Uranium isotopes in marine carbonates as a global ocean paleoredox proxy: A critical review

The protracted oxygenation of the ocean-atmosphere system is one of the most fundamental changes to the Earth system through its history. The uranium isotopic composition (²³⁸U/²³⁵U, denoted as δ²³⁸U) of marine carbonates has been developed as a proxy to quantitatively track the timing, duration, and extent of global marine redox chemistry changes. This proxy has been applied to many critical evolutionary intervals in the last decade, significantly advancing our understanding of how life on Earth and its environment have co-evolved through geological history. Successful application of the uranium isotope paleoredox proxy requires a thorough understanding of the marine uranium budget, the processes by which seawater U-isotope signatures are recorded in marine carbonates, and the potential for alteration of these primary signatures by syn- and post-depositional diagenetic processes. Here, we provide a critical review of the U isotope proxy in marine carbonates with a focus on the current problems and areas where future work is needed to further develop this proxy. We also use a recently developed global C-P-U cycle model to illustrate that when the carbon cycle is perturbed by volcanic carbon injections, the ensuing transient relationship between seafloor anoxic area and δ²³⁸U can be complex and sometimes counter-intuitive.