Authigenic uranium isotopes of late Proterozoic black shale

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).…”

Environmental controls on very high δ238U values in reducing sediments: Implications for Neoproterozoic seawater records

“…The change in U isotope systematics could plausibly reflect a broader chemocline (i. e. less-stratified) or less stable conditions as the stratified anoxic ocean of the Paleo-and Mesoproterozoic gave way to more oxygenated conditions in the late Neoproterozoic. Far less is known about local redox dynamics in the Tonian, but the same interpretation of increased oxygenation could be relevant (Dang et al, 2022), rather than an increase in water column U reduction and higher productivity (Zhang et al, 2022). Indeed, the Tonian δ 238 U carb record shows two intervals of higher values (Zhang et al, 2022) and high Δ 238 U anox have been inferred in more oxygenated sediments throughout the Meso-Neoproterozoic (Dang et al, 2022).…”

“…Far less is known about local redox dynamics in the Tonian, but the same interpretation of increased oxygenation could be relevant (Dang et al, 2022), rather than an increase in water column U reduction and higher productivity (Zhang et al, 2022). Indeed, the Tonian δ 238 U carb record shows two intervals of higher values (Zhang et al, 2022) and high Δ 238 U anox have been inferred in more oxygenated sediments throughout the Meso-Neoproterozoic (Dang et al, 2022).…”

“…It is also unclear how applicable these lake findings are to ferruginous oceans, given the potential effects of U speciation on isotope fractionation (Chen et al, 2016). Moreover, higher Δ 238 U anox values (up to +0.6‰) have been inferred for Neoproterozoic ferruginous marine samples (Dang et al, 2022). Secondly, interpretation using current Proterozoic geochemical frameworks is limited by the ability to identify low levels of dissolved sulfide (see also Scott and Lyons, 2012).…”

“…Finally, the tendency for samples with very high Δ 238 U anox to have lower [U auth ] makes it difficult to identify these samples using bulk measurements. We recommend that leaching approaches (Clarkson et al, 2021a;Dang et al, 2022) be used in order to avoid the uncertainties of detrital corrections for δ 238 U auth estimates.…”