Preprints
https://doi.org/10.5194/egusphere-2026-3519
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2026-3519
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
29 Jun 2026
| 29 Jun 2026
Status: this preprint is open for discussion and under review for Geochronology (GChron).
Precision measurement of δ234U in annually banded tropical corals
Abstract. Tropical corals preserve geochemical 238U/234U ratios that provide valuable records of past seawater uranium isotope compositions. Variations in the coral skeletons and thus seawater are likely indicative of freshwater contributions from submerged groundwater discharge and river runoff, or reflecting coral diagenesis. Advances in multi-collector ICP-MS allow precise determinations of 238U/234U ratios, enabling the reconstruction of subtle (typically >1 ‰) environmental changes in these marine records. In this study, we evaluate the reliability of coral-based δ234U records across multiple genera, sampling strategies, and intra-skeletal variability. Analyses of reference material NBS-CRM-112A demonstrate reproducibility within ±0.4 ‰. Replicate sampling across coral structures and colonies indicates measurable intra-band heterogeneity (±0.6 ‰), although local hydrodynamics and submarine groundwater discharge can introduce small inter-colony offsets. No species-dependent isotope fractionation was detected, underscoring the robustness of δ234U as a geochemical proxy. These findings demonstrate that coral skeletons provide reliable archives of up to sub-annual δ234Usw for detecting subtle climatic and hydrological signals.
How to cite. Greve, S., Frank, N., Warken, S., Montagna, P., Winter, A., Rico-Esenaro, S. D., Carricart-Ganivet, J. P., Sanchez-Cabeza, J.-A., Ruiz-Fernández, A. C., Alonso-Hernández, C., Gomez–Batista, M., Taviani, M., and Hetzinger, S.: Precision measurement of δ234U in annually banded tropical corals, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-3519, 2026.
Competing interests: One of the coauthors is a member of the editorial board of Geochronology.
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Sahra Greve
CORRESPONDING AUTHOR
Institute of Environmental Physics, Heidelberg University, 69120 Heidelberg, Germany
Norbert Frank
Institute of Environmental Physics, Heidelberg University, 69120 Heidelberg, Germany
Institute of Earth Science, Heidelberg University, 69120 Heidelberg, Germany
Sophie Warken
Institute of Environmental Physics, Heidelberg University, 69120 Heidelberg, Germany
Institute of Earth Science, Heidelberg University, 69120 Heidelberg, Germany
Paolo Montagna
Institute of Polar Sciences, ISP-CNR, Via Gobetti 101, 40129, Bologna, Italy
Stazione Zoologica ‘Anton Dohrn’, Naples, Italy
Amos Winter
Department of Earth and Environmental Systems, Indiana State University, Terre Haute, IN USA
Serguei Damián Rico-Esenaro
Laboratorio de Esclerocronología de Corales Arrecifales, Unidad Académica de Sistemas Arrecifales, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, Puerto Morelos, Q. Roo 77580, México
Departamento El Hombre y su Ambiente, Universidad Autónoma Metropolitana Unidad Xochimilco, Coyoacán, Cd. de México 04960, México
Juan P. Carricart-Ganivet
Laboratorio de Esclerocronología de Corales Arrecifales, Unidad Académica de Sistemas Arrecifales, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, Puerto Morelos, Q. Roo 77580, México
Joan-Albert Sanchez-Cabeza
Unidad Académica Mazatlán, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, 82040 Mazatlán, Mexico
Ana Carolina Ruiz-Fernández
Unidad Académica Mazatlán, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, 82040 Mazatlán, Mexico
Carlos Alonso-Hernández
IAEA Marine Environment Laboratories 4 Quai Antoine 1er, MC-98000 Monaco, Principality of Monaco
Miguel Gomez–Batista
Centro de Estudios Ambientales de Cienfuegos (CEAC), Cienfuegos, Cuba
Marco Taviani
Stazione Zoologica ‘Anton Dohrn’, Naples, Italy
Institute of Marine Sciences, ISMAR-CNR, Via Gobetti 101, 40129, Bologna, Italy
Steffen Hetzinger
Christian-Albrechts-Universität zu Kiel, Institut für Geowissenschaften, Kiel, Germany
Short summary
Coral skeletons record subtle changes in the uranium composition of seawater, providing a window into past environmental conditions. By comparing different coral species, colonies, and sampling methods, we show that corals can reliably detect very small changes (0.6 ‰) associated with freshwater input and climate variability. This makes them a valuable tool for reconstructing past changes in reef and coastal environments.
Coral skeletons record subtle changes in the uranium composition of seawater, providing a window...
Greve et al. review
General comments
This paper reports sampling and analytical protocols for conducting high precision δ234U analyses (c. ±0.4 ‰) on tropical, shallow water corals. The authors then investigate variations in δ234U in modern and near-modern corals to determine their utility as environmental and hydrological archives.
Coeval samples of twelve species of coexisting corals are found to faithfully record analytically indistinguishable values of seawater δ234U. Small but resolvable variations in δ234U (up to c. 1‰) are observed in growth bands of corals sampled at sub-annual scales. Larger variations in δ234U (up to several ‰) were observed in decadal-scale time series of coral samples from an area of known submarine groundwater discharge (SGD), with distinct values of δ234U recorded in neighboring coral colonies. The authors conclude that corals provide reliable archives of δ234U for detecting subtle climatic and hydrological signals. Much of this is unsurprising but nonetheless it will be useful to those working with coral archives to incorporate these carefully measured results into their thinking.
Line-by-line comments follow, with specific comments followed by technical corrections.
Specific comments
Line 24. On glacial-interglacial timescales, variations in seawater 238U/234U are expected (as the authors are aware) and such variation should be mentioned here.
Line 52. Secular variation in the past 234U/238U ratio of seawater on the multi-ka timescale is now well documented (e.g., Chutcharavan et al., 2018) so not all pristine corals are expected to yield initial 234U/238U activity ratios of ~1.45. The authors are clearly aware of this given later sentences, but the statement starting on line 52 needs to be corrected.
Line 55. The phrase “U-series open system behavior…in well-preserved corals…” is self-contradictory; please rewrite.
Also, since young corals have d234U ≈ 450 versus the d234U of CRM-122A ≈ -38.5, it’s important to test that the normalization based on CRM-122A is appropriate for the coral analyses. The authors have done so using their seawater standard, although their results are not mentioned until line 228. It would be helpful to expand line 194-195 to include words to the affect that “The reproducibility was also assessed with repeated measurement of an in-house seawater standard, which yields a mean δ234U value of 145.7‰ with a standard deviation of ±0.3‰, thus comparable in precision to NBS-CRM-112A”, with appropriate repositioning of call outs for Figs. 2 and 3.
Line 189. The authors state that “all isotope measurements were normalized to the NBS-CRM-112A standard”. The authors should briefly describe how this was done and how large the normalization factors were.
Line 206. Please specify the signal levels (in amps or volts with respect to a 10E11 ohm amplifier) typically obtained for 238U in Fig. 1 or state your typical sensitivity in volts/ppb. Such signal levels are more useful than concentrations for assessing mass spectrometer performance and analytical uncertainties. Also please give the approximate amount of 238U consumed during typical analyses.
Line 217. (a) The certified value of CRM-112A should be added to Fig. 2 to illustrate this statement.
Lines 217-219. (b) The authors state that “The methodology applied here allows theoretically for ε-level precision for δ234U measurements …”However, this statement is not supported by the data presented since the scatter observed for replicates of CRM-112A is too high to support ε-level precision. It would be appropriate to omit this statement unless it can be supported by measurements.
The authors go on to state that although the instruments used in this study and that of Hu et al. (2025) are similar, runs of impractical length (c. 3 hours) would be required to achieve ε-level precision with the author’s protocol. But there are some salient differences in the measurement and data reduction protocols in the two studies. For example, Hu et al apparently use much higher intensity 238U beams — ~400 volts versus mostly ~80 volts or less in the current study (judging by Fig. 1). Other things equal, the large beams used by Hu et al provide a 5X improvement in signal/noise ratios on Faradays. This may help explain why excessively long runs would be necessary to attain ε-level precision with the author’s protocol. Another notable difference in the two studies is how corrections for peak-tailing on 234U are made, with measurements at distinct e/m positions and different curve-fitting approaches being used. It’s possible that larger samples are not the sole reason, or even the main one, for the higher precision reported by Hu et al. for CRM-112A and other materials.
In any case, as the authors point out, an external precision of ~ ±0.3–0.4‰, which they demonstrate for CRM-112A, their in-house sea water standard, and replicate coral analyses is adequate to assess variable 234U/238U in corals at the c. 1‰ level, which is the topic of this paper.
Line 226. It’s hard to reconcile a “a total measurement duration of 4 minutes” with an “integration time of 4 s and 60 scans”, which to me implies an on-peak measurement time of 240 s. Please clarify.
Lines 235-238. How do the authors interpret the larger deviations observed for three samples out of 20?
Lines 240-242. Do the authors think they have detected true intra-band variability in the TM2020 coral?
Lines 356-375. This section could be reduced by 50% or more. The main point — that SGD, already a known feature of the region, provides a plausible explanation for the differences in the d234U time series of the two corals — can be stated more succinctly.
Technical Corrections
Line 22. Can delete “geochemical”
Line 27. Add “characterize”; i.e., characterize intra-skeletal variability.
Line 28. Specify confidence level of quoted reproducibility.
Line 34. “Elemental and isotopic compositions…” instead of “Geochemical element and isotope composition…”.
Line 35 “Coral-based”, not “Coral based”. Delete “over”.
Line 42. “freshwater discharge from rivers and submarine groundwater may locally modify marine 234U/238U ratios.”
Comment: chose either 234/238 or 238/234 and then be consistent. Since d234U is defined in terms of the 234/238 ratio, that is probably the better choice.
Line 44. Delete “today”.
Line 47. Sentence beginning with “Lastly…” is vague. Can you be more specific or omit?
Line 49. Should read “absence of, or accurate correction for, initial 230Th.
Line 61. Suggest starting a new paragraph here, where you switch to discussing methods.
Line 86. Are the “skeletal structure replicates” sampled from the same growth band at different positions on the colony? If so, please so state.
Line 89. Delete “and” in “and past seawater…”
Line 169. “interspersed” rather than “interspaced”
Line 170, 171. Ohm is not capitalized when used a unit of measurement. Also, it seems a word is missing after electronic; e.g., electronic source…
Line 262. That these samples are coeval should be stated right away; i.e., Eleven coeval samples….
Line 304. Minor or subtle, not both.
Line 320. “high resolution uranium isotope…”
Line 380. No capital needed for uranium.