• journal_title：Bulletin de la Societe Geologique de France
• Contributor：Laurent Emmanuel ; Maurice Renard ; René Cubaynes ; Marc de Rafelis ; Michael Hermoso ; Laurence Lecallonnec ; Antoine le Solleuz ; Jacques Rey
• Publisher：Societe Geologique de France
• Date：2006-
• Format：text/html
• Language：en
• Identifier：10.2113/gssgfbull.177.5.237
• journal_abbrev：Bulletin de la Societe Geologique de France
• issn：0037-9409
• volume：177
• issue：5
• firstpage：237
• section：Articles

The geochemical events (negative shifts in δ¹³C and δ¹⁸O, 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 δ¹³C. 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 (δ¹⁸O) appears associated with this event. Finally, the apparent diachronism of geochemical events in the Boreal and the Tethyan realms is discussed.