Quantification des teneurs en opale biogene des sediments de l'Ocean Austral par diffractometrie X | Clay Minerals | Cambridge Core (original) (raw)

Résumé
References

Résumé

Le dosage de l'opale biogène dans les sédiments marins peut être effectué par diffractométrie X par la mesure des surfaces des bandes de diffusion et par la mesure des surfaces des pics de cristobalite. La surface de la bande de diffusion, observée sur les diagrammes X de sédiments non chauffés, caractérise la concentration en composés amorphes (organiques et inorganiques). Elle permet de quantifier l'opale biogène de sédiments ne contenant que peu ou pas d'amorphes non biogènes. La surface du pic de cristobalite, obtenu par transformation de l'opale biogène par chauffage à 1050–1100°C pendant 12 h, permet de doser la silice biogène de divers sédiments. Ces deux approches méthodologiques ont été appliquées aux sédiments d'une carotte (MD 84–527) prélevée dans le secteur ouest-Indien de l'Océan Austral. Ce matériel, qui a enregistré la sédimentation des 40,000 dernières années, ne renferme qu'accessoirement des constituants amorphes volcaniques. Les concentrations en silice biogène sont obtenues avec une précision de ±10%. Le comptage des diatomées par observation au microscope a été également réalisé sur les mêmes sédiments. Les courbes des teneurs en opale biogène obtenues par les deux méthodes sont comparables et se corrèlent bien à l'abondance totale des diatomées.

Abstract

The XRD peak of cristobalite (4·05 Å) and diffuse X-ray scattering bands can be used for quantitative analysis of biogenic opal in marine sediments. For non-heated samples, diffuse bands are due to X-ray scattering by the whole amorphous fraction (organic and inorganic). For samples which contain little or no inorganic amorphous components, these diffuse bands can be used for quantitative analysis of biogenic opal. Conversely, heating at 1050–1100°C for 12 h causes biogenic silica to transform into cristobalite, and by measuring the area of the cristobalite peak, the amount of biogenic silica can be measured whatever the nature of the sediment. The two approaches (natural versus heated samples) have been used to study sediments cored in the western Indian part of the Southern Ocean. These sediments encompass the last 40,000 years and do not contain amorphous volcanic components. The content of biogenic silica has been obtained with an accuracy of ±10%. The curves of opal abundance variation along the core obtained from the two X-ray methods and micropaleontologic counting are very similar.

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Research Article

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