On the microstructure, growth pattern and original porosity of belemnite rostra: insights from calcitic Jurassic belemnites

Calcitic belemnite rostra are usually employed to perform paleoenvironmental studies based on geochemical data. However, several questions, such as their original porosity and microstructure, remain open, despite they are essential to make accurate interpretations based on geochemical analyses.This paper revisits and enlightens some of these questions. Petrographic data demonstrate that calcite crystals of the rostrum solidum of belemnites grow from spherulites that successively develop along the apical line, resulting in a “regular spherulithic prismatic” microstructure. Radially arranged calcite crystals emerge and diverge from the spherulites: towards the apex, crystals grow until a new spherulite is formed; towards the external walls of the rostrum, the crystals become progressively bigger and prismatic. Adjacent crystals slightly vary in their c-axis orientation, resulting in undulose extinction. Concentric growth layering develops at different scales and is superimposed and traversed by a radial pattern, which results in the micro-fibrous texture that is observed in the calcite crystals in the rostra.Petrographic data demonstrate that single calcite crystals in the rostra have a composite nature, which strongly suggests that the belemnite rostra were originally porous. Single crystals consistently comprise two distinct zones or sectors in optical continuity: 1) the inner zone is fluorescent, has relatively low optical relief under transmitted light (TL) microscopy, a dark-grey color under backscatter electron microscopy (BSEM), a commonly triangular shape, a “patchy” appearance and relatively high Mg and Na contents; 2) the outer sector is non-fluorescent, has relatively high optical relief under TL, a light-grey color under BSEM and low Mg and Na contents. The inner and fluorescent sectors are interpreted to have formed first as a product of biologically controlled mineralization during belemnite skeletal growth and the non-fluorescent outer sectors as overgrowths of the former, filling the intra- and inter-crystalline porosity. This question has important implications for making paleoenvironmental and/or paleoclimatic interpretations based on geochemical analyses of belemnite rostra.Finally, the petrographic features of composite calcite crystals in the rostra also suggest the non-classical crystallization of belemnite rostra, as previously suggested by other authors.

Afectividad y creaciones estéticas: intervención social con jóvenes no cualificados

Los jóvenes no cualificados, que participan en los dispositivos educativos de segunda oportunidad, recorren itinerarios de transición al mercado laboral cada vez más complejos. Por lo que conocer sus expectativas y su capacidad de agencia resulta ineludible para mejorar nuestro acompañamiento como profesionales de la intervención sociolaboral. En este artículo exploramos la adaptación de esos jóvenes a una exigencia creciente de cualificación, que los está excluyendo laboralmente. Frente a este proceso, observamos que contraponen una resistencia basada en su capital simbólico, a través de la afectividad y la estética. Nuestro interés por estas respuestas nace de la desorientación percibida entre los profesionales de los programas educativos. Incapaces de comprender las trayectorias divergentes que estos jóvenes plantean, ante los itinerarios homogéneos que se les ofrecen para su inserción social. Tras el análisis de las creaciones artísticas y estéticas de los adolescentes, intuimos que hay dos factores que activan su resistencia a las limitaciones de sus expectativas de éxito. Por un lado, una variable institucional, que apunta a una deficiente planificación de los itinerarios formativos, que los jóvenes perciben que les aboca a la infracualificación y a un mercado laboral precario e inestable. Y por otro lado, una resistencia a la exclusión, que se enraíza en un imaginario de éxito y de movilidad social propio de una juventud globalizada.