Weak and intermittent anoxia during the mid-Tournaisian (Mississippian) anoxic event in the Montagne Noire, France

Abstract

The mid-Tournaisian black radiolarian cherts of the Lydiennes Formation are exposed in deep-shelf successions of the Puech de la Suque and Col des Tribes sections of the Mont Peyroux Nappe area in the Montagne Noire, southern France. This interval represents the mid-Tournaisian anoxic event that is also termed the Lower Alum Shale Event. This event is associated with a global marine transgression that was characterized by increased productivity and drastic facies changes from pelagic carbonate sedimentation to the widespread deposition of black organic-rich siliceous shales and radiolarites in many parts of the world. In the present study, high-resolution inorganic geochemistry and framboidal pyrite analyses were employed to decipher changes in depositional conditions during the mid-Tournaisian anoxic event in the Montagne Noire. The results show that the total organic carbon contents of sediments associated with the Lower Alum Shale Event vary from 0.09 to 1.9 wt %. These low to moderate total organic carbon contents, high U/Th, low Corg/P and intermediate V/Cr ratios, enrichment in redox-sensitive trace elements, such as U, Mo and V, as well as varying sizes of pyrite framboids, indicate periodic dysoxic to anoxic bottom-water conditions during deposition of the studied sediments. Anomalous Hg spikes (>500 ppb) are also reported in the mid-Tournaisian deep-water marine succession of the Montagne Noire in the present study, which confirm a possible influence of increased regional volcanic activity during this environmental turnover.

Chemotrophy-based phosphatic microstromatolites from the Mississippian at Drewer, Rhenish Massif, Germany

“…The Lower Alum Shale Event is also represented by black shales and cherts in the Holy Cross Mountains in Poland and from the Lydiennes Formation in the Montagne Noire, southern France. These black shale deposits provide evidence for dysoxic to anoxic conditions in bottom waters, as well as photic zone euxinia (Rakocinski et al 2021, 2023), probably favoured by enhanced productivity in surface waters. Since the same environmental constraints are probably applicable to the area of the Drewer locality, it suggests that the prolonged sea-level highstand with eutrophication in the photic zone (cf.…”

“…2002; Rakocinski et al . 2023). These conditions are similar to present-day upwelling zones, where the lack of oxygen in sediments and bottom waters facilitates the accumulation of phosphorus released during organic matter remineralization (Lomnitz et al .…”

“…The Lower Alum Black Shale represents a period of reduced carbonate and clastic sedimentation, as well as high degrees of eutrophication and marine organic productivity leading to anoxic conditions in the sediments and possibly in the bottom waters (Siegmund et al 2002;Rakocinski et al 2023). These conditions are similar to present-day upwelling zones, where the lack of oxygen in sediments and bottom waters facilitates the accumulation of phosphorus released during organic matter remineralization (Lomnitz et al 2016).…”

The Požáry section in the Prague Synform – a possible candidate auxiliary section for the basal Emsian GSSP redefinition

“…The moderate correlation between Pb and Al (r = 0.52), indicate that the distribution of this element might be controlled by other factors besides the siliciclastic input -Pb was used as a palaeoproductivity or redox proxy in some studies (e.g. Kumpan et al 2019;Rakociński et al 2021Rakociński et al , 2023. At the Požár 3 section a higher Pb EF values can be observed within and above the BGE interval, i.e.…”