the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Oct 2024
Cryptotephra in the East Antarctic Mount Brown South ice core
Abstract. Ice cores contain stratified layers of impurities scavenged from the atmosphere, which are a vital tool for investigating the Earth system. Reconstructing past eruption records by way of ice core tephrochronology can help us understand ash dispersal, atmospheric circulation processes, and the impacts of volcanic eruptions on climate. This study presents the coastal East Antarctic Mount Brown South (MBS, 69.11° S, 86.31° E; 2084 m ASL) ice core as an untapped tephrochronological archive. We utilize a novel cryptotephra sampling plan, integrating ice core data, HYSPLIT air parcel trajectories, and known eruption records, and identify two distinct cryptotephra horizons at ∼13.3 and ∼17.9 m depth in the MBS-Alpha ice core. We also find sparse tephra grains throughout the core. Through geochemical characterization with electron probe microanalysis (EPMA), we correlate the two cryptotephra horizons with the 1991 eruption of Cerro Hudson and the continuous eruptions of Mt. Erebus throughout the mid-1980s. The volcanic horizons identified here underscore the role of MBS in extending the regional volcanic record, helping to constrain ice core dating efforts, and enhancing understanding of volcanic ash dispersal to East Antarctica.
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RC1: 'Comment on cp-2024-64', Anonymous Referee #1, 25 Nov 2024
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This manuscript presents new tephra measurements in the upper ~20 m of two Mount Brown South ice cores from coastal East Antarctica spanning the recent ~40 years. The authors first conduct a broad, lower resolution investigation of tephra particles in the main MBS ice core before doing more high-resolution geochemical analysis on samples on an adjacent firn core to fingerprint specific eruption sources.
Overall, I think the methods and results are described well, and the geochemical data, and therefore ties to known volcanoes, are robust. While the authors identified a wide range of sparse tephras in their samples, the interpretation is focused (in my view, rightfully so) on the two clusters of tephra identified in the two samples with the largest number of shards. For these two events, the authors identify appropriate candidate eruptions and then logically rule out unlikely eruptions to conclude that Cerro Hudson and Erebus as the sources for the tephra layers they identify in 1991 and 1985, respectively. One aspect that could be better presented is the analytical uncertainty of the geochemical measurements- often these are included as crosses on the major element diagrams (bottom 9 panels of Figs. 7 and 9, for example). I’d suggest adding these uncertainties if possible, though the analytical error is in a supplementary data table.
While the tephra data and attribution are robust, I did not find the modeling component or some aspects of the discussion to be as convincing and have the following questions/suggestions for those aspects of the manuscript:
First, the HYSPLIT modeling is used to develop air mass back trajectories for the two eruptions to show they are within the probable source region for the MBS site. Given that long-range transport is implicit to most aerosol records in Antarctica, I do not think that using such back trajectories to show that these volcanoes are within the source region is particularly useful since it seems plausible that nearly all volcanoes in the high latitude Southern Hemisphere could result in tephra deposition in Antarctica. To me, it seems the power of concretely attributing tephra to a specific volcanic source is the possibility for modeling emissions scenarios specific to that volcano. Since these eruptions are relatively well documented down to the specific eruption dates, I think running HYSPLIT in forwards mode with ash emissions for each eruption would be very useful to understand if modeled ash dispersion is consistent with the tephra identified at MBS. It may be worth mentioning that HYSPLIT back trajectories are air mass trajectories and therefore do not specifically consider aerosol transport/scavenging (which would impact tephra), but the forwards dispersion model can be specifically run for volcanic ash, though I am not sure how sophisticated the transport scheme is.
Second, I did not find that most of the discussion was justified given the limited findings of the manuscript specific to these two modern eruptions. The ice core dating section (5.1) focuses on detailing the uncertainties about ice core sulfate records and presents tephra as a more reliable means of developing tie points. I strongly disagree with this. Volcanic synchronization of ice cores using sulfate has led to the development of extremely accurate ice core chronologies (the volcanic chronologies in Sigl et al., 2014 and 2015 being some of the best recent examples) despite some of the complications associated with these records. While tephra can certainly be a powerful tool to identify specific eruptions in certain ice core records, because it is an insoluble particle it will always be much more heterogeneously distributed in the atmosphere than sulfate and therefore have much more limited utility for synchronizing geographically widespread ice core records. Furthermore, low tephra counts, reworked sparse tephras, and large sample volume required for analysis can hinder cryptotephra work. Maybe this section would be more meaningful if the authors identified these specific tephras in all four cores at MBS as well as in other regions of Antarctica to show a viable widespread signal, but identifying these shards in a single shallow core does not justify this broader discussion of ice core dating. Lastly, sections 5.2 and 5.3 seemed almost too hypothetical to be meaningful. Both sections describe how a longer MBS cryptotephra could be insightful and potentially linked to large-scale climate and teleconnections, which doesn’t particularly relate to the findings of this study. Additionally, the authors do not even show that the tephra record from the main MBS record is valid given the limited amount of available sample. All tephra results in this manuscript are from the shallow core where they were able to obtain larger sample volumes, so the sampling approach presented in this study is not applicable to the main MBS core. Overall, I suggest the authors reframe the discussion to be more pertinent and specific to their results presented in the manuscript.
Minor notes
Page 2 line 53: Very nitpicky, but I’d say a 295 m ice core is intermediate, not deep.
Page 3 line 64-65: I don’t think you justify why MBS would be more advantageous than other Antarctic sites for tephra studies. It seems like a coastal site could be even more complicated than interior sites as it would be more influenced by marine biogenic sulfur complicating the link to sulfur peaks and a coastal site presumably would be closer to exposed land and therefore sources of reworked tephra?
Page 4 line 75: Define IE (I don’t think I saw ice equivalent anywhere else before this).
Page 5 lines 87-89: what do you mean by this? Weren’t all depths sampled in the MBS main core and used to guide more detailed analysis on the shallow core? How does that link to moisture sources?
Page 6 section 2.3- this seems like a very long-winded way to say that approximate timing of eruptions was guided by linear interpolation between austral summer peaks in sulfate/Cl and austral winter peaks in Na.
Page 7 line 163- any particular reason that backtrajectories initiated at 1500 m AGL?
Page 16 Fig 7- Why is the polygon/region on the TAS diagram for Cerro Hudson a different shape on this plot than on Fig 6?
Page 17 line 306- how would sulfur isotopes allow for disentangling two coincident eruptions that appear as a single mixed sulfate peak in the ice core record? I’m not sure that is possible. I’d just remove the mention of sulfur isotopes or explain it.
Page 18 Fig 8- I’m not sure the AVHRR data appeared as intended. It seems extremely blurry.
Page 21 Fig 10- Why the switch to trajectory frequencies instead of backtrajectories? I actually find this approach more useful than individual backtrajectories so I wonder how this would look on Fig 8.
Page 25 line 481- be consistent with the number of shards. These are different than Table 1 and those stated on lines 417-418.
Citation: https://doi.org/10.5194/cp-2024-64-RC1 -
RC2: 'Comment on cp-2024-64', Anonymous Referee #2, 07 Dec 2024
reply
The manuscript by Harlan et al. presents a complete and thoughtful investigation of cryptotephra preserved in Mount Brown South ice core layers. They assess the presence of volcanic ash in ice layers, propose potential volcanic sources and aim to characterise transport pathways in a data-sparse region of Antarctica. This manuscript is interesting and relevant and should be considered for publication in Climate of the Past. However, some points need to be discussed before acceptance for publication in Climate of the Past.
Main comments:
- A low-resolution screening was first applied to identify the presence of tephras in bulk samples which analysed a small cross-section (2cm^2) of the MBS-Main core. Then, only samples with tephras were explored in the same layers of MBS-Alpha, this time analysing a larger cross-section (10cm^2). The manuscript discusses in depth the layers with cryptotephras in MBS-Alpha, however, there is no mention of the number of cryptotephras found in MBS-Main or discussion about potential misses of cryptotephra layers in MBS-Main due to the small cross-section analysed. Presenting the results from MBS-Main and discussing potential limitations due to the small cross-section used for the low-resolution screening will help to improve the manuscript.
- The manuscript describes the method used to sample cryptotephras. Samples were melted inside Whirl-pack bags and then centrifuged inside centrifuge tubes. While moving samples with low-concentration insoluble particles from one container to another, there is always a risk of leaving some of these particles behind, potentially biasing the sample that will finally be analysed. Were there any measures in place to ensure that cryptotephras were not left behind while preparing the samples? Adding this information or including this subject as a potential limitation will help improve the manuscript.
- The manuscript discusses the presence of small numbers of cryptotephras found in scattered MBS-Alpha layers. The temporality of these layers indicates these particles are not linked to specific eruptions. Based on this evidence, the authors classify them as background cryptotephra. While these particles could be background cryptotephra, in the absence of more information, they could also be contamination introduced while handling or in the lab environment. Were there any measures or protocols followed while processing the samples which can ensure that those particles were originally inside the ice? Adding this information or including this subject as a potential limitation will help improve the manuscript.
- Throughout the manuscript, it is highlighted that MBS has strong teleconnections with low latitudes and is well connected with the southern Indian Ocean. If MBS is so well connected with the Southern Indian Ocean and low latitudes, why are there no cryptotephra layers found for the mentioned Heard Island eruption (1997) and Pinatubo (1991)? Currently, there is no discussion in the manuscript addressing this issue. The unexpected presence of Erebus cryptotephra is discussed and highlighted as an anecdotic case. However, the wide presence of unexpected Erebus cryptotephra highlights more the absence of the eruptions that “should” have been present in MBS layers. The absence of these expected eruptions highlights the transport of cryptotephra to MBS may be highly biased by the occurrence of favourable atmospheric conditions. Incorporating this topic into the discussion section will help improve the manuscript.
Minor comments:
Line 26: for reference, specify the distance you are considering when classifying something as distal or ultra-distal
Line 37-38: The Chichon (1982) Eruption is mentioned here but not mentioned again throughout the text. Given its magnitude and relevance, I thought this eruption was going to be discussed as a potential source for the 1985 layer.
Line 44: higher resolution or higher accumulation?
Line 48 and throughout the text: usually the satellite era is considered as the interval from 1979 to present
Figure 1: The text mentions many sites which are not included in this map (e.g. Siple, Vostok, Talos, Law Dome, Puyehue). I suggest incorporating them for the reader to understand where those sites are located.
Lines 73-74: specify that the units are “meters depth” for all cores, as done for the MBS-Main core
Lines 75-76: what does IE stand for? Ice equivalent? Please, specify.
Lines 79-84 and throughout the text: Please, specify that you are referring to “stable water isotopes”
Line 90: Please specify if you refer to marine fertilization or soil fertilization
Line 91: towards
Lune 94: ~20 meters depth
Line 106: please, add further details about the containers where the samples were placed. Were those bottles new? Were they pre-cleaned?
Figure 2a: apparently there is no point (4) in the diagram
Line 147: Please, specify which 14 major and minor elements were measured
Line 157: the text specifies that totals below 60% are considered with caution. It would be useful if those samples were specifically labelled in the text and figures
Line 163: Is there a particular reason why using 1500 AGL instead of other elevation?
Line 169: The line starts stating that there were glass shards in 48 out of 70 samples, however, after SEM analyses, this number fall to 29. This suggests that the initial 48 samples had, in fact, “potential glass shards”. Please, consider correcting this at the beginning of the paragraph
Figure 5: there are no details about how the nssSO4 was calculated. if it was calculated using Na+ or Cl-
Line 261: for temporal reference, consider adding the dates when the eruptions happened
Line 291: identified, instead of identify
Lines 309-331: If MBS is so well connected with the Indian Ocean, why are there no cryptotephras from Pinatubo or Heard island?
Line 398: were instead of are
Line 432: Consider specifying a year for Krakatau and a year for Tarawera, as they happened during different years
Line 442: consider adding “is viable” after MBS
Lines 444-470: Evidence presented in this manuscript supports Erebus cryptotephra made it to MBS. Did other, closer cores (e.g. Talos, Taylor, Gv7) have 1985 Erebus cryptotephra?
Line 463: consider replacing “satellite image evidence” with “remote sensing evidence”
Line 471: tephra or cryptotephra?
Line 481: The line states that there is a cluster of 14 shards, but Table 1 shows only 13
Citation: https://doi.org/10.5194/cp-2024-64-RC2 -
RC3: 'Comment on cp-2024-64', Anonymous Referee #3, 14 Dec 2024
reply
I enjoyed reading about the novel approach applied to the Mount Brown South core array to discover two cryptotephra horizons in the MBS-Alpha surface firn core, and the integration of seasonal glaciochemical signals, atmospheric modelling, and geochemical fingerprinting to support the proposed source attributions. The manuscript presents a thorough method for sample preparation and a well-considered rationale for source attribution that acknowledges the challenges of identifying (crypto)tephra in Antarctic ice cores. The reported discovery of cryptotephra horizons derived from the 1991 CE Cerro Hudson (Chile) and 1985 CE Mt. Erebus (Antarctica) eruptions establishes promising new satellite era time-stratigraphic markers for East Antarctica, and highlights the value of future investigation of the MBS-Main ice core to further enhance the emerging tephrochronological framework for Antarctica.
Overall the content of this manuscript is logically presented and well-written, however there are some minor errors and inconsistencies that should be addressed to improve the presentation of key data and subsequent discussion. In particular, I would like to draw attention to the following areas:
Core selection and justification: It is unclear from the main text (sections 2.1-2.2) why the MBS-Alpha firn core was selected for examination instead of MBS-Bravo or MBS-Charlie. Was this selection based on differences in accumulation rates? The Figure 1 caption suggests that the MBS-A core was chosen "due to larger available sample volume", please clarify.
Sample selection and SEM-EDS data: Please clarify why 19 samples from the MBS-Alpha firn core were not selected for further analysis by EPMA-WDS (Figure 2a-7; lines 169-171). Was there glass identified in these samples but deemed too small for analysis? Are the results of the SEM-EDS analyses conducted (Figure 2a-7) available, and if so do they concur with the EPMA-WDS results presented for samples that progressed (Figure 2a-8)? Considering the use of a small beam diameter of 2 µm for EPMA-WDS please clarify why a further 17 samples were not analysed by EPMA-WDS (Figure 2a-8), were there no glass shards identified or was the material too small for analysis (lines 171-172; cf. line 179)? Define what is meant by "tephra grains of suitable size" (line 123), which would help justify the reasons for why these samples may have been suitable for further analysis.
EPMA-WDS analytical conditions and Na loss: Please provide some further clarification and justification (line 145) as to why these analytical conditions were used for EPMA-WDS. For example, Innes et al. (2024) propose that a 3-μm EPMA beam is suitable for use on all glass compositions provided that the beam current is reduced to 1 nA. Given the use of the "broad beam overlap" method of Iverson et al. (2017) why not use a larger beam of ≥ 5 μm in diameter for larger microparticles identified? This would have reduced the need for multiple analyses of individual glass shards from MBS-A sample 14-1 that likely increased the likelihood of Na loss. On this point, I disagree with the statement made on lines 277-281 and consider that the lower Na2O values obtained relative to the published values of Cerro Hudson eruptive material is instead indicative of minor Na loss (visible in Figure 7). Consider the effect of successive analyses of particles by SEM-EDS and EPMA-WDS (inferred from Figure 2), as well as the effect of measuring multiple spots (two to three) per glass shard, the majority of which showed a decline in Na2O wt. % (see supplementary materials). It would be interesting to see the individual data, rather than averaged values, plotted against the Cerro Hudson fields to see if there is a better correlation. Presented values may also be low because of the small beam size and current used. The minor Na loss can be explained and does not adversely affect your correlation or source attribution. Please offer some consideration of these points in the text.
Minimum threshold for acceptable analytical totals: Given the literature cited (lines 154-157), please justify why analytical totals of at least 50 % were presented in this study. Without adequate justification, it would be preferable to remove values that have analytical totals <67 % given the use of the "broad beam overlap" method of Iverson et al. (2017). However, considering the very small beam diameter used (2 µm) it may be even better to instead apply a higher minimum threshold for acceptable analytical totals (e.g., 90 %), or to discuss why lower totals were produced from shards measuring up to 15 µm in diameter.
Complete reporting of the point-by-point data: Please correct Table 1 to present the identification of 3 rhyolite shards and 1 rhyolite shard in samples 8-5 and 17-1, respectively. The analytical totals presented in the supplementary materials should also be included in Table 2, along with the exact number of analyses completed per shard for sample 14-1. This data is very important to include within the main text, particularly with regard to the consideration of possible Na-loss that may explain the discrepancies in correlation to the published data for the 1991 CE Cerro Hudson eruption visible in Figure 7.
MBS-A glass morphology: The images presented in Figure 3 illustrate the sparsity of microparticles present in samples 14-1 and 17-9, however it is very difficult to see individual particles and inspect the general size and morphology of the volcanic glass shards present. Consider replacing these images, or including additional panels that present magnified images of some of the shards found in each sample. This would better support your descriptions of glass shard morphologies in section 3.1. From the current images presented, it appears like there is more material that could have been analysed. Furthermore, it would be helpful to see images of the sparse rhyolite shards, which could be included as additional panels or in the supplementary materials.
Source eruption dynamics: The discussion of source attribution could be further improved by providing more details about the proposed source eruptions for both englacial cryptotephra horizons discovered in MBS-A. For example, what was the estimated or known duration of these recent eruptions, maximum plume heights, and total amount of ash ejected? Some of this information is presented, such as in section 4.2.1, but could be summarised earlier when first mentioning the potential source eruptions. Differences in eruption duration, style, and magnitude between the proposed source eruptions (i.e., line 313 ) could be better used to support the source attributions, complementing the geochemical correlations and atmospheric modelling presented.
Presentation of literature values: In the supplementary materials, please include the published EPMA data that was used to create the geochemical fields and reference points presented in Figures 6, 7, and 9. Where possible include the beam diameter and analytical totals for these published major element analyses. This information would be very useful to better understand the correlative similarities and discrepancies between the published reference data and new data presented by this study, for example, the discussion of rhyolitic glass compositions in section 4.1 (lines 224-228) and similarity to shards produced by the 1991 CE Pinatubo eruption.
Minor Comments:
Title: Consider referring to the Mount Brown South ice core array, acknowledging the work undertaken on both the MBS-Main and MBS-Alpha cores.
Line 13: Make consistent with the abstract, consider changing to "Earth system".
Line 17: Suggest inserting a full stop after eruption histories, and begin the next sentence with "Analyzing".
Line 23: Include a reference to non-sea-salt conductivity (see Winstrup et al., 2019) as another soluble tracer used to identify volcanic horizons.
Line 24: Replace "eruptions events" with "eruption events".
Line 26: Tephra and cryptotephra.
Line 33: Studies reported in this paragraph mostly refer to the discovery of cryptotephra rather than tephra. The terms are used interchangeably throughout the text, creating some confusion about the nature and number of (crypto)tephra horizons discovered in Southern Ocean-Antarctic palaeoarchives.
Line 35: Consider a reference to the identification of (crypto)tephra in blue ice areas, which are emerging as critical archives to help further develop and refine regional tephrochronological frameworks.
Line 36: Consider replacing "eruptions" with "sources". Remove the forward slash between "Aotearoa" and "New Zealand" (here and throughout the text), and include the macron in "Taupō".
Line 44: Remove "short", as there are studies cited in this sentence that investigated a range of intermediate and deep ice cores, as noted in Abbott et al. (2024). Consider including explicit reference to some of the ice cores from higher resolution sites referred to, such as WDC06A.
Line 48: Consider replacing "seen in Antarctic records" with "identified in Antarctic ice core records".
Line 50: Replace "Aoteroa" with "Aotearoa".
Lines 51-52: I would recommend clarifying here that only some of the volcanoes from the common source regions for volcanic products identified in Antarctic ice have been active in the satellite era. The previous sentence and cited literature refer to volcanic sources that have not been active in the satellite era or even past 1,000 years (i.e., Taupō volcano).
Line 53: Replace "deep" with "intermediate" when referring to the MBS-Main ice core, and replace "3" with "three". Round up "25 m" to "26 m", given that the depth of the MBS-Charlie firn core extends to 25.89 m depth.
Figure 1: Please consider increasing the size of the figure and including the location of other ice cores (e.g., WAIS-Divide, RICE, Law Dome, Siple Dome, Vostok, Talos Dome, Dome Fuji) and other volcanic source regions referred to in the text.
Line 64: Replace "site" with "location", and include an in-text citation to support the proposed claim of "known teleconnections across the region".
Line 65: Replace "Antarctic ice cores" with "the Antarctic ice core array".
Line 74: Mean accumulation rates?
Line 75: How do the mean accumulation rates compare for the MBS-Bravo and MBS-Charlie firn cores?
Lines 81-82, and throughout: Be consistent with the use of in-text citations throughout the text, see line 80 and line 105 for contrast. Consider removing brackets around year of publication and replacing the comma with a colon, for example: "(chemistry/trace impurities: Vance et al., 2024b; Harlan et al., 2024b)".
Line 98: Fine tune.
Lines 99-100: Good consideration, was MBS-Alpha therefore chosen out of the three surface firn cores to be used for a particular reason? Provide an example of future analyses that may require large sample volumes, such as the analysis of sulfur isotopes, iron fertilisation, or trace metals.
Line 105: First mention of the age model, consider replacing with "on the MBS2023 chronology". Please clarify in the text whether this chronology can be applied to all four cores.
Figure 2: Missing step 4. Please correct the units used for cross-sectional area (e.g., step 1 [~2 cm2] and step 5 [~10 cm2]), and replace "Alpha" with "MBS-Alpha" in the step 3 caption. Insert a hyphen between "MBS" and "Alpha" in step 5. Replace "MBS main core" with "MBS-Main ice core" in the figure caption.
Line 112 and throughout the text: Insert hyphen, please check that this is consistent throughout the text and figures.
Line 123: Considering the use of a small beam diameter of 2 µm for EPMA-WDS please clarify what is meant by "tephra grains of suitable size". This will will help inform why some samples were discarded for further analysis.
Lines 123-125, and throughout the text: Please check the text to ensure that acronyms are defined on their first use. The acronym EPMA is first used in the Figure 2 caption and again in line 123 without being defined until line 125, whilst the acronym SEM is not currently defined in the text. Please insert "scanning electron microscopy by energy dispersive spectroscopy (SEM-EDS)" in line 125, and clarify where the FEI MLA 650 ESEM was used (see lines 143-146 for contrast).
Lines 130 and 132: Please define what is meant by high(er) resolution.
Lines 139-141: Consider inserting a reference to the work of Winstrup et al. (2019), which quantified the seasonality of impurity influx to Roosevelt Island visible in the RICE CFA records using the RICE17 timescale. Further comparison could be drawn between MBS and Roosevelt Island given the coastal location of both sites, and the inherent challenges faced in locating volcanic horizons.
Line 145: Five WDS spectrometers (see Table 2 caption for comparison).
Lines 147-148: Please state here which elements were analysed.
Line 149: Provide a reference for the secondary glass standards used.
Lines 152-153: Please revise this statement, as 27 of the 52 EPMA-WDS analyses presented in the supplementary materials have analytical totals <90 %.
Line 158: Replace "based on the as recommended" with "as recommended".
Line 161: HYSPLIT acronym first used in section 2.2 on line 92.
Line 170: BSE.
Line 174: Consider referring to the sample ID rather than sample depths, for example: "Glass shards were most abundant in sample 14-1 (13.28-13.34 m depth)".
Line 175, and throughout the text: Be consistent with units, replace micron(s) with µm.
Lines 176-178: Please clarify as it is uncertain from the current wording if only one of the glass shards identified in this sample was found to be >10 µm in diameter (in this case ~15 µm).
Table 1 Caption: Please clarify in the caption that this is a "Summary of the 12 MBS-Alpha core samples analyzed by EPMA-WDS", as a total of 48 of the 70 prepared samples were reported to contain volcanic glass shards. Here and throughout the text, please be consistent with the chosen expanded abbreviation for TAS (contrast presentation in the caption for Table 1 with that of Figures 4, 6, 7, and 9, as well as line 183).
Line 185: Consider reordering in order of particle abundance.
Line 186: One andesite.
Line 187, and throughout the text: Ensure capitalisation for in-text references to the Tables, as with Figures.
Line 190, and throughout the text: Please be consistent with reference to tables and figures, contrast with line 197 (Fig. vs. Figure).
Line 199: Trace element chemistry?
Line 204: Replace "non-seasalt sulfate" with "non-sea-salt sulfate (nssSO4-2)", and revise accordingly in the text caption for Figure 5 and line 245.
Table 2: In the caption, replace "glass tephra shards" with "volcanic glass shards". Please consider using another symbol for particle 18-1_014.
Line 212: Majority? Only 12 samples analysed by EPMA-WDS of the initial 70. Does this refer to data from SEM-EDS not presented in this study?
Figure 4: Why not present sample ID rather than mean depths? The symbol used here for sample 18_1 is different to Table 2 but much easier to read.
Figure 5 Caption: Water isotopes.
Lines 242-243: Interesting discussion in section 4.1, which could be better supported by providing images of some of the rhyolite glass analysed by EPMA-WDS to illustrate their size and morphology in comparison to the proposed primary dacite and phonolite populations.
Line 247: Delete additional space in "sample , we".
Line 257: What literature values? Please provide a reference.
Line 271: Remove "13.3" and replace with "1991".
Line 274: Reported literature values.
Line 295: Please revise the Cerro Hudson eruption dates to "August 8-15, 1991".
Lines 328-331: I would also recommend referring to the age of the ice that the dacite cryptotephra was found within relative to the nssSO42- peak of Pinatubo (i.e., the MBS2023 chronology and trace impurities from Figure 5). These are convincing lines of supporting evidence for your proposed source attribution.
Line 333: Replace "volcanoes" with "volcanic eruptions".
Figure 9: It is difficult to read and distinguish some of the published values from the presented values. Please consider including the cited data used here within a supplementary file.
Line 374: The 1985 cryptotephra, please be consistent throughout (see line 376).
Lines 375-377: Excellent point, but it is likely that the shards are too small to analyse for trace element compositions by LA-ICP-MS.
Line 411: Such as? Please state what eruptions you are referring to here - are they the eruptions presented in line 432?
Line 416: MBS as a cryptotephra archive.
Section 5.3. There are some intriguing claims and findings proposed, however I find this section of the discussion difficult to follow as it lacks the clarity and structure of previous sections.
Line 480: 42 volcanic glass shards.
Line 481: Please revise this sentence as only 13 shards were reported from sample 14-1 (1991 CE) and this is not an entirely homogenous population (with 12 dacite and 1 andesite shards analysed).
Reference List: Please check each reference to ensure that no key elements are missing, such as author names, the DOI, page numbers, volume number, and journal titles. There are inconsistencies and minor errors throughout the reference list that need to be addressed.
Supplementary Materials, MBS-Alpha Glass Worksheet: Please revise the caption to reflect that the analyses were completed on samples from the MBS-Alpha core. Apply subscript for element titles, align the column headings with the data presented below, and please make the cited reference consistent with the presentation and format used in the main text. Is there a data point missing for sample 14-1_025a?
Supplementary Materials, Secondary Standards Worksheet: Could you please clarify which MBS-A EPMA datasets the three analytical sessions and therefore secondary standards data presented align with.
Citation: https://doi.org/10.5194/cp-2024-64-RC3
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