HR: 0800h
AN: PP31C-0531    [Abstracts]
TI: Deciphering the Geochemistry of Calcareous Pelagic Producers: Beyond Bulk Carbonate Analyses...
AU: * Minoletti, F
EM: minolet@ccr.jussieu.fr
AF: Université Pierre et Marie Curie (Paris 6) - JE 2477 Biominéralisations et Paléoenvironnements, Case postale 116, 4 place Jussieu, PARIS, 75005, France
AU: Hermoso, M
EM: hermoso@ccr.jussieu.fr
AF: Université Pierre et Marie Curie (Paris 6) - JE 2477 Biominéralisations et Paléoenvironnements, Case postale 116, 4 place Jussieu, PARIS, 75005, France
AU: Gressier, V
EM: gressier@ccr.jussieu.fr
AF: Université Pierre et Marie Curie (Paris 6) - JE 2477 Biominéralisations et Paléoenvironnements, Case postale 116, 4 place Jussieu, PARIS, 75005, France
AB: Despite the dominance of calcareous nannofossils in pelagic sediments, their specific geochemistry is still underexploited because of their minute dimensions. Instead, isotopic analyses are performed on bulk carbonate, which is a mixture of calcareous nannofossils and variable amounts of other calcareous components (diagenetic precipitations). The bulk thus records a mean value for different isotopic signatures, i.e. different calcareous components grown in distinct geochemical environments (different calcification depths within the photic zone, deep water masses, interstitial fluid). In order to study the geochemical signatures of each micrometric calcareous component, we propose a new protocol enabling their separation into different subsamples : calcareous nannofossils, planktic and benthic foraminifera, monocrystals. Particles are separated according to (1) size and (2) resistance to ultrasonication related to their microstructure. (1) The granulometric separation is achieved with sequential filtration steps (from 60 down to 2 microns) which allow the recovery of granulometric fractions containing few different particle types (for example mono- and oligospecific assemblages of coccoliths). (2) When various components share their dimensions but exhibit distinct microstructures (imbrication of crystal units, porosity), a strong ultrasonication is applied. During such a treatment, resistant particles remain unaltered while polycrystalline nannofossils are broken. Both particles can be further recovered by filtration. This methodology was applied to various paleoenvironments and lithologies. Oligospecific nannofossil assemblages from Neogene calcareous oozes lighten the influence of specific vital effects on the coccolith isotopic signatures. For lithified sediments, we present monospecific assemblages of coccoliths and nannoliths from three intervals/lithologies: Cretaceous-Paleogene transition clays, Early Toarcian Black Shales and Early Cretaceous marls. The comparison between the geochemical evolution of the photic-zone and the diagenetic overprint offers a better understanding of paleoceanographical changes during these three key-periods.
DE: 0419 Biomineralization
DE: 0452 Instruments and techniques
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 3022 Marine sediments: processes and transport
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting