the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Aug 2026
Analytical strategies for 207Pb/235U carbonate geochronology
Abstract. In carbonate U–Pb geochronology, unknown initial 234U/238U disequilibria can be debilitating for the accuracy of 206Pb/238U dates. Even if residual 234U/238U disequilibrium can be measured precisely enough to perform accurate corrections, beyond ca. 1.5 Ma (depending on the magnitude of initial excess/deficit of 234U) the expansion of uncertainties due to the disequilibrium correction may make carbonate 206Pb/238U dates prohibitively imprecise. An alternative approach utilising the 207Pb/235U system is arguably more accurate, however the lower abundance of 235U and 207Pb relative to 238U and 206Pb inevitably results in larger analytical data-point uncertainties. Here we explore analytical strategies that maximise the potential of the 207Pb/235U system for carbonate geochronology. ID-TIMS and LA-ICPMS are considered the most useful, and complementary, techniques. ID-TIMS offers the ultimate precision and accuracy combined with the ability to filter data for contamination using 208Pb/204Pb, but it requires mg-sized samples that limit the spatial resolution and the spread of data along the isochron. Optimised LA-ICPMS with large spot sizes may be a quick alternative with better spatial control and enhanced spread of data along the isochron, but 208Pb normalisation imposed by the difficulty of analysing 204Pb may add noise to the results. Overall, precise 207Pb/235U carbonate geochronology is feasible and with minor analytical adjustments it can be adopted broadly instead of the 206Pb/238U method. Future work on carbonates should obtain additional compositional information that will allow robust identification of domains contaminated with extraneous Pb.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Geochronology.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 22 Sep 2026)
- RC1: 'Comment on egusphere-2026-4403', Anonymous Referee #1, 19 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-4403', Ariela Mazoz, 08 Sep 2026
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Dear Editor Sandra Kamo,
I have reviewed the manuscript by Szymanowski et al. entitled “Analytical strategies for 207Pb/235U carbonate geochronology.”
This manuscript presents a useful evaluation of the 207Pb/235U approach for carbonate geochronology and provides a valuable comparison with the more commonly used 206Pb/238U system. The topic is relevant, and the results presented are generally convincing and potentially useful for future U–Pb dating studies of carbonate materials. I particularly appreciate the comparison between LA-ICP-MS and ID-TIMS data and the discussion of the factors that can influence the precision and interpretation of carbonate U–Pb ages.
I have only a few minor comments and suggestions that I believe should be addressed before the manuscript can be published. These mainly concern clarification of some statements, presentation of the statistical results, references, and a few typographical and figure-related issues. I believe addressing these points will improve the clarity of the manuscript and make the main conclusions easier for the reader to follow.
I have attached a PDF with the relevant text highlighted in light purple for reference to the general comments.
General comments:
L6–10: I agree with introducing the problem of the 206Pb/238U approach in the abstract, but I think this could be better explained. At the moment, the discussion focuses mainly on 234U/238U disequilibrium, whereas there are other factors that can affect the accuracy of 206Pb/238U ages, such as common Pb and open-system behaviour. I would suggest briefly mentioning these other limitations as well, so that the motivation for exploring the 207Pb/235U approach is not presented as being solely related to 234U/238U disequilibrium.
L18: The statement that 207Pb/235U can be “adopted broadly instead of” 206Pb/238U is quite strong. Since the abstract already points out that 206Pb/238U ratios can be imprecise, it would be helpful to also briefly highlight the main advantages of the 207Pb/235U approach. This would make the motivation for proposing 207Pb/235U as an alternative clearer to the reader.
L122-125: Please add some references here.
L126–127: This is correct, but I suggest clarifying that the “greater variability” refers to the spread of the individual data points along the isochron, rather than variability caused by processes such as U incorporation or Pb loss. This would make the meaning of variability clearer to the reader.
L184: Please add more references here. The work from Silva et al. (2026) is a recent one where they developed a calcite RM (RioM-1) of ca. 65 Ma. You can also find more references there.
L234: Please change “Uranium” to “U” to keep the consistency.
L278-279: Please add the reference for NIST 614 glass reference material.
L289: Please change to “Our new and reprocessed ID-TIMS results (Table 2)”.
L291: Please cite Fig. 4b in “The Hoogland X4 sample (Fig. 4b) returned…”. Because you are describing this sample first, I suggest swapping the Fig. 4a and 4b panels, so that the Hoogland X4 sample is shown as Fig. 4a.
Also add the MSWD values in “The Hoogland X4 sample returned 207Pb–235U isochron dates of 5.29 ± 0.15 Ma (filtered, MSWD = 0.71, n = 5/8, details of filtering in section 4.3 below) and 5.35 ± 0.13 Ma (MSWD = 2.2, n = 8/8)”.
L293-294: Please cite Fig. 3c and also add the MSWD values in “are indistinguishable from our uncorrected 206Pb–238U isochron date of 5.349 ± 0.085 Ma (Fig. 3c; MSWD = 35, n = 5/8)”. In “indicating that initial 234U/238U disequilibrium was likely negligibly small”, I see the data from figures 4b and 3c are consistent within uncertainties, but the MSWD for figure 3c is quite high (MSWD = 35). So, can we really say that any disequilibrium was negligible? Perhaps I have missed something, but maybe the authors could expand this a little bit to make it clearer.
L302: In “Reprocessed RA138 data (Fig. 4)”, change to “4a” following your current Figure 4. But as I previously suggested above, the RA138 panel should become “Fig. 4b”. Please also add the MSWD value in “207Pb–235U isochron dates of 324.0 ± 2.3 Ma (MSWD = 0.99, n = 15/21) and 323.0 ± 3.9 Ma (MSWD = 2.9, n =21/21)”.
L306: Same comments as above, please add the respective MSWD values.
L383: Please cite “Fig. 4” as suggested in “Finally, the variability of 208Pb/204Pb illustrated here (Fig. 4)…”.
L398: I agree that the ASH15 date is consistent with the ID-TIMS age, but given the relatively high MSWD (3.5), I think it is important to highlight the overdispersion rather than describing the agreement as “excellent.” e.g.: “Finally, data for ASH15 were slightly overdispersed (MSWD = 3.5, n = 50/50) but resulted in an isochron date of 3.04 ± 0.25 Ma, consistent with the ID-TIMS date (Fig. 4)”.
L402-402: In “lower than the corresponding Tera-Wasserburg concordia intercept dates…” did you mean the diagram in Fig. 6? If so, please cite Fig. 6 at the end of this sentence to make it clearer for the readers.
L411: Please make it clear that 208Pb is used as the numerator on the y-axis specifically for the LA-ICP-MS analyses. I was briefly confused by this wording and thought that the same approach was also being used for the ID-TIMS isochron, which is not the case.
L423: Another small typo here. Please change “results is no improvement” to “results in no improvement”.
I also have a minor suggestion here, which the authors can decide whether to accommodate. In Fig. 6a, removing the 12 outliers identified in Tera–Wasserburg space seems to help constrain the lower-intercept age, with a good MSWD. However, in Fig. 6b, the MSWD is higher than for the isochron in Fig. 5c. Perhaps it would be interesting to include a third panel (Fig. 6c), showing the effect of excluding the 12 outliers together with a few additional spots (e.g., 3 or 4) whose error ellipses do not overlap the isochron. I recognise that selecting additional spots would be somewhat like “cherry picking”, but this could provide an alternative view of the data and show whether removing a few poorly aligned analyses further improves the MSWD. It could also help demonstrate the value of the isochron after removing the 12 outliers identified in the Tera–Wasserburg diagram.
L451: There is a small typo here. Please change “than can be used” to “that can be used.”
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Reviewer comments on Szymanowski et al. - egusphere-2026-4403
General Comments:
RC1: This manuscript presents protocols for 235U-207Pb analysis and data reduction for measuring ages on Pleistocene hydrogenic minerals. It is a worthwhile study that should find important use, especially in the field of early hominid development. The authors include one of the foremost analysts in the field as well as the foremost expert on geochronology data statistics so I have no doubt that the data and interpretations are reliable. I would like to think, however, that neither actually wrote the manuscript. While the organization and composition are generally good, I have made suggestions for improvement in an annotated copy of the manuscript and below. Many of the suggestions in the manuscript are personal preferences in grammar that I leave up to the discretion of the authors. More seriously, I think that the authors unintentionally misrepresent the ‘normal method’ of dating, which in my case at least, created some confusion. Detailed remarks are given below. I apologize if I have misunderstood anything.
To review the background, because of the U double decay system (238U - 206Pb, 235U - 207Pb) U-Pb measurements can be compared to a curve (concordia) showing isotopic equilibrium and be used to give more information than just the crystallization age. This can include the age and degree of isotopic disturbance in the case of a highly radiogenic mineral like zircon, or the proportion of common Pb in the case of other minerals where there is no disturbance. The daughter/parent age calculation assumes that the radioactive decay chain was in isotopic equilibrium from the beginning, which is never the case, but effective equilibrium is established rapidly enough that this complication can be ignored for minerals with ages of a few Ma or more. For younger samples, correcting the 238U – 206Pb system can be difficult. The 235U-207Pb decay system by itself is simpler because there is only one relatively long-lived intermediate radionuclide, 231Pa, which is insoluble. Therefore, one can safely assume that it was not present during crystallization of a hydrogenic mineral like calcite, which should make the disequilibrium correction reliable.
Specific comments
RC2: I found the introduction to be confusing in places. The authors contrast their approach of using only the 235U-207Pb system with the ‘default method’ (T-W concordia plot). This uses both U decay systems but the wording seems to imply that it uses only the 238U-206Pb system (lines 18, 25 reproduced below). I am not aware of any published method that uses only the 238U-206Pb system on carbonates. The 238U-206Pb system is commonly used to determine ages on relatively young (Phanerozoic) radiogenic minerals like zircon but this is in a different context. The issue here is that for very young samples that are likely to have had 234U/238U ratios out of equilibrium, 238U-206Pb is of limited use despite being relatively easy to measure, which also compromises the concordia (2 decay system) method. Therefore, it is best to rely on only the 235U-207Pb system.
An argument that might be anticipated against this is that 207Pb must be uniquely relied on and this is the smallest isotope except for 204Pb and 235U, which it is not practical to measure. 208Pb works as an effective proxy for 204Pb in hydrogenic minerals and the natural range of variation of 238U/235U is negligible in this context so 238U can be a proxy for 235U. The concordia method also depends on measuring 207Pb but may be more tolerant of errors on this peak (see discussion below).
RC3: Line 18: ‘Overall, precise 207Pb/235U carbonate geochronology is feasible and with minor analytical adjustments it can be adopted broadly instead of the 206Pb/238U method’
Again, there is no commonly used 206Pb/238U method for carbonates. I can see a reason to adopt the 207/235 system broadly if you add: ‘on young samples’, but this raises an interesting question in view of the last sentence in RC2. To what degree does the 207/235 information in a normal concordia analysis provide a worse constraint on the age than the information from both isotopic systems? Obviously, it would depend on how radiogenic the data are. Highly radiogenic data (close to concordia) should give much more precise 206/238 ages whereas this is less clear for data near the 207/206 axis in the T-W plot. I would like to see more discussion of this, exploring the conditions over which 207/235 may be more, or at least as useful as, the concordia method.
RC4: Line 25 ‘The default method (Roberts et al., 2020) uses the 206Pb/238U system which is more accessible analytically than 207Pb/235U’. Again, the default method uses both systems. It requires measuring 207 as well as 206.
RC5: Line 111 ‘Even if this assumption is wrong, the maximum bias of an inaccurate 231Pa correction is < 50 ka’.
It is unclear how this is determined. One would normally assume that initial Pa/U was zero for a hydrogenic mineral but if not, it could theoretically be high enough to account for almost all radiogenic 207Pb, so the lower age error could be 100%.
RC6: Line 122 ‘Carbonates have much lower U/Pb ratios than U-rich accessory minerals, which generally precludes calculation of single-analysis dates’
Confusing at first glance. Low U/Pb could just imply an old sample. You should state that carbonates have a significant common Pb component, whose isotopic composition adds a second unknown besides the age. This precludes calculation of single-analysis dates.
RC7: Fig. 2B caption should explain that the numbers represent laser beam diameters in microns.
RC8: Line 266 on mass selection: The object in analytical work is to maximize the information collected from the ablation. This should mean not collecting data on isotopes that are not needed and adjusting relative collection periods to be proportional to the square root of the inverse isotopic ratio, as much as possible (e.g. if the ratio is 9, the smaller isotope should be measured 3 times as long as the larger one). Why collect data on 202Hg and 204Pb, especially as it was argued not to do this earlier in the manuscript? These peaks are so small that useful results would require them to take up most of the measurement cycle. Similarly, why measure 235U since according to lines 253-255 the natural range of 238/235 variation is negligible? If this is in question, it would be better to perform a dedicated measurement of 235/238 on an adjacent sample within the same sedimentary horizon. 232Th needs to be monitored but at a minimum dwell time to confirm that it is near zero. That leaves 206Pb, 207Pb, 208Pb and 238U, of which 206Pb is assumed to be unreliable so can be omitted. 207Pb is the smallest of the remaining isotopes and should have a significant common Pb component so it needs to be measured as precisely as possible to limit error propagation after subtracting the common Pb component using the 208Pb proxy. My point is that the measurement protocol is extremely important and should be carefully thought through.
RC9: Line 274: Again, the two references given are on using information from both U-Pb decay schemes, not just 206Pb/238U.
RC10: Fig 4: The caption should state which of the two results shown in 4a-4c represent filtered and which are unfiltered data. Are the uncoloured ellipses filtered out? Similarly for 4d-4f.
RC11: Line 361: It should be noted that Th can be taken as a proxy for non-hydrogenic detrital components. Analyses with significant Th should be edited out of the data.
RC12: Line 423: The contrast in consistency of the anomalous (low 207/206) data with the double-decay isochron (Fig 6a) and the 235-207 decay isochron (Fig 6b) seems odd at first glance. Tera-Wasserburg Y-axis intercepts (207Pb/206Pb) that are significantly below the value expected from the Stacey and Kramers model may be due to mobilization of radiogenic (high 206) Pb from high-U minerals and incorporation into the hydrogenic mineral when it formed. In fact, Fig 6a could be interpreted as showing two similar-age isochrons: the normal one and a scatterchron below it with a lower initial 207/206 intercept but a similar age. As a contaminant, radiogenic Pb has about an order of magnitude lower 207 than 206 so it will have less effect on the 235-207 system than on 238-206. This is another important argument that the 235U-207Pb system can be more robust than the double decay system.
RC13: Line 431: Signal spikes are a problem that bedevilled us when we first started analyzing calcite, which requires large laser spots. They occur because large particles get lodged on the walls of the tubing and the nebulizer (He-Ar mixing chamber) before the input to the plasma. These should be clearly visible if you place a sheet of black paper beneath the nebulizer. We have found that they are almost completely eliminated after ultrasonic cleaning of the nebulizer and replacement of the tubing. The only reason I can think of for a sample-specific spike problem would be if ablation of some samples produces larger particles than from others. Ablation of calcite using a 213 nm (solid-state) laser creates fractured (jagged) ablation pits presumably because CO2 release explodes the target before complete melting. Pits ablated at 193 nm are much smoother looking, perhaps because melting is more rapid and complete. Perhaps the pits from samples where this is a problem should be examined to see if they look any different.