the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Feb 2025
Accuracy and validity of maximum depositional ages in light of tandem (laser ablation + isotope dilution) U–Pb detrital zircon geochronology, including n = 1 results from northern Alaska
Abstract. Sound geologic reasoning underpins detrital zircon (DZ) maximum depositional ages (MDAs) via the principle of inclusions, although interpreting in situ U–Pb date distributions requires many geologically, analytically, and statistically driven decisions. Existing research highlights strengths and challenges of various algorithm approaches to deriving MDAs from DZ dates, yet community consensus on best practices remains elusive. Here, we first present new laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb geochronology for five DZ samples from a ~1 km thick section of mid-Cretaceous strata in Alaska’s Colville foreland basin. Youthful DZ yields are extremely sparse, and the MDAs are n = 1. LA-ICPMS and CA-ID-TIMS dates from the same grains (i.e., tandem dating) adhere to a uniform pattern: laser ablation dates are younger than paired isotope dilution dates, with in situ offsets ranging from –0.3 % to –6.4 %. Existing biostratigraphic constraints suggest a ~110–94 Ma sedimentation window for the sampled section, but the CA-ID-TIMS MDAs reduce by ~8.5 Myr the maximum geologic time recorded by the stratigraphy. A simple age–depth analysis incorporating the CA-ID-TIMS MDAs and correlation of a new CA-ID-TIMS tephra zircon age yields geologically reasonable minimum stratigraphic accumulation rates, but an LA-ICPMS-based interpretation would render a geologically improbable and geochronologically inaccurate chronostratigraphy. We then explore the new tandem data and two previously published Mesozoic tandem DZ datasets for their broader MDA research implications, focusing on tandem-date-pair relations rather than conducting the typical MDA algorithm outputs assessment. Percent-offset plots document impactful (~2–3 % on average) and pervasive (~87–100 % of pairs per study) young bias for the laser ablation dates, likely reflecting a complex combination of analytical dispersion, low-temperature Pb-loss, and matrix effects, which are topics we review in detail. Definitively deconvolving offset sources without elaborate geochronologic experiments is difficult, but our tandem-date analysis provides critical context, and follow-up CA-ID-TIMS can diminish or eliminate analytical, systematic, and geologic offset sources. We also redefine the reference value for MDA accuracy as the crystallization age of the youngest analyzed DZ population in a sample and reframe LA-ICPMS-based DZ MDA algorithm evaluations around validity—how capable are the metrics at accurately measuring what they are intended to measure?—rather than MDA benchmarking by existing age constraints. These new perspectives follow straightforward geochronologic and stratigraphic principles, and our synthesis intends to identify and clarify opportunities to further refine DZ MDA research.
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RC1: 'Comment on egusphere-2025-727', B. Schoene, 17 Apr 2025
Herriott et al. presents an analysis of detrital zircon (DZ) U-Pb geochronologic data as a means of determining maximum depositional ages (MDAs) for strata by comparing the youngest zircons measured from a bed by LA-ICPMS with the same grains measured by ID-TIMS. It is a thorough analysis that focuses on one dataset, and it also compiles data from a few other recent studies that have both ICPMS and TIMS paired analyses. They show there is a semi-systematic offset between the ICPMS and TIMS data where the ICPMS data is younger – often beyond analytical uncertainty – than the TIMS date from the same zircon. They discuss the possible causes and implications of this in detail.
I was a bit surprised to find out that all the data in this paper are already published in an Alaskan Survey publication that is easily obtainable and contains some of the same figures and tables (in addition to all the raw data tables). I speculate that perhaps the great state of Alaska requires this form of publication first, as that first publication reads like more of a data dump. The current manuscript is perhaps seen as an opportunity to analyze the data in more detail and in a way that would appeal to a broader readership, but I can't help wonder if they both need to exist.
I must admit that I’m not entrenched in the DZ literature and therefore am somewhat ignorant to published discussions of the precision and accuracy of DZ MDAs. To be honest, I have just always assumed MDAs in general should be taken with a grain of salt, and those determined by ICPMS are likely inaccurate by some unknown amount, and that playing statistical games doesn’t get you much closer to assessing their accuracy. I have also noticed numerous examples in my own lab where ID-TIMS dates on the same zircons dated by LA-ICPMS are older, sometimes by tens of Myr or more in the Paleozoic or Ediacaran, so I bring this observation to every MDA DZ dataset I see (i.e., big grain of salt). Given that predisposition, I find the results and conclusions of Herriott et al. not too surprising and found parts of the paper to be overly verbose in coming to their conclusions. So, I guess assessing the value of this contribution would be best done within the context of someone who uses DZ MDAs, or wants to, but is less aware of the challenges of doing so. Herriott et al. cites numerous papers that appear to provide reviews of or new approaches to analyzing MDAs, so I would hope one of those folks could also chime in on this paper to assess its novelty. Regardless, I see an obvious contribution from this paper is the reiteration that without paired LA-ICPMS and CA-ID-TIMS dates, you can’t have much confidence that n=1 zircon MDAs determined without chemical abrasion to remove Pb-loss are likely accurate. Whether or not that matters of course depends on the question being asked in a given study.
For revisions, perhaps the authors could add some statements in the abstract and intro that delineate the novelty of this analysis. I also suggest shortening parts of the manuscript. For example, I bet sections 2.4.3., 3.1.1., 3.1.2. could be considerably condensed (by 50%?). I would also include some text near the beginning that clearly states what data from this paper are already published and why those data need to be revisited here, in order to alleviate potential concerns about, or impressions that you are, double-dipping.
Here's some line-by-line comments:
27: I find the sentence beginning on this line fairly confusing. Try rewriting or breaking into two sentences.
34: I’ve never heard the word “stratal” used in this way before. But I guess you mean “same age as the bed the zircons are found in”?
54: the previous two paragraphs assume people already know this, so move it up or perhaps it’s ok to delete.
176: “…to the work by LePain…” could be interpreted to mean that LePain did the correlation, though I think you mean you did the correlation between your samples and those in LePain.
199: I’m not sure what you mean by “in quadrature” here. Maybe just cite the papers with the algorithms used.
212: If your intended audience includes geochronologists, you might report the MSWD statistic when discussing equivalence of MSWDs.
229-232: I have personally used the term accuracy to also include data interpretation, i.e., how well does a date measure the process you’re interested in. But I like the suggestion here to use validity in that it separates the two, perhaps in useful ways.
240: out of curiosity, why did the ID-TIMS dates fail for these?
260: could you actually state what the kernel bandwidth is?
Fig 4: I’d put the dates for individual zircons on the figure beneath the sample name, and include you’re preferred depositional interpretation of the tephra samples as well.
Fig. 5 and associated: I must say that after reading the abstract I was expecting the LA data to be resolvably and consistently younger than the TIMS data. However, when I look at Fig. 5 it seems 6/10 single analysis pairs agree within 2sigma. Some LA people may call that a win, whereas it’s being pitched here (at least how I read it) as a loss. I’d recommend revising some of the language with a focus on accurately describing the data comparison given analytical uncertainties. One could raise a similar question for Fig. 9, which may or may or may not look so bleak if analytical uncertainties were included. But don’t get me wrong, I agree the dates are shifted young and I appreciate the cause is likely to be Pb-loss but for most readers, esp. of DZ data, they want to know if the mean overlaps with the true or not, not if the mean is shifted slightly younger but overlapping with the true given the uncertainty.
Fig. 6: these aerial photos are amazing.
Fig. 7: I don’t think you should be plotting a KDE with the intent of comparing precision and accuracy of two datasets that measure the same thing with different instruments. The KDE ignores actual measured uncertainties and therefore is not going to serve your purpose. I’d plot a PDF in (b) instead.
337: I’d delete “suggest and” and replace with “and are”
374: “very good goodness” sounds funny
514-529: I don’t feel like I have enough information about what Copeland 2020 is proposing for me to get much out of this paragraph.
593-595: I think this sentence is important, and what I was thinking while reading the previous few pages…
670: it would be useful to describe what the MLA algorithm does. For me, I’ve never used it, so the following discussion about it is rather black box.
720, Fig. 9: I could have used a better description of what the nth Youngest Tandem LA-ICPMS date means. I got lost here.
724: typo
Citation: https://doi.org/10.5194/egusphere-2025-727-RC1 -
AC1: 'Reply on RC1', Trystan Herriott, 10 May 2025
We thank Dr. Schoene for reviewing this manuscript. The review’s main aspects are addressed below, and the comments will be integrated in revision, pending provisional acceptance of the manuscript.
The Alaska Division of Geological & Geophysical Surveys Raw Data File (Herriott et al., 2024) serves as this manuscript’s supplemental material (line 1007). Funding entities (e.g., NSF, USGS), research agencies (e.g., geological surveys), and journal publishers (e.g., Copernicus) now typically require or encourage that data be archived in publicly accessible repositories, and journal supplementals may not comply with these directives. Herriott et al. (2024) provide the data and metadata necessary to validate the research findings presented in section 2 of this manuscript. The RDF does not include a chronostratigraphic analysis or discussion of tandem date relations or implications thereof on the broader field of MDA research. In preparing the RDF, we followed widely available recommendations for separately archiving data that are paired with a companion research article, and such approaches are not double-dipping. This will be clarified in revision.
We understand the general sentiment that ICPMS MDAs may be inaccurate to some unknown extent. However, the DZ MDA Google Scholar search terms of Coutts et al. (2019; their fig. 1) for 2024 yield ~450 results, the majority of which are ICPMS-only studies. Authors of such papers are likely striving to be objective and careful in their work. But we agree that statistical games aren’t helpful, so, to this end, our evaluation of tandem date relations provides key context for choices, including experimental design or MDA algorithm selection, that are routinely made in case studies. Some hedging is expected in interpretations, but there usually isn’t much room for a big grain of salt in scholarly publishing (for better or worse), even for low-n, ICPMS-based DZ MDAs.
TIMS zircon geochronologists may not be surprised by our conclusions regarding Pb-loss. Yet many researchers employing ICPMS DZ MDAs are not geochronologists (TIMS or otherwise) and may not appreciate the limits of discordance filtering for young zircon, potential mechanisms or pervasiveness of Pb-loss, etc. And even if a result is anticipated, documenting and interpreting data relations and synthesizing relevant information are necessary steps toward advancement. Also, the ubiquity of ~2–3% average young offset for 206/238 ICPMS dates of Fig. 10 is arguably a bit surprising and suggests systematic bias, such as matrix effects (3.1.3), should not be discounted.
Overlap at uncertainty relations are important considerations in this work (e.g., 3.1.1; lines 250, 306, 665, 697, 760, 770, 776; paragraphs at lines 445, 923; Figs. 5, 7, 8), and we can further bolster these comparisons in revision. The convention to regard data as accurate if a measured result overlaps a reference value at reported uncertainty is reasonable. However, for the tephra sample, despite a majority of pairs overlapping at 2σ, a sensible weighted mean approach yields an ICPMS date that does not overlap at 2σ at Y the TIMS age (Fig. 7). Even though most of the DZ date pairs from Slope Mountain overlap at 2σ (e.g., Fig. 5), standing up an accurate ICPMS-based DZ MDA chronostratigraphy is unlikely (paragraph at line 445; Fig. 8). And, in a larger context, an overlap-at-uncertainty-criterion for accuracy could effectively mask chronostratigraphically impactful young bias for ICPMS (as noted above), with offset averages perhaps commonly lying within—yet near the negative edges of—the ±2σ envelopes (e.g., lines 664, 696, 769, 775). We anticipate that 1) some researchers won’t be satisfied with where the ICPMS dates lie in offset plots of this study and of Howard et al. (2025) and 2) community efforts will improve accuracy of ICPMS zircon geochronology.
We also appreciate the recommendations regarding how to further clarify and condense this contribution to improve its impact.
We again thank Dr. Schoene for reviewing this preprint.
Regards,
Trystan M. Herriott and co-authors
Citation: https://doi.org/10.5194/egusphere-2025-727-AC1
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AC1: 'Reply on RC1', Trystan Herriott, 10 May 2025
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CC1: 'Comment on egusphere-2025-727', Mike Eddy, 29 Apr 2025
Overview
The manuscript by Herriott et al. uses U-Pb zircon geochronologic data from a Cretaceous sedimentary sequence in northern Alaska to assess the quality of maximum depositional ages (MDAs) that are derived from different approaches. Importantly, all of the zircon used in MDA determinations were dated using both laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and chemical abrasion-isotope dilution-thermal ionization-mass spectrometry (CA-ID-TIMS). This tandem dating approach allowed the team to compare approaches that use only LA-ICP-MS data (typical in most detrital zircon studies) to those that utilize the more precise (and presumably accurate) CA-ID-TIMS measurements. They find that all MDA estimates derived from LA-ICP-MS data are younger than those derived using CA-ID-TIMS data. In the case of the northern Alaska sedimentary section, relying on MDA estimates based only on the LA-ICP-MS data leads to geologically implausible sediment accumulation rates. They go on to document that systematically younger LA-ICP-MS MDA estimates are pervasive in previous studies that have dated detrital zircon by both LA-ICP-MS and CA-ID-TIMS. The manuscript discusses two possible sources for this systematic offset: Pb-loss and matrix effects during laser ablation. Ultimately, the manuscript concludes that CA-ID-TIMS analyses should be used to benchmark MDAs.
This manuscript should be of wide interest because it highlights a pervasive issue within a widely used technique: MDA estimation using LA-ICP-MS detrital zircon geochronology. Documentation that MDA estimates from routine LA-ICP-MS measurements are systematically too young is, therefore, an important point. The manuscript also gives two areas where the problem might arise: (1) Pb-loss and (2) differences between reference and unknown zircon that lead to systematic biases. Both issues are tractable. Indeed, I think that the most important point of this manuscript is that improved MDA estimations will only come from careful analytical work, not through the development of a silver bullet algorithm.
The detrital zircon data has been previously published as part of a raw data file for the Alaska Department of Natural Resources. The document has minimal discussion about data interpretation and I don’t see an issue with a new publication that focused on interrogating the data more thoroughly. There are plenty of publications that conduct meta-analyses of data that was originally presented elsewhere in a different context.
The main weakness of the manuscript is its length. In general, the manuscript’s message is simple (MDAs generated via routine LA-ICP-MS are likely inaccurate due to matrix effects that preferentially bias Mesozoic and Cenozoic unknowns and/or Pb-loss.). I fear that this message will be lost because readers won’t make it through the entire document. I recommend that the authors make significant cuts and have outlined areas where I think they could be made below. However, authors should ultimately decide how to present their work and I see no issues with the data, interpretations, or discussion that should preclude its publication in a long format. They should feel free to incorporate my suggestions at their own discretion.
Thank you for the opportunity to look over this research.
Mike Eddy
Suggested Cuts/Edits
Section 3.1.1 – This section ultimately is used to argue that the MDAs derived from routine LA-ICP-MS analyses are inaccurate and biased toward being too young. This is evident in Figure 9 and Figure 10. I think the manuscript could be shortened by several pages by reducing this section and the introductory paragraphs in 3.1 to just a few sentences that point to Figures 9 and 10 and simply state that ‘all MDA approaches are biased toward dates that are too young’. This will allow the manuscript to quickly move to the more interesting discussion about why they are too young.
Figure 9A& 9B – It would be helpful to incorporate uncertainty in these plots. Maybe the easiest way would be to put a gray bar behind all of the data that is centered on 0.0% relative offset and extends to the typical LA-ICP-MS uncertainty. This would allow the reader to quickly see whether the uncertainty in LA-ICP-MS dates encompasses the corresponding CA-ID-TIMS date.
Figure 10 – At this scale it is really hard to see the systematic offsets. Maybe do insets of each data set or three different plots so that it is more clear.
Section 3.2 – I think that this section could be simplified to just a few sentences that culminate in the recommendation that MDAs are benchmarked with CA-ID-TIMS.
Section 4 – The summary section has redundant information and could probably be cut.
Citation: https://doi.org/10.5194/egusphere-2025-727-CC1 -
AC2: 'Reply on CC1', Trystan Herriott, 10 May 2025
We thank Dr. Eddy for reviewing this manuscript. The review’s suggested revisions are briefly addressed below and, pending provisional acceptance of the manuscript, will be incorporated in revision.
Regarding Figure 9, incorporating uncertainty into the offset plots is consistent with a comment by Dr. Schoene, and the gray bar suggestion is a good one. We will update these plots in revision. For Figure 10, we will replot this to improve readability and provide uncertainty context as well.
Dr. Eddy’s suggested approaches to reducing the manuscript’s length are also appreciated.
We again thank Dr. Eddy for reviewing this submission.
Regards,
Trystan M. Herriott and co-authors
Citation: https://doi.org/10.5194/egusphere-2025-727-AC2
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AC2: 'Reply on CC1', Trystan Herriott, 10 May 2025
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RC2: 'Comment on egusphere-2025-727', Anonymous Referee #2, 30 Apr 2025
Review of Herriott et al.: Accuracy and validity of maximum depositional ages in light of tandem (laser ablation + isotope dilution) U–Pb detrital zircon geochronology, including n = 1 results from northern Alaska
The authors present new tandem U-Pb dates from Alaska and discuss their results both in the context of regional (AK) geological implications as well as much broader discussions about the applications of LA-ICPMS and CA-ID-TIMS geochronology for interpreting maximum depositional ages (MDAs). This is a carefully and thoughtfully crafted manuscript which serves as a case study for competing methods and procedures to constrain depositional ages. It is thoroughly referenced and well-written. Moreover, this is an important paper integrating the detrital zircon LA-ICPMS community with the ID-TIMS geochronology community toward a better approach for interpreting max depo ages that is not reliant on selecting from a menu of calculation metrics (or formulas). Rather, this case study demonstrates the utility of CA-TIMS for confidence in youngest single grain dates for MDA interpretations in samples with very few young zircon.
I think the text is longer than it needs to be. Due in part to the authors trying to account for both the local geological considerations from their new data and the review-style discussion that is intended for a broader audience. Consequently, it feels a bit disjointed at times going back and forth between broader/philosophical discussions and the results of the study from AK. Moreover, many of the sections sort of have their own introductions, which adds up.
I only have a few general comments/suggestions that might improve the manuscript for the broader community to understand the implications of the results and any implicit recommendations by the authors.General comments/suggestions.
Small point (take or leave): Is there a better word than “stratal”? I presume the authors mean a near-depositional age when referring to “near stratal age”. Not sure I’ve seen this usage before. I think of strata as ‘layers of rock’ and thus strata are not deposited. Sediment is deposited and eventually becomes strata. Nit-picky, I know. I can adjust.Matching the data presented with the main message(s) of the paper: The introduction nicely frames the background and review of the paper’s topic(s), but falls a bit short in setting up the motivation for the new data pursued/presented in Northern AK. In other words: the problem is presented well, but why is the Alaska dataset the go-to for solving this problem? For instance, if the introduction was more focused on the issues/challenges around interpreting sediment accumulation rates of clinothems then I could see how/why the new analyses from Torok and Nanushuk detrital (and ash) zircons would be pursued. Or if the intro focused more on use of the YSG and how many grains/dates are needed – thereby setting up the potential benefits of one good/reliable CA-TIMS date – that would set up the value of a suite of samples with few young grains that could be leveraged for the benefits of tandem dating. More of a thought/comment than a tangible suggestion. Obviously, this is up to the authors’ discretion, but improvements could be as simple as condense/focus the first four paragraphs of the introduction to set up the benefits of the AK dataset.
Correlating the clinothem (timelines vs. facies): I wonder about the validity of using samples that have been correlated ~200km apart to calculate sediment accumulation rates on a time-transgressive clinothem. Does this correlation honor Walther’s Law or is it correlating facies rather than timelines (Figure 2)? In the subsurface some of these clinoforms have 100’s of meters of relief. For the sake of demonstrating how CA-TIMS dates can be used for estimation of sediment accumulation rates, I just want to confirm that the expected lateral facies variations are being accounted for when correlating timelines ~200km (dip view-ish) along a delta front clinoform. Or is this assumed to be negligible? Also, are there any other implications for revising the interpreted minimum sedimentation rates beyond achieving more realistic values?
Clarification of benchmarking with CA-ID-TIMS: What is the goal of this section? Are the authors recommending the community to benchmark DZ MDA results using CA-TIMS? If so, can this be clarified/specified for a broad audience (including DZ aficionados and such)? Some studies may seek to collect detrital zircon U-Pb dates exclusively for the purpose of constraining depositional ages and others may primarily be interested in provenance considerations. If someone collects ~300 detrital zircon dates from a sample for provenance information, are the authors suggesting new limits of interpretability of the upper bounds on depositional age if the sample has not been benchmarked by CA-TIMS? If one yields 3 to 4 young dates (or more?) from that sample, they will have to decide if it is worth an additional $500-$1000 (lab depending) to get accurate and precise results for a “half-age”. I certainly agree with the authors that if one wishes to rely on a limited number of young dates/grains to push a young MDA, then they would be wise to explore CA-TIMS. This dataset, and the compilation by Howard et al. (2025) make that point clear. I think the benchmarking section can be condensed into another section. If it remains, I suggest clarification for a broad readership. I was encouraged to see (in the conclusion section) acknowledgement of other considerations one must make when deciding to pursue CA-ID-TIMS with their detrital zircon results (e.g., feasibility, budget, accessibility, etc.).
A refined analysis of tandem date offsets? The authors present a comparison of tandem date pairs from their study, Herriott et al. (2019), and Rasmussen et al (2021). It’s not clear to me how this comparison differs from Howard et al (2025), which I think includes the Herriott et al (2019), Rasmussen et al (2021), and many more. Unless the authors have strong feelings about this comparison, this seems like another part of the paper that can be condensed. Perhaps focus on if/how their new comparison differs from the comprehensive dataset present by Howard et al (2025). In Figure 10 it is not obvious that all the data points fall significantly off the 1-1 line – suggest showing the 3 datasets separately so the offset can be expanded (zoomed in).
Line 565: Should it be”… older than THE stratal age”?
Line 571: Should chronostratigraphy be pluralized as “chronostratigraphies”? Chronostratigraphic records?
Line 814: Should that be “rendering”?
Line 853: Maybe clarify if both standards and unknowns were thermally annealed in the Herriott et al (2019) study so that it follows previous discussion.Thank you for the opportunity to review the manuscript and thanks to the authors for clean, high-quality submission. Best wishes in getting it through to publication.
Citation: https://doi.org/10.5194/egusphere-2025-727-RC2 -
AC3: 'Reply on RC2', Trystan Herriott, 10 May 2025
We thank the anonymous referee for reviewing this manuscript. The review’s main aspects and suggested revisions are addressed below, and their comments will be integrated in revision, pending provisional acceptance of the manuscript.
Regarding time-transgressive relations in the Nanushuk–Torok clinothem and correlating the Ninuluk Bluff tephra zircon age nearly 200 km to Slope Mountain, note that the correlation is not associated with regressive clinothem growth, which is highly time-transgressive, but rather with termination of clinothem growth via ~apparently synchronous transgression (see Lease et al., 2022; paragraph at line 156). The correlation is also broadly an along-depositional-strike (marine) topset tie, albeit likely along an “L” shaped trend that probably mirrored clinothem regression/growth patterns (see paleo-shelf margins of Fig. 1a), so along-clinoform-dip complications should largely be avoided. Further, the treatment of the tephra result from Ninuluk Bluff as a minimum age after its ~shoreline parallel projection to Slope Mountain recognizes uncertainty in the details of the transgressive topset relations (paragraph at line 400; regional framework of Fig. 2). These stratigraphic considerations and, critically, the minimum age treatment post-projection (and derivation of minimum accumulation rates) should prevent running afoul of time-line issues relative to Walther’s law (see line 424). We can clarify this in revision.
The TIMS-based minimum stratigraphic accumulation rates are fundamentally reflecting a reduced sedimentation window (e.g., line 16) in the newly established chronostratigraphy, with the bulk of Nanushuk at Slope Mountain likely being Cenomanian rather than Albian (Fig. 8). These results complement the work by Lease et al. (2022) by providing additional Nanushuk–Torok age constraints that bear on rates of clinothem growth (in the case of Slope Mountain, the vector being quantitatively constrained is aggradation); this is relevant to northern Alaska specifically and in foreland basins generally. As in any basin, improved age control has many implications, and another one we note is that the major incision surface of Fig. 6 may be associated with the globally significant Albian–Cenomanian boundary event (paragraph at line 415; e.g., Lease et al., 2024). That potential tie, which is testable with further research, would have been difficult to recognize, let alone justify, prior to this study.
The goal of section 3.2 is to note the strengths and limitations of TIMS, which directly relates to the justification for benchmarking ICPMS dates of unknown zircon with tandem TIMS dates. These two paragraphs contain what is likely common knowledge to zircon geochronologists. However, our experience suggests that many practitioners who use ICPMS-based DZ MDAs in their research are not familiar with TIMS zircon geochronology. MDA researchers in general (not just geochronologists) should have the opportunity to evaluate assumptions that are baked into ICPMS date offset perspectives. We will clarify this intent in revision.
Also, regarding limits on establishing MDAs based on sparsely vs. densely sampled (by ICPMS) youthful DZ populations, there are benefits to tandem dating in both scenarios (paragraphs at lines 604, 747, 757, 972). Our recommendation is to use methods that are validly positioned to answer research questions of interest (e.g., paragraphs at lines 972, 983).
The recent paper by Howard et al. (2025) is a significant contribution to the field of ICPMS geochronology, and their tandem-date-pair compilation and ICPMS offset analysis included 206/238 and 207/206 dates from detrital, igneous, and metamorphic zircon. Howard et al. (2025) note the implications of their work on DZ MDAs, but our manuscript is focused on DZ MDAs. We provide geologic context for a few tandem datasets from Mesozoic basins, highlight chronostratigraphic impacts of ICPMS date offsets, and emphasize the relevance of offsets on DZ MDA research relative to the array of available algorithms, precision, accuracy, and validity. We also place significant emphasis on Pb-loss (3.1.2) and matrix effects (3.1.3), and some benchmarking of analytical dispersion must be reflected in the percent offset plots as well (3.1.1). Ultimately, we consider this manuscript as a complement to—and in part building on—the work of Howard et al. (2025)
We also appreciate the reviewer’s suggestions regarding how to reduce the length of the manuscript, and those comments complement the recommendations by Dr. Schoene and Dr. Eddy.
Thanks again to this referee for reviewing the preprint.
Regards,
Trystan M. Herriott and co-authors
Citation: https://doi.org/10.5194/egusphere-2025-727-AC3
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AC3: 'Reply on RC2', Trystan Herriott, 10 May 2025
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