کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
5783590 | 1637954 | 2017 | 68 صفحه PDF | دانلود رایگان |
عنوان انگلیسی مقاله ISI
Uranium stable isotope fractionation in the Black Sea: Modern calibration of the 238U/235U paleo-redox proxy
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کلمات کلیدی
موضوعات مرتبط
مهندسی و علوم پایه
علوم زمین و سیارات
ژئوشیمی و پترولوژی
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چکیده انگلیسی
The isotopic compositions of redox-sensitive metals, including uranium (U), in marine sediments have recently emerged as powerful diagnostic tracers of the redox state of the ancient ocean-atmosphere system. Reliable interpretation of sedimentary isotopic information requires a thorough understanding of the environmental controls on isotopic fractionation in modern anoxic environments before being applied to the paleo-record. In this study, the relationship between ocean anoxia and the isotopic fractionation of U was investigated in the water column and sediments of the Black Sea, the world's largest anoxic basin. Paired measurements of 238U/235U and U concentration, supported by other redox parameters, were obtained for water column and sediment samples collected during the 2013 GA04N GEOTRACES expedition to the Black Sea. Removal of U from the water column occurs during the redox transition of soluble U(VI) to relatively insoluble U(IV), resulting in up to 43% of U being removed from solution in euxinic bottom waters. Uranium reduction and removal is accompanied by a progressive shift in 238U/235U towards isotopically light values in the water column as heavier 238U is preferentially exported to sediments over lighter 235U. This gives rise to apparent isotope enrichment factors of ε = â0.63 ± 0.09â° and ε = â0.84 ± 0.11â° when U removal is modelled by Rayleigh and closed system equilibrium isotope fractionation, respectively. These ε values fall within the range determined for bacterial U reduction experiments, and together with a striking correlation between the distributions of U and H2S, implicate microbially-mediated U(VI)-U(IV) reduction as the primary mechanism controlling U isotopic shifts in the Black Sea. The 238U/235U of underlying sediments is related to the the 238U/235U of Black Sea bottom waters through the isotope enrichment factor of the U reduction reaction but the relationship between sedimentary and water column 238U/235U is complicated by mass transport processes, and the modelled ε values may underestimate the true isotope enrichment factors. These new results for the Black Sea provide important constraints on the use of 238U/235U as a proxy of the redox state of ancient oceans.
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Geochimica et Cosmochimica Acta - Volume 203, 15 April 2017, Pages 69-88
Journal: Geochimica et Cosmochimica Acta - Volume 203, 15 April 2017, Pages 69-88
نویسندگان
John M. Rolison, Claudine H. Stirling, Rob Middag, Micha J.A. Rijkenberg,