A new vanadium species in black shales: Updated burial pathways and implications

Examining vanadium (V) geochemistry in organic-rich rocks might provide new insights into the redox evolution of paleo ocean and atmosphere through Earth's history. Previous work, mainly based on experiments and thermodynamic predictions, suggested that V is mainly bound to oxygen (O) atoms and/or O[sbnd]N (nitrogen) groups with oxidation states ranging from +III to +V, when removed from the water column. This conceptual model represented our understanding of the burial processes of V in sedimentary archives. However, until our study, the speciation of V had not been investigated in ancient sediments. For the first time, micro-focused X-ray Absorption Near Edge Structure (µ-XANES) analysis is applied to characterize V speciation in the late Cambrian – Early Ordovician, organic-rich Alum Shale of the Scandinavian region. The result shows the presence of a new V(+IV)–S structure that largely dominates the V speciation (>80%), subordinated by a V(+III)–O structure (<20%) in our samples deposited under euxinic conditions. The V(+IV)–S/∑V ratio seems to be influenced by the intensity of euxinic conditions. We suggest that the new V(+IV)–S structure may have been formed under strongly sulfidic conditions. Organic matter is most likely to be the dominant host phase for this newly identified species. Additionally, we propose an updated model describing the processes involved during the burial of V under a wide range of redox conditions. This model shows that the V speciation has the potential to provide a more nuanced picture of the redox conditions that prevailed at the time of deposition.