EGU General Assembly 2021
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Record of the Toarcian oceanic anoxic event in the Grands Causses Basin (southern France) and its implications for vertebrate preservation

Brahimsamba Bomou1, Guillaume Suan2, Jan Schlögl3, Anne-Sabine Grosjean4, Baptiste Suchéras-Marx5, Thierry Adatte1, Jorge Spangenberg6, Stéphane Fouché7, Axelle Zacai8, Corentin Gibert8, Jean-Michel Brazier9, Vincent Perrier2, Peggy Vincent10, Kévin Janneau11, and Jeremy E. Martin2
Brahimsamba Bomou et al.
  • 1University of Lausanne, Institute of Earth Sciences, Lausanne, Switzerland (
  • 2Université de Lyon, UCBL, ENSL, CNRS, UMR 5276 LGL-TPE, 69622 Villeurbanne, France
  • 3Department of Geology and Paleontology, Comenius University, Mlynska dolina G, 84215, Bratislava, Slovakia
  • 4Paleorhodania, 69006 Lyon - France
  • 5Aix Marseille Univ, CNRS, IRD, INRAE, Coll France, CEREGE, Aix-en-Provence, France
  • 6Institute of Earth Surface Dynamics (IDYST), University of Lausanne, Geopolis, CH-1015 Lausanne, Switzerland
  • 7Musée de Lodève, 1 place Francis Morand, 34700, Lodève, France
  • 8PALEVOPRIM Laboratoire Paléontologie Evolution Paléoécosystèmes Paléoprimatologie UMR CNRS 7262, Université de Poitiers, 86073 Poitiers Cedex 9, France
  • 9Institute of Applied Geosciences, Graz University of Technology, Rechbauerstrasse 12, A-8010 Graz, Austria
  • 10Sorbonne Université (CNRS-MNHN-UPMC) – Muséum national d'Histoire naturelle, Département Origines et Evolution, UMR CNRS-MNHN-UPMC 7207, Centre de Recherche en Paléontologie-Paris (CR2P), 57, rue Cuvier - 75231 Paris Cedex 05, France
  • 11Université de Strasbourg (UNISTRA), Jardin des sciences, 12 Rue de l'Université, 67000 Strasbourg, France

Paleontological excavations realized by our group in Toarcian shales (Lower Jurassic) of the Grands Causses Basin in Roqueredonde (Hérault, France), yielded several specimens of marine vertebrates. The newly discovered specimens are partly or entirely preserved in anatomical connection and include a partial ichthyosaur skeleton with soft tissues, and a 4 m-long thalattosuchian longirostrine marine crocodile. A multi-proxy approach has been developed (XRD-bulk and clay mineralogy, Rock-Eval pyrolysis, phosphorus and mercury contents) in order to replace these findings in a well-defined temporal and paleoenvironmental context, and hence constrain the factors that led to their remarkable preservation. The fossiliferous succession exposes a 3 m-thick upper Pliensbachian interval of marl and nodular carbonate beds, overlain by a 3 m-thick interval of lower Toarcian laminated shales and limestone beds. Our high-resolution ammonite biostratigraphy, combined with inorganic and organic carbon isotope chemostratigraphy, shows that the fossiliferous Toarcian strata were deposited at a time of global warming and major carbon cycle perturbation known as the Toarcian Oceanic Anoxic Event (T-OAE). The studied succession shows several similarities with the classical coeval fossiliferous levels of the Posidonia Shale in SW Germany, including high organic matter and hydrocarbon contents as well as extremely reduced sedimentation rates. These results indicate that the unusual richness in well-preserved vertebrates of the studied site can be explained by a combination of warming-induced, low salinity and stratified waters, prolonged seafloor anoxia and reduced dilution by low carbonate and terrigenous input due to rapid sea-level rise. Our results also reveal a significant peak in mercury at the base of the T-OAE interval, consistent with that recorded in several coeval sections (e.g. Portugal, Morocco, Argentina, Chile). This mercury anomaly, most likely resulting from intense volcanic activity Karoo-Ferrar large igneous province, suggests that widespread exceptional vertebrate preservation during the T-OAE was initiated by a suite of severe environmental perturbations ultimately triggered by intense volcanic emissions.

How to cite: Bomou, B., Suan, G., Schlögl, J., Grosjean, A.-S., Suchéras-Marx, B., Adatte, T., Spangenberg, J., Fouché, S., Zacai, A., Gibert, C., Brazier, J.-M., Perrier, V., Vincent, P., Janneau, K., and Martin, J. E.: Record of the Toarcian oceanic anoxic event in the Grands Causses Basin (southern France) and its implications for vertebrate preservation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11835,, 2021.


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