The uranium isotopic record of shales and carbonates through geologic time

In the modern ocean, U reduction and incorporation into anoxic sediments imparts a large isotopic fractionation of approximately vertical bar 0.6 parts per thousand that shifts the seawater delta U-238 value (U-238/U-235, expressed as delta U-238 per mil deviation relative to CRM-112a) relative to continental runoff. Given the long residence time of U in the modern oceans (similar to 400 kyr), the isotopic composition of carbonates (taken as a proxy for seawater) reflects the global balance between anoxic and other sinks. The U isotopic composition of open-marine carbonates has thus emerged as a proxy for reconstructing past changes in the redox state of the global ocean. A tenet of this approach is that the delta U-238 values of seawater and anoxic sediments should always be fractionated by the same amount.