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

Abstract : 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.
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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, Elsevier, 2018, 481, pp.133 - 145. ⟨10.1016/j.chemgeo.2018.01.032⟩. ⟨hal-01879415⟩

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