{"refrec":{"BRefID":361185,"RR":"<b>Clarkson, M.O.; Sweere, T.C.; Chiu, C.F.; Hennekam, R.; Bowyer, F.; Wood, R.A.</b> (2023). Environmental controls on very high δ238U values in reducing sediments: Implications for Neoproterozoic seawater records. <i>Earth-Sci. Rev. 237</i>: 104306. <a href=\"https://dx.doi.org/10.1016/j.earscirev.2022.104306\" target=\"_blank\">https://dx.doi.org/10.1016/j.earscirev.2022.104306</a>","BEntID":358900,"PublicFlag":1,"CheckedFlag":0,"wosflag":1,"vabbflag":1,"RefStringPartII":". <i>Earth-Sci. Rev. 237</i>: 104306. <a href=\"https://dx.doi.org/10.1016/j.earscirev.2022.104306\" target=\"_blank\">https://dx.doi.org/10.1016/j.earscirev.2022.104306</a>","DocTypID":8,"DocType":"Journal article","MarineFlag":0,"FreshFlag":0,"BrackishFlag":0,"TerrestrialFlag":0,"Authorstring":"Clarkson, M.O.; Sweere, T.C.; Chiu, C.F.; Hennekam, R.; Bowyer, F.; Wood, R.A.","OrigTitleTranslFlag":0,"Authorstringtrunc":"Clarkson, M.O. <i>et al.</i>","Englishabstract":"<p>    Uranium isotopes (δ<sup>238</sup>U) are a widely applied tool for tracing    global changes in oceanic anoxia. Interpretation of seawater values and    trends, often reconstructed from carbonates, requires knowledge of the U    isotope fractionation that occurs during U reduction, typically favouring    the heavier <sup>238</sup>U isotope relative to <sup>235</sup>U. Yet the    environmental controls on the expression of isotope enrichment during    reduction (Δ<sup>238</sup>U<sub>anox</sub>) are poorly understood, leading    to large uncertainties in interpretation of seawater records. This is    particularly limiting for the Neoproterozoic, where exceptionally lowinferred seawater δ<sup>238</sup>U requires very high Δ<sup>238</sup>U    <sub>anox</sub>, which are rarely seen in modern sediments. Here we present    a compilation of authigenic δ<sup>238</sup>U from modern and recent    (Mediterranean sapropel) reducing settings to better constrain the first    order controls on the expression of large U isotope enrichments.    Accompanying geochemical data help identify the dominant mechanisms    responsible for high Δ<sup>238</sup>U<sub>anox</sub>, suggesting they are    an expression of limited sedimentary U reduction in weakly euxinic settings    or temporally dynamic reducing environments. Such environments are    characterised by lower to intermediate organic carbon and uranium    accumulation rates (OCAR, UAR) where U reduction appears dominated by    non-diffusion-limited processes at the sediment-water interface, on sinking    organic matter or within the water column itself. Conversely, under    strongly euxinic conditions with higher OCAR and UAR, U reduction occurs    mainly under a diffusion-limited regime in the sediment. These findings    suggest that the very low seawater δ<sup>238</sup>U of the Neoproterozoic    may be a result of progressive ocean oxygenation and temporally dynamic    expansions of anoxia, or the development of weakly euxinic conditions,    rather than more widespread or ‘intense’ anoxia as previously inferred.    Such a revised interpretation is more consistent with other geochemical and    paleontological records from this time and is critical for understanding    the relationship of anoxia to the rise of complex life.</p>","AbstractOtherLang":null,"BibLvlCode":"AS","StandardTitle":"Environmental controls on very high δ238U values in reducing sediments: Implications for Neoproterozoic seawater records","OrigTitleLangCode":"en","OrigTitleLangCodeExtended":"eng","OrigTitleLangID":15,"DateLastModified":{"date":"2026-04-19 01:32:18.955525","timezone_type":1,"timezone":"+02:00"},"UserAccessRight":null,"UserAccID":null,"AuthorKeywords":"Ediacaran; Tonian; Uranium; Molybdenum; Sapropels; Metal Isotopes","OtherDescriptors":null,"Notes":null,"AnaPub":2023,"MonPub":null,"DateUpdate":"2023-02-16","DateCreate":"2023-02-16","SecASFANote":null,"ConfID":null,"PeerRev":1,"VlizCoreFlag":1,"WoScode":"WOS:000974746200001","VABBcode":null,"OpenAcc":1,"DOI":"10.1016/j.earscirev.2022.104306"},"refs":null,"anarec":{"AnaID":361185,"PubliDate":2023,"Pagination":"104306","XtraPublOfAnaID":null,"ISBN":null,"Volume":"237","Issue":null,"BRefMon":null,"BRefMonRR":null,"BRefXtra":null,"BRefXtraRR":null,"SerBRefID":196914,"SerRR":"Earth-Science Reviews. Elsevier: Amsterdam; Lausanne; London; New York; Oxford; Shannon.  ISSN 0012-8252; e-ISSN 1872-6828","StandardTitleSer":"Earth-Science Reviews","ISSN":"0012-8252","AbbrevSer":"Earth-Sci. 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