US EPA

3703970 

Journal Article 

Uranium and molybdenum isotope systematics in modern euxinic basins: Case studies from the central Baltic Sea and the Kyllaren fjord (Norway) 

Noordmann, J; Weyer, S; Montoya-Pino, C; Dellwig, O; Neubert, N; Eckert, S; Paetzel, M; Boettcher, ME 

2015 

Yes 

Chemical Geology
ISSN: 0009-2541 

396 

182-195 

10.1016/j.chemgeo.2014.12.012 

WOS:000351670200016 

Recent investigations have revealed significant fractionation of U-238/U-235 between organic-rich sediments of anoxic marginal seas and seawater, indicating redox-dependent U isotope fractionation. This study explores the conditions controlling U isotope fractionation in selected modern anoxic basins (Baltic Sea: Landsort and Gotland Deeps and the Kyllaren fjord in Norway) and compares U with Mo isotope fractionation. Therefore, the concentrations and isotope compositions of dissolved U and Mo from the water column and organic-rich sediments from three currently anoxic basins have been analysed. The water column samples from the Kyllaren fjord display a moderate depletion of U and a strong depletion of Mo with increasing depth. These variations are associated with a decrease in delta U-238 and an increase in delta Mo-98 with depth, from -0.35 parts per thousand to -0.70 parts per thousand and from 2.4 parts per thousand to 2.6 parts per thousand, respectively. From the U isotope composition of the deep euxinic water column of the Kyllaren fjord, a minimum value of Delta U-238(red) approximate to 0.7 parts per thousand for the U isotope fractionation during reduction is inferred. Due to the high sedimentation rate in the Kyllaren fjord, surface sediment samples are only moderately enriched in U and Mo(similar to 4 mu g/g and 6 mu g/g to 37 mu g/g, respectively) and display delta U-238 and delta Mo-98 averages of -0.26 parts per thousand +/- 0.10 parts per thousand and 2.18 parts per thousand +/- 0.21 parts per thousand, respectively. Water column samples from the Baltic Sea display a minor decrease in the U and Mo concentrations and only negligible U and Mo isotope fractionation compared to open seawater. The sediment layers from the Baltic Sea (Landsort Deep), which were deposited under mostly anoxic conditions, are moderately enriched in U (up to 8 mu g/g) and highly enriched in Mo (up to 222 mu g/g). However, U and Mo isotopic compositions are significantly lighter (with delta U-238 of -0.36 parts per thousand +/- 0.08 parts per thousand and delta Mo-98 of -0.04 parts per thousand +/- 0.11 parts per thousand) than those of typical organic-rich sediments from anoxic basins. These findings reveal that compared to Mo, the U isotopic composition of sediments in restricted and strongly euxinic basins like the Kyllaren fjord is significantly more dependent on (1) the extent of U removal from the water column, which is lower than that of Mo, and on (2) the sedimentation rate (i.e., the fraction of authigenic U relative to detrital U in the sediment), which is also lower compared to that of Mo. In more open and only temporarily euxinic basins such as the investigated basins of the Baltic Sea, strong Mo is coupled with weak U isotope fractionation between water and sediment. These signatures were likely the result of isotope fractionation under weakly sulfidic conditions and the generation of isotopically light Mo during frequently occurring flushing events with oxygen-rich water from the open sea. This implies that Mo and U isotope signatures of sediments only record paleo-water column redox conditions of restricted basins if the water column was permanently stratified. (C) 2014 Elsevier B.V. All rights reserved. 

Uranium isotope fractionation in anoxic basins; Molybdenum isotopes; Baltic Sea; Kyllaren fjord; Uranium isotope geochemistry