Abstract The 1.1 Ga Midcontinent Rift (MCR) is a thick volcanic-sedimentary succession that forms a curvilinear belt through central North America and crops out along its northern apex around Lake Superior. Sedimentary units of the MCR have been long interpreted as fluvial-lacustrine and invited a number of studies on the early evolution of life in non-marine habitats. One of the key units is the siliciclastic Nonesuch Formation, thought to record deposition in a large lake. However, recent sedimentological observations indicate the presence of marine incursions. To further test this interpretation, we analysed trace element abundances in a broad suite of samples from multiple drill cores through the Nonesuch Formation. We aimed to differentiate geochemical influences of sediment provenance from post-depositional hydrothermal overprint and thereby identify authigenic enrichments in fluid-mobile elements that are indicators of primary environmental conditions. Our results reveal discrete enrichments in Mo and U in organic- and sulphide-rich horizons, which are most parsimoniously interpreted as marine signatures. This conclusion is supported by Sr/Ba ratios, which suggest mixing between freshwater and saltwater, and by rare cm-thick gypsum in the upper Copper Harbor Formation immediately below the Nonesuch rocks. The gypsum displays δ34S values of +25.9 ± 0.6‰, consistent with input of marine sulphate at least during parts of the basin's history. Collectively, our geochemical data support the sedimentological interpretation that this portion of the MCR archives a marine-influenced estuarine system. Although this conclusion rules out that microbial life documented from the MCR was living in exclusively freshwater habitats, the Nonesuch Fm and associated rocks still hold important clues about organisms that were capable of withstanding salinity gradients and bridging the gap between the marine and non-marine environments of the mid-Proterozoic.

中文翻译：

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北美中元古代晚期中大陆裂谷海水的地球化学指纹：海陆分界处的生命

摘要 1.1 Ga 中部大陆裂谷 (MCR) 是一个厚厚的火山沉积层序，它形成了一条穿过北美中部的曲线带，并沿着苏必利尔湖周围的北端出露。MCR 的沉积单元长期以来一直被解释为河流湖泊，并邀请了许多关于非海洋栖息地生命早期进化的研究。关键单元之一是硅质碎屑非此类地层，被认为记录了一个大湖的沉积。然而，最近的沉积学观察表明存在海洋入侵。为了进一步检验这种解释，我们分析了从多个钻芯到 Nonesuch 地层的广泛样本中的微量元素丰度。我们旨在区分沉积物来源的地球化学影响与沉积后热液叠加，从而确定流体流动元素中的自生富集，这些元素是主要环境条件的指标。我们的结果揭示了富含有机物和硫化物的层中 Mo 和 U 的离散富集，这被最简单地解释为海洋特征。这一结论得到了 Sr/Ba 比率的支持，这表明淡水和咸水之间存在混合，以及紧邻 Nonesuch 岩石下方的上铜港组中罕见的厘米厚的石膏。石膏显示的 δ34S 值为 +25.9 ± 0.6‰，至少在盆地历史的某些部分与海洋硫酸盐的输入一致。总的来说，我们的地球化学数据支持沉积学解释，即 MCR 的这一部分记录了受海洋影响的河口系统。尽管这一结论排除了 MCR 记录的微生物生活仅生活在淡水栖息地中，但 Nonesuch Fm 和相关岩石仍然拥有关于能够承受盐度梯度并弥合海洋和非海洋环境之间差距的生物的重要线索中元古代的。