The record of the redox changes and submarine volcanic activity during the OAE 2 event (Cenomanian-Turonian boundary) in the Gubbio area (Apennines, Italy)

The Bonarelli event (OAE 2; ~94 Ma) is the second-order extinction event with ~ 26% of marine genera loss, starting from single-celled foraminifera, numerous marine invertebrates, and ending with marine reptiles top predators - ichthyosaurs. Increased submarine volcanic activity is believed to have been the main cause for global climate warming and palaeoceanographic change. Many magmatic centres were active during this period, such as the Caribbean-Columbian Large Igneous Province (LIP), the High Arctic LIP, the Madagascar LIP, the Kerguelen LIP and the Ontong-Java LIP. The close age correspondence between LIPs and biotic overturn suggests that large-scale volcanism could be the main driver of mass extinction. The aim of our research was classic outcrop of the Bonarelli level lying in the Bottaccione Gorge, near Gubbio (Apennines, Italy). To deciphering redox changes at Umbria-Marche basin the U/Th and V/Cr ratios as well as concentration of redox sensitive elements were used (e.g. Mo, U, V, Ni, Cu, Pb, Zn, Se). The values of U/Th ratios in the Bonarelli level (OAE 2) varying from 0.42 to 4.23, while the V/Cr range from 0,59 to 12.48, which is indicative for variable redox conditions ranging from oxic to anoxic-euxinic conditions. More restricted redox conditions in the OAE 2 interval are confirm by enrichments in U (avg. U_(EF) = 19.92, Mo (avg. Mo_(EF) = 568.45) V (avg. V_(EF) = 22.26), Ni (avg. Ni_(EF) = 21.04), Zn (avg. Zn_(EF) = 53,44) and Cu (avg. Cu_(EF) = 47,21. While in the sedimentary background (Scaglia Bianca) values of redox-sensitive trace elements are low e.g. Mo ranging from 0.04to 0.82 ppm, V ranging from 1 to 5 ppb and U often below detection limit (< 0.1 ppm). Volcanic eruptions and submarine hydrothermal activity are the main natural sources of mercury in recent and ancient environments, and are reflected by Hg spikes in sedimentary rocks. We found huge Hg spikes (maximum values >1500 ppb) in the OAE 2 interval with 5 ppb values of sedimentary background of Scaglia Bianca in the classical Bottaccione Gorge. Recently, Hg anomalies were found in the Mentelle Basin, which suggests a regional influence of the Kerguelen LIP located in the Southern Hemisphere. However, our findings of Hg spikes in the Tethys area indicate that the volcanic scenario may be more complex and that the event may be associated with the activity of several, rather than a single, magmatic province. These results are a starting point for research on potential bacterial biomethylation during OAE 2 and the influence of toxic methylmercury on aquatic life, especially top predatory marine reptiles.

 This project was financially supported by the grant of the National Science Centre in Poland (2023/49/B/ST10/00505).