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The end-Ordovician glaciation and the Hirnantian Stage: A global review and questions about Late Ordovician event stratigraphy
No full textThis paper proposes a global review of Hirnantian event stratigraphy. The Hirnantian GSSP in south China is tentatively correlated with latest Ordovician strata from the peri-Gondwanan "glacial" regions. Problems of biostratigraphical correlation are highlighted. At a worldwide scale, the major biostratigraphically useful fossil groups (graptolites, chitinozoans, brachiopods, conodonts, acritarchs) are analysed and their limits for global correlation of the uppermost Ordovician are discussed. Palaeobiogeographical disparities are invoked as the primary cause of the difficulty in establishing an effective Late Ordovician global biostratigraphical scheme. As an alternative correlative tool, the HICE (Hirnantian Isotopic Curve Excursion) event is often put forward in the literature. However, carbon isotope chemostratigraphy shows, like biostratigraphy, some limits to the present state of knowledge. No good independent biostratigraphical control of the HICE exists in both shallow carbonate deposits and deeper shaly ones. Recent studies have also demonstrated inconsistencies between carbon isotopic signals obtained from organic (delta(13)C(org)) and inorganic (delta(13)C(carb)) carbon species, further complicating the use of the HICE as an isochronous benchmark. All of these difficulties for Hirnantian event stratigraphy are discussed in detail in order to enable them to be overcome in the future. Precise Late Ordovician and early Silurian event stratigraphies are essential for the understanding of the mechanisms linked to the first of the "Big Five" extinctions. (C) 2009 Elsevier B.V. All rights reserved
Abnormal forms of acritarchs (phytoplankton) in the upper Hirnantian (Upper Ordovician) of Anticosti Island, Canada
No full textA detailed study of Late Ordovician-early Silurian acritarchs (Palaeozoic phytoplankton) from Anticosti Island (Quebec, Canada) revealed an unusually high abundance of abnormal forms from the upper Hirnantian carbonate strata (uppermost Ordovician) of the Ellis Bay Formation in the western part of the island (member 6, Spinachitina taugourdeaui chitinozoan Biozone). The objective of this paper is to describe these abnormal forms in detail. Two species are particularly affected: Disparifusa psakadoria Loeblich and Tappan, 1978 presents abnormally hypertrophied central vesicles, whereas Peteinosphaeridium laframboisepointense nov. sp. has appendices that are fused along their length. The abnormal forms of acritarchs occur in rocks deposited during periods that are near time-equivalents to those of maximum ice-sheet extensions on Gondwana during the Hirnantian glaciation. Although this stratigraphic level corresponds to an interval of strong perturbations of the global carbon cycle, the exact factors causing the observed malformations remain unknown. (C) 2011 Published by Elsevier B.V
Acritarchs from the Ordovician-Silurian boundary beds of the Valga-10 drill core, southern Estonia (Baltica) and their stratigraphical and palaeobiogeographical implications
No full textFourteen samples of the Valga-10 drill core, south Estonia, from the lower Jelgava Formation (middle Pirgu Regional Stage, Upper Katian) to the lowermost Ohne Formation (lowermost Juuru Regional Stage, Lower Rhuddanian) were investigated for acritarchs. The section is biostratigraphically and chemostratigraphically well constrained; it comprises the rugata, taugourdeaui and scabra chitinozoan zones, the ordovicicus and giradeauensis conodont zones and the Hirnantian Isotopic Carbon Excursion (HICE). The good preservation allowed the identification of three prasinophyte phycomata and 52 acritarch species including the four new species Evittia porkuniensis, Helosphaeridium tongiorgii, Nexosarium leherissei and ?Veryhachium bulliferum. One new combination is proposed: Poikilofusa obliquipunctata (Uutela & Tynni 1991) comb. nov. Comparison with contemporaneaous palynofloras shows that eastern Laurentia and Baltica share a high number of species during the latest Katian-Hirnantian. Some of these species show a potential for future improvement of biostratigraphical correlation between the uppermost Katian-Hirnantian strata of low to mid-latitude carbonate platforms in eastern Laurentia and Baltica. Conversely, significant taxonomic differences exist between the assemblage studied and typical Gondwanan palynofloras. These results suggest that the Laurentian/Baltic and Gonwanan phytoplanktonic palaeoprovinces existed during latest Ordovician times. Published data reveal similar distribution pattern for chitinozoans and graptolites during the Hirnantian. A bathymetric ridge rise associated with the opening of the Rheic Ocean, coupled with the Hirnantian glacially-driven sea-level fall might have prevented water mass exchange and mixing/migration of phytoplankton between Gondwana and Laurentia/Baltica, thus being at the origin of the observed acritarch bioprovincialism. Additionally, distribution and diversity patterns of acritarchs are compared to those of other microfossils in the Valga-10 section. Near the base of the Hirnantian (Porkuni Regional Stage), benthic organisms (ostracods and scolecodonts) and phytoplankton (acritarchs) show increasing diversity with appearances of new taxa and new morphologies. Planktonic (chitinozoans) and nektonic (conodonts) organisms show a different pattern, with a decline in diversity during the earliest Hirnantian and a marked increase in the later part of the stage. Two alternative hyptotheses are proposed to explain these differences in diversification patterns: (1) the development of a shallower, proximal environment in the locality studied during the Hirnantian glaciation may have been more favourable to the diversification of benthonic organisms; (2) the planktonic and nektonic organisms suffered the effects of glaciation more than benthonic ones
Systematic occurrences of malformed (teratological) acritarchs in the run-up of Early Palaeozoic δ 13C isotope excursions
No full textCryptospore assemblages from Upper Ordovician (Katian-Hirnantian) strata of Anticosti Island, Quebec, Canada, and Estonia: Palaeophytogeographic and palaeoclimatic implications
No full textRich palynological assemblages have been recovered from deposits of Hirnantian age in Anticosti Island (Quebec, Canada), and in borehole Valga-10 in southern Estonia. The assemblages are well preserved, and include acritarchs, chitinozoans. and cryptospores. The age of the deposits is well constrained by means of palynomorphs (acritarchs and chitinozoans) as well as sequence stratigraphic and chemostratigraphic correlations. Cryptospore assemblages from the two localities are similar and are also broadly comparable to the few known coeval assemblages described elsewhere. They include 11 genera and 20 species, and testify to the presence of an extended and diverse flora during Hirnantian times in Laurentia and, for the first time, also in Baltica. The present findings contribute to an improved knowledge of origin and early development of vegetative cover. The recovery of diverse and abundant cryptospores in Himantian deposits may be related to increased input of land-derived sediment during the global sea-level fall linked to the Late Ordovician glaciation, but it also demonstrates that the early land plants may have tolerated a wide range of climatic conditions. (C) 2011 Elsevier B.V. All rights reserved
Phytoplankton dynamics across the Ordovician/Silurian boundary at low palaeolatitudes: Correlations with carbon isotopic and glacial events
No full textThe Late Ordovician culminated in a major glacial period that has been related to one of the strongest mass extinctions recorded during the Phanerozoic. During this interval, Anticosti Island (Quebec, eastern Canada) was located at low to intermediate palaeolatitudes (15-30 degrees S) on the eastern margin of Laurentia. It displays a relatively complete section across the Ordovician-Silurian (O/S) boundary. Upper Ordovician to Lower Silurian strata of Anticosti studied here comprise the Vaureal (Katian), Ellis Bay (Hirnantian) and Becscie (latest Hirnantian-Rhuddanian) formations. Phytoplankton dynamics inferred from the acritarch content of these strata are locally compared with new geochemical environmental proxies (delta(13)C), with recent palynological data from Baltica and also with phytoplankton dynamics as described from near-polar, high latitude localities of the Gondwana region. Two positive excursions of the carbon isotope record have been identified in the lower and upper Ellis Bay Formation. Overall, phytoplankton trends are as follows: (1) as with chitinozoans, conodonts, brachiopods and corals, acritarchs from Anticosti display a major turn-over during the early Hirnantian (uppermost Vaureal Formation-lower Ellis Bay Formation), with the appearance of taxa that exhibit Silurian affinities (e.g.. Tylotopalla sp., Ammonidium sp., Oppilatala sp., Evittia sp., Dilatisphaera spp.). High polymorphism for some common taxa is observed in the interval corresponding to the first delta(13)C positive excursion. Using sequence stratigraphy, this period may be related to a first glacially driven sea-level drawdown corresponding to terrestrial ice-sheet growth on Gondwana, such as recorded in Morocco. (2) Subsequently, during the late Himantian of Anticosti (uppermost Ellis Bay Formation), phytoplanktic communities suffered a crisis, exemplified by the disappearance of several typical, large Late Ordovician taxa (i.e., Orthosphaeridium spp., Peteinosphaeridium spp., large Baltisphaeridum spp., Sacculidium sp.). The lower Becscie strata, corresponding to the time of deglaciation at the O/S boundary interval, only contain a low diversity assemblage dominated by large acritarchs of the genus Hoegklintia and other morphologically related forms, and a few tolerant and long-ranging taxa (e.g., Evittia remota, Micrhystridium spp., leiospheres). Despite a possible taphonomic bias in the uppermost Ellis Bay Formation, the major crisis in the late Himantian coincides with the second delta(13)C excursion and a second, stronger glacially-driven sea-level fall corresponding to the time of the maximum extend of the ice sheets on Gondwana. (3) Such phytoplankton events have also been detected in Gondwanan ice-center area, but with "typical Ordovician taxa crises" occurring apparently later when plotted against positive carbon isotope excursions, as also observed in Baltica. This time-lag may be linked to a basin configuration of the studied areas, where environmental stress consequently differed or linked to the presence of omission surfaces coupled with preservation bias of acritarch assemblages in the uppermost Ordovician strata of Anticosti. In addition to their biostratigraphic and palaeobiogeographic utility, acritarchs turn out to be more specific in highlighting palaeoenvironmental conditions. (C) 2011 Elsevier B.V. All rights reserved
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
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