Abstract The U isotope composition of ancient marine carbonates has been used to determine the δ238U of past oceans based on the premise that the δ238U signatures of marine carbonates reflect the δ238U of seawater at the time of deposition. However, recent work from the Bahamas shows that diagenesis alters the δ238U 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 δ238U 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 δ238U signatures can record the U isotope composition of the seawater in which they lived. To explore whether calcitic brachiopods can more robustly record seawater δ238U 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 δ238U values were relatively stable and lower than present day (δ238U = –0.58 ± 0.11‰, 2sd, n = 17), indicative of a more anoxic ocean state across the positive CIE. We also find that carbonate matrices display a distinctive positive δ238U offset from contemporaneous seawater that varies between beds. Some beds show a ca. +0.30‰ 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 δ238U 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 δ238U 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 δ238U of ancient oceans and the style of diagenesis of U in marine carbonates.

中文翻译：

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解开海洋碳酸盐中铀同位素的成岩历史：使用方解石腕足动物壳追踪晚志留纪海洋的 δ238U 组成的案例研究

摘要 基于海相碳酸盐的δ238U特征反映沉积时海水的δ238U这一前提，古海相碳酸盐的U同位素组成已被用于确定过去海洋的δ238U。然而，巴哈马最近的工作表明，成岩作用通过有效地从海水中产生正和可变的偏移量来改变大块碳酸盐的 δ238U 特征。因此，找到一个能够可靠地记录和保存来自海水的 δ238U 信号的沉积档案变得至关重要。方解石腕足动物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 δ238U 特征可以记录它们生活的海水的 U 同位素组成。为了探索方解石腕足类动物是否能比大块碳酸盐更可靠地记录海水 δ238U，我们分析了志留纪“Lau”碳同位素偏移 (CIE) 上五个不同地层水平的腕足动物壳及其宿主碳酸盐基质。使用腕足动物壳的数据，我们的结果表明海水 δ238U 值相对稳定且低于当前值 (δ238U = –0.58 ± 0.11‰, 2sd, n = 17)，表明在正 CIE 期间海洋状态更加缺氧。我们还发现碳酸盐基质显示出独特的正 δ238U 偏移量，与同期海水不同，不同床层之间存在差异。一些床显示约。+0.30‰ 偏移，类似于巴哈马描述的；而其他人则显示与推断的海水 U 同位素组成没有偏移。我们建议观察到的 δ238U 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。减少的 U(VI) 交换将定义成岩作用的封闭系统样式，并使沉积物和同期海水之间表达的净同位素偏移量减弱。影响 U(VI) 交换程度的一些因素包括进入沉积物的有机物质通量和成岩流体流速，它们可能因相对海平面变化而变化。因此，观察到的被分析碳酸盐基质样品的 δ238U 变异性可能是由所研究地层区间描述的海平面波动引起的环境变化的结果。