Microbial activity is often invoked as a direct or indirect contributor to the precipitation of ancient chemical sedimentary rocks such as Precambrian iron formations (IFs). Determining a specific metabolic pathway from the geological record remains a challenge, however, due to a lack of constraints on the initial conditions and microbially induced redox reactions involved in the formation of iron oxides. Thus, there is ongoing debate concerning the role of photoferrotrophy, that is the process by which inorganic carbon is fixed into organic matter using light as an energy source and Fe(II) as an electron donor, in the deposition of IFs. Here, we examine ~2.74-Ga-old Neoarchean IFs and associated carbonates from the Carajás Mineral Province, Brazil, to reconstruct redox conditions and to infer the oxidizing mechanism that allowed one of the world's largest iron deposits to form. The absence of cerium (Ce) anomalies reveals that conditions were pervasively anoxic during IF deposition, while unprecedented europium (Eu) anomalies imply that Fe was supplied by intense hydrothermal activity. A positive and homogeneous Fe isotopic signal in space and time in these IFs indicates a low degree of partial oxidation of Fe(II), which, combined with the presence of ¹³C-depleted organic matter, points to a photoautotrophic metabolic driver. Collectively, our results argue in favor of reducing conditions during IF deposition and suggest anoxygenic photosynthesis as the most plausible mechanism responsible for Fe oxidation in the Carajás Basin.

中文翻译：

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无氧光合作用与 Carajás（巴西）的新太古代铁形成有关

微生物活动通常被认为是古代化学沉积岩（如前寒武纪铁层 (IF)）沉淀的直接或间接贡献者。然而，由于缺乏对初始条件和参与氧化铁形成的微生物诱导的氧化还原反应的限制，从地质记录中确定特定的代谢途径仍然是一个挑战。因此，关于光致铁营养作用的争论一直存在，即使用光作为能源和 Fe (II) 作为电子供体将无机碳固定到有机物质中的过程，在 IFs 的沉积中。在这里，我们检查了来自巴西 Carajás 矿产省的 ~ 2.74-Ga 新太古代 IF 和相关碳酸盐，重建氧化还原条件并推断允许形成世界上最大的铁矿床之一的氧化机制。没有铈 (Ce) 异常表明在 IF 沉积期间条件普遍缺氧，而前所未有的铕 (Eu) 异常意味着 Fe 是由强烈的热液活动提供的。在这些 IF 中，空间和时间上的正且均匀的 Fe 同位素信号表明 Fe (II) 的部分氧化程度较低，这与¹³ C 耗尽的有机物，指向光合自养代谢驱动因素。总的来说，我们的结果支持减少 IF 沉积过程中的条件，并表明无氧光合作用是负责 Carajás 盆地 Fe 氧化的最合理机制。