²³⁸U, ²³⁵U and ²³⁴U in seawater and deep-sea corals: A high-precision reappraisal

Uranium isotope ratios are widely utilized in paleoceanography. The ²³⁸U/²³⁵U ratio (expressed as δ²³⁸U) is leveraged as a proxy for the areal extent of anoxic seafloor, and the ²³⁴U/²³⁸U ratio (expressed as δ²³⁴U_sec) tracks riverine and estuarine inputs to the ocean, in addition to featuring prominently in U-series geochronology. Both of these ratios are thought to be recorded by biological carbonates precipitating from seawater, with corals serving as one of the most commonly-used archives of seawater U isotope ratios in the past. The utility of the U isotope proxy in biological carbonate archives relies not only on this faithful archiving of ambient seawater signatures, but also on the homogeneity of the seawater U isotope composition, which enables samples to be leveraged as proxy for the entire ocean.

Here we revisit the foundational assumption of homogeneity of the marine U reservoir, and the capacity of deep-sea corals to record the U isotopic composition of ambient seawater. We begin by evaluating the analytical limits of precision and accuracy achievable for both δ²³⁸U and δ²³⁴U_sec analysis by MC-ICP-MS. We then report data for 45 seawater and 26 deep-sea coral samples from multiple sites around the world. We find subtle δ²³⁸U and δ²³⁴U_sec heterogeneity that correlates with U concentrations, which allows us to calculate new salinity-normalized global mean seawater values for δ²³⁸U (−0.379 ± 0.023 ‰) and δ²³⁴U_sec (+145.55 ± 0.28 ‰). At each site, biological carbonates act as precise archives of the seawater δ²³⁸U value. The same is true for δ²³⁴U_sec, with a few exceptions where samples appear to show vital effects that cause intra-sample ²³⁴U/²³⁸U re-partitioning. In sum, these observations support deep-sea corals as a robust archive of seawater U isotope ratios, but highlight the importance of utilizing multiple sample sites and replicate analyses to overcome coral vital effects (for δ²³⁴U_sec) and subtle marine U isotopic heterogeneity.