The prominent negative stable carbon isotope excursion in both carbonate and organic carbon recorded in organic-rich sediments deposited during the Toarcian oceanic anoxic event (OAE) has commonly been explained by recycling of ¹³C-depleted CO₂ (the so-called Küspert model). More recently, the massive release of ¹³C-depleted methane or other forms of ¹³C-depleted carbon was also proposed to account for the observed negative δ ¹³C excursions in organic carbon of terrigenous as well as of marine origin. The occurrence of diagenetic products of the carotenoid isorenieratene (isorenieratane and other aryl isoprenoids) in Toarcian black shales has been regarded as supporting evidence for the Küspert hypothesis as they point to strong stratification of the epicontinental seas. A section of a drill core straddling the Toarcian of the Paris Basin (Cirfontaine-en-Ornois) contained intact isorenieratane, providing evidence that photosynthetic green sulphur bacteria were present at the time of deposition, even prior to the OAE. However, the isorenieratane abundances are very low in the section where the negative δ ¹³C excursion in organic carbon and phytane, a chemical fossil derived from chlorophyll, occurs. The abundance of the isorenieratene derivatives increases, once the δ ¹³C records have shifted to more positive values. The δ ¹³C of isorenieratane (generally circa −13.1 ± 0.5 ‰) indicates that the respired CO₂ contribution at the chemocline was low and is thus not likely to be the main cause of the prominent up to 7‰ negative δ ¹³C shift recorded in Toarcian organic carbon records.