SELECTIVE CHELATION OF METAL SPECIES TO FOSSIL PORPHYRINS AND THEIR PALEOENVIRONMENTAL SIGNIFICANCE: INSIGHTS FROM VANADYL, NICKEL, GALLIUM, AND IRON PORPHYRINS DISTRIBUTIONS IN BLACK SHALES

Abstract

An integrated geochemical investigation of sedimentary porphyrin biomarkers reveals critical insights into metal chelation preferences and their implications for reconstructing paleoenvironmental conditions of black shale deposits. Using reverse-phase liquid chromatography quadrupole time-of-flight mass spectrometry (RPLC-ESI-qTOF-MS), this research examines the distributions of vanadyl (VO), nickel (Ni), gallium (Ga), and iron (Fe) porphyrins in some representative shale sequences, including the late Devonian – early Mississippian Woodford Shale, mid Cretaceous – mid Paleogene Demerara Rise, and the late Cretaceous Eagle Ford Formation. The work identifies a pervasive metal chelation bias, notably VO favoring bicycloalkanoporphyrins (BiCAP) versus Ni complexing with etioporphyrins (Etio), which has the possibility to affect interpretations of the redox-sensitive metal ratios commonly used in paleoenvironmental reconstructions. Further, the study elaborates on the geochemical behavior of gallium and iron porphyrins, explores the geochemical conditions inferred from the occurrence of metalloporphyrins and carotenoids, and expands the understanding of the mechanisms of metalloporphyrin chelation. These findings revise metalloporphyrin based proxies by demonstrating metal and porphyrin chelation selectivity which can potentially affect environmental interpretations of the sedimentary record.