HR: 0800h
AN: PP11A-0235    [Abstracts]
TI: Uranium Migration In Organic Rich Sediments
AU: * Gourgiotis, A
EM: alkis.gourgiotis@lsce.cnrs-gif.fr
AF: Laboratoire des Sciences du Climat et de l'Environnement CEA-CNRS-UVSQ, Bat 12 Av de la Terrasse Gif sur Yvette cedex, Paris, 91198, France
AU: Reyss, J
EM: Jean-Louis.Reyss@lsce.cnrs-gif.fr
AF: Laboratoire des Sciences du Climat et de l'Environnement CEA-CNRS-UVSQ, Bat 12 Av de la Terrasse Gif sur Yvette cedex, Paris, 91198, France
AU: Frank, N
EM: Norbert.Frank@lsce.cnrs-gif.fr
AF: Laboratoire des Sciences du Climat et de l'Environnement CEA-CNRS-UVSQ, Bat 12 Av de la Terrasse Gif sur Yvette cedex, Paris, 91198, France
AU: Gurriaran, R
AF: Institut de Radioprotection et de Surete Nucleaire, Bois des Rames Bat 501 Orsay cedex, Paris, 91400, France
AU: Guihou, A
AF: Laboratoire des Sciences du Climat et de l'Environnement CEA-CNRS-UVSQ, Bat 12 Av de la Terrasse Gif sur Yvette cedex, Paris, 91198, France
AU: Guihou, A
AF: Laboratoire des Sciences de la Terre, UMR CNRS 5570, Ecole Normale Superieure de Lyon, Universite Claude Bernard Lyon 1, 46 Av d'Italie, Lyon, 69364, France
AB: Sediments cores from the eastern Mediterranean Sea contain numerous dark-colored layers that are rich in organic carbon, the sapropels. The organic matter traps uranium (U) leading to concentrations of up to 50ppm. Consequently those layers are interesting for U series dating and/or to investigate U as a productivity proxy. The exceptionally high organic carbon concentration of S5 sapropel limits post-depositional oxidation and make this sapropel one among the best preserved. Here we measured U concentrations and activity ratios of ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U and for the first time also ²³¹Pa/²³⁵U in different sediment cores of the eastern Mediterranean Sea by using gamma spectrometry and mass spectrometry. Compared to a sea water origin of uranium and a closed system evolution we find that measured ratios in the center of S5 are depleted for ²³⁴U/²³⁸U and elevated for ²³⁰Th/²³⁴U and ²³¹Pa/²³⁵U ratios. Because Th and Pa can be considered immobile the observed systematic deviation from closed system evolution across S5 is not only due to radiogenic ²³⁴U migration as previously stated, but also to U (²³⁵U and ²³⁸U) migration. To constrain effective diffusion coefficients for radiogenic ²³⁴U and U we have developed a 1D box model, which accurately fits the measured radionuclide profiles. Thus, U migration has to be taken into account to improve the interpretation of U profiles as a productivity proxy or to establish U series dating of organic rich sediments.
DE: 1040 Radiogenic isotope geochemistry
DE: 1115 Radioisotope geochronology
DE: 1120 Isotopic disequilibrium dating
DE: 1165 Sedimentary geochronology
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting