the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jun 2026
Plutonium isotopes as time-markers in South American lake sediments: identifying pre- and post-1959 and French test periods
Abstract. In regions where rapid and extensive environmental changes have occurred, particularly in South America, reliable time markers are essential for dating sediment sequences and quantifying environmental degradation. In this respect, the use of 239Pu and 240Pu isotopes in South American sediments may help to identify distinct sources of fallout radionuclides, including radioactive fallout from French atmospheric nuclear weapons tests (NWTs) conducted between 1966 and 1974 at the Mururoa and Fangataufa atolls (French Polynesia). Here, we present post-1900 continuous records of 240Pu/239Pu isotope ratios in sediments cores from sites located between 32° and 52°S latitude: Lakes Natri and Laja in Chile; Lakes La Barrancosa, Melincué, Ñe Luan, and Roca in Argentina; and the Rincón del Bonete Reservoir and La Estanzuela Pond in Uruguay. Depth profiles revealed two 240+239Pu activity peaks, from which the more recent is not concomitant to the 137Cs maximum activity peak dated back to 1964-1965. The low 240Pu/239Pu atom ratio (< 0.08) associated with this more recent Pu peak confirms a contribution from French fallout, dated to the late 1960s to early 1970s. The investigated lakes exhibited similar patterns in Pu isotope ratios: (i) an initial phase dominated by the U.S. NWTs signature (240Pu/239Pu > 0.20, also often referred to pre-moratorium), followed by (ii) increasing Pu activities characterized by a Pu isotopic signature consistent with global fallout (240Pu/239Pu ~ 0.18) and, finally, (iii) a period of increased Pu activities from French NWTs fallout (0.03 <240Pu/239Pu < 0.08). The 239Pu and 240Pu isotopes revealed a consistent nuclear source pattern with a distinct French fallout contribution, confirming their suitability as an additional time marker for environmental reconstruction in South America.
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Status: open (until 20 Aug 2026)
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CC1: 'Comment on egusphere-2026-3019', Sergio Ribeiro Guevara, 12 Jun 2026
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AC1: 'Reply on CC1', Floriane Guillevic, 10 Aug 2026
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Many thanks for your comment and support. We agree with your suggestion and will update the citation at l.78 and l.165 accordingly. We also agree that we can add also Ribeiro Guevara and Arribére (2002) in lines 93–94 to corroborate our conclusion.
Citation: https://doi.org/10.5194/egusphere-2026-3019-AC1
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AC1: 'Reply on CC1', Floriane Guillevic, 10 Aug 2026
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RC1: 'Comment on egusphere-2026-3019', Anonymous Referee #1, 15 Jul 2026
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The paper presents a comprehensive dataset on the presence of plutonium isotopes, Pu‑239 and Pu‑240, in eight sedimentary reservoirs from different regions of South America located within the 30–60°S latitudinal band. The authors also provide dating results based on the Pb‑210 technique, complemented by Cs‑137 data, which together allow the interpretation of Pu profiles in relation to periods of atmospheric nuclear testing.
A broad Cs‑137 peak is observed in all cores, and most of them display both a primary and a secondary Pu peak. The most recent years, up to the 1970s, consistently show the lowest Pu‑240/Pu‑239 ratios, indicating a strong influence from the French nuclear tests conducted in French Polynesia up to 1974. The interpretation of the results is very thorough, the figures are of high quality, and the overall manuscript is solid. In my view, the paper could be published in its current form with only minor revisions and incorporating some suggestions from my side.
General suggestions:
- I recommend adding a table summarizing the dates and yields of the French nuclear tests in French Polynesia, which would serve as a useful reference when interpreting the Pu profiles.
- The Bristish tests in the premoratorium period in Australia and in the Christmas Islands should also be named and considered in the Introduction.
- There is a recent paper reporting the Pu‑239 signal in Antarctic ice cores with sub‑annual resolution: Shin et al., Sci. Adv. 11, eadv1172 (2025). This study provides complementary information to Koide et al. and could be used to support the authors’ conclusions and offer additional evidence regarding the influence of French tests in the Southern Hemisphere.
- I have serious concerns regarding the chemical procedure applied to the samples. It appears that, after the leaching or fusion step, no coprecipitation step was performed prior to Pu purification using TEVA resin. Such a coprecipitation step is critical for removing potential interferences in mass spectrometry and for achieving reliable chemical yields. The authors should clarify why this step was omitted. Additionally, quality control of the results, e.g., through the analysis of IAEA reference materials or other well‑characterized samples with similar matrices and Pu concentrations, should be reported to validate the results.
- Another issue concerns the nature of Pu contamination in the samples. Several studies reporting Pu concentrations in samples from South America and Africa have shown that contamination can be highly heterogeneous, meaning that duplicate aliquots of the same sample may yield different Pu concentrations (see, for instance, (Chamizo et al., 2011, https://doi.org/10.1016/J.NIMB.2011.04.021) and (Chamizo et al., 2020, https://doi.org/10.1016/j.scitotenv.2020.139993). Have the authors investigated the nature of contamination in their sediment profiles by analyzing duplicate samples, particularly in those layers exhibiting lower Pu ratios?
- A further question relates to the consistently low Pu‑240/Pu‑239 ratios associated with French tests, even those with the highest yields. Low ratios are observed across different sediment cores throughout the entire French testing period. Is there an explanation for this? Tests conducted by other countries generally show a relationship between yield and Pu ratios, with higher yields corresponding to higher ratios. A comment on this would be welcome.
Minor remarks:
Graphical abstract: What is the meaning of the grey bar in Lake Natri and Lake Naja plots?
Line 95: When naming previous works on the presence of FF in South America, please include: 10.1016/J.NIMB.2011.04.021
Line 165: Was Am241 detected in the cores by gamma spectrometry? What activity levels were measured?
Line 211: The GF average value for the Southern Hemisphere should be used here. As far as I know, this integrated value for the Pu240/Pu239 ratio includes also the contribution from the premoratorium period. This is just the integrated signature from soils that are assumed to represent and average value for the whole nuclear era.
Lines 225-230: The uncertainties formula should be moved to SI. On the other hand, I think a subscript is missing in the last “Ds” of the formula in line 230.
Line 240: Are the Cs137 profiles corrected by decay using the dating of the core?
Figure 2: “R-FR” should be changed by “R-FF” for consistency.
“Grey horizontal bands mark instantaneous events, which do not represent continuous sedimentation”. What is it the meaning of this?
It would be helpful including the latitude of the cores below the names in each plot, to facilitate the interpretation of the data.
Line 389: It is not clear the meaning of the Ipeak. Does it mean the integrated inventory for the peak in the 1970s?
Line 391: Does the Ipeak appears in Table S2?
Fig. S3: Please, change the unit of liter by L (not “l”).
Table S2: How is it calculated, the SF factor for Th and U? Why is it so different between samples? Is there a correlation between bad SFs and Pu ratios?
Citation: https://doi.org/10.5194/egusphere-2026-3019-RC1 -
AC2: 'Reply on RC1', Floriane Guillevic, 10 Aug 2026
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Many thanks for these constructive and encouraging comments! If the editor decides so, we will provide a revised version of the manuscript incorporating all the requested modifications.
You will find our responses to your comments in the attached file.
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RC2: 'Comment on egusphere-2026-3019', Anonymous Referee #2, 29 Jul 2026
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The authors describe measurements of Pu isotopes in lake sediments on a longitudinal transect in South America, to resolve different eras of nuclear bomb testing. This is to produce a better time calibration for dating sediment cores from which to reconstruct environmental records.
Using the distinct Pu isotopic composition produced by different iterations of nuclear bomb production authors successfully constrain three periods of Pu deposition to the study region. I agree with the authors stated goals and conclusions that this can refine our use of fallout radionuclides to provide age constraints on environmental records.
Some specific line comments follow which may be helpful for the authors in revising a final version for publication,
Line 34: unsure of intent here, is it that such extensive environmental changes are particular to South America, or that reliable time markers are essential for South America? I would prefer the latter if this can be clarified
Line 36: little unclear how we have gone from needing reliable tracers to identifying distinct fallout sources…
Line 38: and then continue to describe only the fallout sources, and no link back to how this helps towards reliable time marker and then to understanding environmental change.
Line 62: is broad Cs peak attributable to post-depositional mobility? Maybe clarify if this is the case? (I believe that it is).
Line 147: geometry of vial/ volume, and typical sample masses should be reported as well.
Line 188: I’ve not seen this expression, surface activity, before. More commonly called inventory, stock, or areal activity? In line 196 you use inventories, or is this only for “total” inventory?
Line 198: Can we calculate a reliable flux without taking account of dispersion? I.e., the core profiles are not true histories even if the peak position is reliable because dispersion broadens the peak, and reservoir effect (in-lake or watershed) leads to long tailing of fallout (often) into contemporary sediments.
Line 204: please cite the number of reference soil sites.
Line 210: endmember is a key word that could be used here, some readers (including myself) would think about the distinct sources this way.
Line 231: brief justification for how 7% was arrived at? This is based on volume of the cylindrical sediment plug?
Line 240: comparison between Cs and Pu profiles is not articulated clearly. Should we expect two Cs peaks as well? Is the single peak then due to higher dispersion/diffusion of Cs?
Line 248: there should be citations available here I believe, consider …
Line 250: the verification should be quantified. What are 210Pb ages of Cs maximum? Include this in Table 1?
Line 251: why choose CFCS over CRS when it’s assumptions would appear to be more restrictive?
Fig. 2 is a tough read. Consider label each pair a-h
Fig. 3: I like this Figure, it is effective. I wonder, while we see a shift to low Pu ratios corresponding to FF , most cores see a gradual increase in ratios to present. Why is this? Is this due to much larger GF total deposition and slow mobilization of Pu from watershed?
Line 389: Peak inventories meaning only the Pu in the main peak, but not in the entire core? Is this intended to reduce noise from post-depositional reservoir effect i.e. long tailing of Pu into recent sediments? Maybe explain the significance of Ipeak?
Fig. 5: having trouble with the y-axis not being continuous.
The Pu inventories might be corrected for sediment focusing, if the true atmospheric flux is desired, e.g. using 210Pbex if those numbers in soils are better known, or can be estimated from rainfall, i.e., Pu-corrected = Pu-lake*Pb-soil/Pb-lake.
Line 420: re-evaluation?
Citation: https://doi.org/10.5194/egusphere-2026-3019-RC2 -
AC3: 'Reply on RC2', Floriane Guillevic, 10 Aug 2026
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We thank you for your time and for your overall positive assessment of our manuscript. We also appreciate your constructive suggestions which would help to clarify some statements if the editor invites us to resubmit the manuscript.
Please find attached our replies to your comments.
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AC3: 'Reply on RC2', Floriane Guillevic, 10 Aug 2026
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Very interesting work; congrats to all authors. Just a few comments. I think that the citation Guevara et al. (2003) in lines 78 and 165 should be changed to Ribeiro Guevara and Arribére (2002). According to the results reported there, the period of maximum fallout extends to 1964-1966 (not 1964-1965), with the highest 137Cs fallout at 1966, along with an increase in 90Sr. This is coincident with the start of French NWTs, confirmed with the detection of short-lived fission products in precipitation, milk, and air (131I, 140Ba, 103Ru, 95Zr+95Nb). Also, this reference could add something in lines 93-94. Moreover, this work shows a secondary 137Cs fallout peak at 1969-1973, consistent with the results highlighted in the conclusions of the present research.
Ribeiro Guevara S., Arribére, M. A. 137Cs dating of sedimentary cores from lakes of Nahuel Huapi National Park, Patagonia, Argentina: historical records and profile measurements. Journal of Radioanalytical and Nuclear Chemistry, 252(1) (2002) 37–45. https://doi.org/10.1023/A:1015275418412