Depositional settings and speciation control vanadium isotopic fractionation in black shales

Vanadium (V) stands out as a relatively abundant and remarkably redox-sensitive metal that can be found in multiple oxidation states. In this study, we combine X-ray Absorption Near Edge Structure (XANES) speciation analyses and stable V isotope composition measurements with vanadium bulk concentration data. By applying this approach to samples from the extensively studied Cambrian-Ordovician Alum Shale Formation in Baltic Paleobasin, we refined our understanding of the V use as a paleo-redox proxy. Vanadium concentrations in the studied samples ranged from 798 to 2286 mg/kg, whilst bulk δ51V values were highly variable, ranging from −0.01‰ to −0.95‰. XANES analyses revealed two main V species, V(+IV)-S and V(+III)-O, whereas ca. 26% of the latter is present in an early diagenetic V-rich illite-type clay mineral structure. In addition, we report the first direct V(+IV)-O-porphyrin detection in black shale using XANES analysis. Our results show a negative correlation between δ51V values and V(+III)-O species, and a positive correlation between δ51V values and V(+IV)-S species that also covaries positively with the increasing basinal restriction. This highlights the effects of depositional conditions and burial pathways on V isotope fractionation in shales. Additionally, our data imply the presence of two distinct environmental zones in the eastern facies of the Alum Shale formation in Estonia: in the west zone the sediments were initially formed under mild to weakly euxinic conditions, which later became more oxic; in the east zone, sedimentation was primarily controlled by the combination of increasing basinal restriction and temporal redox changes.