Redox changes and mercury signature during the Lower Alum Shale Event (mid-Tournaisian, Mississippian) in the Rhenish Massif: Implications for oxygenation history and volcanism in southern Laurussian shelf and Palaeotethys Ocean

Abstract

The mid-Tournaisian Event, also called the Lower Alum Shale Event (LASE), was a global anoxic event that occurred ca. 355 Ma ago. This event is connected to drastic facies changes from pelagic carbonate sedimentation to widespread black organic-rich siliceous shales and radiolarites in many parts of the world. For the first time, high-resolution inorganic geochemistry and framboidal pyrite analyses were applied to decipher depositional condition changes during the mid-Tournaisian anoxic event in the Rhenish Massif, then situated on southern shelf of the Laurussia continent. The lower and middle parts of the LASE interval in the study area were deposited under anoxic conditions at the bottom and water column, while the upper part was deposited under restricted but better‑oxygenated conditions. Additionally, a minor Hg anomaly was found during the LASE deposition in the Rhenish Massif, confirming volcanic overprint during the event. The scenario of palaeoenvironmental changes obtained in the Rhenish Massif is compared to other areas where the LASE was recorded and studied, providing a wider, panregional picture of changes in marine settings during that important Mississippian event. The results obtained in the present study show that the conditions during the LASE in pelagic settings were not uniform, with variable bottom-water oxygenation and different proximity and influence of volcanic activities. The numerous regional magmatic centers are considered as potentially responsible for the drastic depositional changes at a local scale and the bioproductivity increase at the global scale. However, their total contribution caused the climatic turnover responsible for the Gondwana deglaciation and resultant global transgression leading to development of anoxia in many parts of the world during this mid-Tournaisian anoxic event.