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

Duc Huy Dang (1, 2) , R. Douglas Evans , Wei Wang (3) , Dario Omanović (4) , Amonda El Houssainy (5, 6) , Véronique Lenoble (1, 5, 6) , Jean-Ulrich Mullot (7) , Stéphane Mounier (1, 5, 6) , Cédric Garnier (1, 5, 6)

1 PROTEE - Processus de Transfert et d'Echanges dans l'Environnement - EA 3819
2 School of the Environment, University of Trent
3 NTNU - Norwegian University of Science and Technology [Trondheim]
4 Rudjer Boskovic Institute [Zagreb]
5 MIO - Institut méditerranéen d'océanologie
6 UTLN - Université de Toulon
7 LASEM - Laboratoire d'Analyses de Surveillance et d'Expertise de la Marine

By assessing U geochemistry as well as U isotopic composition in marine sediments (Toulon Bay, NW Mediterranean Sea), authigenic U accumulation in sediments was found to be tightly linked to that of Mo and V with a slight difference in accumulation rate depending on sediment redox conditions and with a typical U loss in re-oxidized sediments. In sediments collected on a transect along a river plume, the authigenic accumulation of these redox-sensitive elements appears to be linked to the sediment grain size which probably drives the redox status of the sediments. The U isotopic composition in Toulon Bay sediments showed enrichment of the heavy isotope (δ238U =-0.12±0.12 ‰ relative to CRM-145 in the surface sediments). However, while U isotopic fractionation reaches half fractionation factor (Δ238U = 0.6 ‰) in some sediment cores, similar to typical values observed in the literature, other cores show a full fractionation factor (Δ238U = 1.2 ‰). In parallel, 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 below 10 cm that is probably linked to the biotic reoxidation of authigenic U(IV). The released U could be further scavenged by a competition between UP precipitation and biotic reduction which is most likely driven by diagenetic reactions via porewater acidification and release of chelators and phosphorus.

Mots clés: Uranium; Coastal sediments; Diagenesis; Uranium isotope composition; Multiple regression

Domaines: Géochimie

Duc Huy Dang, R. Douglas Evans, Wei Wang, Dario Omanović, Amonda El Houssainy, et al.. Uranium isotope geochemistry in modern coastal sediments: Insights from Toulon Bay, France. Chemical Geology, 2018, 481, pp.133 - 145. ⟨10.1016/j.chemgeo.2018.01.032⟩. ⟨hal-01879415⟩