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Published on: July 1, 2014
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Uranium isotope fractionation by abiotic reductive precipitation
Shaun T Brown1,2, Anirban Basu3,2,4, Xin Ding3
1Energy Geosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720; stbrown@lbl.gov.
Summary
Abiotic reduction of uranium (U) causes significant U isotope fractionation, contrary to previous beliefs. This process, observed during reductive precipitation onto iron monosulfide, is influenced by removal rates and calcium-uranium-carbonate species.
Area of Science:
- Geochemistry
- Environmental Science
- Isotope Geochemistry
Background:
- Uranium (U) isotope fractionation was previously thought to be exclusively mediated by biological processes.
- Abiotic U reduction in natural environments is common, but its potential for isotope fractionation was largely unexplored.
Purpose of the Study:
- To investigate whether abiotic reduction of aqueous uranium can lead to significant isotope fractionation.
- To identify the environmental factors controlling the extent of U isotope fractionation during abiotic reduction.
Main Methods:
- Experiments involving reductive precipitation of aqueous U onto synthetic iron monosulfide.
- Controlled variation of U removal rates and aqueous Ca-U-CO3 species concentrations.
- Measurement of U isotope ratios in solution and solid phases.
Main Results:
- Significant U isotope fractionation was observed during abiotic reductive precipitation.
- Fractionation magnitude increased with decreasing U removal rates.
- Increasing concentrations of neutrally charged Ca-U-CO3 species enhanced U isotope fractionation.
Conclusions:
- Abiotic U isotope fractionation is a significant process in reducing environments, particularly those with Ca concentrations ≥ 1 mM.
- Major ion concentrations, by influencing U speciation, directly impact the extent of U isotope fractionation.
- This finding has critical implications for interpreting past anoxic conditions in Earth's history, especially in ancient oceans.
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