Million-year secular variations in the elemental geochemistry of Devonian marine records and a link to global climate and bioevents: Prague Basin, Czechia

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