the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-5052', Anonymous Referee #1, 01 Dec 2025
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RC2: 'Comment on egusphere-2025-5052', Anonymous Referee #2, 02 Mar 2026
This manuscript presents a high-resolution annual δ234U record from a Cuban coral covering the past 237 years, with particular focus on the end of the Little Ice Age. I find the dataset valuable and timely. Records from this time interval provide a rare opportunity to explore the short- to decadal-scale behavior of seawater δ234U and its coupling to hydrological variability. The study offers an interesting perspective on how coral δ234U may respond to both local and advected freshwater sources, and it potentially broadens our understanding of the variability of δ234U within the modern oceanic range. In that sense, I consider this work promising and worth publishing after clarification of several issues.
My first concern relates to the definition of δ234U and the role of HU-1. The manuscript states that measurements were bracketed to HU-1 and that HU-1 was treated as secular equilibrium. The authors then re-normalized the δ234U values to another reference. This workflow is confusing to me. If HU-1 is primarily used for instrumental drift correction and mass bias normalization, there is no need to assume that it represents secular equilibrium in a physical sense. For instrumental purposes, it would be sufficient to fix a reference 234U/238U ratio and normalize all measurements relative to that value.
At the same time, δ234U is calculated using specific decay constants, which places the data on an activity-based scale (in the present manuscript the authors adopt the values from Cheng et al., 2013). It is therefore unclear whether the reported δ234U values are intended as purely relative values referenced to HU-1, or as absolute activity deviations tied to the adopted decay constants. In fact, the results of Cheng et al. (2013) suggest that HU-1 is not strictly at secular equilibrium. These approaches are conceptually different, and I think the manuscript should explain more clearly how they are reconciled.
The second issue concerns the calibrated δ234U value of CRM112A. After correction, the manuscript reports −38.1 ± 0.3‰. In Figure 2 the error bars appear much smaller (closer to ±0.1‰). My assumption is that the manuscript reports 2σM for its own measurements (?) whereas some of the literature values shown in the same figure (e.g., Cheng et al., 2013) likely report 2σ uncertainties. If different uncertainty definitions (2σ vs. 2σM) are plotted together without being clearly distinguished, this could easily create a misleading visual comparison and should be clarified explicitly. Also, looking at Figure 2 as presented, the reported value appears to only just overlap with Cheng et al. (2013) at the edge of uncertainty, and it does not overlap at all with Pourmand et al (2014) (I think the error presented is also not correct). More recent high-precision work from the same research lineage as Cheng et al. (2013), namely Hu et al. (2025), reports a value closer to −38.49 ± 0.09‰. Compared to that, the value presented here appears slightly less negative. I wonder whether there are any possible reasons that could explain these less negative values, such as measurement corrections or normalization procedures. Although this difference is small and does not affect the discussion of the 1–3‰ variability in the coral record, it does raise questions about absolute accuracy of the methodology. In addition, since δ234U depends directly on the adopted decay constants, I would also suggest that the authors consider recalculating their data using the most recent half-life values (Hu et al., 2025) to ensure full comparability with current high-precision studies. A clearer explanation of the uncertainty definitions, the normalization procedure, and the adopted decay constants would strengthen confidence in the analytical scale used in this study.
Finally, while the discussion of Little Ice Age variability is thoughtful and physically plausible, it remains largely interpretative rather than tightly constrained. The manuscript presents several reasonable mechanisms to explain the enhanced variability during 1778–1846, including extreme local runoff events, Mississippi River input enriched in 234U, changes in Loop Current dynamics, reduced Florida Strait throughflow, and land-use changes affecting terrestrial uranium flux. Each of these mechanisms is plausible, and the overall narrative is coherent. However, I do not see any independent constraints that allow one mechanism to be clearly favored over the others. The interpretation therefore remains a set of competing but largely untested hypotheses. Given the availability of other high-resolution LIA hydroclimate records in the Caribbean and Gulf of Mexico, a more systematic comparison or a clearer prioritization of one dominant process would strengthen the conclusions. At present, the manuscript convincingly demonstrates increased δ234U variability during the end of the LIA, but it does not yet identify the primary controlling mechanism with sufficient certainty.
Overall, this is a promising contribution that provides a valuable new dataset. Addressing the issues above would significantly strengthen the manuscript and enhance its impact.
Citation: https://doi.org/10.5194/egusphere-2025-5052-RC2
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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.