104,879 research outputs found
Biogeodynamics of Cretaceous marine carbonate production
No full textWe 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
First B-organyloxy-substituted iminoboranes: Preparation, stabilization, and reactivity
No full textThe amino(halo)(organyloxy)boranes ROB(X)N(H)R' (R = 2,6-(Bu2C6H3)-Bu-t, X = Cl, R' = Bu-t (7), SiMe3 (8), 2,6-(Pr2C6H3)-Pr-i (9), 2,4,6-Me3C6H2 (10), 2,4,6-(Bu3C6H2)-Bu-t (11); R = 2,6-C6H5)(2)C6H3, X = Cl, R' = Bu-t (12); R = 2,6-(Bu2C6H3)-Bu-t, X = F, R' = Bu-t (13), 2,6-(Pr2C6H3)-Pr-i (14); R = (Bu3C)-Bu-t, X = F, R' = Pr-i (15), Bu-t (18), SiMe3 (17)) were dehydrohalogenated with tert-butyllithium at low temperature. The reaction involved the intermediate formation of an aryloxy(imino)borane in the case of 7, 9, 11, and 12. The diazadiboretidines [ROB=NR'](2) (R = 2,6-(Bu2C6H3)-Bu-t, R' = Bu-t (18), SiMe3 (19), 2,6-(Pr2C6H3)-Pr-i (20), 2,4,6-Me3C6H2 (21); R = (Bu3C)-Bu-t, R' = Pr-i (22), SiMe3 (23)) were isolated as the ultimate products of the reactions. The aryloxy(imino)borane 2,6-(Bu2C6H3)-Bu-t-O-B=N-Bu-t (derived from 7) was reacted with BEt3 and Me3SiN3 to give the corresponding 1,2-addition products across the (B=N) triple bond (25 and 28, respectively). The compounds were characterized by NMR spectroscopy (H-1,B-11, C-13, Si-29), MS (ET and FIS, and elemental analyses. X-ray structure determinations are presented for 18, 22, and 26
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Evidence for a Novel Binding Site Conformer of Aldose Reductase in Ligand-Bound State
No full textHuman aldose reductase (ALR2) has evolved as a promising therapeutic target for the treatment of diabetic long-term complications. The binding site of this enzyme possesses two main subpockets: the catalytic anion-binding site and the hydrophobic specificity pocket. The latter can be observed in the open or closed state, depending on the bound ligand. Thus, it exhibits a pronounced capability for induced-fit adaptations, whereas the catalytic pocket exhibits rigid properties throughout all known crystal structures. Here, we determined two ALR2 crystal structures at 1.55 and 1.65 A resolution, each complexed with an inhibitor of the recently described naphtho[1,2-d]isothiazole acetic acid series. In contrast to the original design hypothesis based on the binding mode of tolrestat (1), both inhibitors leave the specificity pocket in the closed state. Unexpectedly, the more potent ligand (2) extends the catalytic pocket by opening a novel subpocket. Access to this novel subpocket is mainly attributed to the rotation of an indole moiety of Trp 20 by about 35 degrees . The newly formed subpocket provides accommodation of the naphthyl portion of the ligand. The second inhibitor, 3, differs from 2 only by an extended glycolic ester functionality added to one of its carboxylic groups. However, despite this slight structural modification, the binding mode of 3 differs dramatically from that of the first inhibitor, but provokes less pronounced induced-fit adaptations of the binding cavity. Thus, a novel binding site conformation has been identified in a region where previous complex structures suggested only low adaptability of the binding pocket. Furthermore, the two ligand complexes represent an impressive example of how the slight change of a chemically extended side-chain at a given ligand scaffold can result in a dramatically altered binding mode. In addition, our study emphasizes the importance of crystal structure analysis for the translation of affinity data into structure-activity relationships
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe 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
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
How Reliable Are Current Docking Approaches for Structure-based Drug Design? Lessons from Aldose Reductase
No full textTwo related inhibitors were flexibly docked into different conformers of aldose reductase. Although the overall binding topologies were roughly matched, significant deviations are observed in the subsequently determined crystal structures. Flexible redocking into the crystallographically observed protein conformers achieves, however, perfect binding-position predictions
Dispelling the Myths Behind First-author Citation Counts
No full textWe 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
GLEASON SCORE UPGRADING AT TIME OF PATHOLOGICAL GLEASON SCORE OCCURS MORE FREQUENTLY IN EXTERNAL BASED BIOPSIES THAN IN UNIVERSITY-BASED BIOPSIES
No full textThe construction of Karen Karnak: The multi-author-function
No full textThis thesis is situated within the comparatively recent developments of Web 2.0 and the emergence of interactive WikiMedia, and explores the mode of authorship within a Read/Write culture compared to that of a Read/Only tradition. The hypothesis of this study is that the role of the audience has become merged with the author, and as such, represents new functions and attributes, distinct from a more conventional concept of authorship, in which the roles of audience and author are more separate. Read/Write and participatory culture, as defined by this study, is focused on collaboration, and includes the influences of D.I.Y. culture, Open-Source practices and the production of text by multiple authors. Multi-authorship presents a re-thinking of several concepts which support the notion of the individual author, since the focus of multi-authorship is not on attribution and ownership of a finished text, but on the continued malleability of a text. Modes of multi-authorship, demonstrated in the use of the pseudonyms Alan Smithee and Karen Eliot, represent declarative authors whose names signify multiple origins, whilst concurrently indicating a distinct body of work. The function of these names form an important context to this study, since primary research involves the construction of an experimental mode of multi-authorship utilising WikiMedia technology and the interaction of thirty nine participants, who are invited to create a body of work under the collective pseudonym Karen Karnak. The data generated by this experiment is analysed using aspects of Michel Foucault's author-function to identify and determine power structures inherent in the WikiMedia context. The interplay of power structures, including concepts such as identity, ownership and the body of work, affect the resulting mode of authorship and contribute to the construction of Karen Karnak, suggesting further areas of research into the emerging multi-author
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