Fluctuating seafloor oxygenation before, during and after the Frasnian/Famennian crisis: Quantitative colour analysis and geochemistry of Upper Devonian marine red beds, Montagne Noire, S. France

Early diagenetic precipitation of red iron oxide (hematite) in most marine red beds (MRB) makes these sediments suitable archives for seafloor redox conditions. A composite succession of upper Frasnian to lower Famennian red pelagic limestones was studied at two sections in the Montagne Noire, South of France including the Coumiac base of the Famennian GSSP section containing the Upper and Lower Kellwasser (LKW and UKW) horizons of global dysoxia/anoxia. Data from facies analysis, gamma-ray spectrometry, magnetic susceptibility, Vis-diffuse reflectance and X-ray fluorescence spectroscopy with 5 to 25 cm resolution at the section were supplemented by optical and scanning electron microscopy, electron microprobe analysis and laser-ablation inductively coupled mass spectrometry of thin sections. Submicron hematite crystals in the red facies are closely associated with authigenic clays, pitted microspar, microstromatolites, microborings, and Frutexites microproblematica indicating a microbial control on its precipitation at or very close to the sediment–water interface. The hematite precipitation was driven by iron cycling accompanied by cycling of redox-sensitive elements (Mn, Cu, V, Mo, U) along (micro-)redox gradients within the sediments under oxic seafloor conditions, supported by low sedimentation rates. The red stratigraphic succession is punctuated by eight, ~ 0.5 to ~ 1.5 m thick horizons including the LKW and UKW, with non-red colours and elevated concentrations of redox-sensitive elements reflecting various stages of bottom dysoxia to anoxia. The horizons show a recurrence time of ~ 0.85 Myr to ~ 2.0 Myr and they are interpreted as repeated phases of seafloor dysoxia/anoxia due to vertical fluctuations of the oxygen minimum zone culminating during the Frasnian/Famennian crisis.