Iron speciation in ancient sedimentary rocks is widely used to reconstruct oceanic redox conditions over geological time, specifically to assess the extent of oxic, euxinic (anoxic containing sulfide), and ferruginous (anoxic containing iron) conditions. We explore how post-depositional sedimentary processes can skew particular geochemical signals in the rock record. One such process is when aqueous sulfide—including that produced in the sediment column—reacts with sedimentary iron, converting non-sulfide, highly reactive iron minerals to iron-sulfide minerals; this can lead to increased preservation of iron as pyrite and an overestimation of seafloor euxinia. We show that sedimentary rocks with higher (>5 wt%) total iron content are more buffered to this effect and thus are a more reliable indicator of true water-column euxinia. When considering this effect in the geological past, we estimate that true euxinia in the mid-Proterozoic may have been as much as fourfold less than previously thought—more in line with other recent paleoredox proxies not based on iron minerals. Marine iron and sulfate concentrations were more equivalent in Proterozoic–Neoproterozoic oceans, suggesting this time period was particularly susceptible to this post-depositional alteration, explaining the extent of euxinia suggested for this geological interval.

中文翻译：

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成岩作用对中元古代沉积岩古氧化还原记录影响的量化

古代沉积岩中的铁形态被广泛用于重建地质时期的海洋氧化还原条件，特别是评估好氧、富氧（含硫化物缺氧）和含铁（含铁缺氧）条件的程度。我们探索沉积后沉积过程如何扭曲岩石记录中的特定地球化学信号。一种这样的过程是当含水硫化物（包括在沉积柱中产生的硫化物）与沉积铁反应，将非硫化物、高活性铁矿物转化为硫化铁矿物；这可能导致铁作为黄铁矿的保存增加，并高估了海底euxinia。我们表明，总铁含量较高（> 5 wt%）的沉积岩更能缓冲这种影响，因此是真实水柱euxinia 更可靠的指标。在考虑地质历史中的这种影响时，我们估计中元古代的真正的euxinia 可能比以前认为的少四倍——更符合最近其他不基于铁矿物的古氧化还原替代物。海洋中的铁和硫酸盐浓度在元古代-新元古代海洋中更为相似，这表明这个时期特别容易受到这种沉积后蚀变的影响，从而解释了这个地质区间所建议的euxinia 的范围。