1,721,194 research outputs found
Quantitative evaluation of microplankton palaeobiogeography in the Ordovician-Early Silurian of the northern Trans European Suture Zone: implications for the timing of the Avalonia-Baltica collision
No full textQuantitative analysis of assemblage similarity among chitinozoan and acritarch associations recovered from various sedimentary sequences across the Trans European Suture Zone (TESZ: southern Baltic Sea and northern Germany region), permits evaluation of changes in microplankton palaeobiogeography during the Ordovician in the study area. The present data confirm strong palaeobiogeographic differences between the lower Ordovician of the Rugen area, and the coeval domains of the East European Platform (EEP), corroborating the idea that the subsurface of Rugen should be considered palaeogeographically as the eastern extension of Avalonia. Cluster analysis of chitinozoan assemblages from numerous wells in the Rugen area, and one well from the southern margin of the EEP indicates that chitinozoan bioprovincialism reached its maximum during the Llanvirn; during this period, the Rugen microplankton communities were clearly Gondwanan in character. Calculations using the coefficient of similarity support the conclusion of a high similarity between Llanvirn acritarch assemblages from the Rugen subsurface and from coeval Perigondwana localities (e.g. Tunisia). Since the early Caradoc, this Gondwanan affinity of the Rugen microfossils starts to lessen, and becomes negligible during the late Caradoc. During latest Caradoc-early Ashgilll through Llandovery times the chitinozoan assemblages from either side of the TESZ are undistinguishable. If palaeobiogeographical differentiation is primarily related to palaeolatitudinal distance, then the present data support closure of the Tornquist Ocean during late Caradoc-Ashgill times. The presence of reworked Llanvirn acritarchs of Perigondwanan affinity in middle Ashgill sedimentary sequences at the southern margin of the EEP, clearly shows that by this time erosion of an uplifted area was taking place. Accordingly, the closure of the Tornquist Ocean, and consequent Avalonia-Baltica collision must have taken place during the time interval between the middle Caradoc and the early middle Ashgill (Rawtheyan). New chitinozoan data from boreholes H 2 and K 5, offshore of Rugen Island, northern Germany, allow for precise biostratigraphic dating. In the pre-Devonian part of H 2, Siphonochitina formosa indicates an Abereiddian (Early Llanvirn) age, facilitating correlation with boreholes Binz 1/73 and Lohme 2/70 of Rugen. In borehole K 5, the occurrence of Belonechitina robusta and Tanuchitina bergstroemi permits attribution of the investigated pre-Carboniferous sequence to the middle-late Caradoc, and, possibly, early Ashgill. (C) 2001 Elsevier Science B.V. All rights reserved
Reworked acritarchs as provenance indicators in the Lower Palaeozoic of Denmark
Get PDFProfuse and well-preserved acritarchs were recovered from subsurface Lower Palaeozoic successions cored by the boreholes Slagelse-1 and Pernille-1 (Danish-North German Basin). Together with Llandovery in situ microphytoplankton, reworked Cambrian and Ordovician species occur. The reworked Ordovician acritarchs show a clear Perigondwanan palaeobiogeographic affinity and indicate elastic sedimentary input from a Perigondwanan-related terrane located south of the East European Platform. Microfloral similarity enables identification of the detrital source area with the Avalonia Terrane. The present data also suggest that development of a foreland basin marginal to the Caledonian Deformation Front in the Danish-North German Basin started in Early Silurian times
Ordovician-Silurian palynostratigraphy (Chitinozoa and acritarchs) of the G14-1/86 borehole, southern Baltic Sea
No full textBorehole G 14-1/86, located in the Baltic Sea between the Tornquist Zone and the Caledonian Deformation Front/Thor Suture, penetrates a Lower Palaeozoic sedimentary succession deposited on the south-western margin of the East European Platform. Detailed palynological analysis of the Ordovician and Silurian successions enables precise dating and facilitates provenance interpretation of the sedimentary sequence. The following chitinozoan biozones are identified: Cyatho-chitina primitiva Biozone (early - early late Arenig); Laufeldochitina striata Biozone (late Abereiddian - late Llandeilan) and Ancyrochitina merga Biozone (Cautleyan - Rawtheyan, middle Ashgill). A further chitinozoan assemblage is possible to be correlated with the global Spinachitina maennili Biozone (uppermost Rhuddanian - lower Telychian). A relatively diversified acritarch assemblage of Ashgill age (independently dated by Chitinozoa) also occurs. This assemblage contains numerous reworked forms of Llanvirn, Tremadoc and late Cambrian ages. The reworked Llanvirn acritarchs are typical of the high-latitude Perigondwanan micro-phytoplankton palaeobioprovince and testify to detrital input of Gondwanan-related source during the Ashgill, suggesting that by this time the Tornquist Ocean was closed
Micropalaeontology of a Moroccan Ordovician deposit yielding soft-bodied organisms showing Ediacara-like preservation
No full textA newly discovered locality of an hitherto unknown age, but now estimated to the Late Ordovician, in south-eastern Morocco has yielded numerous specimens of problematic soft-bodied metazoans preserved in coarse sandstones. The excellent preservation of the recovered specimens permits an improved understanding of both the palaeobiology of these Ordovician metazoans and the taphonomic processes associated with them. This style of soft-tissue preservation is closely comparable to that of the late Neoproterozoic problematic Ediacaran organisms, and is virtually absent in the Phanerozoic. Palynological analysis (chitinozoans and acritarchs) of shaly intercalations in an ophiuroid-bearing sandstone slab facilitates accurate identification of the site as belonging to the Upper Ktaoua Formation and thus a relative dating of the associated fossils. The chitinozoan assemblage is relatively well preserved, moderately diverse and closely comparable to the previously described chitinozoan fauna from the Upper Ktaoua Formation (Rawtheyan, late middle Ashgill age). Acritarchs are abundant but mostly ill-preserved; nonetheless they clearly indicate an undifferentiated Ashgill age, consistent with the chitinozoan chronostratigraphic result. These datings prove that the newly discovered Moroccan metazoan assemblage contains one of the oldest known paropsonemids, and possibly some of the youngest protolyelloids and aglaspidids. The new finds also show that Ediacara-like coarse-elastic preservation did not completely vanish with the onset of the Phanerozoic. (C) Etditions scientifiques et medicales Elsevier SAS
Timing of the Avalonia-Baltica plate convergence as inferred from palaeogeographic and stratigraphic data of chitinozoan assemblages in west Pomerania, northern Poland
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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
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