Quantification des teneurs en opale biogene des sediments de l'Ocean Austral par diffractometrie X

A B S T RA C T: The XRD peak of cristobalite (4.05 ~) 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~ 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.