Uranium isotope systematics of ferromanganese crusts in the Pacific Ocean: Implications for the marine ²³⁸U/²³⁵U isotope system

Variations of ²³⁸U/²³⁵U ratio (δ²³⁸U) in sedimentary rocks have been proposed as a possible proxy for the paleo-oceanic redox conditions, although the marine δ²³⁸U system is not fully understood. Here we investigate the spatial variation of δ²³⁸U in modern ferromanganese (Fe-Mn) crusts by analyzing U isotopes in the surface (0-3mm depth) layer of 19 Fe-Mn crusts collected from 6 seamounts in the Pacific Ocean. δ²³⁸U values in the surface layers show little variation and range from -0.59‰ to -0.69‰. The uniformity of δ²³⁸U values is consistent with the long residence time of U in modern seawater, although the δ²³⁸U values are lighter than that of present-day seawater by ~0.24‰. The light δ²³⁸U values are consistent with the isotope offset observed in previously reported adsorption experiment of U to Mn oxide. These results indicate that removal of U from seawater to Mn oxide is responsible for the second largest U isotope fractionation in the modern marine system, and could contribute to isotopically heavy U to seawater. Depth profiles of U isotopes (δ²³⁴U and δ²³⁸U) in two Fe-Mn crusts (MR12-03_D06-R01 and MC10_CB07_B), dated by Os isotope stratigraphy, were investigated to reconstruct the evolution of the oceanic redox state during the Cenozoic. The δ²³⁸U depth profiles show very limited ranges (-0.57‰ to -0.67‰ for MR12-03_D06-R01 and -0.56‰ to -0.69‰ for MC10_CB07_B), and have values that are similar to those of the surface layers of Fe-Mn crusts. The absence of any resolvable variation in the δ²³⁸U depth profiles may suggest that the relative amounts of oxic and reducing U sinks have not varied significantly over the past 45Myr. However, the δ²³⁴U depth profiles of the same samples show evidence for the possible redistribution of ²³⁴U after deposition. Therefore, the depth profile of δ²³⁸U in Fe-Mn crusts may have been also overprinted by later chemical exchange with pore-water or seawater, and may not reflect the paleo-oceanic environmental changes. To assess the potential role of U removal by Mn oxides on seawater δ²³⁸U, we calculated seawater δ²³⁸U with different U sink fluxes into Mn oxides using a simple mass balance model. The results of these calculations suggest that seawater δ²³⁸U could have varied significantly throughout Earth's history due to changes in the accumulation rate of Mn oxides.