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 redoxsensitive 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 (δ 238 U =-0.12±0.12 ‰ relative to CRM-145 in the surface sediments). However, while U isotopic fractionation reaches half fractionation factor (Δ 238 U = 0.6 ‰) in some sediment cores, similar to typical values observed in the literature, other cores show a full fractionation factor (Δ 238 U = 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.

238U, 235U and 234U in seawater and deep-sea corals: A high-precision reappraisal

“…To confirm that our approach gives accurate results, we compare our d 238 U data from several replicate digests of the BCR-2 geostandard to all previously published values (Weyer et al, 2008;Brennecka and Wadhwa, 2012;Cheng et al, 2013;Murphy et al, 2014;Iizuka et al, 2014;Dahl et al, 2014Dahl et al, , 2017Dahl et al, , 2019Tissot and Dauphas, 2015;Holmden et al, 2015;Goldmann et al, 2015;Brennecka et al, 2015Brennecka et al, , 2018Andersen et al, 2015;Tissot et al, 2016Tissot et al, , 2018Tissot et al, , 2019Lau et al, 2016Lau et al, , 2017Wang et al, 2018;Avanzinelli et al, 2018;Dang et al, 2016Dang et al, , 2018Dang et al, , 2022Freymuth et al, 2019;Merle et al, 2020). We find that our data (d 238 U = -0.260‰ ± 0.016‰, ndigests = 11 separate digests, nanalyses = 142 replicate analyses) closely agree with prior work (d 238 U = -0.262‰ ± 0.005‰, ndigests = 33).…”

Authigenic uranium isotopes of late Proterozoic black shale

“…More details are provided in Table S4. External precision of our dataset is usually better than 0.09 ‰ (±2 SD) based on duplicate analyses of reference materials and samples (here and those previously published (Dang et al, 2016;Dang et al, 2018b;Dang et al, 2018a).…”

“…The method for U isotope analysis has been described in Dang et al (2018bDang et al ( , 2018aDang et al ( , 2016. Briefly, U was purified from sample matrices using TRU resin (100-150 μm, Eichrom).…”

“…Uranium isotope ratios were measured on a MC-ICP-MS (Thermo Scientific Neptune) at the Trent Water Quality Center. Technical details have been described previously (Dang et al, 2018b;Dang et al, 2018a). Each sample was bracketed by two double-spiked standards (prepared in HCl/HF 0.1M/0.3M solution); the U concentrations were 33 μg L -1 in both samples and standards, yielding a signal of ca.…”

“…However, the impact of ferruginous conditions (i.e., with abundant reactive ferrous iron species) on U isotope systematics must be considered in the context of mass balance and the specific fractionation factors for U removal by this pathway. For instance, it is inferred that the U isotope fractionation factor for water column U(IV) removal may be significantly larger than from U reduction from pore-waters within sediment, where this reaction is more likely to be limited by availability of electron donors and diffusion (Andersen et al, 2014;Dang et al, 2018a). However, if U removal is largely quantitative, the expressed U isotope fractionation would be muted.…”