Cuban coral traces annual hydrologically driven variability in δ²³⁴U values since the end of the Little Ice Age

Abstract. The natural uranium isotope ratio of ²³⁴U/²³⁸U in seawater behaves conservatively at basin scale, yet it can be regionally affected by continental freshwater discharge at decadal to centennial timescales. Here, we analyse annual variations in the ²³⁴U/²³⁸U isotope ratio, expressed as ‰-deviation from radioactive equilibrium as δ²³⁴U, of a coral from Cuba. Over the past 237 years, the mean δ²³⁴U value of the coral was 145.6 ± 0.1 ‰ (1 σ_M), which is identical to that of modern open ocean seawater, whereas the average variation over the past century has been ± 3.7 ‰. This moderate variability is, however, significantly greater than the external precision and reproducibility of measurements of ± 0.55 ‰ (2 σ_M). Moreover, the δ²³⁴U values coincide inversely with regional precipitation, suggesting excess ²³⁴U contribution from regional freshwater runoff. The most important finding, is a strong increase in annual δ²³⁴U variability to ± 8.1 ‰ during the end of the Little Ice Age (LIA, 1778–1847). We suggest that the increased δ²³⁴U dynamics reflect substantial excess ²³⁴U contributions from the Mississippi, far greater variability in the local freshwater fluxes to the Gulf of Mexico, and/or reduced advective currents during the LIA. This study demonstrates that yet unexplored variability in coral δ²³⁴U records within the presently known range of seawater δ²³⁴U may be attributed to local and advected freshwater sources, which opens a new pathway for reconstructing these processes over time. Moreover, it places strong constraints on the initial δ²³⁴U variability of fossil corals in light of ultrahigh-precision ²³⁰Th/U dating.