Co-application of rhenium, vanadium, uranium and molybdenum as paleo-redox proxies: Insight from modern and ancient environments

Abstract

Numerous approaches have been developed for determining past redox conditions in marine settings (e.g., Fe speciation, redox sensitive trace metal (RSTM) systematics, pyrite morphologies, I/(Ca + Mg) ratios), enabling a broad range of redox conditions, from fully oxic to euxinic, to be potentially identified. However, many points along this spectrum remain difficult to constrain, including dysoxic and highly versus weakly euxinic conditions. This limits the broader scale inferences that can be drawn from paleoredox studies, including links between oxygen availability and biological evolution, and the potential for isotope systems (e.g., Mo) to record water column signals. Here, we develop a new approach using RSTM ratios (Re/Mo, Re/U, Re/V, Mo/U), in combination with modified RSTM enrichment factors (EF^⁎) and Mo_EF^⁎-U_EF^⁎ cross-plots, that potentially enables a robust, highly resolved reconstruction of ancient water column redox conditions to be achieved. We initially document the differential behaviour of RSTM EF^⁎ values and ratios in modern settings that range from fully oxygenated, through weakly (30–90 μM O₂) and highly (<30 μM O₂) dysoxic, to anoxic non-sulfidic and euxinic conditions. This redox behaviour is further resolved when the drawdown mechanisms for Mo are evaluated by Mo_EF^⁎-U_EF^⁎ cross-plots. We subsequently ground-truth this approach by considering samples from the Carboniferous Bowland Basin, which have previously been studied for redox conditions via independent geochemical and mineralogical techniques, as well as the Jurassic Kimmeridge Clay Formation, where redox conditions have been defined based on paleoecological characteristics. A strong degree of consistency between RSTM behaviour in modern and ancient settings highlights that weakly to highly dysoxic conditions are characterized by increases in Re/Mo, Re/U and Re/V ratios, accompanied by RSTM EF^⁎ values that only become notably enriched under highly dysoxic conditions. Non-sulfidic water column anoxia is indicated by increased U_EF^⁎ values and low Re/Mo ratios, while euxinia is readily identified by high Mo/U, low Re/U and very low Re/Mo ratios, alongside high Mo_EF^⁎ values. In addition, highly euxinic conditions may be distinguished from weak euxinia by particularly high Mo/U ratios and Mo_EF^⁎ values. This combined approach has the potential to provide a hitherto unprecedented level of insight into paleodepositional redox conditions, and consequently the chemical evolution of the biosphere.