Molybdenum speciation as a paleo-redox proxy: a case study from Late Cretaceous Western Interior Seaway black shales

Molybdenum (Mo) geochemistry is widely used to reconstruct ocean oxygenation throughout Earth’s history. However, gaps in our fundamental knowledge of Mo burial within sediments hinder the utility of Mo as a proxy for paleo-redox reconstruction. To improve our understanding of Mo burial pathways and sedimentary Mo speciation, we present combined geochemical analysis and X-ray absorption fine structure spectroscopy from black shales deposited in the North American Western Interior Seaway (WIS) during the Late Cretaceous. Molybdenum was present in multiple phases during deposition of the black shales, probably as a combination of Mo(IV) and Mo(VI) species with no more than 54% of Mo associated with Mo-sulfide phases. These Mo molecular geochemistry results indicate that sediments were not deposited under permanent euxinic conditions, but rather weakly euxinic, periodically euxinic, or anoxic conditions (with sulfide restricted to pore waters). This is the case even in samples where other redox indicators would indicate strongly euxinic conditions. Additionally, differences in Mo speciation between our two localities highlight variability in the intensity of oxygen limitation within the WIS, providing a more nuanced redox reconstruction from sections deposited in conditions between weakly and strongly euxinic, suggesting that Mo molecular geochemistry may allow for more detailed spatial reconstructions of past stratified anoxic basins and oxygen minimum zones.