Environmental controls on very high 8238U values in reducing sediments: Implications for Neoproterozoic seawater records

Uranium isotopes (8238U) are a widely applied tool for tracing global changes in oceanic anoxia. Interpretation of seawater values and trends, often reconstructed from carbonates, requires knowledge of the U isotope fractionation that occurs during U reduction, typically favouring the heavier 238U isotope relative to 235U. Yet the environmental controls on the expression of isotope enrichment during reduction (A238Uanox) are poorly understood, leading to large uncertainties in interpretation of seawater records. This is particularly limiting for the Neoproterozoic, where exceptionally low inferred seawater 8238U requires very high A238Uanox, which are rarely seen in modern sediments. Here we present a compilation of authigenic 8238U from modern and recent (Mediterranean sapropel) reducing settings to better constrain the first order controls on the expression of large U isotope enrichments. Accompanying geochemical data help identify the dominant mechanisms responsible for high A238Uanox, suggesting they are an expression of limited sedimentary U reduction in weakly euxinic settings or temporally dynamic reducing environments. Such environments are characterised by lower to intermediate organic carbon and uranium accumulation rates (OCAR, UAR) where U reduction appears dominated by nondiffusion-limited processes at the sediment-water interface, on sinking organic matter or within the water column itself. Conversely, under strongly euxinic conditions with higher OCAR and UAR, U reduction occurs mainly under a diffusion-limited regime in the sediment. These findings suggest that the very low seawater 8238U of the Neoproterozoic may be a result of progressive ocean oxygenation and temporally dynamic expansions of anoxia, or the development of weakly euxinic conditions, rather than more widespread or 'intense' anoxia as previously inferred. Such a revised interpretation is more consistent with other geochemical and paleontological records from this time and is critical for understanding the relationship of anoxia to the rise of complex life.