Abstract The sedimentary record is affected by periodic and/or random variations in Earth's near-surface conditions, such as the Milankovitch band variations, which are responsible for small-scale cyclicity in distal marine settings. However, the controlling factors of normal field-scale sedimentary cycles operating on Myr-scale that dominate the stratigraphic record remain largely enigmatic. The Lower to Middle Devonian succession (Lochkovian to Givetian) of the Prague Basin, Czechia, is represented by an alternation of largely deep-marine carbonates and shales with typical duration of ~5–7 Myr. We studied the elemental geochemistry of bulk rock samples supported by XRD mineralogy and TOC concentrations. Stratigraphic distribution of the organic-productivity proxies, redox proxies, Ce/Ce* and Pr/Pr* anomalies, and TOC, supported by lithology, allochem composition and field gamma-ray spectrometry, indicates that the Prague Basin was governed by two alternating depositional modes. The oligotrophic mode is characterised by low values of productivity proxies and U/Th ratios, good bottom water oxygenation, heterotrophic skeletal producers, and facies typical of homoclinal ramp settings. The mesotrophic mode is characterised by high U/Th, elevated organic productivity, less oxic bottom conditions, relatively abundant autotrophic skeletal producers and deposition on a distally steepened ramp. The modes were in phase with the Devonian climatically driven environmental changes. We assume that the elevated silicate weathering rates during warmer periods, as previously reported, resulted in higher nutrient input to the seas setting the marine system into the mesotrophic mode. Cooler periods led to oligotrophic water conditions. The global Devonian bioevents show recurrence intervals of the same order as the trophic modes but they typically coincided with the transition between the modes. It is suggested that marine biotic assemblages and the associated carbonate production became unstable during perturbations of the carbon cycle.

中文翻译：

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泥盆纪海洋记录元素地球化学的百万年长期变化以及与全球气候和生物事件的联系：捷克布拉格盆地

摘要 沉积记录受地球近地表条件的周期性和/或随机变化的影响，例如米兰科维奇带变化，这是造成远端海洋环境中小尺度周期性的原因。然而，在 Myr 尺度上运行的正常现场尺度沉积旋回的控制因素在地层记录中占主导地位，这在很大程度上仍然是个谜。捷克布拉格盆地的下到中泥盆纪序列（Lochkovian 到 Givetian）的代表是大部分深海碳酸盐岩和页岩的交替，典型的持续时间为~5-7 Myr。我们研究了 XRD 矿物学和 TOC 浓度支持的大块岩石样品的元素地球化学。由岩性支持的有机生产力指标、氧化还原指标、Ce/Ce* 和 Pr/Pr* 异常以及 TOC 的地层分布，异体化学成分和场伽马射线光谱法表明，布拉格盆地受两种交替沉积模式控制。贫营养模式的特点是生产力代理值和 U/Th 比值低、底部水氧合良好、异养骨骼生产者和典型的同斜斜坡设置相。中营养模式的特点是 U/Th 高、有机生产力提高、底部含氧量少、自养骨骼生产者相对丰富，并且沉积在远端陡峭的斜坡上。这些模式与泥盆纪气候驱动的环境变化同步。我们假设，如先前报道的那样，温暖时期硅酸盐风化率升高导致向海洋输入更多营养物质，从而使海洋系统进入中营养模式。较冷的时期导致贫营养水条件。全球泥盆纪生物事件显示出与营养模式相同顺序的重复间隔，但它们通常与模式之间的过渡重合。这表明海洋生物组合和相关的碳酸盐生产在碳循环的扰动期间变得不稳定。