| 24 Oct 2025
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 σM), 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 σM). 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.
The paper by Greve et al presents a very neat record of ∂234U variability within a coral core from northern Cuba, whose variability is interpreted to reflect hydrological changes. The manuscript includes an analytical validation of the methodology, to verify that the observed variability of few permil is beyond the analytical uncertainty and are accurate. The authors also compare the results to a contemporaneous ∂18O record from a stalagmite collected from a nearby cuban cave.
I find the work very interesting and worth publishing, yet I find that most of the hypothesis about the potential processes controlling ∂234U in the coral somewhat speculative and more evidence about the processes that might be controlling ∂234U in seawater would be required to make a stronger case. It would be interesting, for example, to compare to other geochemical proxies in the coral itself, like REE or other trace elements to verify or rule out other processes.
Whilst the comparisons with the d18O record from the stalagmite and the other rainfall records are interesting, I find it not very compelling and stronger evidence would enhance the author's conclusions. The fact that the period with highest ∂234U is not correlated with the rainfall record deserves further exploration.
The authors make a strong case to explain who the elevated ∂234U from the Mississippi river are being diluted and only during times of significant runoff increments the high ∂234U values might reach the reef area. However, it would be interesting to determine if this is because increased sediment load into the reef, dissolved sediment (or both, I assume that Th was also measured in some samples?). I would also hypothesize that the increased ∂234U at the end of the Little Ice Age might be the result from increased runoff from the Mississippi from melting ice and snow at higher latitudes in continental North America, whilst the increased ∂234U at the end of the end of the XX century might be the result form increasingly larger sediment loads from increased oil extraction in the Gulf of Mexico (which could be tested with Ba analyses in the core as BaSO4 is usually used as lubricant during oil drilling.
I would love to see this record published, but with more geochemical backing of the potential processes modulating the ∂234U variability.