and Coulter counter data
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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