CARBON AND NITROGEN ISOTOPIC COMPOSITIONS OF ALKYL PROPHYRINS 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 C31 and C32 butanoporphyrins, values of deltaC-13PDB and deltaN-15AIR averaged -24.0 parts per thousand and -3.1 parts per thousand. In contrast, the C31 and C32 methylpropanoporphyrins, DPEP, and a C30 13-nor etioporphyfin had deltaC-13 and deltaN-15 values averaging -27,5 and -3.3 parts per thousand, 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 parts per thousand. The butanoporphyrins apparently derive from a biological source different from that giving rise to the other porphyrins, their C-13 enrichment not being related to carbon isotopic fractionation accompanying diagenetic reactions. The deltaN-15 values for all the porphyrins indicate that the depletion of N-15 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 N2 may have been the main cause of N-15 depletion.