Carbon and nitrogen isotopic compositions of alkyl porphyrins from the Triassic Serpiano oil shale

The carbon and nitrogen isotopic compositions of seven of the most abundant alkylporphyrins from the Serpiano oil shale (marine, Triassic) were determined. For the C 31 and C 32 butanoporphyrins, values of δ 13 C PDB and δ 15 N AIR averaged −24.0‰ and −3.1‰. In contrast, the C 31 and C 32 methylpropanoporphyrins, DPEP, and a C 30 13-nor etioporphyrin had δ 13 C and δ 15 N values averaging −27.5 and −3.3%., respectively. Carbon and nitrogen isotopic values for kerogen averaged −30.8 and −0.9, whereas those for total extract averaged −31.6, and −4.0‰. The butanoporphyrins apparently derive from a biological source different from that giving rise to the other porphyrins, their 13C enrichment not being related to carbon isotopic fractionation accompanying diagenetic reactions. The δ 13 N values for all the porphyrins indicate that the depletion of 15N observed in the kerogen is of primary origin. Consistent with the very high abundance of hopanoids and methyl hopanoids in the aliphatic hydrocarbon fraction, it is suggested that cyanobacterial fixation of N 2 may have been the main cause of 15N depletion.