Uranium isotope fractionation induced by aqueous speciation: Implications for U isotopes in marine CaCO₃ as a paleoredox proxy

Natural variations of ²³⁸U/²³⁵U in marine CaCO₃ rocks are being explored as a novel paleoredox proxy to investigate oceanic anoxia events. Although it is generally assumed that U isotopes in CaCO₃ directly record ²³⁸U/²³⁵U of seawater, recently published laboratory experiments demonstrate slight U isotope fractionation during U(VI) incorporation into abiotic calcium carbonates. This fractionation is hypothesized to depend on aqueous U(VI) speciation, which is controlled by pH, ionic strength, pCO₂ and Mg²⁺ and Ca²⁺ concentrations. Secular variation in seawater chemistry could lead to changes in aqueous U(VI) speciation, and thus, may affect the extent of U isotope fractionation during U(VI) incorporation into CaCO₃. In this study, we combine estimates of seawater composition over the Phanerozoic with a model of aqueous U speciation and isotope fractionation to explore variations in the expected offset between the U isotope composition of seawater and primary marine CaCO₃ through time. We find that U isotope fractionation between U in primary marine CaCO₃ and seawater could have varied between 0.11 and 0.23‰ over the Phanerozoic due to secular variations in seawater chemistry. Such variations would significantly impact estimates of the extent of marine anoxia derived from the U isotope record. For example, at the Permo-Triassic boundary, this effect might imply that the estimated extent of anoxia is ∼32% more extreme than previously inferred. One significant limitation of our model is that the existing experimental database covers only abiotic carbonate precipitation, and does not include a possible range of biological effects which might enhance or suppress the range of isotopic fractionation calculated here. As biotic carbonates dominate the marine carbonate record, more work is need to assess controls on U isotopic fractionation into biotic marine carbonates.