Towards A Modern Calibration Of The 238U/235U Paleoredox Proxy: Apparent Uranium Isotope Fractionation Factor During U(VI)-U(IV) Reduction In The Black Sea

Abstract:

The isotopic compositions of redox-sensitive metals, including uranium (U), in marine sediments have recently emerged as powerful diagnostic tracers of the redox state of the ancient ocean-atmosphere system. Interpretation of sedimentary isotopic information requires a thorough understating of the environmental controls on isotopic fractionation in modern anoxic environments before being applied to the paleo-record. In this study, the relationship between ocean anoxia and the isotopic fractionation of U was investigated in the water column and sediments of the Black Sea. The Black Sea is the world’s largest anoxic basin and significant removal of U from the water column and high U accumulation rates in modern underlying sediments have been documented. Removal of U from the water column occurs during the redox transition of soluble U(VI) to relatively insoluble U(IV). The primary results of this study are two-fold. First, significant ²³⁸U/²³⁵U fractionation was observed in the water column of the Black Sea, suggesting the reduction of U induces ²³⁸U/²³⁵U fractionation with the preferential removal of ²³⁸U from the aqueous phase. Second, the ²³⁸U/²³⁵U of underlying sediments is related to the water column through the isotope fractionation factor of the reduction reaction but is influenced by mass transport processes. These results provide important constraints on the use of ²³⁸U/²³⁵U as a proxy of the redox state of ancient oceans.