Marine red beds are usually interpreted as indicating water column oligotrophy, good bottom-water oxygenation and redox conditions. Lower Devonian successions of the Prague Basin, Czechia, exhibit a distinct centimetre to metre-scale alternation of layers of marine red beds, grey carbonates, marls and black shales. In order to understand why the redox potential fluctuated so rapidly, reflectance spectroscopy, microscopy, elemental geochemistry data and stable isotopes of Mo have been analysed in this paper. Whilst the grey and black facies only contain goethite, the marine red beds are enriched with synsedimentary and early diagenetic, submicronic hematite, which is present in micrite, skeletal interiors, microstromatolites and oncoids. It was formed by microbially mediated precipitation, the replacement of detrital Fe phyllosilicates, and/or by the oxidation of microbially precipitated Fe-bearing aluminosilicate precursors. The marine red beds are frequently enriched in Fe, depleted in U, V, Mo and Cu, and show negative δ⁹⁸Mo values indicating oxic conditions. Peloidal micrite, microbial coatings and cements with the marine red beds exhibit positive (up to 9) Ce/Ce* anomalies. The non-red facies show opposite patterns. This geochemical variability is probably related to Mn oxyhydroxide cycling and organic matter remineralization along the sediment subsurface redox gradient, particularly by reactions between pore water and various elemental pools. These patterns, combined with the centimetre-scale colour alternation of the sediments, may reflect redox zonation that has been preserved beneath the ancient seafloor. Four zones are recognized: (i) the oxic zone of Fe-oxide precipitation (marine red beds); suboxic zones of (ii) Fe enrichment, and (iii) U-Mo enrichment; and (iv) suboxic–anoxic zone of Cu, V (± Mo) enrichment. The presented model of a marine red bed origin from redox reactions in the sediment subsurface contradicts models of the formation of marine red beds through iron enrichment from Fe²⁺ supersaturated ocean waters following periods of ocean anoxia.

中文翻译：

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海洋红层地下氧化还原梯度的地球化学记录：以捷克泥盆纪布拉格盆地为例

海洋红层通常被解释为表明水体贫营养、良好的底水氧化和氧化还原条件。捷克布拉格盆地的下泥盆统层序显示出明显的厘米到米级海相红层、灰色碳酸盐岩、泥灰岩和黑色页岩层的交替。为了理解氧化还原电位波动如此之快的原因，本文对Mo的反射光谱、显微镜、元素地球化学数据和稳定同位素进行了分析。虽然灰色和黑色相仅含有针铁矿，但海相红层富含同沉积和早期成岩的亚微米赤铁矿，存在于泥晶、骨骼内部、微叠层石和巨粒陨石中。它是由微生物介导的沉淀形成的，取代碎屑铁页硅酸盐，和/或通过微生物沉淀的含铁铝硅酸盐前体的氧化。海相红层常富集 Fe，贫 U、V、Mo 和 Cu，δ 呈负值⁹⁸Mo 值表示好氧条件。具有海洋红层的球状泥晶、微生物涂层和胶结物表现出正（高达 9）Ce/Ce* 异常。非红色相显示出相反的模式。这种地球化学变化可能与 Mn 羟基氧化物循环和沿沉积物地下氧化还原梯度的有机物再矿化有关，特别是通过孔隙水和各种元素池之间的反应。这些图案与沉积物厘米级的颜色交替相结合，可能反映了古老海底下保存的氧化还原带。识别出四个区域：(i) 氧化铁沉淀的好氧带（海洋红层）；(ii) Fe 富集和 (iii) U-Mo 富集的低氧区；(iv) Cu、V (± Mo) 富集的低氧-缺氧区。海洋缺氧期后²⁺过饱和海水。