Iron mobilization during continental weathering was pervasive before the Great Oxidation Event (GOE) that started at around 2.43 billion years (Ga) ago, due to the soluble nature of reduced iron. However, various geochemical proxies indicate transient oxygenation during deposition of the Mesoarchean (∼2.95 Ga) Pongola Supergroup, South Africa, which suggests that continental weathering could have also occurred under transiently oxic conditions before the GOE. We analyzed trace elemental and Fe, Ti, and Cr isotopic compositions of the ca. 2.95 Ga Denny Dalton paleosol in the Pongola Supergroup to better understand continental weathering and redox conditions in the ancient critical zone, and the nature of geochemical fluxes from the continents to the oceans and marine sediments. Iron isotope systematics are consistent with a model where Fe was released during intense leaching from the paleosol to concentrate in Fe-rich groundwaters in the deeper part of the soil horizon. We show for the first time that Fe isotopic fractionation during Mesoarchean continental weathering was limited, and Fe enrichments and depletions are coupled with those of divalent transition metals, Co, Ni, and particularly Zn. This suggests that Fe redox cycling was not involved in paleosol formation, and Fe²⁺ was mobilized under anoxic conditions. Chromium isotopes are also unfractionated relative to the parent igneous rock in this paleosol, which precludes removal of isotopically heavy Cr⁶⁺ and thus supports anoxic continental weathering. We show that previously reported Cr isotopic fractionation in another Denny Dalton paleosol profile does not follow a Cr⁶⁺ leaching trend, but instead scales with Cr enrichment and may reveal Cr enrichment from post-burial fluids. Thus, there is no clear evidence for an oxidative continental weathering during deposition of the Pongola Supergroup. Titanium isotopes are not significantly fractionated in the paleosol, suggesting that continental weathering and erosion in the Archean did not fractionate Ti isotopes. Similarly, Ni/Co and Th/Sc ratios are reasonably conserved, which validates their use as a robust proxy for upper continental crust composition in shales, whereas La/Sc, Cr/Zn and Cr/U ratios are highly variable relative to the provenance composition, which suggests that caution should be used when applying these ratios in shale studies of the ancient upper continental crust composition.

由于还原铁的可溶性，大风化事件（GOE）始于大约24.3亿年（Ga）之前，大陆风化过程中的铁动员已经很普遍。然而，各种地球化学指标表明，在南非Mesoarchean（〜2.95 Ga）Pongola超群沉积过程中发生了短暂的氧化作用，这表明在GOE之前的短暂有氧条件下也可能发生了大陆风化。我们分析了痕量元素和铁，钛和铬的同位素组成。2.95位于Pongola超群中的Ga Denny Dalton古土壤，以更好地了解古代临界区的大陆风化和氧化还原条件，以及从大陆到海洋和海洋沉积物的地球化学通量的性质。铁同位素系统学与一个模型相吻合，在该模型中，Fe从古土壤大量浸出时被释放出来，集中在土壤深层的富铁地下水中。我们首次表明，在美拉奇海大陆风化过程中，铁同位素分馏作用是有限的，并且铁的富集和富集与二价过渡金属，钴，镍，尤其是锌的富集和耗竭结合在一起。这表明铁的氧化还原循环不参与古土壤的形成，而铁 特别是锌。这表明铁的氧化还原循环不参与古土壤的形成，而铁 特别是锌。这表明铁的氧化还原循环不参与古土壤的形成，而铁在缺氧条件下动员²⁺。铬同位素相对于该古土壤中的母火成岩也是未分离的，这排除了同位素重质Cr ^6+的去除，因此支持了缺氧性大陆风化。我们表明，先前在其他Denny Dalton古土壤剖面中报告的Cr同位素分馏并未遵循Cr ⁶⁺浸出趋势，但随着Cr富集而增加，并可能显示出后埋藏液中Cr的富集。因此，没有明确的证据表明在Pongola超群的沉积过程中存在氧化性大陆风化。钛同位素在古土壤中没有显着地分馏，这表明太古代的大陆风化和侵蚀并未使Ti同位素分馏。同样，合理地保留了Ni / Co和Th / Sc的比值，这证实了它们可作为页岩上大陆壳成分的有力替代品，而La / Sc，Cr / Zn和Cr / U的比值相对于物源变化很大组成，这表明在对古代上陆壳成分的页岩研究中应用这些比率时应谨慎行事。