Natural variations of the 238U/235U isotope composition: A new paleoredox tracer?
Zusammenfassung der Projektergebnisse
The overall goal of the project was to test whether isotopic signatures of uranium, i.e. variations in 238U/235U (reported as □238U), are suitable to be used as a paleo redox proxy. To address this question, and to further improve the modern U isotopic mass balance, we performed the following investigations: (1) We analyzed □238U of organic-rich sediments, and also of the dissolved U of the water column, from several modern anoxic basins (initial analyses on sediments from the Black Sea displayed the largest U isotope fractionation and the heaviest □238U relative to seawater). Our findings revealed that strong U isotope fractionation only occurs in permanently anoxic basin and that the extent of isotope fractionation depends on local redox conditions and likely also on basin restriction. (2) We analyzed the U isotope composition of several rivers, granites, and other rocks representative for the continental crust, to investigate U isotope fractionation during weathering and transport. Only some minor rivers with low U concentrations display significant U isotope fractionation, towards high □238U, which may be generated by U isotope fractionation between dissolved and particulate U. However, rivers in average display only minor U isotope fractionation relative to the continental crust. (3) We analyzed □238U of altered basalts (relative to that of fresh basalts), of U-rich carbonate veins and of hydrothermal springs (all from mid-oceanic ridges) to investigate whether U isotopes are fractionated during hydrothermal alteration, resulting in the removal of U from seawater. Though 238 individual basalts and carbonates display significant U isotope variations towards high and low □238U, hydrothermal waters are only slightly isotopically lighter than seawater, indicating only minor U isotope fractionation during hydrothermal alteration. (4) We compared the U isotope composition of ancient black shales, notably with those from oceanic anoxic events (OAEs), with that of modern organic-rich sediments (mostly from the Black Sea) to investigate whether there are systematic differences according to the global expansion of seafloor anoxia or to the respective local redox conditions. The latter investigations have been performed in close cooperation with another DFG project, where Mo isotope signatures have been used to investigate the expansion of seafloor anoxia during Mesozoic OAEs, and where the suitability of Mo and U isotopes as paleo redox tracers could be compared. One of the most exciting results of our studies was that Mo and U isotopes are frequently negatively correlated in restricted ancient and modern anoxic basins, likely as a result of variable local redox conditions. Thus, combining Mo and U isotopes appear to be the most reliable paleo redox proxy, indicating significantly enhanced seafloor anoxia during the mid-Cretaceous (around OAE 2) and during the Toarcian OAE.
Projektbezogene Publikationen (Auswahl)
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Variations of the 238U/235U isotope composition in rivers and hydrothermal waters. (GV meeting), 2009
Noordmann, J., Weyer, S., Sharma, M. and Georg, R.B.
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Variations of the 238U/235U isotope composition in rivers. Geochimica et cosmochimica acta, Vol. 73 page A951, 2009
Noordmann, J., Weyer, S., Sharma, M. and Georg, R.B.
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(2010). Global enhancement of ocean anoxia during the OAE-2: A quantitative approach using U isotopes. Geology 38, 315-318
Montoya-Pino, C., Weyer, S., van de Schootbrugge, B., Anbar, A.D., Oschmann, W., Arz, H.W., Pross, J.
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(2010). New average values for the 238U/235U Isotope Ratios for Natural Uranium Standards. International Journal of mass spectromentry (short communications) 295, 94-97
Richter S., Eykens R., Kühn H., Aregbe Y., Verbruggen A., Weyer S.
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Constraints on the global cycle of 238U/235U. (DMG meeting), 2010
Noordmann, J., Weyer, S., Sharma, M., Georg, R.B., Rausch, S., Bach, W.
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Fractionation of 238U/235U in rivers and hydrothermal systems: Constraints for the oceanic U isotope cycle. Eos Trans. AGU, Fall Meet. Suppl., Abstract V31B-2330, 2010
Noordmann, J., Weyer, S., Sharma, M. and Georg, R.B., Rausch, S., Bach, W.
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(2011). Uranium isotope fractionation during adsorption to Mn-oxyhydroxides. Environmental Science and Technologies 45, 1370-1375
Brennecka G.A., Wasylenki L.E., Bargar J.R., Weyer S., Anbar A.D.
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Fractionation of 238U/235U during weathering and hydrothermal alteration. Geochimica et cosmochimica acta, Vol. 76, page A1548, 2011
Noordmann, J., Weyer, S., Sharma, M., Georg, R.B., Rausch, S., Bach, W.
