Carbon cycle perturbation and paleoenvironmental changes during the Lower Alum Shale Event (mid-Tournaisian, Mississippian) in southern Euramerican shelf and Palaeotethys Ocean

The Mississippian constituted a time of important global changes in marine environments. 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 related to global transgression connected with increased productivity, sedimentary starvation, collapsed carbonate production, and drastic facies changes from pelagic carbonate sedimentation to the widespread onset of organic-rich black shales, often with phosphate nodules, followed by black siliceous cherts and lydites in pelagic settings in many parts of the world. In contrast to the younger lower-Mississippian event, the Tournaisian Isotope Carbon Excursion (TICE), the LASE was connected with greenhouse climatic conditions associated with increased volcanic activity. The trigger for the mid-Tournaisian event is still a matter of debate, but intense volcanism (including submarine arc and explosive type) and related climate change seems to be a good causes of these environmental perturbation. The LASE interval was previously investigated in terms of high-resolution inorganic geochemistry and framboidal pyrite analyses in the Carnic Alps (Austria), Montagne Noire (France), Rhenish Massif (Germany) and Holy Cross Mountains (Poland), as well as in terms of organic geochemistry in the last area. Paleoenvironmental and paleooceanographic changes during the LASE event must have influenced the global carbon cycle. However, in contrast to inorganic geochemistry, the data on changes in C_org and C_carb isotope signatures were sparse and of low resolution. The aim of our study was to fill this gap. Positive carbon anomalies were often associated with oceanic water eutrophication, however some of recent studies provide new perspectives for decipher changes in δ¹³C_org record, and several negative isotope anomalies have been reinterpreted as a primary signal associated with large-scale thermogenic degassing of light isotopically carbon (¹²C isotope), due to increased volcanic activity. Our isotopic data reveal negative shifts in the Carnic Alps, Montagne Noire, at the beginning of the LASE interval, reflecting a volcanic impact on the global carbon cycle. The record of stable carbon isotopes presents an extremely similar trend in the isotopic curve in the studied sections (except for France), with a negative shift in the lower part of the LASE horizon, and a positive shift in the upper part. The previous results show that on a regional scale, the LASE in the pelagic setting was not uniform, both in terms of redox changes and intensities of volcanism and styles of magmatism. Several regional magmatic centers are considered as potentially responsible for the drastic depositional changes on a local scale and for the bioproductivity increase on a global scale. Their total contribution led to a maximum of climatic warming after the D-C boundary glacial episode, resultant global transgression, and to the development of anoxia in many parts of the world during the mid-Tournaisian, causing extinctions and faunal turnovers in fossil groups that had just recovered from the global Hangenberg Crisis.

This project was financially supported by the grants of the National Science Centre in Poland no. 2023/49/N/ST10/00857.