Distributions of vanadyl and nickel porphyrins in the Woodford Shale and selective chelation of metal species by different tetrapyrrole configurations

Abundant vanadyl (VO) and nickel (Ni) porphyrins were detected in the Late Devonian–Early Mississippian Woodford formation shales using a recently developed reverse-phase liquid chromatography quadrupole time-of-flight mass spectrometry with electrospray ionization (RPLC-ESI-qTOF-MS) method. This Woodford shale sequence is characterized by the presence of sustained photic zone euxinia (PZE), as indicated by the occurrence of green sulfur bacteria-produced isorenieratane. The distribution of VO- and Ni-porphyrins, expressed as the VO/(VO + Ni) ratio, reflects a generally reducing/sulfidic condition throughout most of the Woodford deposition. However, in the later Upper Woodford, there is a significant drop in the VO/(VO + Ni) ratio, marking the collapse of anoxia/euxinia due to gradual oxygenation. The depth profiles of VO- and Ni-porphyrin concentrations exhibit a remarkable increase during the peak of PZE in the Middle Woodford, implying that Ni-porphyrin is not completely limited by sulfide. All three categories of porphyrin structures, namely bicycloalkanoporphyrin (BiCAP), deoxophylloerythroethioporphyrin (DPEP), and etioporphyrin (Etio), occurred in VO- and Ni-porphyrins. VO²⁺ exhibits a preferential chelation to BiCAP over the other two types of porphyrins. In contrast, Ni-porphyrins are predominantly composed of Etio structures. Due to such selective binding of metal species to specific porphyrin structures, the relative abundance of VO- and Ni-porphyrins (VO/Ni) exhibits a strong positive correlation (R² > 0.83) with the ratio of BiCAP to Etio (BiCAP/Etio). These findings suggest that the distribution of VO- and Ni-porphyrins in sediments is not solely controlled by the availability of metal ions. When both VO²⁺ and Ni²⁺ are not limited, the molecular configurations of porphyrins determine the relative abundance of VO- and Ni-porphyrins. Further investigations into the influence of biological and diagenetic processes on the production of BiCAP and Etio are crucial and warrant future research endeavors.