High abundances of the diaromatic carotenoid isorenieratene derived from photosynthetic green sulfur bacteria (Chlorobiaceae) were found just below the chemocline in an anoxic fjord in Norway, throughout the annual cycle. The stable carbon isotope composition of this carotenoid co-varied with the δ 13 C of CO 2 (aq) and is independent of the CO 2 and isorenieratene concentration. This constant isotopic fractionation ɛ p of isorenieratene versus CO 2 , 4±1‰, was subsequently used in the reconstruction of δ 13 C of CO 2 at the chemocline in ancient oceans and lakes. These reconstructions indicate that δ 13 C of CO 2 at the chemocline is often influenced by isotopically light CO 2 , formed by remineralization of organic matter. This process can, depending on the depth and stability of the chemocline, also effect the isotopic composition of the phytoplankton and, thus, isotopic records of sedimentary inorganic and organic carbon.