Natural variations of the 238U/235U isotope composition: A new paleoredox tracer?

我们建议在陆地近地表环境中研究238U/235U同位素比值的分馏。主要目标是评估观察到的U同位素组成的变化是否可以作为古氧化还原示踪剂。在过去的十年里，人们发现，在自然界中，一度被认为仅限于轻元素的质量分馏，在元素周期表中无处不在。据推测，对于非常重的元素，依赖于体积而不是依赖于质量的同位素分馏可能是主要的机制，因此是每毫微克水平（例如Tl和Hg）观测到的自然变化的原因。我们是世界上第一个在对自然样品进行初步调查期间确实检测到U同位素分馏，最高可达每毫升水平的实验室。U是一种氧化还原敏感元素，在含氧环境中很好溶解，在现代海洋中有很长的停留时间。在海洋减少的时候，它的溶解度和海洋停留时间可能会大大减少。理论考虑和我们自己的初步研究表明，这种氧化还原过程导致铀的显著同位素分馏，因此铀同位素组成可以作为重建古氧化还原条件的示踪剂。铀同位素的其他应用可能在环境科学（铀流动性）或经济地质学（铀矿床的形成）领域。值得注意的是，变化的U同位素组成也可能影响U- pb定年的精度。本研究的目的是：(1)探讨缺氧-缺氧环境中铀还原过程中铀同位素的分馏机制；(2)研究热液活动过程中铀同位素的分馏；(3)寻找合适的古海水U同位素组成记录仪；(4)回溯古环境U同位素与现代环境U同位素的变化对比；(5)将U同位素信息与其他古氧化还原示踪剂的信息进行比较。

We propose to investigate the fractionation of the 238U/235U isotope ratio in terrestrial near surface environments. The major goal will be to evaluate if observed variations in the U isotope composition can serve as a paleoredox tracer. Within the last decade it has been revealed that mass fractionation in nature, once thought to be confined to light elements, is ubiquitous across the periodic table. It has been theorized that for the very heavy elements, volume-dependent rather than mass-dependent isotope fractionation is likely to be the dominant mechanism and thus responsible for the observed natural variations at per mil levels (e.g. for Tl and Hg). We are the first laboratory world-wide that indeed detected U isotope fractionation, up to a per mil level, during preliminary investigations on natural samples. U is a redox-sensitive element, which is well soluble in oxygenated environments and has a long residence time in modern oceans. In times of reduced oceans its solubility and ocean residence time was likely significantly reduced. Theoretical considerations as well as our own preliminary investigations indicate that such redox processes result in significant isotope fractionation for U. Thus U isotope compositions may serve as a tracer to reconstruct paleoredox conditions. . Other applications of U isotopes may be in the fields of environmental sciences (U mobility) or economic geology (formation of U deposits). Notably, variable U isotope compositions may also affect the precision of U-Pb dating. Aims of this study will be (1) to investigate mechanisms of U isotope fractionation during U-reduction in suboxic to euxinic environments, (2) to investigate U isotope fractionation during hydrothermal activity (3) to search for suitable paleo-seawater recorders of the U isotope composition (4) to go back in time and compare U-isotope variations of ancient with modern environments and (5) to compare information from U isotopes with that of other paleoredox tracers.