Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Résumé Figures Only Full Text Full Text (PDF) Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Emmanuel, L. Articles by Rey, J. Search for Related Content GeoRef GeoRef Citation

The "Schistes carton" of Quercy (Tarn, France): a lithological signature of a methane hydrate dissociation event in the Early Toarcian. Implications for correlations between Boreal and Tethyan realms

¹ Laboratoire Biominéralisations et Paléoenvironnements – J.E. 2477, Univ. P. & M. Curie, 4 place Jussieu, 75252 Paris cedex 05. laurent.emmanuel{at}upmc.fr
² Univ. Paul Sabatier, Lab. Dynamique des Bassins Sédimentaires, 39 allées Jules Guesde, 31062 Toulouse cedex 4.
³ Univ. H. Poincaré-Nancy 1, UMRCNRS 7566 G2R, bd des Aiguillettes, BP 239, 54506 Vendoeuvre-les-Nancy cedex.

The geochemical events (negative shifts in ¹³C and ¹⁸O, and Mn peak) associated with the "Schistes carton" of Quercy (SW France) do not appear to be due only to lithological variations and a differential diagenesis. They correspond to the Lower Toarcian global event and seem to be connected to a destabilization phase of gas hydrates. During its oxidation by seawater, the release of methane gas with a very low carbon isotopic ratio (– 60%) led to the production of CO₂ and carbonates with a negative ¹³C. The consumption of oxygen in the seawater resulted in conditions that were first dysoxic and then anoxic. The reduction of burrowing and bioturbation associated with this oxidation resulted in the laminated appearance of the "Schistes carton". The decrease in the oxygen content led to a decrease in MnO₂ microparticles present in the water column and sediments. This created a complementary source of Mn²⁺ that was incorporated into the carbonates produced during this event. Furthermore, an increase in the average seawater temperature (¹⁸O) appears associated with this event. Finally, the apparent diachronism of geochemical events in the Boreal and the Tethyan realms is discussed.

Key Words: Toarcian • Anoxia • Methane hydrates • Stable isotopes • Carbon • Oxygen • Manganese • Subsidence