Behavior studies of natural uranium radioactive families descendants in organic rich sediments: the sapropels

The element uranium with the particular oxido-reducing properties is often associated with environments rich in organic matter; this is why several authors have proposed to use it as tracer of paleo-productivity in marine sediments. This work describes the distribution of the uranium natural families' radionuclides in organic rich Mediterranean sediments: the sapropels. Several techniques of measurements were used such as mass spectrometry (TIMS, ICP-QMS), alpha and gamma spectrometry. Activity ratios 234U/238U as well as the ages U-Th of the sapropels present irregular profiles which do not correspond to the assumptions which had been made to explain their formation. Using an 1D diffusion model we have showed that these profiles result from the migration of the radionuclides out of the sapropels. We validated these observations by analyzing several levels of sapropels presenting a spatio-temporal variability. Our study confirms the migration of radiogenic uranium 234Urad, which is produced in situ by his father the 238U, as well as the migration of the 226Ra. However the mobility of radiogenic uranium (234Urad) is not sufficient to explain the drift of the 230Th/238U and 231Pa/235U activity ratios in the S5 sapropel. An important result is that authigenic uranium also migrates, but with lower effective diffusion coefficients than those of the 234Urad. Because of this mobility, the use of U authigenic of the sediments as an indicator of paleo-productivity must thus be used with precaution. (author)