Untangling the diagenetic history of uranium isotopes in marine carbonates: A case study tracing the δ238U composition of late Silurian oceans using calcitic brachiopod shells
Untangling the diagenetic history of uranium isotopes in marine carbonates: A case study tracing the δ238U composition of late Silurian oceans using calcitic brachiopod shells
复制标题
DOI：
10.1016/j.gca.2020.06.002
复制
发表时间：
2020-10-15
影响因子：
5
通讯作者：
Dahl, Tais W.
中科院分区：
文献类型：
--
作者：
del Rey, Alvaro;Havsteen, Julius C.;Dahl, Tais W.
古海相碳酸盐岩的U同位素组成被用来确定过去海洋的U-238三角洲，其前提是海相碳酸盐的U-238三角洲特征反映了沉积时海水的U-238三角洲。然而，巴哈马最近的研究表明，成岩作用通过有效地与海水产生正的和可变的偏移量，改变了块状碳酸盐的三角洲U-238特征。因此，找到可靠地记录和保存来自海水的U-238三角洲信号的沉积档案就变得至关重要。钙质腕足类贝壳可以提供这样的档案，因为它们的壳结构可以承受成岩作用的影响，因此，它们的增量U-238特征可以记录它们生活的海水的铀同位素组成。为了探索钙质腕足类是否能比块状碳酸盐更有力地记录海水三角洲U-238，我们分析了上志留统‘lau’碳同位素漂移(CIE)上5个不同地层层次的腕足类贝壳及其宿主碳酸盐基质。利用腕足类贝壳的数据，我们的结果表明，海水的U-238值相对稳定，比现在低(U-238=-0.58+/-0.11 ppm，2SD，n=17)，表明CIE处于更低的缺氧状态。我们还发现，碳酸盐基质显示出与不同层位不同的同期海水的独特的正增量U-238偏移。有些岩层显示出约为千分之0.30的偏移量，类似于巴哈马群岛的描述；而另一些岩层则与推断的海水铀同位素组成没有偏移量。我们提出，观测到的U-238偏移量的可变性是缺氧孔隙水和海水之间U(VI)交换程度的函数。铀(VI)交换的减弱将定义一种闭合系统的成岩作用，并减弱沉积物和同期海水之间的净同位素偏移量。影响铀(VI)交换程度的一些因素包括进入沉积物的有机质通量和成岩流体流动速度，这些因素可能会因相对海平面的变化而变化。因此，观测到的碳酸盐岩基质样品的U-238变异性可能是所研究地层段所描述的海平面波动引起的环境变化的结果。我们认为，钙质腕足类贝壳为评价古海洋U-238三角洲和海相碳酸盐岩中U的成岩类型提供了重要的档案。(C)2020爱思唯尔有限公司。保留所有权利。
The U isotope composition of ancient marine carbonates has been used to determine the delta U-238 of past oceans based on the premise that the delta U-238 signatures of marine carbonates reflect the delta U-238 of seawater at the time of deposition. However, recent work from the Bahamas shows that diagenesis alters the delta U-238 signatures of bulk carbonates by effectively creating a positive and variable offset from seawater. Therefore, finding a sedimentary archive that reliably records and preserves the delta U-238 signal from seawater becomes fundamental. Calcitic brachiopod shells may provide such an archive, as their shell structure can endure the effects of diagenesis and, thus, their delta U-238 signatures can record the U isotope composition of the seawater in which they lived. To explore whether calcitic brachiopods can more robustly record seawater delta U-238 than bulk carbonates, we analyzed brachiopod shells and their host carbonate matrices from five different stratigraphic levels across the upper Silurian 'Lau' Carbon Isotope Excursion (CIE). Using the data from brachiopod shells, our results suggest that seawater delta U-238 values were relatively stable and lower than present day (delta U-238 = -0.58 +/- 0.11 parts per thousand, 2sd, n = 17), indicative of a more anoxic ocean state across the positive CIE. We also find that carbonate matrices display a distinctive positive delta U-238 offset from contemporaneous seawater that varies between beds. Some beds show a ca. +0.30 parts per thousand offset, similar to what is described in the Bahamas; whereas others display no offset from the inferred seawater U-isotope composition. We propose the variability of the observed delta U-238 offset to be a function of the degree of U(VI) exchange between anoxic porewaters and seawater. A diminished U(VI) exchange would define a closed-system style of diagenesis and mute the net isotope offset expressed between sediments and contemporaneous seawater. Some of the factors affecting the degree of U(VI) exchange include organic matter fluxes into the sediments and diagenetic fluid flow rates, which could vary due to relative sea-level changes. As a result, the observed delta U-238 variability of the analyzed carbonate matrix samples could be the result of the environmental changes caused by the sea-level fluctuations described for the studied stratigraphic interval. We conclude that calcitic brachiopod shells provide an important archive for evaluating the delta U-238 of ancient oceans and the style of diagenesis of U in marine carbonates. (c) 2020 Elsevier Ltd. All rights reserved.