Controls on the stratigraphic distribution and nitrogen isotopic composition of zinc, vanadyl and free base porphyrins through Oceanic Anoxic Event 2 at Demerara Rise

The formation of the metal complexes had a N isotope effect. We observed significant differences in the δ15N values of the three most abundant BiCAPs. VO BiCAPs were systematically 15N-depleted by an average of 2.5 ± 1.5‰ relative to FBs, and Zn BiCAPs were equivalent to FB BiCAPs within error (15N enriched by + 0.1‰) but were variable (±1.5‰). The δ13C values of the Zn, VO and FB BiCAPs were equivalent, suggesting that the three compounds share a common chlorophyll source, in agreement with structural data. The significant difference in δ15N values between the VO and FB BiCAPs suggests that δ15N values of VO porphyrins are controlled by N isotopic effects during VO porphyrin formation. Under experimental conditions, the formation of Zn octaethylporphyrin had a fractionation (εproduct/reactant −3.9) consistent with the differences between FB and VO BiCAPs, supporting a diagenetic origin for the differences in δ15N. The δ15N values of the porphyrins in these ancient sediments have been used to reconstruct the δ15N value of the primary phototrophic biomass and the ecological balance of chlorophyll a producing organisms via the εpor proxy. If the δ15N values of porphyrins are indeed altered by metal-ligand reactions, the application for paleoenvironmental reconstruction may require a more complete analysis of the range of metalloporphyrins in a given structural class. The covariance of bulk, BiCAP, porphyrin class and kerogen δ15N through OAE 2 suggests that the variability in δ15N reflects primary N cycle dynamics and that the εpor proxy can be viable in ancient sediments despite these complexities.