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

过去海洋234U/238U同位素比

• 批准号: 312721254
• 负责人: Professor Dr. Norbert Frank
• 金额: --
• 依托单位: Institut für Umweltphysik
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2016
• 资助国家: 德国
• 起止时间: 2015-12-31 至 2018-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/312721254?language=en
• 关键词: marine; 234U; 238U; isotopic; ratio

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.

234U/238U的海水同位素比率通常被假设为恒定且处于平稳状态，因为U在海水中的表现就像一种类似盐的元素，其来源和汇与U的溶解量相比都很小。Dunk和同事（2000,Chemical Geology）已经证明，海洋U循环具有许多且部分非常大的不确定性，可能存在海水U同位素组成的时间变化。气候驱动的海水234U/238U的变化可能由以下因素驱动：淡水通量及其各自的u同位素组成的变化、陆架地区（特别是红树林）暴露导致的海平面变化、物理和化学风化速率变化导致的大陆侵蚀。此外，碳汇也可能因u的氧化还原敏感性而发生变化。以前的工作（以及本文所包括的数据汇编）表明，在大约2万年前的末次冰期结束时，一定发生了几毫微米的微小变化。然而，由于现有资料存在很大的不确定性和缺乏几乎连续的高分辨率记录，就上述过程的气候驱动变率而言，最难以评估这些变化。此外，234U/238U比值作为u系列测年的质量控制，因此，为了进一步提高海平面重建或海洋放射性碳定标，迫切需要高分辨率、高精度的海水234U/238U时间记录。因此，在这里，我们建议使用一组独特的沉积物岩心，其中包含超过40万年的化石珊瑚碎片，以产生这样一个独特的记录。高精度将通过尖端的多收集器ICPMAS技术实现。将根据上述影响海洋U收支的过程的可能时间和幅度，探讨已记录的同位素变率。