1,721,162 research outputs found
Episodes of last interglacial aridity of Mediterranean coastal sites: a contrast to freshwater discharge hypothesis?
No full textEarly taphonomic processes in a microbial-based sedimentary system from a temperate salt-pan site (Cervia salterns, Italy)
Full text linkIn the only salt evaporation pond retaining its natural setting of the historic Salina di Cervia (Italy), the northernmost salterns of the Mediterranean area, a number of potentially preservable textures derive from the interactions between photosynthetic mat producers and the sedimentary substrate. These morphologies occur at the beginning of the taphonomic processes when repeated emerged-submerged conditions take place. In these conditions the cohesive nature of the diatom- and cyanobacterial-derived mucilage favours the stabilization of otherwise ephemera structures. Surface micromorphologies for which diatoms and cyanobacteria have played some active role when still living in the soft microlayer and down to the sediment-water interface, such as during the gliding motility, can overcome the surface layer of most intense mixing (i.e., the taphonomically active zone) and keep traces of them in the fossil record either as body fossils or as texture contributors. Tiny microbial-derived remnants, such as filaments and biofilm strands of halotolerant microorganisms, while fragile upon their formation, can therefore stabilize as biosignatures when combined with salt precipitation. Halophilic and halotolerant ecosystems are models for life in extreme environments (analogue sites) with similarity to those strongly suspected to occur and/or have occurred on Mars and on other planetary bodies. The study of hypersaline systems such as Salina di Cervia which harbour diverse and abundant microbial life, can be relevant for astrobiology since it allows the investigation of potential biosignatures and their preservation, and of further understand the range of conditions and the planetary processes sustaining potentially habitable systems
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
Layered mound, inverted channels and polygonal fractures from the Makgadikgadi pan (Botswana): Possible analogues for Martian aqueous morphologies
No full textLayered mounds and inverted channels with polygonal fractures from the Ntwetwe Pan in the Makgadikgadi Basin (central Botswana) have been herein investigated. These morphologies are from an evaporitic basin (the Makgadikgadi Basin) that is the remnant of an ancient Pleistocene lake and is currently part of the world's largest evaporitic system.The mounds in the Ntwetwe Pan are characterized by a layered structure and low relief (max. 5 m above the pan floor) and can be in excess of 2 km wide. The mounds consist mainly of loose (non-lithified) sand and silt with high moisture contents, even during the dry season. Geophysical investigations have shown that groundwater processes, particularly those related to the capillary fringe that rises and conveys moisture through the mounds, are factors that make mound sediments resistant to wind erosion.The inverted channels, identified in the southern part of the Ntwetwe Pan, are characterized by gentle reliefs and depressions, which depend upon the distribution of calcretes and indurated sediments. Large scale (up to 100 m wide) polygonal fractures localized at the front of the channels, disappear at the transition with the present-day pan floor.We consider that these particular mounds, within the Ntwetwe Pan, are remnants of the strandline of the paleo-Makgadikgadi Lake, and that the inverted channels represent distributary channels of a relict fan delta, formed by an ephemeral river, most likely the paleo-Boteti River, during a Lake Paleo-Makgadikgadi highstand stage. We consider that large scale (up to 100 m wide) polygonal fractures, located on the channel-mouth lobes, represent large-scale desiccation cracks formed by rapid water evaporation from delta deposits.The results of this investigation highlight the importance of the paleo-drainage system and its interactions with the water table and wind-deflation as main geomorphological factors within salt pan environments. The mounds in the Makgadikgadi pans also show strong geomorphic similarities to spring mounds on the surface of Mars, localized in equatorial layered deposits (ELDs). These ELDs mounds are considered to result from cyclical groundwater upwelling, evaporation and wind deflation. The geological processes that resulted in the formation of mounds within the Makgadikgadi pans may, therefore, help to explain how similar layered deposits formed on Mars and confirm existing theories
Microbial geomorphologies in modern salt flats and their astrobiological significance
No full textFollowing the discovery of Martian mineral deposits with an assumed evaporite origin (especially sulfates), salt flat areas from hot and cold arid environments (such as sabkhas and playa lakes) are currently investigated for comparative analysis between terrestrial salt-bearing settings and their Martian environmental analogs. Similarly to the Earth, evaporite deposits seem widespread on Mars where their stratigraphic setting and composition suggest relationships with aqueous environments and, therefore, they represent a useful indicator of habitability.
Among the traces of biological activity retained in salt flat settings those produced by surface to near surface microbial activity deserve a special astrobiological interest. These microbial geomorphologies derive from the interaction between physical and biological agents, and can impact on the development and stabilization of surface morphologies with some fossilization potential. Therefore, once established a clear relationship between micro-geomorphologies and micro-biota, their recognition at the surface on Martian terrains may represent unambiguous indicators of biogenicity.
In the sabkha Oum Dba, near the Atlantic coast of the Western Sahara (southern Morocco), thick microbial mats and mineral precipitations (carbonates and sulfates) are in progress and actively interact in an ecosystem where living and fossilized sectors coexist. It, therefore, represents an ideal natural site for detecting type, evolution, and stabilization of microbial geomorphologies in a typical arid salt flat area
Texto, imagen y tiempo
Full text linkBARBIERI, Roberto (ed.) Atlas histórico de la cultura medieval Madrid, 2007, Ed. Paulinas, 280 pp
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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