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 (0-3 mm 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 45 Myr. 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.