1,720,972 research outputs found
Dextral to sinistral coiling switch in planktic foraminifer Morozovella during the Early Eocene Climatic Optimum
No full textCoiling direction is a basic characteristic of trochospiral planktic foraminifera. Modifications in the coiling direction within ancient planktic foraminiferal populations may reflect important changes in evolution or environment, yet they remain scarcely discussed. Here we investigate fluctuations in the coiling direction within Morozovella assemblages from sections that span the interval of peak Cenozoic warmth, the Early Eocene Climatic Optimum (EECO; ~53–49 million years ago, Ma), at Atlantic Ocean Drilling Program (ODP) sites 1051, 1258 and 1263. The surface-dwelling genus Morozovella is of particular interest because it dominated tropical-subtropical early Paleogene assemblages then suffered an abrupt and permanent decline in abundance and taxonomic diversity at the start of the EECO. At all ODP sites, morozovellids display a dominant dextral coiling preference during the interval preceding the EECO. However, all the Morozovella species at all sites modify their coiling from preferentially dextral to sinistral coiling within the EECO, <200 kyr after the K/X event (~52.8 Ma), providing a new biostratigraphic tool for correlation. We also document that before the major shift in morozovellid coiling, transient excursions to higher abundances of sinistral tests occurred in conjunction with negative carbon isotope excursions. Significantly, carbon isotope data reveal that sinistral morphotypes belonging to the same morphospecies typically have lower δ13C values. The dominance of sinistral morphotypes, at the expense of dextral forms within the EECO, coupled with the lower δ13C signatures of the former, suggests that the sinistral forms were less dependent on their photosymbiotic partnerships and thus able to adapt more readily to paleoceanographic change at the EECO. The observed sinistral and dextral coiling of morozovellids can be a genetically heritable characteristic that lies within cryptic speciation across multiple morphologically defined species. Alternatively the coiling changes were exclusively ecophenotypic responses whereby different species were able to preferentially adopt sinistral coiling in reaction to the changed conditions in the mixed-layer during the EECO. Previous interpretations of coiling flips in planktic foraminifera in the early Eocene, especially including morozovellids, have favoured a genetic explanation rather than an ecological response. Our present data cannot validate or disprove this idea, but should stimulate renewed thought on the matter
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
Demise of the planktic foraminifer genus Morozovella during the early eocene climatic optimum: New records from ODP Site 1258 (Demerara Rise, western equatorial Atlantic) and Site 1263 (Walvis Ridge, south Atlantic)
No full textHere we present relative abundances of planktic foraminifera that span the Early Eocene Climatic Optimum (EECO) at Ocean Drilling Program (ODP) Site 1258 in the western equatorial Atlantic. The EECO (~53.3−49.1 Ma) represents peak Cenozoic warmth, probably related to high atmospheric CO2, and when planktic foraminifera, a dominant component of marine sediment, exhibit a major biotic response. Consistent with previous work, the relative abundance of the genus Morozovella, which dominated early Paleogene tropical-subtropical assemblages, markedly and permanently declined from a mean percentage of ~32% to less than ~7% at the beginning of the EECO. The distinct decrease in Morozovella abundance occurred at Site 1258 within ~20 kyr before a negative excursion in δ13C records known as the J event and which defines the beginning of EECO. Moreover, all morozovellid species except M. aragonensis dropped in abundance permanently at Site 1258, and this is related to a reduction in test-size. Comparing our data with that from other locations, the remarkable switch in planktonic foraminifera assemblages appears to have begun first with unfavourable environmental conditions near the Equator and then extended to higher latitudes. Several potential stressors may explain observations, including some combination of algal photosymbiont inhibition (bleaching), a sustained increase in temperature, or an extended decrease in pH
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
Planktic foraminiferal response to the Early Eocene Climatic Optimum (EECO, ca 53-49Ma): biostratigraphy and quantitative abundances from Hole 762C (Exmouth Plateau, Indian Ocean)
No full textThe EECO (ca 53-49 Ma) is a crucial interval of time to explore the marine biota response to past global warming as it records the Earth temperatures and pCO2 peak of the entire Cenozoic Era. The unicellular planktic foraminifera are a major group of open-marine calcifiers to investigate in this view as a group extremely sensitive to paleoenvironmental changes and largely utilized in biostratigraphy. Recent studies highlight that the EECO significantly impacted the abundance and diversity of the symbiont-bearing genus Morozovella. This genus, close to the carbon isotope excursion (CIE) known as J event, markedly and permanently decreased in abundance and diversity in the Atlantic and Pacific Oceans whereas abundance and diversity of genus Acarinina concomitantly increased. In addition, Morozovella species switched their coiling direction (the ability to add chambers in clock or counter clock-wise) from dominantly dextral to dominantly sinistral within 200-400 kyrs after the CIE K/X event, whereas Acarinina maintains both below and within the EECO rather proportional dextral and sinistral coiling direction. The detailed record from Atlantic and Pacific Oceans also underlines diachroneities among planktic foraminiferal biohorizons. We decided to explore planktic foraminiferal biostratigraphy, quantitative abundance and coiling direction from Site 762 C (Exmouth Plateau). This site, though affected by some core breaks, records several CIEs below and within the EECO. The study of this site is essential to outline a global perspective of planktic foraminiferal response to the EECO due to its far southern high latitude location of northwest margin of Australia. Our dataset provides new biostratigraphic data suitable for a required Eocene zonal scheme revision and new insights on the resilience of planktic foraminifera from southern Indian Ocean, essential for a more comprehensive understanding of past global warming events in light of the current climate changes
Successive extinctions of muricate planktonic foraminifera (Morozovelloides and Acarinina) as a candidate for marking the base Priabonian
No full textThe formal placement of the Global Stratotype Section 25 and Point (GSSP) for the base of the Priabonian (Upper Eocene) is currently under discussion. We suggest that two closely spaced extinctions of planktonic foraminifera at ~38 Ma offer excellent potential for long-distance correlation in marine strata and is the most suitable level to place the GSSP. The double extinction, which occurred within 11 kyr, involved the loss of the distinctive ‘muricate’ taxa Morozovelloides and the large acarininids. We present detailed biostratigraphic analyses from the Adriatic Sea and re-evaluate the magnetobiochronology of the extinction of these muricate taxa from previous studies from Ocean Drilling Program Site 1052 (western North Atlantic) and the Alano di Piave Section (NE Italy). We show these bioevents are robust, synchronous across the Atlantic Ocean and among the best calibrated and most easily recognised foraminiferal biohorizons for the entire Cenozoic. The two separate but very closely spaced bioevents provide a means for testing for completeness in the proposed stratotype and other sections that contain these fossils. The muricate extinctions coincide with a large turnover in radiolarians, within the short Subchron C17n.3n, providing distinct correlation horizons in siliceous and terrestrial sediments.
We propose that the lithological level at 57.62 m in the Alano Section associated with the HO of Morozovelloides crassatus define the base of the Priabonian and the Middle/Upper Eocene boundary
- …
