Mo‐Isotopic Variation in the Mississippian Bowland Shale (Craven Basin, UK) Controlled by Glacio‐Eustasy and Fe‐Shuttling

Molybdenum (Mo) isotopes can be used to provide insights into water-column redox variability and trace-metal fluxes and their underlying climato-environmental controls. Here, we present Mo isotopic compositions (δ98Mo) for the Bowland Shale Formation in three Visean-Serpukhovian (Upper Mississippian) sections of the Craven Basin, UK, that were influenced by glacial-interglacial cycles of the Late Paleozoic Ice Age. Abrupt shifts of δ98Mo values between different redox and salinity conditions in the basinal watermass likely indicate control by sea-level elevation and marginal sill depth: (a) sea-level falls resulted in reduced seawater inflow over marginal sills (hence, lower salinity) coupled to progradation of freshwater from nearby deltas, weakening redox and salinity gradients, which facilitated transfer of 98Mo-depleted molybdenum by an active Fe-oxide particulate shuttle to the seafloor; and (b) sea-level rises produced the opposite pattern, in which establishment of a relatively stable, saline, strongly reducing deep watermass induced reductive dissolution of Fe-oxide particulates in the water column, inhibiting accumulation of adsorbed 98Mo-depleted molybdenum in the sediment. In the uppermost part of the study succession, a shift to brackish and oxic bottom-water conditions accompanied by near-detrital δ98Mo values records the gradual progradation of the Pendle Paleodelta. The peak δ98Mo value of +1.75‰ represents a minimum estimate for the Mo isotopic composition of contemporaneous seawater and may indicate that the average redox state of the global ocean was more reducing in the Carboniferous than in the modern, providing insight into the long-term oxygenation history of the Earth's oceans.