The 2.4Ga Hotazel Formation is a cyclically interlayered sequence of banded iron formation (BIF) and manganese-rich sedimentary rock at the uppermost part of the Neoarchaean-Palaeoproterozoic Transvaal Supergroup in South Africa. It represents an unusual stratigraphic association in the context of the origin of BIF and the coevolution of oxygen and life on early Earth and hence bears special relevance to the environmental conditions and processes that characterized the period leading up to the Great Oxidation Event (GOE) at ca. 2.3Ga. The mineral assemblages that characterize the Hotazel rocks are dominated by carbonate, silicate and oxide minerals, which are traditionally interpreted as predominantly diagenetic in origin, particularly the carbonates. By contrast, primary mineral assemblages are inferred to have been dominated by ferric oxyhydroxides and tetravalent manganese oxides, which show no preservation in the rock record and consequently hinder reconstruction of environmental conditions during sedimentation. Here, we revisit the Hotazel succession with a focus on its bulk-rock and carbonate-specific mineralogical, geochemical and stable isotope (C, Fe) composition by applying for the first time a high-resolution stratigraphic approach to sampling and analysis. Our main aim is to constrain the precursor mineralogy to the Fe- and Mn-rich facies in the Hotazel strata in order to unravel the redox conditions behind the massive cyclic deposition of Fe and Mn at the onset of the GOE. Our carbonate-specific results question traditional diagenetic models for the development of the carbonate fraction of the rocks and instead place the origin of much of the present mineralogy on water-column processes in a stratified basin characterized by successive redox pathways with changing water depth. These pathways exploited a series of thermodynamically predictable electron acceptors for organic carbon recycling, which included – probably for the first time in Earth history – aqueous Mn(III) and O₂ as electron acceptors for the oxidation of both Fe(II) and organic carbon. The emergence of Mn(III) was also critical for the development of a Mn redox shuttle, which led to effective water-column stratification between aqueous Mn and Fe in the depositional basin. We conclude that the first known record of Mn(II) to Mn(III) oxidation as recorded in the Hotazel Formation must be a fundamentally diagnostic step in the redox evolution of the oceans and atmosphere in the lead-up to the GOE.

2.4Ga Hotazel​​ 地层是位于南非新太古代-古元古代德兰士瓦超群最上部的带状铁地层 (BIF) 和富锰沉积岩的循环夹层序列。它代表了 BIF 起源和地球早期氧气与生命共同演化背景下不寻常的地层学关联，因此与导致大氧化事件 (GOE) 的时期特征的环境条件和过程具有特殊相关性约2.3Ga。Hotazel​​ 岩石的矿物组合以碳酸盐、硅酸盐和氧化物矿物为主，传统上认为这些矿物主要是成岩作用，尤其是碳酸盐。相比之下，推断原生矿物组合以氢氧化铁和四价锰氧化物为主，它们在岩石记录中没有保存，因此阻碍了沉积过程中环境条件的重建。在这里，我们通过首次应用高分辨率地层学方法进行采样和分析，重新审视 Hotazel​​ 层序，重点关注其大块岩石和碳酸盐特定的矿物学、地球化学和稳定同位素（C、Fe）组成。我们的主要目标是将前体矿物学限制在 Hotazel​​ 地层中的富铁和富锰相，以揭示 GOE 开始时铁和锰大量循环沉积背后的氧化还原条件。我们的碳酸盐特定结果对岩石碳酸盐部分发育的传统成岩模型提出了质疑，而是将大部分现有矿物学的起源置于层状盆地的水柱过程中，该盆地的特征是连续的氧化还原途径随水深变化。这些途径利用一系列热力学可预测的电子受体进行有机碳循环，其中包括——可能是地球历史上的第一次——含水 Mn(III) 和 O 我们的碳酸盐特定结果对岩石碳酸盐部分发育的传统成岩模型提出了质疑，而是将大部分现有矿物学的起源置于层状盆地的水柱过程中，该盆地的特征是连续的氧化还原途径随水深变化。这些途径利用一系列热力学可预测的电子受体进行有机碳循环，其中包括——可能是地球历史上的第一次——含水 Mn(III) 和 O 我们的碳酸盐特定结果对岩石碳酸盐部分发育的传统成岩模型提出了质疑，而是将大部分现有矿物学的起源置于层状盆地的水柱过程中，该盆地的特征是连续的氧化还原途径随水深变化。这些途径利用一系列热力学可预测的电子受体进行有机碳循环，其中包括——可能是地球历史上的第一次——含水 Mn(III) 和 O₂作为 Fe(II) 和有机碳氧化的电子受体。Mn(III) 的出现对于 Mn 氧化还原梭的发展也至关重要，这导致沉积盆地中 Mn 和 Fe 水溶液之间的有效水柱分层。我们得出结论，在 Hotazel​​ 地层中记录的第一个已知的 Mn(II) 到 Mn(III) 氧化记录必须是 GOE 前海洋和大气氧化还原演化的基本诊断步骤。