We would like to thank Kristi Wallace for her thorough and constructive review of our manuscript, with comments focusing on the tephra data and the implications of this northern Alaska cryptotephra record. We agree with and will incorporate the suggested editorial comments, and have highlighted responses to the main suggestions/criticisms here:

1. "There needs to be more discussion about homogenous vs heterogenous glasses so make this point as not many Alaska eruptions are homogenous … The manuscript would benefit from more discussion regarding the limited suite of reference materials available and evaluation of the quality of the correlations made."

Discussion of available reference data will be added, specifically mentioning the limited available data for Holocene deposits in Alaska and that heterogenous deposits seem quite common in this region.

We would like to note that the confident correlations that underpin the chronology (that the reviewer does note they largely agree with) are to tephra that do have detailed geochemical datasets (either in published work or new data that are included here) and have been widely identified in other depositional records both intra- and extra-regionally. We will clarify this where necessary to emphasize these points.

2. “How do the glass shard morphologies compare, glass characteristics (microlitic, clean etc.)? … If there were more discussion regarding the “robustness” of each correlation it would allow the reader to evaluate if the age model is really improved by the addition of the tephra correlations. I suggest the authors review the community established best practice recommendations for correlation.”

The basis for tephra correlation will be discussed explicitly, with added comments that evaluate the relative strength of our correlations. To achieve this, we will add:

• Shard morphology, characteristics and size data, to show that our tephra correlations are based on multiple parameters
• Proper references to the community established best practice recommendations for correlation
• A clarification on how independent published ages and tephrostratigraphies support the correlations made her

3. “I am skeptical about the Ruppert correlations as it implies an unknown source that erupted in the late Holocene but that has only been identified as cryptotephra in very distal areas and a very large footprint yet no visible layers or source has been identified. This would make sense where ice has removed older records, but this is a young deposit where preservation should not be a problem and we should see this layer more often in Holocene sediment sequences.”

We agree that the lack of identified visible deposits for Ruppert tephra is strange, but the geochemical correlation with deposits in eastern Canada (where it was first documented in Pyne-O’Donnell et al. 2012) and USA hold true as evidenced in Jensen et al. (in review). In addition, its presence in Alaska is firmly established regardless of the furthest afield correlations. This tephra is named after Ruppert Lake in the southern Brooks Range where it was reported by Monteath et al. (2017), only ~120 km south of Cascade Lake. The age of the tephra Rupert Lake is estimated at 2700-2300 cal yr BP, which is compatible with the age modelled in Jensen et al. (in review) that is used here.

As well as the additional clarification of correlations discussed in the previous point, we will also add a more thorough discussion of the independent ages of this tephra and explicitly mention the lack of visible deposits as an unusual situation.

We would like to thank the anonymous Reviewer 2 for their comprehensive review of our manuscript and their positive comments. We agree with and will incorporate their editorial suggestions, and have highlighted responses to the three main points they raised:

1. “The methods conducted by the authors is definitive though additional detail could be added to some sections to allow clear reproduction of certain processes and provide additional information as to why a certain process were chosen. … The discussion section brings together all the important points in a concise format. Additional information and references would provide further context for the reader to understand the points made”

More details and references will be added to the methods and discussion sections as suggested in RC2’s editorial comments.

2. “For each cryptotephra layer discussed in the remainder of this section - It would be advantageous to include additional information on glass shard morphology and size for every tephra cryptotephra layer discussed in this section ... This would further emphasise the point that these peaks are homogenous and are primary (if being similar in size and morphology) as well as help with the correlation to which eruption.”

“Closely space eruptions are not the only reason for this type of shard concentration profile. The possibility of taphonomic processes e.g. reworking, bioturbation are also a possibility. This needs to be emphasised here in a short sentence before the detailed discussion in the next session.”

Shard morphology and size data will be added, as also mentioned by RC1. Reference will be made to other potential causes of heterogenous shard deposits, although it should be noted that there are no additional data that can be used here to discuss taphonomic processes such as bioturbation or secondary inwash at Cascade Lake specifically.

3. "It is also typical within tephra papers that geochemical data is presented as normalised within the main paper and non-normalised within supplementary material.”

Raw geochemical data will be added to the SI tables, as commented on by RC1& RC2.

We would like to thank the anonymous Reviewer 3 for their positive comments on our manuscript and their thoughtful attention to our dual submission with Steen et al. We will incorporate their editorial suggestions and have detailed our response to their suggestions about the dual submission below.

General Summary:

Overall, the three reviewers were positive about our manuscript, expressing that this is an important study with a key dataset and age model that will be helpful for other researchers going forward. We thank the three reviewers for their time spent in writing these reviews; we agree with many of the suggested revisions and will be incorporating them.

The major point to address is the recommendation of RC3 (as well as RC1/RC2 of Steen et al.) that this paper either: (1) incorporate Steen et al.’s (the companion paper) PSV data into this manuscript, or (2) remove all references to Steen et al. and focus on revision of the radiocarbon chronology.

Suggestion (1) response: Prior to submitting the papers, and now following this review, the authors of both papers have considered how the papers could be combined into one. We have made a concerted effort but cannot find a path that enables us to present all the relevant data. We also note that while RC3 suggests this, they note that this is not the best alternative.

We have submitted these two manuscripts to Geochronology because we aim to convey the important details about the individual dating methods to this specialized audience. We believe that attempting to do this with a single paper would be unwieldy for readers: both manuscripts date sediments from this high-latitude lake using two completely unrelated methods and reporting this in a meaningful fashion requires two separate papers that can focus on properly presenting the methods and results for these independent approaches.

Suggestion (2) response: one of the motivations behind this study was to test the results from Steen et al. Removing all references to Steen et al. would also remove one of the major contributions of this paper: proving that Steen et al.’s preferred PSV model does indeed date (at least) the mid to late Holocene, which will assist in the development of a reference PSV curve for this region.

The suggestion to focus on the radiometric dates and cryptotephra tie-points is echoed by RC1 and RC2 of Steen et al.: both reviewers suggest creating an age model using the 14C and cryptotephra tie-points that can then be tested by the PSV (excerpts of these comments are included at the end here for reference). While this approach would remove the duplication of the radiometric data, the resulting age model would only cover 15 ka and three 14C dates identified as outliers would then not be excluded. A second model iteration with the PSV dates added would need to be created by Steen et al., and this would not be any different from the final Bayesian model we have presented here. It’s unclear how publishing two models for the site in this fashion would be beneficial.

Hence, we still believe the dual submission format is the best approach here and do not believe either of these suggested approaches will be viable for these manuscripts.

Excerpt from RC3 (anon.): (numbers refer to additional author responses below)

“The ongoing gchron discussion about the Steen et al. paper includes some negative criticism of the paleomagnetic data (quantity and quality). There are large uncertainties in the palaeomagnetically derived ages that rely on subjective tie points [1] between inclination trends in the composite record from Cascade Lake, a nearby radiocarbon-dated record from Burial Lake and the predictions (note, not “calculations”) of time-varying geomagnetic field models. The two submissions cross-reference each other, and it is difficult to understand how by much the conclusions of Davies et al. are dependent on the conclusions of Steen et al. (and vice versa) [2]. There is duplication of data sets in the two submissions (e.g. radiocarbon ages) that should be addressed [3]. Davies et al. state that there is more detail about the construction of the age-depth model in Steen et al., but in fact the latter contains less detail than one would expect [4]. 

My advice would be for Davies et al. to remove the references to Steen et al. and use the known ages of the detected cryptotephra to point out the offset in ages. Subsequently, ages obtained through PSV correlation could be used to test the radiocarbon- tephrochronology based age-depth model in a separate, future improved manuscript by Steen et al. An alternative would be to combine the data into one paper, but that will not remove the underlying uncertainty in the PSV data from Cascade Lake and the correlations to other PSV data & model predictions [2].”

The following are excerpts from RC1 and RC2 of Steen et al. that overlap with RC3 statements and are referred to in our response:

Excerpt from RC1 of Steen et al.:

“(9) P. 16, “5.3 Age model discrepancy” The authors mention that four tephra ages published in the companion paper by Davies et al. provide strong evidence for the discrepancy. If they clearly show the discrepancy without help of PSV data, the authors can construct a new age model with selected measured 14C dates and the tephra ages [2]”

Excerpts from RC2 of Steen et al.:

“A general problem is that the two manuscripts frequently cross- reference each other and it is quite difficult to understand how either one can stand alone. Some raw data are put into both manuscripts (e.g. the 14C dates and tables) which gives future writers an unnecessary choice about which source to cite [3]. Davies et al. state that details of the radiometric age-model construction are in Steen et al., but there are more details in the former than the latter [4]. The editor could consider asking the authors to combine the papers into one so that problems that arise from twin-submissions are negated.”

“Section 4.7 To avoid duplicating raw data I suggest either combining to the two manuscripts or allowing Davies et al. to present the age model in detail, which Steen et al. test using the paleomagnetic data in a subsequent manuscript. The data in Table 1 show the 210Pb activity for the upper 3.6 cm, which is not useful for the PSV data set (no PSV data are from the short core) [2].”

“Davies et al. refer to a submitted manuscript by Jensen et al., which might be crucial because it concerns the age of a cryptotephra [5].”

Response from authors to RC3 (anon.) and overlapping comments from RC1 and RC2 of Steen et al.:

1. We believe the criticisms of the PSV data can be addressed, especially, for example, by improving the strength of the PSV tie-point correlations, and they do not therefore need to be removed from this manuscript.

This argument is more fully addressed in the response to reviewers by Steen et al., but, for example, we propose to highlight three subsections of the final age model in Davies et al. with different levels of certainty (depending on the number of chronological methods used to construct each section). This will explicitly show where data are in good agreement vs. where they are more limited and producing a robust chronology is more challenging. While we accept that there are limitations to some of the PSV interpretations (especially in the lower part of the record where they are not validated by other methods) we also see value in the data.

2. This point may partially be addressed by clarifying in the manuscript one of the reasons this study was undertaken in the first place. Steen et al. produced one of the first PSV lake records in this region – an important contribution because PSV is one of the few methods that has the potential to date these difficult sedimentary records – and cryptotephra could test their model.

The study of Davies et al. therefore had two goals; (1) explore if cryptotephra were present in lake sediments in the north slope in detectable and analysable quantities to provide chronologic control and (2) if these cryptotephra could be used to test Steen et al.’s conclusions. In that sense, these two papers are indelibly intertwined (hence the dual submission) but their conclusions are not dependent on one another. In this manuscript we show that cryptotephra are present and clearly helpful in dating these sediments and support Steen et al.’s assessment that several ¹⁴C dates are anomalously old.

3. The duplication of the datasets is minimal, and appears restricted to only the radiometric dates, which are important for both papers. If removed from either, each paper will not be able to stand alone. We will place the 14C data into a supplement for this paper and cite Steen et al. as the source so the repetition is less obvious. We will also review both manuscripts again to ensure there is no unnecessary duplication.

4. This comment is similar to one made by RC2 for Steen et al., and we believe this is a misreading of Lines 130-131 and 145-149 that say that there are more details about the radiometric dates and the composite inclination record and PSV age model in Steen et al and not the construction of the Bayesian age-depth model (which is detailed in section 2.4 of this manuscript).

5. The submitted Jensen et al. manuscript is an invited review and, following its scheduled timeline and assuming its eventual acceptance, it should be publicly available by the time this manuscript may be published. However, this information is not ‘crucial’ for the tephra story here. The paper’s importance lies in the reanalysis of samples discussed in previously published papers (e.g., Pyne-O’Donnell et al., 2012; Monteath et al., 2017) – demonstrating that these disparate samples do geochemically correlate. We can demonstrate this correlation to the Alaska samples using the data from Monteath et al., but correlations to sites on the East Coast rely on the data in Jensen et al. (in review).

Jensen et al.  also contains the updated age estimate for this Ruppert tephra used here in the Davies et al. age model, but if the submitted paper is not published in time, then an alternative age with a larger uncertainty can be used from Monteath et al. (2017). While this is not ideal it, again, is not ‘crucial’ to our age model as Ruppert lies in the middle of the four dated tephra tie-points, and if given a larger uncertainty or even if removed, the resulting model will not change substantially.

This is illustrated by this zoomed in snapshot of Figure 6 (below): the Ruppert cryptotephra tie-point (highlighted with a dashed box) has depths with PSV and cryptotephra tie points within ± 30cm above and below.

Modelled ages for Ruppert tephra from two cores taken at Ruppert Lake (Monteath et al., 2017) are plotted in pink for comparison. The younger age range (reported in the paper as the preferred age) overlaps with the age estimate from Jensen et al. (in review) and the 95.4% confidence range of the current age model for Cascade Lake. However, the Jensen et al. age is more robust as it is modelled using dates from five different sites.

Figure: Snapshot of Figure 6 from Davies et al. The position of Ruppert tephra is highlighted; reported age estimates for the tephra from Monteath et al. (2017) are shown in pink.