Uranium isotope geochemistry in modern coastal sediments: Insights from Toulon Bay, France

By assessing U geochemistry in marine sediments of Toulon Bay (NW Mediterranean Sea), authigenic U accumulation was found to be tightly linked to that of Mo and V with slight differences in accumulation rate depending on sediment redox conditions. In sediments collected on a transect along a river plume, the authigenic accumulation of these redox-sensitive elements appears to be linked to sediment grain size which probably drives the redox status of the sediments. A typical U loss in re-oxidized sediments was observed in sediments that had been previously disturbed. However, the dissolved U profiles do not show a simple and typical depletion trend but rather a depletion in the top 10 cm followed by release in deeper pore waters that could be linked to a potential reoxidation/mobilization of authigenic U(IV). The released U could be further scavenged as a U-P precipitate. The U isotopic data (delta U-238), which are the first reported for the coastal hypoxic sediments of Toulon Bay, average -0.12 +/- 0.12% in surface sediments. However, sediments situated in front of river mouths show higher values of delta U-238 (up to +0.75%); these high values are reported for the first time in sediments below an oxic water column.