The marine 234U/238U isotopic ratio in the past

海水中234 U/238 U的同位素比值通常被认为是恒定的，并且处于稳定状态，因为U在海水中的行为类似于盐元素，其源和汇与U的溶解量相比很小。Dunk及其同事（2000年，化学地质学）已经证明，海洋铀循环具有许多和部分非常大的不确定性，海水铀同位素组成可能会随时间变化。气候驱动的海水234 U/238 U的变化可能是由淡水通量及其各自的U同位素组成的变化，海平面的变化，由于暴露的大陆架地区，特别是在红树林森林，大陆侵蚀由于物理和化学风化率的变化。此外，汇可能有变化方面的氧化还原敏感性的U。先前的工作（以及这里包括的数据汇编）表明，在大约20000年前的末次冰期结束时，一定发生了几个千分之一的微小变化。然而，由于现有数据的不确定性很大，而且缺乏几乎连续的高分辨率记录，因此很难从气候驱动的上述过程的变率方面评估这种变化。此外，234 U/238 U比值还被用作U系定年的质量控制，因此，为了进一步提高海平面重建或海洋放射性碳校准，迫切需要高分辨率高精度的海水234 U/238 U时间记录。因此，在这里，我们建议使用一套独特的沉积物芯轴承化石珊瑚碎片，年龄跨度超过40万年，以产生这样一个独特的记录。高精度将通过尖端的多集电极ICPMAS技术实现。记录的同位素变异性将探讨上述过程影响海洋铀收支的可能时间和幅度。

The seawater isotopic ratio of 234U/238U is often assumed to be constant and in stationary condition, since U behaves in seawater like a salt like element for which its sources and sinks are small compared to the dissolved amount of U. Dunk and co-workers (2000, Chemical Geology) have demonstrated that the marine U cycle bears numerous and partly very large uncertainties with the possibility of temporal changes of the seawater U isotopic composition. Climate driven changes of seawater 234U/238U might be driven by changes of freshwater fluxes and its respective U-isotopic composition, sea level variations due to the exposure of shelf areas in particular in mangrove forests, continental erosion due to change in the rate of physical and chemical weathering. Also sinks might have changes with respect to the redox sensitive nature of U. Previous work (and the here included data compilation) indicate that small changes of a few permil must have occurred at the end of the last glacial some 20000 years ago. However, due to the large uncertainties of the available data and the lack of a nearly continuous high resolution record it is most difficult to assess such changes in terms of climate driven variability of the above processes. Moreover, the 234U/238U ratio is used as quality control of U-series dating and thus to further improve the reconstruction of sea level or the marine radiocarbon calibration, a highly resolved high precision temporal record of sea water 234U/238U is urgently needed. Here we thus propose to use a set of unique sediment cores bearing fossil coral fragments that span in ages over 400000 years to produce such a unique record. High precision will be achieved from cutting edge Multi-collector ICPMAS technology. The documented isotopic variability will be explored with respect to the possible timing and amplitude of the above processes affecting the marine U budget.