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

Abstract

The Cenomanian–Turonian black shales of the Demerara Rise (DR) contain a wide range of tetrapyrroles and allow porphyrin analysis through Oceanic Anoxic Event 2 (OAE 2), making this site unique. The sediments provide an excellent opportunity to study the factors controlling the stratigraphic distribution and N isotopic composition of porphyrins. Three C33 bicycloalkanoporphryins (BiCAPs) are present as free bases (FBs) and as complexes with Zn and VO in quantities sufficient for compound-specific isotope analysis (CSIA) and were the focus of this study. The stratigraphic distribution of BiCAPs is controlled, foremost, by metal availability rather than sedimentary Eh/pH conditions or post depositional maturity. Titration of the local water column metal reservoir with sulfide and organic matter during OAE 2 resulted in a high concentration of FB BiCAPs and a very low concentration of metallo-BiCAPs. Conversely, high metal concentration occurs in sediments above and below the OAE, and Zn and VO porphyrin abundances mirror bulk metal concentration. The highest total concentration of porphyrins was where metal concentration was highest, suggesting that porphyrin preservation is enhanced by the increased stability of metal complexes. Unexpectedly, the total concentration of porphyrins was lowest during the heart of OAE 2, in an interval of higher total organic carbon (TOC) content, where enhanced preservation would be expected; we suggest this was the result of decreased preservation of extractable tetrapyrroles in the absence of available metals. The high reactivity of Zn2+ with sulfide limited the formation of Zn complexes to the non-sulfidic regions in the water column or at the sediment/water interface. The formation of VO porphyrins occurred exclusively within the sediments from available FBs or through transmetallation with other metallo-BiCAPS (Zn, Ni, Cu).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.