Cuban coral traces annual hydrologically driven variability in <i>δ</i> ²³⁴ U 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 145.7±0.1 ‰ (1σM), 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 ‰ (2σM). 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 ultra high-precision 230Th/U dating.