Authigenic uranium isotopes of late Proterozoic black shale
Authigenic uranium isotopes of late Proterozoic black shale
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DOI：
10.1016/j.chemgeo.2021.120644
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发表时间：
2021-11
期刊：
影响因子：
3.9
通讯作者：
D. H. Dang;W. Wang;T. Gibson;M. Kunzmann;M. Andersen;G. Halverson;R. D. Evans
中科院分区：
文献类型：
--
作者：
D. H. Dang;W. Wang;T. Gibson;M. Kunzmann;M. Andersen;G. Halverson;R. D. Evans
早期生命的演化与地球环境的变化，特别是大气和海洋中氧气的积累密切相关。然而，中元古代中晚期的环境O2丰度记录很少且存在争议，在此期间出现了许多新的真核生物谱系。在这里，我们展示了加拿大西北部、加拿大北极地区（巴芬岛）、斯瓦尔巴群岛和格陵兰岛晚元古代页岩的铀（U）同位素记录，以及推断自生 U 同位素值（δ238U 自生）的新方法。由我们的新数据和现有文献数据（854 个 δ238U 自生值）组成的汇编表明，在大氧化事件之后，δ238U 自生值持续上升。 δ238U 的逐渐增加可以解释为瞬态氧合事件频率的增加，也可以解释为自生吸收和海水之间 U 同位素分馏因子的变化 (Δ238U)，与地球历史上发生的不同氧化还原条件相关。结合 U 同位素特征，我们使用之前发布的样品中 Fe 形态数据来推断 U 掺入和同位素分馏的局部控制。结果表明，晚元古代海洋主要是铁质的，不时有短暂的氧化期。在这些短暂的含氧条件下，高 U 同位素分馏导致 Δ238U 值相对于 δ238U 地壳高达约 1.2‰。然而，在含铁条件下，较小的同位素分馏导致 Δ238U 值<0.6‰。将我们的研究结论与其他地球化学研究相结合表明，在元古代晚期，全球范围内发生了几次空间局部氧化事件。这些结论有助于在复杂生命早期进化的背景下更好地整合地球化学和化石记录。
The evolution of early life is intimately related to environmental changes on Earth, and in particular, the accumulation of oxygen in the atmosphere and oceans. However, the record of environmental O2abundance in the middle to late Proterozoic Eon, during which many new eukaryotic lineages emerged, is sparse and controversial. Here we present a uranium (U) isotope record from late Proterozoic shales from northwestern Canada, Arctic Canada (Baffin Island), Svalbard, and Greenland, coupled with a novel approach for inferring authigenic U isotope values (δ238Uauthigenic). A compilation comprising our new data and available literature data (854 δ238Uauthigenicvalues) through geologic time indicates a consistent rise in δ238Uauthigenicvalues following the Great Oxidation Event. This gradual increase in δ238U can be interpreted as an increase in the frequency of transient oxygenation events and also as a variation of U isotope fractionation factors between authigenic uptake and seawater (Δ238U) associated with different redox conditions occurring over the Earth's history. In conjunction with the U isotopic signature, we used previously published Fe speciation data from our samples to infer local controls on U incorporation and isotopic fractionation. The results suggest that late Proterozoic oceans were dominantly ferruginous, punctuated by periods of transient oxygenation. During these transient oxic conditions, high U isotope fractionation resulted in Δ238U values as high as ~1.2‰ relative to the δ238Ucrust. However, under ferruginous conditions, smaller isotopic fractionation led to Δ238U values <0.6‰. Integrating conclusions from our study with other geochemical studies suggests the occurrence of several spatially localized oxygenation events across the globe during the late Proterozoic. These conclusions help to better integrate geochemical and fossil records in the context of early evolution of complex life.