983 resultados para 154-926
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Includes index.
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Cha ben shu wei you Qing Guangxu zhong Guang ya shu ju yuan kan hui yin.
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Shipping list no.: 90-394-P.
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To better understand the links between the carbon cycle and changes in past climate over tectonic timescales we need new geochemical proxy records of secular change in silicate weathering rates. A number of proxies are under development, but some of the most promising (e.g. palaeoseawater records of Li and Nd isotope change) can only be employed on such large samples of mono-specific foraminifera that application to the deep sea sediment core archive becomes highly problematic. "Dentoglobigerina" venezuelana presents a potentially attractive target for circumventing this problem because it is a typically large (> 355 ?m diameter), abundant and cosmopolitan planktic foraminifer that ranges from the early Oligocene to early Pliocene. Yet considerable taxonomic and ecological uncertainties associated with this taxon must first be addressed. Here, we assess the taxonomy, palaeoecology, and ontogeny of "D." venezuelana using stable isotope (oxygen and carbon) and Mg/Ca data measured in tests of late Oligocene to early Miocene age from Ocean Drilling Program (ODP) Site 925, on Ceara Rise, in the western equatorial Atlantic. To help constrain the depth habitat of "D." venezuelana relative to other species we report the stable isotope composition of selected planktic foraminifera species within Globigerina, Globigerinoides, Paragloborotalia and Catapsydrax. We define three morphotypes of "D." venezuelana based on the morphology of the final chamber and aperture architecture. We determine the trace element and stable isotope composition of each morphotype for different size fractions, to test the validity of pooling these morphotypes for the purposes of generating geochemical proxy datasets and to assess any ontogenetic variations in depth habitat. Our data indicate that "D." venezuelana maintains a lower thermocline depth habitat at Ceara Rise between 24 and 21 Ma. Comparing our results to published datasets we conclude that this lower thermocline depth ecology for the Oligo-Miocene is part of an Eocene-to-Pliocene evolution of depth habitat from surface to sub-thermocline for "D." venezuelana. Our size fraction data advocate the absence of photosymbionts in "D." venezuelana and suggest that juveniles calcify higher in the water column, descending into slightly deeper water during the later stages of its life cycle. Our morphotype data show that d18O and d13C variation between morphotypes is no greater than within-morphotype variability. This finding will permit future pooling of morphotypes in the generation of the "sample hungry" palaeoceanographic records.
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Esta investigación analiza la producción científica presentada en los congresos de la Asociación Española de Investigación de la Comunicación con el fin de comprender su desarrollo y su estado actual. Para ello, se han analizado 715 comunicaciones con veintidós variables, entre las que se encuentran: el autor, el género, la vinculación institucional, la nacionalidad, el tipo de comunicación, la metodología empleada, la réplica de contenidos, la tipología descriptiva y la finalidad de las comunicaciones. Entre otros hallazgos, los resultados muestran una mayoría femenina en la contribución académica, pero no entre los cargos académicos de mayor rango; la preferencia de los objetos de estudio empresariales, pero no profesionales o laborales; una tendencia hacia la mayor participación de doctorandos; y una elevada concentración de la producción científica en cuanto a la temática y la distribución territorial.
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