Abstract Despite iron and trace element proxies informing much of our insight into Earth’s oxygenation history, the processes that controlled their accumulation and distribution in ancient sedimentary environments are not fully understood. Furthermore, deciphering between primary, depositional signals and oxidative weathering alteration poses a substantial challenge to reconstructing Earth’s redox history using its sedimentary record. Here, we present a multi-proxy geochemical investigation of three transgressive shale intervals from the ca. 811 Ma Reefal assemblage (upper Fifteenmile Group) in Yukon, Canada to help resolve these issues. Systematic stratigraphic trends from low to high FeHR/FeT and FeT/Al compositions across shale intervals represent shoaling of a discrete redoxcline that separated oxygenated surface waters from anoxic, ferruginous deep waters during marine transgression. In some cases, these trends are followed by a symmetric fall, which represents a full “transgressive-regressive cycle” of the redoxcline. The high proportion of glauconite to total illite indicate short-term oscillating redox conditions as the redoxcline migrated across the sediment-water interface during relative sea-level rise. These conditions invigorated dissimilatory iron reduction (DIR) within anoxic sediment pore waters, which released Fe(II) into the overlying water column where it was oxidized to iron oxy(hydr)oxide and shuttled downslope. High rates of DIR within sediment replete with highly reactive iron oxy(hydr)oxide and organic matter removed isotopically light, bio-available iron, rendering the residual sediment depleted in FeT/Al relative to the detrital baseline and enriched in 56Fe. The low iron content of the detrital flux to the basin (Fe/Al ∼ 0.3) rendered the sediment susceptible to changes in its bulk δ56Fe composition by modification of its authigenic components alone and enabled fractionation from DIR to drive bulk-rock δ56Fe values up to 0.8‰. Anomalously high concentrations of redox-sensitive elements in three samples (up to 5792 ppm Cr, 586 ppm Mn, 726 ppm Mo, and 3509 ppm Ni) and their relative distribution patterns show similarities to co-enrichment due to particulate shuttling within a weakly restricted basin. However, SEM-EDS element maps and large-area BSE image mosaics show the remains of framboidal pyrite “ghosts” with secondary enrichment of redox-sensitive elements. Thus, these geochemical trends in redox-sensitive trace metals most likely reflect mobilization and re-accumulation following pyrite dissolution during outcrop weathering. The low primary pyrite content and local transformation of pyrite iron to iron oxy(hdr)oxide suggest that δ56Fe and FeHR/FeT data were unaltered despite indications of pyrite weathering. Ultimately, this dataset elucidates key aspects of sedimentary redox cycling directly preceding evolutionary milestones and a major perturbation to the global carbon cycle and also provides a template for evaluating the effects of outcrop weathering on commonly used sedimentary redox proxies.

中文翻译：

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加拿大育空地区 811 Ma 十五英里群海侵页岩层段的地球化学特征：从风化蚀变中解开沉积氧化还原循环

摘要 尽管铁和微量元素替代物为我们提供了对地球氧化历史的大部分见解，但控制它们在古代沉积环境中积累和分布的过程尚未完全了解。此外，破译原始沉积信号和氧化风化改变对利用其沉积记录重建地球的氧化还原历史构成了重大挑战。在这里，我们提出了一个多代理地球化学调查从大约三个海侵页岩层段。位于加拿大育空地区的 811 Ma Reefal 组合（上 Fifteenmile Group）帮助解决这些问题。整个页岩层段从低到高 FeHR/FeT 和 FeT/Al 组成的系统地层趋势代表了离散氧化还原线的浅滩，该氧化还原线将含氧地表水与缺氧、海侵期间的含铁深水。在某些情况下，这些趋势之后会出现对称下降，这代表了氧化还原反应的完整“海进-回归循环”。海绿石占总伊利石的高比例表明，在相对海平面上升期间，随着氧化还原碱迁移穿过沉积物-水界面，存在短期振荡氧化还原条件。这些条件激发了缺氧沉积物孔隙水中的异化铁还原 (DIR)，将 Fe(II) 释放到上覆水柱中，在那里它被氧化成铁氧（氢）氧化物并穿梭下坡。沉积物中高速率的 DIR 充满了高反应性的氧化铁氧（氢）氧化物和有机物，以同位素轻的、生物可利用的铁去除，使残留沉积物相对于碎屑基线耗尽 FeT/Al 并富含 56Fe。流向盆地的碎屑通量的低铁含量 (Fe/Al ∼ 0.3) 使沉积物容易通过单独改变其自生成分而对其大块 δ56Fe 组成发生变化，并能够从 DIR 进行分馏以驱动大块岩石 δ56Fe 值高达0.8‰。三个样品中异常高浓度的氧化还原敏感元素（高达 5792 ppm Cr、586 ppm Mn、726 ppm Mo 和 3509 ppm Ni）及其相对分布模式显示出由于颗粒在弱限制条件下穿梭而导致的共富集盆地。然而，SEM-EDS 元素图和大面积 BSE 图像马赛克显示了具有氧化还原敏感元素二次富集的枝状黄铁矿“幽灵”的遗迹。因此，这些对氧化还原敏感的痕量金属的地球化学趋势很可能反映了露头风化过程中黄铁矿溶解后的流动和再积累。低原生黄铁矿含量和黄铁矿铁向氧化铁氧（hdr）氧化铁的局部转化表明，尽管有黄铁矿风化迹象，但 δ56Fe 和 FeHR/FeT 数据没有改变。最终，该数据集阐明了直接在进化里程碑之前的沉积氧化还原循环的关键方面以及对全球碳循环的主要扰动，并且还提供了用于评估露头风化对常用沉积氧化还原代理的影响的模板。低原生黄铁矿含量和黄铁矿铁向氧化铁氧（hdr）氧化铁的局部转化表明，尽管有黄铁矿风化迹象，但 δ56Fe 和 FeHR/FeT 数据没有改变。最终，该数据集阐明了直接在进化里程碑之前的沉积氧化还原循环的关键方面和对全球碳循环的主要扰动，并提供了评估露头风化对常用沉积氧化还原代理的影响的模板。低原生黄铁矿含量和黄铁矿铁向氧化铁氧（hdr）氧化铁的局部转化表明，尽管有黄铁矿风化迹象，但 δ56Fe 和 FeHR/FeT 数据没有改变。最终，该数据集阐明了直接在进化里程碑之前的沉积氧化还原循环的关键方面以及对全球碳循环的主要扰动，并且还提供了用于评估露头风化对常用沉积氧化还原代理的影响的模板。