Unexpected occurrence and significance of zinc alkyl porphyrins in Cenomanian–Turonian black shales of the Demerara Rise

We analyzed alkyl porphyrins in acetone extracts of Cenomanian–Turonian black shales from the Demerara Rise using reverse phase high performance liquid chromatography and liquid chromatography–tandem mass spectrometry. The major alkyl porphyrins comprise mixtures of free base and metallo porphyrins of the C33 bicycloalkanoporphyrin (BiCAP) structural type. The most abundant porphyrins in the sediments are vanadyl complexes, occurring with varying relative amounts of free base porphyrins, nickel complexes and, unexpectedly, zinc complexed C33 alkylporphyrins. The geochemical conditions that favor production of vanadyl, zinc/nickel and free base porphyrins are very different. Although the geochemical conditions that controlled metal availability were highly variable spatially and temporally over the sample interval, the dominant precursor chlorophyll(s) appears to have remained constant, giving rise to limited structural variation with a dominance of BiCAP structures.