The most notable trend in the sedimentary iron isotope record is a shift at the end of the Archean from highly variable δ⁵⁶Fe values with large negative excursions to less variable δ⁵⁶Fe values with more limited negative values. The mechanistic explanation behind this trend has been extensively debated, with two main competing hypotheses: (i) a shift in marine redox conditions and the transition to quantitative iron oxidation; and (ii) a decrease in the signature of microbial iron reduction in the sedimentary record because of increased bacterial sulfate reduction (BSR). Here, we provide new insights into this debate and attempt to assess these two hypotheses by analyzing the iron isotope composition of siderite concretions from the Carboniferous Mazon Creek fossil site. These concretions precipitated in an environment with water column oxygenation, extensive sediment pile dissimilatory iron reduction (DIR) but limited bacterial sulfate reduction (BSR). Most of the concretions have slightly positive iron isotope values, with a mean of 0.15‰ and limited iron isotope variability compared to the Archean sedimentary record. This limited variability in an environment with high DIR and low BSR suggests that these conditions alone are insufficient to explain Archean iron isotope compositions. Therefore, these results support the idea that the unusually variable and negative iron isotope values in the Archean are due to dissimilatory iron reduction (DIR) coupled with extensive water column iron cycling.

中文翻译：

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高氧环境中的底栖铁循环：对太古宙沉积铁同位素记录的解释。

在沉积铁同位素记录最显着的趋势是在太古代的从高度可变δ端部的移位⁵⁶的Fe的值具有大的负偏移到更小可变δ ⁵⁶Fe值具有更有限的负值。对此趋势背后的机理解释进行了广泛的辩论，其中有两个主要的相互竞争的假设：（i）海洋氧化还原条件的转变和向定量铁氧化的转变；（ii）由于细菌硫酸盐还原作用（BSR）的增加，沉积记录中微生物铁还原作用的减少。在这里，我们为这场辩论提供了新的见解，并试图通过分析石炭纪马宗溪化石遗址中菱铁矿凝结物的铁同位素组成来评估这两个假设。这些固结物在水柱充氧，大量沉积物堆异化铁还原（DIR）但细菌硫酸盐还原（BSR）受限的环境中沉淀。大多数固结体的铁同位素值略为正，与太古代沉积记录相比，平均值为0.15‰，铁同位素变异性有限。在高DIR和低BSR的环境中，这种有限的可变性表明，仅凭这些条件不足以解释太古宙铁同位素组成。因此，这些结果支持了太古代中异常可变的铁同位素值和负铁同位素值是由于异化铁还原（DIR）以及广泛的水柱铁循环而导致的。