IMIS

Uranium isotopes (δ²³⁸U) 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 (Δ²³⁸U_anox) are poorly understood, leading to large uncertainties in interpretation of seawater records. This is particularly limiting for the Neoproterozoic, where exceptionally lowinferred seawater δ²³⁸U requires very high Δ²³⁸U _anox, which are rarely seen in modern sediments. Here we present a compilation of authigenic δ²³⁸U 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 Δ²³⁸U_anox, 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 non-diffusion-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 δ²³⁸U 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.