Dear Anonymous Referee #1,

Thank you for a detailed review of our manuscript. Your comments have been very helpful and we hope to have improved our manuscript by addressing your concerns.

The construction of the GICC21 timescale has been a long work and we are glad to see our efforts recognized. We are satisfied to see that all Referees agreed on our uncertainty estimation to be valid.

We agree that the manuscript ended up being quite long, however GICC21 needs thorough explanation and documentation. In response to your comment and to those by An.Ref.#2 and An.Ref.#3, we have concluded that the manuscript will gain more readability by shortening some parts, in order to refocus the manuscript around the GICC21 timescale, i.e. we intend to cut out the discussion of volcanic cooling and Mediterranean volcanic events (sections 5.2 and 5.3, Appendix A and B, and the relevant parts of abstract and conclusion). We will continue the investigation on these topics in our future work. We thank you for the comments about these sections, which we will consider when working onward on these topics.

All ‘technical corrections’ have been included into the revised manuscript.

Our replies to your specific comments:

>>L18: Radiocarbon dated evidence. please clarify. What kind of evidence? Evidence for what?

Reply: The evidence from eruption sites such as the Olive branch from Thera. This sentence as well as most of the last paragraph of the abstract will be removed as a consequence of removing sec 5.2 and 5.3.

­>>L24: Late Holocene – please give time frame

Reply: From 4.2 ka b2k to today (Walker et al., 2012).

>>L33: a short description of the general differences in dating methods, stratigraphic and relative vs. absolute age markers and their pros and cons in terms of error would be helpful.

Reply: We agree. We have added a few sentences on the matter in the revised manuscript (MS).

>>L47: That is quite a bold statement. We know that isotopes can be used as temperature proxies but only to a very limited extend and under certain circumstances, also keeping in mind all the post-depositional effects. Please weaken this statement and also cite some more recent studies, for example: Laepple, T. et al. On the similarity and apparent cycles of isotopic variations in East Antarctic snow pits, The Cryosphere, 12, 169–187, 2018.

Reply: We will weaken our statement and include the proposed citation.

>>L52: Reference for high and stable snow deposition?

Reply: The stability of Greenland Holocene climate is demonstrated by the quite constant isotope values (with the exception of the cooling events older than 4.2 ka b2k) (Vinther et al., 2009). The description of  “high” accumulation is subjective and we will rephrase it to be about the relative change of layer thickness compared to glacial values (Cuffey & Clow, 1997; Rasmussen et al., 2006).

>>L108: why do nitrate peaks coincide with ammonium peaks? Please explain, give reference.

Reply: We apologize that this sentence was not substantiated with proof and references. This observation was made during the dating work, although it did not provide an additional base for the matching. We will add a figure in the Supplement to illustrate the coincidence of the peaks in the two species. Nitrate is described in Legrand et al. (2016) as a potential proxy for bio-mass burning and the frequent co-occurrence of nitrate and ammonium is documented in this publication, hence we will cite their study in this context. However, the use of NO^-₃ as a proxy for bio-mass burning is questioned in that same study because of the influence of other sources, so we refrained from using NO^-₃  peaks as tie-points. We remark, however, that in the absence of NH⁺₄ in one ice core because of a data gap, one could use NO^-₃ tentatively, if the other cores showed co-registration of NO^-₃  and NH⁺₄ peaks.

(In Supplement: Figure 1 Example of the co-occurrence of NH⁺₄ and NO3 peaks in the NEEM ice cores.)

>>L110ff: please add the tritium peak as a radiometric marker

Reply: We will add it.

>>L157: what bias do you refer to? Overcounting, undercounting or something else? Please clarify.

Reply: As you say, a dating bias can be caused by several aspects of the dating process. However, it is hardly ever caused by the counting in itself but rather by mistakes in the layer-identification rules. As stated in Vinther06 and in Rasmussen at al., 2006, the MCE accounts for uncertainties because of layers that were hard to interpret, but it does not account for the bias related to the layer-identification criteria used by each observer. Hence, no account of this type of bias is used to quantify the MCE. In our error estimation, on the other hand, we are somewhat trying to estimate the bias the MCE did not account for, by looking at the systematic undercounts/overcounts produced by StratiCounter. We will expand on this in the revised MS.

>>L175: Please define WDC

Reply: Will be added in the revised MS: West Antarctic Ice Sheet (WAIS) Divide Ice Core (WDC).

>>Table 1: Accumulation given in m total or m.w.e.?

Reply: The accumulation is given in m ice equivalent.

>>L252ff: can you please give depth resolutions for the NEEM and NEEM-2011-S1 data?

Reply: The resolution of NEEM is 1 mm, and that of NEEM-2011-S1 is 1 cm.

>>L265ff: Depth resolution for GRIP data?

Reply: For the isotopes it’s 2 cm , for ECM 1 cm

>>L275ff: Depth resolution for DYE-3 data?

Reply: For the isotopes it’s 1 cm , for ECM 1 cm

>>L283-290: Nice overview of the data processing steps

Reply: Thank you.

>>L306f: Please clarify that sentence. What do you mean exactly by pre-processing? What was the control data?

Reply: We will explain this better in the revised MS: StratiCounter has an inbuilt pre-processing option that allows the user to select the pre-processing that SC should apply to the data in order to count the layers at the best of its possibilities (e.g. log-transformation of Na and Ca data). By control data we mean the ice-core data between the Laki and Samalas eruptions. The way we selected the appropriate pre-processing steps was by  studying which one produced the layer count closest to the true number of years between the two eruptions.

>>L337-340: I don’t really understand the knowledge gain by inverting and log transforming the ECM into pseudo NH4+. In my opinion this does not give any additional information and rather creates apparent alignments which are completely artificial. Either clarify the benefits of the pseudo-ammonium or take it out (also in Fig. 2), because I think it could be misleading.

Reply: We observed in our work that log-transformed and inverted ECM aligns well with NH⁺₄ peaks, which is a consequence of NH⁺₄ neutralizing the acidity of the ice core. The alignments were seen within the same ice core, i.e. between the log-inverted-ECM and NH⁺₄ of EastGRIP, NEEM, and NorthGRIP. The use of log-inverted-ECM  is hence extended to those cores for which NH⁺₄ wasn’t measured.

In the revised MS we will rectify that not all ECM dips are caused by NH⁺₄ peaks, since there are other positive ions in ice cores. To explain better, we intend to add an illustrative figure in the supplement, and we will perform a statistical test of correlation.

(In Supplement: Figure 2 Alignment of log-inverted ECM (pseudo-NH⁺₄) peaks and true NH⁺₄ peaks in the NEEM ice core.)

>>L341: Clarify reference datum

Reply: We will explain this more in the revised MS as follows. Dating the ice cores down from the surface is often not practical or not feasible because the very top layers are made of loose firn and have very variable width. Moreover, most deep-ice-core data start after some depth, e.g. the main EastGRIP core was only drilled from 14 m down. Therefore, we chose a reference datum that is both well established and not too far from the surface, but deep enough for the data quality to be sufficient. The Laki eruption of 1783 CE is well documented in all ice cores and was used as reference datum, by which we mean the absolute date to which the rest of the timescale is anchored to.

>>L345-355: Using both timescales (b2k and CE) can be confusing for the reader. I would recommend to use b2k throughout the MS and only additionally refer to CE for some of the historical events where the CE age is well known.

Reply: We agree and we will make the changes.

>>Fig.2: would be good to either use more distinct colors for pseudo NH4+ and NH4+ or take out the pseudo signal completely (see comment above). Please add NH4+ on the y-axis, not just ppb. Why was the big ammonium peak between 214 and 216 m in North GRIP not used for matching? Seems like an unusual gap and as if you could find matching features at least in GRIP and DYE. Contrarily some match points are not very convincing, e.g. nr. 5 in EastGRIP and GRIP.

Reply: Because the peak doesn’t appear in all cores, not even in years close by. Hence, we did not use it as a tie-point. A gap of 20 years between tie-points such as the one between n. 10 and 11 is not unusual in the timescale. The patterns formed by nr 8,9,10 and 11,12,13 provided a match in which we are more confident. We will remove some tie points such as nr.5 for the sake of clarity of our message: it is patterns of ammonium peaks that are matched, not just single isolated peaks.

>>Table 2: I’m not sure if it is helpful to calculate the total score, because for example for Laki the total score is two, but four out of six cores undercounted it. Seems to be misleading and not necessary for the fine tuning procedure in my opinion.

Reply: We agree and we will be removing the total column.

>>L424: I can’t find the 17% improvement in Table 3

Reply: 0.27/ 0.23 = 1.17. We will make an addition to the table to highlight this percentage.

>>L420-425 and Table 3: I would recommend to shorted or take out the comparison between DYE-3 and the SWG Greenland temperatures as well as the correlation study between GRIP and DYE-3 isotopes. I would not call the improvement of correlation “substantially”, even if it is 17% and for the SWG temperatures the authors themselves state that there is no significant change. As it is not relevant for the further discussion I would remove this paragraph.

Reply: We disagree with removing this section as we think it provides additional information to the reader about the timescale. By keeping a high correlation to the SWG temperatures while increasing the correlation between GRIP and DYE-3, we demonstrated that the timescale has improved, since a correlation measurement of a long time-series is unlikely to improve by chance. The SWG temperatures (recorded from 1784 CE) should correlate with the isotope signal, according to the finding by Vinther et al. (2010). Since we are exceeding the uncertainty of GICC05 (MCE~1 year), we have to verify if the correlation with SWG is still valid. Moreover, the correlation between DYE-3 and GRIP could have changed in GICC21, so that this is an independent and objective test of the quality of the timescale.

>>L455: What exactly do you mean by historical evidence? Is that referring to volcanic eruptions?

Reply: Yes, we are referring to historically dated volcanic eruptions.

>>L461: what kind of local effects do you refer to? Please clarify.

Reply: We will explain this more clearly in the revised MS. By local effects we mean the effects that influence the layer identification in one core within neighboring tie points: accumulation fluctuations, uncertain layers across all ice cores, local tie-point uncertainty (e.g. a volcanic marker placed differently across cores), measurement gaps.

>>L467-486: That section needs some clarification in my point of view. First, I think it should be emphasized that for the youngest part the uncertainty is ~2 years, that important statement is somehow lost in the text in line 471. Then follows the calculation of the SC uncertainty compared to the fine-tuned record using convolution of the individual probability distributions of the cores. Looking at Fig. S3 it seems as if there are systematic offsets towards over- or undercounting for certain cores and the differences can be quite significant (e.g. panel d or k in Fig. S3). I am therefore wondering if the convolution that weights each core equally is the best method to assess this uncertainty or if there should be a more individual assessment for each core similar to section 3.3

Reply: We agree with your evaluation of the uncertainty section being very technical and insufficiently explained. Emphasizing the 2-years lower boundary is important and we will do it.

If we didn’t fine-tune the timescale at all, then a convolution of the ice-cores’ StratiCounter probability distributions would have been the natural choice of timescale uncertainty. However, systematic offsets of the SC-count were observed and quantified in Table 2: under-count in the “CFA-cores” and overcount in the “isotope cores”. That is why we do the fine-tuning in the first place. The SC bias is quantified by how much the probability convolution is off with respect to the fine-tuned result: we quantify this as a systematic error of σ_SC = 0.95 years per century, on average. We could have done a separate treatment of the ice-cores, but the average result would still be close or equal to 0.95.

>>L493: This number is the key finding of the section if I understand it correctly and should be emphasized.

Reply: Yes, we stand by this value. We will give more weight to it in the revised text.

>>L495-508, Fig. 4 and Table 5: I think this section could be shortened and needs some clarification. It is currently a little confusing. I don’t think the fine tuning of the obtained prob-SC and the bias correction are necessary, or maybe the conclusion of that correction should be clarified. Also I think Fig. 4 and Table 5 could be moved to the supplement if not discarded. The consequences of the outcome of 0.24 years of uncertainty after bias correction of the SC counting is left uncommented to some extent. It is also somehow unrelated to the final statement in L505ff where the uncertainty of the fine tuning process is very vaguely defined as “smaller than the bias correction itself” and an overall uncertainty of 1 year every 100 layers is deduced. Please clarify and shorten that paragraph with respect to these issues.

Reply: We will try to explain this better in the revised MS. In general, we believe there are two main sources to our local error: a StratiCounter error and a random error given by the statistical fluctuations of the ice core data. These should be uncorrelated sources of error and we assume they can be added in quadrature (σ²_tot=σ²_SC+σ²_rand). We empirically deduce σ_SC  from the SC bias evaluation and we find 0.95 years/century. Having performed the fine-tuning, the ice cores’ distributions of the likely layer count become centered around the fine-tuned value; we then presume that they can be multiplied together to derive σ_rand , which we find to be 0.24 years/century. The fine-tuning we perform resolves issues that could not be solved in a single-core analysis.  Therefore, we ask ourselves how much uncertainty there is in our own fine-tuning, which, by account of the high volume of ice-core data compared and the numerous iterations of fine-tuning applied, we speculate is negligible compared to the σ_SC. What we mean by this is that if we repeated the whole process of fine-tuning again in a year from now, having forgotten our interpretation of the data, we would very likely reach the same timescale because we would again aim to make the number of annual layers agree in the different data sets. So, we argue that there is low uncertainty in the fine-tuning (which in itself tells us little about the accuracy of the timescale, as there is still a risk that GICC21 is wrong). In conclusion, σ_tot = 0.96 years per century, which we round to 1 year.

In conclusion, the entire section L467-508 (i.e. sec 3.5) will be rephrased to make our reasoning clearer to the reader. Following comments by An.Ref. #2, we will also perform a more continuous evaluation of the uncertainty (instead of using only 4 test sections). As a consequence, the exact values presented until now might change slightly.

>>Fig.5: What caused the outlier at about 3500 years?

We will consider this datapoint during the re-writing of this section. However, we are not sure we can classify this datapoint as an outlier, since it lies less than 3σ away from the average.

>>L535 and equation 1: Please explain a little more how equation 1 was constructed. I assume the 2 denotes the error for ages younger than Samalas? Does absolute uncertainty mean the result of eq. 1 is the +/- range of the referred age?

Reply: We will expand on this in the revised MS. When considering the time-dependent uncertainty of the timescale, we linearly add the time-dependent part to the (conservative) 2-year contribution given by the delay of volcanic signals and possible mistakes in tie-point placing. The time-dependent contribution is derived from the 1-year per century estimation we have stated previously. Since we’ve argued that the century-errors are uncorrelated, we apply a quadrature sum: . For convenience, we then hypothesize that the formula can be made continuous, i.e. .

>>L556f and Fig.: 6: What causes the common uncertainty excursion in DYE-3 and EastGRIP around 600 b2k? Is that just coincidence? Could be addressed a little more in the main text and not just in the caption of Fig. 7

Reply: The main excursion is at 400 b2k, and we assume the comment is about this excursion. We think it’s a coincidence: EastGRIP has a tie-point on a broad-shaped eruption that was mismatched previously (Mojtabavi et al., 2020), and since layers of EastGRIP are relatively thin, already 4 layers had been misplaced. For DYE-3 we observe a layer thickness fluctuation that was already seen in Vinther06. The revision of the tie-points in this region caused the excursion of the timescale offset at 400 b2k.

>>L607-641 and Fig. 9a: It would be interesting in this discussion to see a direct comparison between GICC21 and IntCal20. The advantages of GICC21 over GICC05 have been discussed in the section before. Why not show something like GICC21-IntCal20 in Fig. 9a? that would help to understand the interpretation of Fig. 9b

Reply: Repeating the study by Adolphi et al. (2016)  was considered beyond the scope of this work . To do that we would have to repeat the wiggle-matching of GRIP ¹⁰Be data, dated on GICC21, to the IntCal20 ¹⁴C production rate. In alternative, we produced Fig 9b, where we show that the datasets for the wiggle-matching are promising in reducing the offset. The results are very likely to be similar to what we already know, i.e. that the wiggle-matched and our independent timescale compare well within uncertainties. The offsets visible in fig. 9b would probably be less than what Adolphi predicted, thanks to a better alignment between the purple and the green curve.

>>L641: Why do you still refer the uncertainty to IntCal 13 here and not IntCal 20? Please clarify.

Reply: Because it’s essentially the same, but we will rectify this in the revised MS. The datasets underlying IntCal20 in this timeframe have not produced substantial changes in the Late Holocene (Muscheler et al., 2020).

>>L651: That is quite a bold statement. I would recommend to weaken it. As was stated, there are some differences in the production signals and the wiggle matching is not always entirely convincing (e.g. at about 3650 b2k).  I think the underlying processes and uncertainties of IntCal 20 as well as the 10Be record are rather complicated, so I would be careful with the conclusion you draw.

Reply: We agree and we will weaken our statement.

(Comments about sections 5.2 and 5.3 have not been answered but will be considered for future work)

>>L843-844: 3300b2k is younger than Thera. I would be careful to still call the offset negligible in that time frame. Please comment.

Reply:  This sentence will be removed.

Kind regards,

Sinnl et al.

Dear Anonymous Referee #2,

Thank you for your positive and comprehensive review of our manuscript. We hope to have improved our manuscript by addressing your concerns.

As we replied to An.Ref.#1, the construction of the GICC21 timescale has been a long work and we are glad to see our efforts recognized. The new uncertainty formula is the result of long discussions in our group and we are satisfied to see that all Referees agreed on its validity.

We agree that the manuscript ended up being quite long, however GICC21 needs thorough explaining and documentation. In response to your comment and to those by An.Ref.#1 and An.Ref.#3 we have concluded that the manuscript will gain more readability by shortening some parts, in order to refocus the manuscript around the GICC21 timescale, i.e. we intend to cut out the discussion of volcanic cooling and Mediterranean volcanic events (sections 5.2 and 5.3, Appendix A and B, and the relevant parts of abstract and conclusion). However, we feel encouraged to continue our future work on these topics thanks to your positive evaluation of the sections we now intend to remove.

Regarding your primary questions/concerns:

1. ‘The first is about the improved match to IntCal.’ It was not our intention to send the message that GICC21 was born as an independent check of IntCal, or to be forced in alignment with IntCal, and we apologize if our phrasing implied otherwise. This section is meant to be a discussion of what GICC21 can be used for in a broader sense, hence we think it is important to keep it and to rephrase it better. In the revised manuscript we have made sure to clarify that GICC21 is an independent result that applies for Greenland only. The comparison to IntCal is intended as to solidify the link between the Greenlandic climate and the broader Northern Hemisphere region.
2. ‘The second question/concern is the underlying data.’ We understand your concern and we will make sure that in the revised manuscript we explain more clearly where data can be found. We emphasize that all data will be available to the reader on Pangaea, but as Pangaea currently has a rather long processing time, not all data sets may get a doi in time for inclusion in this manuscript. For NEEM and NorthGRIP CFA data, the Pangaea repository listed (https://doi.pangaea.de/10.1594/PANGAEA.935838) already contains data of all the species we have used in our work (ECM, Na, Ca, NH⁺₄, NO3), and the moratorium will be lifted as soon as the corresponding ESSD paper is published.

As for EastGRIP data, the ECM has since been made available at https://www.iceandclimate.nbi.ku.dk/data/. For the EastGRIP CFA, Erhardt will make a Pangaea repository, but preliminary data can be obtained by Erhardt.

The two other ice-cores (DYE-3 and GRIP), for which we only used ECM and isotopes, already have published ECM and decadal-resolution isotopes, and we will make sure to refer to those publications in the revised manuscript. As for the full-resolution isotopes from GRIP and DYE-3, they will be published separately by Rasmussen in a paper that makes data from GICC05 and GICC21 available. The files have already been submitted to Pangaea.

Authors who want to access data sets which are not published when this manuscript is published, will be able to get the datafiles from the author team (with preliminary metadata, as the metadata is subject to review by Pangea editors).

1. ‘The third question/concern is about the ammonium (or -ECM log transform) matches.’

In the revised manuscript we have added more explanation about the NH⁺₄ matches and the log-inverted ECM (pseudo-NH⁺₄). We believe that matching of NH⁺₄ is useful for our construction method. We do not use NH⁺₄-matching (including matching NH⁺₄ peaks to ECM dips) for matching up the cores in the first place, but we use it as a way of matching ice-core data across sections that don’t have any clear volcanic signals. NH⁺₄ has showed high potential for ice-core matching from the beginning of our work, as opposed to Na and Ca, who show very low agreement across ice cores since they are more strongly influenced by local climate. That said, by no means we believe NH⁺₄ to be better than volcanic matching and the use of ammonium spikes as a matching tool is secondary and dependent on the coherence of the volcanic match. Moreover, we have made use of patterns of ammonium matches, rather than of single peaks, in order to tie the ice cores together. This is because we believe that by matching NH4 patterns we can obtain better results than just by interpolation between eruptions. As for the pseudo-NH⁺₄, we will reconsider the nomenclature in order not to imply a closer correlation with NH⁺₄ than there actually is. The agreement with ammonium can be seen visually and can possibly be quantified by a statistical test of correlation, which we aim at providing in the Supplementary Information of the revised manuscript.

Our reply to your specific comments:

>>L69 – add sulfate, black carbon, as well as electrical conductivity measurements, and insoluble particles (i.e. dust). This seems too focused on water isotopes.

Reply: Thank you for the remark, we will do that.

>>L69 – change “heavy” to something that does not imply weight, like “significant”

Reply: We will.

>>Section 1.3 – The first paragraph focuses on sulfate for volcanic event identification, then the second paragraph switches to acidity. I think you are likely referring to ECM rather than a direct measure of acidity. Please add more discussion around how the sulfate (and sulfur), ECM, and acidity measurements are interrelated and used together.

Reply: We will make sure to add more information on the matter.

>>L98 – I understand what you are going for with a volcanic event being recorded differently as various sites, but GRIP and DYE-3 are both about the same distance from an Alaska volcano. Seems like Iceland might be a better example.

Reply: Clausen et al. (1997) explicitly talk about the Katmai eruption (Alaska) of 1912 CE being recorded in GRIP but not in DYE-3, which we also observed. That is why we cited Alaska as an example. We will rephrase the sentence to remove the argument of distance from the source, limiting ourselves to the meteorological factors that can alter the shape of the eruption signals across the ice sheet.

>>L105 – There is no reference for the matching of ammonium records among cores. As indicated above, this portion of the timescale development needs to be better supported. I’m not aware of this being widely used before. If it has been, please add a paragraph with references here. If it has not, please dedicate a section of the paper to the ammonium matches.

Reply: Ammonium events are excellently described by Legrand et al. (2016) as peaks that significantly exceed the background levels, and the importance of NH⁺₄ for reconstructing past forest fire activity is well documented in their review. We were not the first to use NH⁺₄ for ice core matching, since Legrand et al.  also provide a match of NH⁺₄ across NEEM and Summit for the recent 2 centuries, admitting that “peak to peak comparison is rarely perfect”. However, they note that similar events happen within 1-2 years, which is explained as being caused by timescale inaccuracies. Furthermore, they provide clear indication that the ice-core peaks occurred within dating uncertainties of record-years of Canadian forest burning.

In conclusion, we agree with your criticism of our explanation being insufficient and we will add more details about why NH⁺₄-matching is a reliable addition to the volcanic matching and that is has to be considered a secondary tool.

>>L107 – add a reference to Taylor et al., 1996: Biomass burning recorded in the GISP2 ice core: a record from eastern Canada?, The Holocene unless there is an even earlier reference for the ECM/ammonium/biomass burning relationship

Reply: Thank you, we will add this reference.

>>L130 – This paragraph provides a nice description.

Reply: Thank you.

>>L157 – “bias in the counting process” should be changed to “bias in the interpretation process”. The counting has never been the issue – the identification of annual layers is the issue. If you can identify each year, the counting is easy

Reply: We agree on this distinction. We will make the change.

>>L182 – “includes all available data from Greenland deep ice cores” is not true – GISP2 is excluded. An explanation for why GISP2 is excluded, despite having publicly available data that allows annual layer identification, should be added. There should also be a transfer function for GISP2 to GICC21.

Reply: We will weaken our statement, also in view of the comments by Michael Sigl. The GISP2 ice core was not included in the timescale, that is true, and we will rectify this. However, including GISP2 in the timescale at this stage is not feasible because of the lengthy fine-tuning process. When planning the timescale construction, we noted that GISP2 ECM data is degrading in quality (many data gaps after 300 m) and that many ECM peaks cannot be matched easily, hence we desisted from including it in our dataset. Since GISP2 is anyway very close to GRIP it would not have represented an important addition to the geographical coverage of GICC21. We will aim at matching GISP2 in the revised manuscript because we agree that this will complete our discussion of Greenlandic ice cores in the frame of a new chronology.

>>L335 – “major and minor” – I don’t see any distinction in the ammonium matches in the remainder of the manuscript. I also think this is a suspect approach. Since there are lots of wiggles in the ammonium and ECM records, it seems like a lot of “matches” could just be of noise. Figures 2 and 3 do not convince me that the ammonium matches are anything more than non-unique wiggles that happen to fall at about the right age. As mentioned above, more analysis needs to be provided to support these matches.

Reply: the distinction between major and minor tie-points was removed from the manuscript in an earlier version. We will edit it out completely and apologize for the confusion. We will aim at making Fig 2 and Fig 3 more credible and expand on NH⁺₄.

>>L480 – I don’t understand why the uncertainty estimates are based on just 4 sections. The straticounter probabilities are available for all sections, as is the manually fine-tuned timescale. This process should be done for all of the timescale.

Reply: It was certainly possible to retrieve a continuous record of the uncertainty data, but we started by only analyzing the 4 test sections since we believed that 4 sections of 300 years (i.e. 1200 years in total) would be representative of the ~3000 years of the timescale before Samalas. Since your review, we have conducted a continuous analysis of the uncertainty and we will add it to the revised MS. As expected, it did not drastically alter the values we have proposed.

>>L485 – convolved not convoluted

Reply: Thank you, we will make this change.

>>L543 – Figure 6 – This figure is the most important in the paper. But it is hard to interpret, particularly the uncertainties. If the primary point of the paper is that a multi- core analysis yields a better timescale, then why plot each of the individual timescales, some of which don’t really exist – i.e. is there really an independent EastGRIP timescale? I think you should make this into a two panel plot. The top panel should have a comparison of only published timescales – GICC05, NS1-2011 (this is the Sigl15 timescale, right?) GICC21, and GISP2 (an oldie but a goodie). This should allow the uncertainties to be better visible, but importantly, the GICC05 uncertainty should be plotted clearly. It is an important point that the GICC21 timescale is a revision that is outside the GICC05 timescale.

Then in a lower plot, you can make a plot that uses the individual timescales as you’ve done.

Reply: We will certainly evaluate if Fig.6 can be improved by dividing it into two panels. We kept the cores separate because they each react differently to the timescale revision, since both tie-points and layers were changed from GICC05. We will highlight the MCE boundaries and the GISP2 timescale will be included.

>>L615 – Of course GICC21 is in better alignment with IntCal; the disagreement, as pointed about by previous work, is why you went to all the effort to update GICC05 in the first place! I think you can rephrase this section to be clear you have developed an annual timescale that reconciles the previous offset within uncertainty. This is an important thing to do!

Reply: Thank you, we will rephrase this section.

>>L642 – I don’t understand this paragraph. I think the main point of Figure 9b is that the purple (Be10 on GICC21) and the black (Be10 on Adolphi-Mueschler16) are very similar. But the main visual is the difference between the green (IntCal production) and the purple/black. The wording in the paragraph is hard to follow. Is the green important? Or could you just plot the purple and black?

Reply: We wanted to show that the wiggle-matching of GICC21 to IntCal (i.e. purple to green) is slightly better than the black-green one, especially in the last 300 years of our timescale. We will rephrase the section to explain this better.

>>L655 – How is the averaging done? Do you perform an initial average for the geographically similar cores, and average those together? I don’t see this in Appendix B either.

Reply: This part will be removed. The data was averaged without considering geographical location.

Kind regards,

Sinnl et al.

Dear Anonymous Referee #3,

Thank you for your positive and detailed review of our manuscript. We hope to have addressed all your comments in the revised manuscript.

We agree that the manuscript ended up being quite long, however GICC21 needs thorough explaining and documentation. In response to your comment and to those by An.Ref.#1 and An.Ref.#2 we have concluded that the manuscript will gain more readability by shortening some parts, in order to refocus the manuscript around the GICC21 timescale, i.e. we intend to cut out the discussion of volcanic cooling and Mediterranean volcanic events (sections 5.2 and 5.3, Appendix A and B, and the relevant parts of abstract and conclusion). We will continue the investigation on these topics in our future work.

In response to your comments:

>>Section 1.3 is important - however please describe more clearly that there is a hierarchy of uniqueness of these tie-points. If a tephra layer in the ice can be geochemically connected to a unique historical eruption (and/or to other dated sites), and if this layer is found and securely uniquely identified in the multiple ice cores, then this tephra layer forms a much more unique signal (and thus securely synchronous tie point) than if the volcanic eruptions are recorded as peaks in SO4, ECM or DEP; the latter peaks are replicated over the core and thus do NOT have unique IDs attached to them. When looked at in isolation, the latter peaks cannot be used as dates, but geochemically ID-ed tephra layers can. Are the shapes of peaks used in tying peaks, and if so, how?

Reply: Thank you for the information, we will clarify this section. The shape of the peaks was certainly used as a matching criterion, we will add that to our description.

>>In the same section, add some more references to tephra layers in ice cores, e.g. Abbott et al. 2021 CP doi:10.5194/cp-17-565-2021. It couldn't hurt to mention in this section already that tephra layer studies have thrown up surprises, in that some acid peaks were previously attributed to the wrong volcanic eruptions (e.g., Plunkett et al. 2021). For a wider context, please also cite Baillie 2008 (doi:10.1029/2008GL034755).

Reply: Thank you for the information. We already acknowledge the erroneous assignment of peaks in GICC05 later in the MS, however we will add more details here and include more references such as the ones suggested.

>>Lines 29/36, "synchronization/synchronized", please reword to "comparison/compared", as the current wording strongly implies adapting site chronologies by aligning/tuning their supposedly simultaneous peaks. Such tuning removes any independence between chronologies, and thus precludes the subsequent investigation of lead-lag dynamics (perhaps for wider context cite Blaauw 2012 doi:10.1016/j.quascirev.2010.11.012).

Such potential problems about the reliability of different peak identifications and alignments are make it more difficult to properly evaluate the important Figure 2.

Reply: We agree with your rephrasing. Thank you for the suggested citation. Our intention throughout the work has always been to obtain an independent Greenland timescale that can later be used to compare Greenlandic climate to other regions.

>> In Table 2, please outline which tephras have been positively and securely geochemically IDd in which core, and which matches instead rely on e.g. matching ECM peaks. For example, were Laki or Oraefajokull tephras geochemically identified in all of the used ice cores?

Reply: We will review the tephra availability for the cited eruptions. For this work we did not present new tephra data, so our identification of ECM peaks relies on published work. To our knowledge, tephra from Öræfajökull for example was reported only in GRIP (Coulter et al., 2012), not in NorthGRIP or other ice cores.

>>Has the manual fine-tuning, including each decision been documented, quantified and motivated? Will the data be made available, including all high-res raw data and decisions? Could it be an idea to put the data on github, with version control and with information on decisions taken?

Reply: Yes, we did keep track of the fine-tuning details, such as the motivation for each intervention on the layer count. We will evaluate if a GitHub repository would be the best option for storing the manual fine-tuning iterations. As we have replied to An.Ref#2, the ice-core data is for the most part already available, but some records (e.g. EastGRIP CFA) have pending publication processes, for which we refer readers to directly contact the reference authors listed in the Data section. We are evaluating if the normalized and preprocessed data we have used as an input to StratiCounter could be published with a separate DOI in the meantime.

In reply to your more technical comments, we have accepted all of them in our revised manuscript. Comments are only relevant for one of these:

>>Beside Fig. 6, could you also show a Figure of the accumulation rates for each core based on the adapted time-scales? Are all resulting accumulation rates and their variability considered to be realistic?

Reply: We have a figure in the supplement about layer thicknesses that provides a first indication of the accumulation rate history of the ice cores. Accumulation rates require thinning models, which are not in the scope of this work. We will refer readers to the most recent publications about thinning models, in the cores where they are available.

Kind regards,

Sinnl et al.

>>L.  674: Have you accounted in your stacked analyses for the fact that volcanic eruptions often formed temporal clusters (e.g. 536/540, 1453/1459, 1809/1815), and that an apparent persistency (i.e. your secondary dip) may be an artifact caused by secondary major eruptions? In tree-ring research on the volcanic climate response, this has been taken into account (Büntgen et al., 2020). In Plunkett et al., (in press) we demonstrate that a decadal scale reduction of d18O from Dye-3, GRIP and NGRIP is explained by the cooling response following a cluster of eruptions dated 536, 540 and 547 CE.

Reply:  We have checked that close eruptions did not affect our results significantly, by separating clustered eruptions and only averaging the signal of the latest one in the cluster. In the end we chose to compute all eruptions in the stack, but this topic will be of course be addressed in our future work on this matter.

>>L.  717: I am not sure one can cite a personal communication from a co-author. The most comprehensive and recent review article of tephra deposits in the Holocene in Greenland is (Plunkett and Pilcher, 2018) which also addresses the situation for tephra from the Mediterranean.

Reply:  We will refrain from citing pers. comm. from co-authors in the future.

>>L.  724: In 2015, no annually dated Northern Hemispheric temperature reconstruction was available, thus we created the N-Tree composite. In the meantime, more comprehensive reconstructions have become available and have already been used to demonstrate the accuracy of the NS1-2011 chronology in the Common Era (Büntgen et al., 2020). Since NS1-2011 and GICC21 are close to each other, I would assume that these results would look the same for GICC21.

>>L.  726: eruptions identified in Greenland ice cores.

Reply: We will edit this.

>>L. 731: Have you analyzed relevant sections from any of the ice cores encompassing 79 CE for the presence of tephra?

Reply:  No, we did not.

>>L.  737: I miss the relevance of the radiocarbon dating of material in context with the exactly dated 79 CE Vesuvius eruption for the problem of identifying volcanic fallout from this eruption in Greenland. I suggest that the point would be better removed, to bring the focus more back to the ice core chronology.

Reply:  We agree that this paragraph can be removed. The intention was to compare Greenland with evidence close to the eruption site, as a proof of concept for the following Thera paragraph.

>>L.  762: Please provide relevant references here.

>>L.  767: Here and again later (e.g. l. 854) you refer to the 3560 year b2k ice-core anomaly. In the supplementary data the rise of the ECM signal is in 3562 year b2k, and the peak in 3561 year b2k. Can  you please clarify the age of the eruption signal in GICC21?

Reply:  The exact age we found for the rise is 3561.4 years b2k (-1562.4 BCE). We think that for most cores the signal is contained within one year, but it is appears that EastGRIP has a smeared peak that lasts into the next year.

>>L.  773: I think it would be appropriate to cite in this section the pioneering work of (Baillie, 2010) who noted a number of the mismatches between tree-ring anomalies and ice-core acidity peaks which now appear to have been corrected.

Reply:  We will keep this in mind.

>>L.  785: If the assumed 10-year cooling would have been caused by a volcanic eruption it would have been through the stratospheric sulfate aerosols. The sharp and short-lived (1 year only) ECM spike characterized by strong spatial variations in Greenland is more likely the result of a tropospheric eruption rather than a major stratospheric eruption. There is no credible evidence in the ECM data and available sulfate records from Greenland (GISP2, (Zielinski et al., 1996)) around 1610 BCE of any significant stratospheric sulfur injection. Linking the drop in d18O to a volcanic eruption is thus only speculation. Internal variability, changes in atmospheric circulation and moisture sources and/or solar activity are just as likely candidates to explain the d18O feature.

Reply:  Thank you for checking this, we will investigate further on the isotope drop in the future.

>>L.  788: It would be helpful for future research if you provided (if possible) metadata on which ice cores and which cross sections (cm²) have been sampled continuously in this time window. To my understanding, NGRIP ice core has been fairly extensively studied by Coulter et al., (2012); GRIP was studied by Hammer et al (1987). NEEM, EastGRIP may have been studied by Eliza Cook, cited before as

personal communications and this paper would greatly benefit from more specific detail about the nature of any tephra work contributing to GICC21. The new timescale revision will certainly stimulate new studies around this time period, and it would be helpful to know the state-of-the-art regarding tephra investigations in this critical period.

Reply:  We will consider making such a file for future studies on the matter.

>>L.  818: This additional ±2 year uncertainty could be reduced for a number of events for which (1) atmospheric transport times are short (<weeks), (2) the timing of tephra arrival is well documented by high-time resolution particle records and (3) independent age constraints (e.g. dendrochronological, documentary or tree-ring evidence) can provide sub-seasonal information. I would argue that Veiðivötn (Feb. 1477 (Abbott et al., 2021)), Tianchi (November 946 (Oppenheimer et al., 2017) whose tephra in Greenland is reported by Sun et al. 2014) and the solar proton events in 993 CE, 774 CE and 660 BCE can all be dated with uncertainties less than ±2 years, and the list could be expanded including Eldgjá 939, unidentified volcanic eruptions in 626, 536 CE (Sigl et al., 2015) and the eruption of Okmok II in winter 44/43 BCE (McConnell et al., 2020). I have, however, no objections if you are leaning towards a more conservative approach regarding the uncertainties for these events.

Reply:  Thank you for this comment, we will consider this in our revision.

>>L.  823: A multi-ice core comparison is favorable rather than essential (see Figure 3). I would further argue that the single ice-core WD2014 chronology is more accurate than the multi ice-core GICC05 chronology over the Holocene (Sigl et al., 2016). It is the quality of the expression of the intra-annual variations that matters most, not the number of ice cores.

Reply:  Yes, the quality of the annual signal is a key ingredient for the ice core timescale. We will rephrase ourselves to include your point. We produced our timescale despite ice core data quality of single ice-cores preventing us to make a single-ice core timescale such as WDC2014. Hence, the number of ice cores is an important feature for the Greenlandic timescale, especially because we continued beyond the 2500 b2k age limit of the NS1-2011 chronology and therefore had much less historical evidence to rely on. For the future of the GICC21 revision, we need to use as much as possible of the existing data.

>>L.  853: Typo: Droughts.

Reply:  Yes.

>>L.  911: All datasets used for the timescale should be made publicly available at high depth resolution to allow for independent validation of the timescale. This was not possible for GICC05, but is now mandatory under FAIR open data principles.

Reply:  We understand your concern and we will make sure that in the revised manuscript we explain more clearly where data can be found. We emphasize that all data will be available to the reader on Pangaea, but as Pangaea currently has a rather long processing time, not all data sets may get a doi in time for inclusion in this manuscript. For NEEM and NorthGRIP CFA data, the Pangaea repository listed (https://doi.pangaea.de/10.1594/PANGAEA.935838) already contains data of all the species we have used in our work (ECM, Na, Ca, NH+4, NO3), and the moratorium will be lifted as soon as the corresponding ESSD paper is published.

As for EastGRIP data, the ECM has since been made available at https://www.iceandclimate.nbi.ku.dk/data/. For the EastGRIP CFA, Erhardt will make a Pangaea repository, but preliminary data can be obtained by Erhardt. 

The two other ice-cores (DYE-3 and GRIP), for which we only used ECM and isotopes, already have published ECM and decadal-resolution isotopes, and we will make sure to refer to those publications in the revised manuscript. As for the full-resolution isotopes and ECM data from GRIP and DYE-3, they will be published separately by Rasmussen in a paper that makes data from GICC05 and GICC21 available. The files have been submitted to Pangaea.

Authors who want to access data sets which are not published when this manuscript is published, will be able to get the datafiles from the author team (with preliminary metadata, as the metadata is subject to review by Pangaea editors).

Kind regards,

Sinnl et al.