Uranium stable isotope fractionation in the Black Sea: Modern calibration of the 238U/235U paleo-redox proxy
Rolison, J.M.; Stirling, C.H.; Middag, R.; Rijkenberg, M.J.A. (2017). Uranium stable isotope fractionation in the Black Sea: Modern calibration of the 238U/235U paleo-redox proxy. Geochim. Cosmochim. Acta 203: 69–88. https://dx.doi.org/10.1016/j.gca.2016.12.014 In: Geochimica et Cosmochimica Acta. Elsevier: Oxford,New York etc.. ISSN 0016-7037; e-ISSN 1872-9533, more |  |
| Author keywords | Uranium isotope fractionation; Microbial U(VI) reduction; Anoxic; Euxinic |
| Authors | | Top | - Rolison, J.M.
- Stirling, C.H.
- Middag, R., more
- Rijkenberg, M.J.A., more
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| Abstract | The isotopic compositions of redox-sensitive metals, including uranium (U), in marine sediments have recently emerged aspowerful diagnostic tracers of the redox state of the ancient ocean–atmosphere system. Reliable interpretation of sedimentaryisotopic information requires a thorough understanding of the environmental controls on isotopic fractionation in modernanoxic environments before being applied to the paleo-record. In this study, the relationship between ocean anoxia andthe isotopic fractionation of U was investigated in the water column and sediments of the Black Sea, the world’s largest anoxicbasin. Paired measurements of 238U/235U and U concentration, supported by other redox parameters, were obtained for watercolumn and sediment samples collected during the 2013 GA04N GEOTRACES expedition to the Black Sea. Removal of Ufrom the water column occurs during the redox transition of soluble U(VI) to relatively insoluble U(IV), resulting in up to43% of U being removed from solution in euxinic bottom waters. Uranium reduction and removal is accompanied by a progressiveshift in 238U/235U towards isotopically light values in the water column as heavier 238U is preferentially exported tosediments over lighter 235U. This gives rise to apparent isotope enrichment factors of e = ?0.63 ± 0.09‰ and e = ?0.84± 0.11‰ when U removal is modelled by Rayleigh and closed system equilibrium isotope fractionation, respectively. Thesee values fall within the range determined for bacterial U reduction experiments, and together with a striking correlationbetween the distributions of U and H2S, implicate microbially-mediated U(VI)–U(IV) reduction as the primary mechanismcontrolling U isotopic shifts in the Black Sea. The 238U/235U of underlying sediments is related to the the 238U/235U of BlackSea bottom waters through the isotope enrichment factor of the U reduction reaction but the relationship between sedimentaryand water column 238U/235U is complicated by mass transport processes, and the modelled e values may underestimatethe true isotope enrichment factors. These new results for the Black Sea provide important constraints on the use of 238U/235Uas a proxy of the redox state of ancient oceans.? 2017 Elsevier Ltd. All rights reserved. |
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