Isorenieratane record in black shales from the Paris Basin, France: Constraints on recycling of respired CO₂ as a mechanism for negative carbon isotope shifts during the Toarcian oceanic anoxic event

Abstract

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 13 C‐depleted CO 2 (the so‐called Küspert model). More recently, the massive release of 13 C‐depleted methane or other forms of 13 C‐depleted carbon was also proposed to account for the observed negative δ 13 C 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 13 C excursion in organic carbon and phytane, a chemical fossil derived from chlorophyll, occurs. The abundance of the isorenieratene derivatives increases, once the δ 13 C records have shifted to more positive values. The δ 13 C of isorenieratane (generally circa −13.1 ± 0.5 ‰) indicates that the respired CO 2 contribution at the chemocline was low and is thus not likely to be the main cause of the prominent up to 7‰ negative δ 13 C shift recorded in Toarcian organic carbon records.