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

摘要

Variations of U-238/U-235 ratio (delta U-238) in sedimentary rocks have been proposed as a possible proxy for the paleo-oceanic redox conditions, although the marine delta U-238 system is not fully understood. Here we investigate the spatial variation of delta U-238 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. delta U-238 values in the surface layers show little variation and range from -0.59 parts per thousand to -0.69 parts per thousand. The uniformity of delta U-238 values is consistent with the long residence time of U in modern seawater, although the delta U-238 values are lighter than that of present-day seawater by similar to 0.24 parts per thousand. The light delta U-238 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 (delta U-234 and delta U-238) 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 delta U-238 depth profiles show very limited ranges (-0.57 parts per thousand to -0.67 parts per thousand for MR12-03_D06-R01 and -0.56 parts per thousand to -0.69 parts per thousand 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 delta U-238 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 delta U-234 depth profiles of the same samples show evidence for the possible redistribution of U-234 after deposition. Therefore, the depth profile of delta U-238 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 delta U-238, we calculated seawater delta U-238 with different U sink fluxes into Mn oxides using a simple mass balance model. The results of these calculations suggest that seawater delta U-238 could have varied significantly throughout Earth's history due to changes in the accumulation rate of Mn oxides. (C) 2014 Elsevier Ltd. All rights reserved.