Carbon isotope characteristics of the diaromatic carotenoid, isorenieratene (intact and sulfide-bound) and a novel isomer in sediments

Messinian marls from evaporitic cycle IV of the Gessoso-solfifera formation (Italy) are known to contain in high abundance the diaromatic carotenoid isorenieratene of green sulfur bacterial (Chlorobiaceae) origin along with a second diaromatic carotenoid. The previous tentative assignment of the latter as a cis diastereomer of isorenieratene has now been confirmed; this indicates that double bond isomerization occurs during early diagenesis and provides further evidence for the pathway proposed previously to link isorenieratene to a number of aromatic isoprenoid hydrocarbon biomarkers. Stable carbon isotopic data confirm the Chlorobiaceae origin of the intact all-trans isorenieratene isomer and indicate that the isomerization is accompanied by little or no kinetic isotope effect. Comparison of the δ13C values of each of the isolated carotenoids (measured as isorenieratane after hydrogenation) with those of their sulfide-bound counterpart in the polar fraction of the extracts (also measured as isorenieratane) indicates that sulfurization resulted in little depletion in 13C.