δ98/95 Mo records in ferromanganese crust suggest downward movement of Oxygen Minimum Zones in the early Eocene

Abstract The seawater Molybdenum isotopic composition has been thought to have undergone little change over the Cenozoic Era, compared to other redox sensitive isotope systems, such as Carbon and Sulfur. However, newly reconstructed Cenozoic Era seawater δ 98/95 Mo values, based on analysis of a hydrogenic ferromanganese (Fe-Mn) crust obtained from the Pacific Ocean, reveal a significant decrease in seawater δ 98/95 Mo values from 2.80% at 53 Ma to 2.34% at 36.7 Ma. By utilizing a steady-state mass balance model, we observed that the primary factor contributing to this decline was likely the shrinkage of oxygen minimum zones (OMZ) in shallow shelf environments. OMZ sediments on shallow shelves typically exhibit lighter Mo isotope signatures than those on deep slopes and reduced burial of lighter Mo isotopes would reduce the seawater δ 98/95 Mo value. Coincidentally, this decline temporally coincided with the initial arc collision and subsequent closure of the foreland basin between India and Asia. Therefore, we consider that the seawater δ 98/95 Mo may reflect the gradual contraction of the shallow shelf and downward migration of sediment along the continental margin in the early Eocene.