The onset of oxidative weathering traced by uranium isotopes
The onset of oxidative weathering traced by uranium isotopes
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DOI：
10.1016/j.precamres.2019.105583
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发表时间：
2020-03-01
影响因子：
3.8
通讯作者：
Weyer, S.
中科院分区：
文献类型：
--
作者：
Brueske, A.;Martin, A. N.;Weyer, S.
光合作用的发明是地球历史上的一个关键时期，引发了海洋和大气演变的重大变化。许多研究表明，早在古元古代大氧化事件（GOE，2.45-2.32 Ga）之前，氧水平就已经提高了。然而，光合氧化的开始时间，以及大气和海洋氧气水平的时空波动是高度争议的。在这里，我们提出了一个新的和大量的铀（U）同位素数据集，包括海洋沉积物，如黑色页岩，碳酸盐岩和富铁沉积岩，从哈默斯利盆地（西澳大利亚州），卡普瓦尔和津巴布韦盆地（南部非洲）的沉积年龄范围从3.4到2.2 Ga。通过分析U-234沿着δ U-238来监测次近期U活动，并通过[Al]/[U]或[Th]/[U]比值来监测碎屑U贡献。相对于后太古代澳大利亚页岩（PAAS），具有显著自生铀富集（EFu > 2）的样品显示自生δ U-238范围为- 0.96至-0.02 ppm（+/- 0.05 ppm，2 s.d.）。长期再现性），这明显偏离大陆地壳的典型三角洲U-238（千分之-0.40至-0.20）。值得注意的是，我们发现增加三角洲U-238的变化在上Ghaap组（2.50-2.47 Ga，碳酸盐和黑色页岩）和较低的比勒陀利亚组（2.42-2.32 Ga，Duitschland组，页岩）的Kaapvaal盆地。这些样品的主要轻U同位素组成可能是由U同位素分馏有关的开始部分U动员在缓慢氧化风化的晶质铀矿不久之前和GOE期间。在Timeball Hill地层（覆盖Duitschland地层）中缺乏低Δ U-238值，表明晶质铀矿和其他含U矿物（具有有限U同位素分馏）在（现代风格）氧化风化过程中发生了定量U动员。这些有限的U同位素变化在古元古代Timeball山组测量进一步表明，在现代海洋相反，在GOE之后的时期，U在很大程度上定量减少，并从德兰士瓦盆地的水柱中去除，主要是在深度缺氧。
The invention of photosynthesis was a key interval in Earth's history, initiating major changes in the evolution of the oceans and atmosphere. Many studies suggest that oxygen levels were already enhanced before the Paleoproterozoic Great Oxidation Event (GOE, 2.45-2.32 Ga). However, the timing of the onset of photosynthetic oxygenation, as well as spatio-temporal fluctuations of atmospheric and oceanic oxygen levels are highly debated. Here, we present a new and substantial uranium (U) isotope dataset comprising marine sediments, e.g. black shales, carbonates and iron-rich sedimentary rocks, from the Hamersley Basin (Western Australia), Kaapvaal and Zimbabwe Cratons (both southern Africa) with depositional ages ranging from 3.4 to 2.2 Ga. Sub-recent U mobilization was monitored by analyzing U-234 along with delta U-238, and detrital U contribution was monitored with [Al]/[U] or [Th]/[U] ratios. Samples with significant authigenic U enrichment (EFu > 2), relative to post-Archean Australian Shale (PAAS), exhibit authigenic delta U-238 ranging from - 0.96 to -0.02 parts per thousand (+/- 0.05 parts per thousand, 2 s.d. long-term reproducibility), which deviates markedly from typical delta U-238 of the continental crust (-0.40 to -0.20 parts per thousand). Remarkably, we find increased delta U-238 variability in the upper Ghaap Group (2.50-2.47 Ga, carbonates and black shales) and the lower Pretoria Group (2.42-2.32 Ga, Duitschland Formation, shales) of the Kaapvaal Craton. The predominantly light U isotope composition of those samples is likely explained by U isotope fractionation related to the onset of partial U mobilization during slow oxidative weathering of uraninite shortly before and during the GOE. The lack of low delta U-238 values in the Timeball Hill Formation (overlying the Duitschland Formation) indicate the onset of quantitative U mobilization during (modern-style) oxidative weathering of uraninite and other U-bearing minerals with limited U isotope fractionation. These limited U isotope variations measured in the Paleoproterozoic Timeball Hill Formation further suggest that, in contrast to modern oceans, in the period following the GOE, U was largely quantitatively reduced and removed from the water column of the Transvaal basin which remained mainly anoxic at depth.