Abstract Marine red beds (MRBs) are common in sedimentary records, but their genesis and environmental implications remain controversial. Genetic models proposed for MRBs variably invoke diagenetic or primary enrichments of iron, with vastly different implications for the redox state of the contemporaneous water column. The Xiamaling Formation (ca. 1.4 Ga) in the North China Platform hosts MRBs that offer insights into the iron cycling and redox conditions during the Mesoproterozoic Era. In the Xiamaling MRBs, well-preserved, nanometer-sized flaky hematite particles are randomly dispersed in the clay (illite) matrix, within the pressure shadow of rigid detrital grains. The presence of hematite flake aggregates with multiple face-to-edge (“cardhouse”) contacts indicates that the hematite particles were deposited as loosely bound, primary iron oxyhydroxide flocs. No greenalite or other ferrous iron precursor minerals have been identified in the MRBs. Early diagenetic ankerite concretions hosted in the MRBs show non-zero I/(Ca+Mg) values and positive Ce anomalies (>1.3), suggesting active redox cycling of iodine and manganese and therefore the presence of molecular oxygen in the porewater and likely in the water column during their formation. These observations support the hypothesis that iron oxyhydroxide precipitation occurred in moderately oxygenated marine waters above storm wave base (likely <100 m). Continentally sourced iron reactivated through microbial dissimilatory iron reduction, and distal hydrothermal fluids may have supplied Fe(II) for the iron oxyhydroxide precipitation. The accumulation of the Xiamaling MRBs may imply a slight increase of seawater oxygenation and the existence of long-lasting adjacent ferruginous water mass.

中文翻译：

---

中元古代华北地台海相红层的形成与铁循环

摘要 海洋红层（MRBs）在沉积记录中很常见，但其成因和环境影响仍存在争议。为 MRB 提出的遗传模型不同程度地引起铁的成岩或初级富集，对同时期水柱的氧化还原状态有很大不同的影响。华北地台的下马岭组（约 1.4 Ga）承载着 MRB，可提供对中元古代时期铁循环和氧化还原条件的见解。在下马岭 MRB 中，保存完好的纳米级片状赤铁矿颗粒随机分散在粘土（伊利石）基质中，处于刚性碎屑颗粒的压力阴影内。赤铁矿片状聚集体与多个面对边缘（“卡房”）接触的存在表明赤铁矿颗粒以松散结合的形式沉积，初级羟基氧化铁絮凝物。在 MRB 中没有发现绿铁矿或其他亚铁前体矿物。MRB 中的早期成岩铁质结核显示非零 I/(Ca+Mg) 值和正 Ce 异常 (>1.3)，表明碘和锰的氧化还原循环活跃，因此孔隙水中存在分子氧，可能在形成过程中的水柱。这些观察结果支持以下假设：在风暴波基部（可能 <100 m）以上的中等含氧海水中发生氢氧化铁沉淀。大陆来源的铁通过微生物异化铁还原而重新活化，远端热液可能为氢氧化铁沉淀提供了 Fe(II)。