Closing in on the marine 238U/235U budget
Closing in on the marine 238U/235U budget
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DOI：
10.1016/j.chemgeo.2015.10.041
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发表时间：
2016-01-20
期刊：
影响因子：
3.9
通讯作者：
Och, L.
中科院分区：
文献类型：
--
作者：
Andersen, M. B.;Vance, D.;Och, L.
最近发现，两种长寿命铀母同位素U-238和U-235之间的比值存在显著的氧化还原驱动变化。因此，U-238/U-235系统提供了一个很有前途的新工具，以评估过去的氧化还原变化的地质记录。然而，为了使这种重建成功，需要对现代海洋铀同位素循环进行量化。在这里，我们编译全球溶解河流铀通量的铀同位素比值。总共测量了30个河流样本，涵盖了各种集水面积、纬度、气候和大陆。虽然观察到的变化，themean溶解河流U通量进入海洋的U-238/U-235是接近大陆地壳的平均值，这表明只有轻微的U-238/U-235分馏过程中U风化和运输。这一河流数据得到了减少沉积物中铀同位素数据的补充，沉积物是主要的海洋铀汇。还原沉积物的氧气最低区的海岸华盛顿州美国，显示自生铀积累与升高的U-238/U-235相比，海水，与其他减少海洋沉积物中的观察结果一致。然而，这些数据也强调，沉积物和孔隙水之间的同位素分配可能会导致累积自生铀的U-238/U-235的变化。这项研究的新数据，在现代海洋铀同位素循环数据不断增长的背景下，表明铀在现代海洋中可能处于稳态。(C)2015 Elsevier B. V.版权所有。
Significant redox-driven variability in the ratio between the two long-lived U parent isotopes, U-238 and U-235, has recently been discovered. Thus, the U-238/U-235 system provides a promising new tool to evaluate redox changes in the past using the geological record. For such reconstruction to be successful, however, the modern marine U isotope cycle needs to be quantified. Here we compile U isotope ratios for the global dissolved riverine U flux. A total of 30 river samples covering a range of catchment sizes, latitudes, climates and continents were measured. Although variability is observed, themean U-238/U-235 in the dissolved riverine U flux entering the ocean is close to the average for the continental crust, suggesting only minor U-238/U-235 fractionation during U weathering and transport. This riverine data is complemented by U isotope data in reducing sediments, the main oceanic U sink. Reducing sediments from an oxygen minimum zone off the coast of Washington State USA, show authigenic U accumulation with elevated U-238/U-235 compared to seawater, consistent with observations in other reducing marine sediments. However, these data also highlight that isotope partitioning between sediment and porewater can cause variability in the U-238/U-235 of the accumulated authigenic U. The new data from this study, placed in the context of the growing body of data on modern marine U isotope cycling, suggest that U could be at steadystate in the modern ocean. (C) 2015 Elsevier B.V. All rights reserved.