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摘要
Uranium isotope fractionation was studied in the sediment and water of Saanich Inlet, a seasonally anoxic fjord on the east coast of Vancouver Island. The concentration of dissolved U is the same above and below the redoxcline at similar to 120 m depth, with an average delta U-238 value of -0.45 +/- 0.06 parts per thousand (2 s.e.), which is indistinguishable from seawater values reported in other studies. This finding is consistent with water renewal times for the inlet that are roughly seasonal in frequency, thus hiding the impact of the U losses to the anoxic sediment of the inlet. Complete digests of the anoxic sediment yielded higher delta U-238 values than seawater (-0.21 +/- 0.11 parts per thousand (2 s.e.)). By contrast, complete digests of the sediment trap samples yielded lower delta U-238 values than seawater (-0.52 +/- 0.10 parts per thousand (2 s.e.)), and marine plankton from the inlet yielded the lowest delta U-238 value of -1.24 +/- 0.17 parts per thousand(2 sigma). Cross-plots of delta U-238 and delta U-234 vs. Th/U reveal strong correlations indicative of two-component mixing when the data from traps and anoxic inlet sediment are combined. One mixing end-member is fractionated uranium of seawater origin, with a delta U-238 value of 0.17 +/- 0.14 parts per thousand(2 sigma). The other is detrital uranium with a delta U-238 value of -0.83 +/- 0.12 parts per thousand(2 sigma). The detrital end-member is lower than the range of delta U-238 values reported in the literature for granitoid igneous rocks (-0.44 parts per thousand to -0.17 parts per thousand, 2 sigma) (Telus et al., 2012), suggesting that continental weathering fractionates uranium isotopes, with preferential release of delta U-238. Development and application of U isotopes as a paleoredox proxy has its basis in the nuclear volume fractionation. The data from Saanich Inlet meets this expectation, with a positive fractionation factor (Delta(sed(IV))(aq(VI))) of 0.62 +/- 0.17 parts per thousand(2 sigma) calculated as the difference in delta U-238 between authigenic U in anoxic sediments and sediment traps (+0.17 parts per thousand) and U dissolved in seawater (-0.45 parts per thousand). However, it is widely believed that U(VI) reduction in the marine environment occurs on the surfaces of particles and that the negative isotope effect associated with U(VI) sorption to plankton (Delta(plankton)(seawater) = -0.79 perpendicular to 0.17 parts per thousand(2 sigma) opens up the possibility that the particulate pool of seawater derived U(VI) is fractionated from the dissolved pool. Accordingly, the reduction of U(VI) in the marine environment might involve two steps: (1) U(VI) sorption to particles with a negative fractionation, and (2) reduction to U(IV) on particles with a positive fractionation. Environmental factors that are not yet well understood may influence the relative reaction rates for the two steps, thus affecting the magnitude and sign of the overall fractionation. This has the potential to explain conflicting results reported for U(VI) reduction experiments in the literature (laboratory and field), where both positive and negative fractionations have been observed, and in some instances, no fractionation at all. Variation in the fractionation factor has the potential to complicate application of U isotopes as a paleoredox proxy, but this study and a study of the Black Sea (Romaniello, 2012) give the same results (within their respective uncertainties) suggesting that 0.62 +/- 0.17 parts per thousand (2 sigma) is a robust assessment of the apparent U isotope fractionation factor associated with reductive deposition of seawater U(VI) in both anoxic and euxinic marine sediments of the present day.
- 出版日期2015-3-15
- 单位Saskatchewan; Saskatoon
