| 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.
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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.