1,721,060 research outputs found
Biogeodynamics of Cretaceous marine carbonate production
Full text linkWe have compiled stratigraphic ranges of genera of calcareous nannofossils, calcispheres, planktonic foraminifers, larger benthic foraminifers, corals and rudists bivalves, and species of dasycladalean green algae. These taxa comprise the main planktonic and benthic carbonate producers of the Cretaceous, a period of exceptionally high sea level and palaeotemperatures that was characterized by unique assemblages of benthic carbonate producers and the significant rise in pelagic carbonate sedimentation. The autecology, physiological control on calcification, and carbonate-production potential of these groups is summarized. The observed diversity patterns are compared with proxy data of Cretaceous climate and seawater chemistry to elucidate the effect of environmental change on carbonate production and sedimentation.
Two characteristic patterns are recognized. Diversity of calcareous nannofossils, calcispheres, planktonic foraminifers and corals trace the evolution of Cretaceous sea-level, while the diversity of dasycladalean algae, larger benthic foraminifers, corals and rudist bivalves show significant reductions at the level of oceanic anoxic events (OAEs). Benthic carbonate producers except for corals thus appear to have been more vulnerable to environmental change, and these general patterns appear to be unrelated to the autecology of the taxa investigated. The expansion of suitable habitats during episodes of high sea level and high temperatures appears to have been a more important control of diversity in calcareous nannofossils, planktonic foraminifers, and corals than changes in seawater chemistry. Aragonitic or aragonite-dominated benthic carbonate producers are most affected during extinction events related to OAEs, and there is a general trend of decreasing aragonite dominance throughout the Cretaceous. This is compensated by the extensive formation of calcitic hemipelagic chalk since the Cenomanian. The trend of decreasing aragonite dominance is independent of the level of biological control on calcification in the different taxa affected. The demise of aragonitic or aragonite-dominated carbonate producers at OAE1a (early Aptian) and OAE2 (Cenomanian–Turonian boundary interval) may be related to short episodes of reduced seawater carbonate-saturation caused by short-lived injections of CO2 from large igneous provinces that initiated OAEs. For OAE1a, this scenario also explains the retreat of carbonate platforms to low latitudes in the early Aptian, as sea-surface water typically has a higher carbonate saturation in warm, lower than in cooler, higher latitude waters. The gradual decrease of aragonite throughout the Cretaceous matches model simulations of seawater carbonate-saturation. An increase in the relative number of azooxanthellate coral genera following OAE1a and OAE2 suggests a disruption of photosymbiosis in the course of these global events due to high temperatures. However, the relative numbers of azooxanthellate genera continued to increase during the Late Cretaceous, when global temperatures declined. Due to the short residence time of major nutrients in seawater, these may have affected carbonate-producing ecosystems regionally. The recent patterns of benthic carbonate production being highest in oligotrophic environments cannot confidently be extrapolated to the Cretaceous.
Our database records ranges of genera at the substage level. Higher-resolution stratigraphical studies of neritic carbonate sequences are required to understand what aspect of environmental change in the sequences are required to understand what aspect of environmental change in the sequence of events that unfolded in the context of OAEs caused the demise of benthic carbonate producers
High-resolution conodont stratigraphy, biofacies, and extinctions around the Hangenberg Event in pelagic successions from Austria, Italy, and France.
No full textBased on high-resolution correlation of condensed pelagic successions of the Carnic Alps (northern Italy), Graz Palaeozoic (Austria), Montagne
Noire (southern France), and French Pyrenees, conodont stratigraphy, extinctions, and biofacies changes were studied around the Devonian/
Carboniferous boundary and compared regionally and internationally. Th e new and previous results allow us to distinguish 11 conodont biozones
from the Lower expansa Zone (upper Famennian) to the quadruplicata Zone (= Upper sandbergi Zone, top of lower Tournaisian). Fifty-six
conodont taxa are documented and assigned to conventional platform element genera. Th eir relative distribution is used to describe the vertical
biofacies evolution. Th e ranges of several species are extended and, in comparison with previous investigations, many new local records are
added. Some new ammonoid data from the Carnic Alps allow a correlation of cephalopod and conodont zones. Th e latter are partly (e.g., Lower
and Middle expansa zones) diffi cult to recognize because the defi ning index species are rare or locally absent; a revision of the zonal scheme is
desirable. New evidence confi rms that the disappearance of Palmatolepis gracilis gonioclymeniae Müller, 1956, is locally very diachronous and
not a meaningful level for correlation. Hence, the Middle praesulcata Zone is abandoned. Some additional conodont species disappear gradually
before the main extinction of the Hangenberg event. Palmatolepis and some dominant Famennian lineages of Bispathodus and Pseudopolygnathus
died out in all sections at the base of the Hangenberg Shale equivalent, at the same level as most ammonoids and benthic faunas of the pelagic
realm. Th e global pelagic conodont extinction rate was ca. 38%, but locally much higher rates are observed in the Prototethys realm. We suggest
using the signifi cant conodont extinction as a zonal boundary; the main part of the Hangenberg Event interval falls in a Bispathodus costatus
– Protognathodus kockeli Interregnum (CKI) that locally can have common Polygnathus inornatus Branson, 1934, or some Protognathodus
meischneri Ziegler, 1969. Th e former Upper praesulcata Zone is renamed the Protognathodus kockeli Zone. Due to the episodic rarity or absence
of siphonodellids close to the boundary, the base of the Carboniferous is alternatively drawn with the entry of Protognathodus kuehni Ziegler &
Leuteritz, 1970
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
Sedimentological and taphonomic geodatabase of rudists from Senonian limestones of the central-southern Apennines, Italy
No full textVariations 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
Marine sepiolite in middle Permian carbonates of South China: Implications for secular variation of phanerozoic seawater chemistry
No full textIt is now widely accepted that the composition of seawater varied significantly during the Phanerozoic, and that the Mg/Ca ratio of seawater is a major factor in synchronized secular oscillations in the mineralogical composition of marine evaporites and nonskeletal as well as biogenic carbonates. However, the nature of the Mg sink is still subject to debate. We describe early diagenetic sepiolite from the Chihsia Formation (middle Permian) of South China. The Mg incorporated in sepiolite likely was furnished by stabilization of high-Mg calcite, and silica was derived from siliceous fossils. Two factors facilitated the accumulation of sepiolite in the Chihsia Formation. The first is a Chihsian depositional environment of an intra-oceanic carbonate platform that lacked detrital terrestrial input, which enhanced the availability of biogenic silica for sepiolite formation. The second is the chemical composition of Permian seawater, as the precipitation of high-Mg calcite was aided by a high Mg/Ca ratio associated with an aragonite sea. The formation of sepiolite is significant in constraining the recycling flux of Mg during recrystallization of carbonate sediments with a high proportion of high-Mg calcite (periods of aragonite seas). Worldwide, major occurrences of marine sepiolite and palygorskite in the Phanerozoic correlate with periods of aragonite seas. This temporal distribution implies that minerals of the sepiolite-palygorskite group play an important role in the geochemical recycling of Mg, and that major sepiolite and palygorskite deposits indicate episodes of high Mg concentrations in seawater. The formation of sepiolite also was probably related to an abundance of silica-secreting organisms, which interacted with the chemical evolution of Phanerozoic seawater. In addition, anthigenic sepiolite and palygorskite are not indicators of and and semi-arid. climates
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
Latest Maastrichtian species-rich rudist associations of the Apulian Margin of Salento (S Italy) and the Ionian Islands (Greece).
No full textNumerical ages have been derived from the Sr-87/Sr-86 values of well-preserved rudist shells from localities in Salento (S Italy) and Lefkas (Ionian Islands, Greece). Samples are from the platform margin (Salento) and from the toe of slope (Lefkas) of the former Apulian carbonate platform. Rudists from both localities yielded a latest Maastrichtian age (66.4 Ma), and their rudist associations are the most species-rich so far reported from the Mediterranean region so close to the Cretaceous-Paleogene (K-P) boundary. The abundance of large-size recumbent morphotypes of a taxon similar to Pseudosabinia is remarkable. Generally, the latest Maastrichtian rudist morphotypes of the Caribbean and of the Mediterranean region are very similar, although the faunas are highly endemicon the genus level. When compared to the Turonian-Campanian, the increased abundance of aragonite in the shells of the dominant rudist taxa, and the higher abundance of other originally aragonitic skeletal components, is discussed with respect to the major-ion composition of Cretaceous seawater. The now well-documented existence of species-rich rudist associations on both sides of the Atlantic argues for a catastrophic rather than a gradual extinction of the group
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
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