The proliferation of eukaryotes preceding the Cryogenian Marinoan glaciation (650–635 Ma) and the subsequent radiation of early animals imply a conceivable linkage between global glaciation and complex life evolution. However, the marine redox condition remains enigmatic during the Cryogenian, impeding our understanding of the role of O on this biological innovation. To fill this gap, we comprehensively reported mineralogy, iron isotope (δFe), and iron speciation in the Cryogenian Nantuo Formation at Liulongshan, South China. This section encompasses glacial marine and non-glacial deposits, where the redbeds situated in the middle of the section, in conjunction with the diamictites adjacent to them, divide the Nantuo Formation into three distinct glacial sedimentary cycles. Micro-particle hematite within the Nantuo Formation indicates that the primary Fe-oxide was authigenic and trapped in water column. Furthermore, the Fe isotopic compositions of these authigenic components (δFe) calculated by mass balance equation reveal the redox state of the ocean coinciding with dynamic glaciation. The lower diamictite shows an increasing trend of δFe, Fe/Al, and Fe/Fe, indicative of gradually more anoxic seawater conditions coupled with ice advance. Although redbeds intervals exhibit higher Fe/Fe ratios (∼ 0.51), traditionally indicative of anoxic environments, our integration of mineralogical evidence suggested that the increased Fe content in redbeds might represent increased oxidation of Fe(II) at the redox chemocline, rather than the redox conditions of the bottom waters. This interpretation is consistent with the observed decreasing trend in our Fe isotope data, as well as the sedimentological evolution. Subsequently, the positive and increasing δFe values suggest that the oceanic redox conditions shifted to anoxic as the second glacial episode re-emerged. These findings suggest dynamic fluctuations in the redox condition of the ocean during the Marinoan Ice Age. Specifically, the formation of the redbeds may reflect a locally oxic environment. A modern analogy resembling this situation is blood falls, where the oxidation of iron-rich brine plumes under the Antarctic ice sheets. Such subglacial environments could have potentially provided a habitable “refuge” for early life to survive during the Neoproterozoic glaciation.

中文翻译：

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马里诺冰川期间的海洋氧化还原波动

成冰期马里诺冰川期（650-635 Ma）之前真核生物的增殖以及随后早期动物的辐射意味着全球冰川期和复杂的生命进化之间可能存在联系。然而，在成冰纪期间，海洋氧化还原条件仍然是个谜，这阻碍了我们对 O 在这一生物创新中的作用的理解。为了填补这一空白，我们全面报道了华南六龙山成冰系南沱组的矿物学、铁同位素（δFe）和铁形态。该剖面包括冰海沉积和非冰川沉积，位于该剖面中部的红层与邻近的混积岩一起，将南沱组划分为三个不同的冰川沉积旋回。南沱组内的微粒赤铁矿表明原生氧化铁是自生的并被困在水柱中。此外，通过质量平衡方程计算的这些自生成分（δFe）的铁同位素组成揭示了与动态冰川作用同时发生的海洋氧化还原状态。下层混积岩显示出 δFe、Fe/Al 和 Fe/Fe 的增加趋势，表明随着冰的推进，海水条件逐渐变得更加缺氧。尽管红床区间表现出较高的 Fe/Fe 比率 (∼ 0.51)，传统上表明缺氧环境，但我们对矿物学证据的整合表明，红床中 Fe 含量的增加可能代表氧化还原趋化素处 Fe(II) 的氧化增加，而不是底层水的氧化还原条件。这种解释与我们在铁同位素数据中观察到的减少趋势以及沉积学演化是一致的。随后，正值且不断增加的 δFe 值表明，随着第二次冰川期的重新出现，海洋氧化还原条件转变为缺氧。这些发现表明马里诺冰河时期海洋氧化还原条件的动态波动。具体来说，红床的形成可能反映了局部含氧环境。与这种情况类似的现代类比是“血液瀑布”，即南极冰盖下富含铁的盐水羽流的氧化。这种冰下环境可能为早期生命在新元古代冰川时期的生存提供了一个可居住的“避难所”。