Terrestrial Cosmogenic Nuclide depth profiles used to infer changes in Holocene glacier cover, Vintage Peak, Southern Coast Mountains, British Columbia

Hawkins, Adam C.; Goehring, Brent M.; Menounos, Brian

doi:https://doi.org/10.5194/egusphere-2024-2900

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https://doi.org/10.5194/egusphere-2024-2900

Preprints

08 Oct 2024

| 08 Oct 2024

Terrestrial Cosmogenic Nuclide depth profiles used to infer changes in Holocene glacier cover, Vintage Peak, Southern Coast Mountains, British Columbia

Adam C. Hawkins, Brent M. Goehring, and Brian Menounos

Abstract. The majority of glaciers in North America reached their maximum Holocene downvalley positions during the Little Ice Age (1300–1850 CE), and in most cases, this expansion also destroyed earlier evidence of glacier activity. Substantial retreat in the 20th and early 21st centuries exposed bedrock that fronts many glaciers that may record early-to-mid Holocene exposure and later burial by ice which can be elucidated using multiple-nuclide cosmogenic surface exposure dating. Furthermore, cores of bedrock allow the measurement of cosmogenic nuclide depth profiles to better constrain potential exposure and burial histories. We collected four bedrock surface samples for ¹⁰Be and ¹⁴C surface exposure dating and shallow bedrock cores from Vintage Peak, in the southern Coast Mountains of British Columbia, Canada. We apply a Monte Carlo approach to generate combinations of exposure and burial duration that can explain our data. Vintage Peak became uncovered by the Cordilleran Ice Sheet between 14.5 and 11.6 ka, though higher reaches on Vintage Peak retained ice until 10–12 ka before retreating to smaller than modern positions. Glaciers on Vintage Peak advanced within 100 m of late Holocene maximum positions around 4–6 ka. Poorly constrained subglacial erosion rates, possible inheritance, and variable mass shielding complicate our ability to more robustly interpret bedrock cosmogenic surface exposure histories. Nine ¹⁰Be ages on late Holocene moraines reveal that glaciers reached their greatest Holocene extents ca. 1300 CE. Our results agree with other regional glacier records and demonstrate the utility of surface exposure dating applied to deglaciated bedrock as a technique to help construct a record of Holocene glacier activity where organic material associated with glacier expansions may be absent or poorly-preserved. Further work to increase exposure/burial history modeling complexity may help to better constrain complex exposure histories in glaciated alpine areas.

Received: 17 Sep 2024 – Discussion started: 08 Oct 2024

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Adam C. Hawkins, Brent M. Goehring, and Brian Menounos

Status: final response (author comments only)

RC1:
'Comment on egusphere-2024-2900', Anonymous Referee #1, 04 Nov 2024

My general comments:
This paper presents TCN-based geochronology, near Vintage Peak in the southern Coast Mountains of British Columbia, Canada, using nine moraine boulder samples, one erratic block, and four shallow bedrock cores. The main purpose of the study was to reveal the early-to-mid Holocene exposure/burial history of the sampled sites using a multiple-nuclide approach (14C, 10Be) and bedrock depth profiles. However, the complex history of ice, possible inherited nuclide inventory, and likely snow/ice/till cover make the study's outcome somewhat ambiguous. However, authors tried to resolve the glacier history at Vintage Peak, and found out (i) major ice sheet deglaciation roughly between 14.5 and 11.6 ka, ii) Holocene advancement at around 4-6 ka, and iii) further advance of ice to maximum position at the LIA (ca 1300 CE). The paper is well-written, concise, and provides all the necessary details for the community. It certainly deserves publication in the Geochronology journal but requires some major revisions, as suggested by my specific comments.

Specific comments:
• Explaining the complex history of alpine ice is a challenging task, but the authors should address and deal with specific complexities individually, such as till cover, inheritance, bedrock erosion, etc. to show the outcome of the study is valid. For instance, till coverage on bedrock surfaces has not been discussed, and may have influence on accumulation of cosmogenic nuclides depending of the thickness and density of till on top of sampled bedrock profiles.
• The Monte Carlo approach has not been discussed in detail. Some information is presented in Ch. 4.4 and Figure 9, but it's method and results are unclear.
• The connection between the LIA extent and Holocene glaciers is poorly explained. The spatial extents should be elaborated, on maps and in the text.
• Cosmogenic 14C bedrock depth profiles have not been discussed in sufficient detail.

Technical corrections:
• Title: Since the depth profiles were taken from the bedrock, you may want to add word “bedrock” after the Nuclide, but before the depth, so that the first line of the title will read “Terrestrial Cosmogenic Nuclide bedrock depth profiles used to infer…” There are not only depth profiles in the study, 9+1 boulders, so, the current title is somewhat misleading.
• L93: The Vintage peak is located at 1800 m. Its impressions look like it is located at 1600 m.
• L95: On the Table 2, where the snow survey data is presented, the altitude of snow stations are different than the numbers here.
• L140: SM Table 1 is related to blank samples, not batch of 10Be samples, so move the referring (SM Table 1) to the end of the sentence
• The erratic boulder (17-VP-07), how would you sure that this is an erratic boulder? a rockfall from top?
• L323: Most of the columns of this table (Table 1) has no units and last four columns of Table 1 have no explanations.
• Table 3: It is not clear what are the numbers at the last 3 columns of Table 3?

Citation: https://doi.org/10.5194/egusphere-2024-2900-RC1
- AC1:
  'Reply on RC1', Adam Hawkins, 22 Dec 2024
  Please see our replies to the reviewer's comments in bold below.
  RC1:
  My general comments:
  This paper presents TCN-based geochronology, near Vintage Peak in the southern Coast Mountains of British Columbia, Canada, using nine moraine boulder samples, one erratic block, and four shallow bedrock cores. The main purpose of the study was to reveal the early-to-mid Holocene exposure/burial history of the sampled sites using a multiple-nuclide approach (14C, 10Be) and bedrock depth profiles. However, the complex history of ice, possible inherited nuclide inventory, and likely snow/ice/till cover make the study's outcome somewhat ambiguous. However, authors tried to resolve the glacier history at Vintage Peak, and found out (i) major ice sheet deglaciation roughly between 14.5 and 11.6 ka, ii) Holocene advancement at around 4-6 ka, and iii) further advance of ice to maximum position at the LIA (ca 1300 CE). The paper is well-written, concise, and provides all the necessary details for the community. It certainly deserves publication in the Geochronology journal but requires some major revisions, as suggested by my specific comments.
  
  We thank the referee for taking time to review our paper and providing useful feedback on how to improve it.
  
  Specific comments:
  Explaining the complex history of alpine ice is a challenging task, but the authors should address and deal with specific complexities individually, such as till cover, inheritance, bedrock erosion, etc. to show the outcome of the study is valid. For instance, till coverage on bedrock surfaces has not been discussed, and may have influence on accumulation of cosmogenic nuclides depending of the thickness and density of till on top of sampled bedrock profiles.
  
  A fair point. While we mention the potential impact from transient till coverage on line 359 and lines 379-381, we recognize this discussion is overly brief. Our revised manuscript will discuss this effect in greater detail.
  
  The Monte Carlo approach has not been discussed in detail. Some information is presented in Ch. 4.4 and Figure 9, but it's method and results are unclear.
  
  Our revised manuscript will provide additional details on our Monte Carlo approach. We will also include additional information about the results in the main section of the paper.
  
  The connection between the LIA extent and Holocene glaciers is poorly explained. The spatial extents should be elaborated, on maps and in the text.
  
  We will review and clarify how we explain the relative extent of glaciers on Vintage Peak and will revise Figure 3 and associated text to make our interpretations of relative ice cover history more clear.
  
  Cosmogenic 14C bedrock depth profiles have not been discussed in sufficient detail.
  
  We agree our current text was too brief when discussing the 14C bedrock depth profiles, their interpretation, and their utility to this study. We will provide additional detail on the first-order observations from the depth profiles and the usefulness for further constraining possible bedrock exposure histories beyond what a surface sample can provide alone.
  
  Technical corrections:
  Title: Since the depth profiles were taken from the bedrock, you may want to add word “bedrock” after the Nuclide, but before the depth, so that the first line of the title will read “Terrestrial Cosmogenic Nuclide bedrock depth profiles used to infer…” There are not only depth profiles in the study, 9+1 boulders, so, the current title is somewhat misleading.
  
  This is a valid point and we will alter our title to include bedrock and address that we sampled multiple types of surfaces for cosmogenic nuclide analysis, to develop our glacier chronology.
  
  L93: The Vintage peak is located at 1800 m. Its impressions look like it is located at 1600 m.
  
  Figure 1 includes labeled contours showing both Lockie’s Table and Vintage Peak are just over 1800 m in elevation. In Line 93, we estimate the average annual temperature at 1600 m, as this is an approximate average elevation from which our bedrock and moraine boulders were sampled. We think Figure 1 and text are clear as written and, unless we are mistaken, we propose to leave the text and figure unchanged.
  
  L95: On the Table 2, where the snow survey data is presented, the altitude of snow stations are different than the numbers here.
  
  Thank you for this correction. In 2015, the elevation of both snow survey sites was changed. We had listed the older elevation in text, not the most recent elevations as reported by the Province of British Columbia. We will note this change in reported elevation in text.
  
  L140: SM Table 1 is related to blank samples, not batch of 10Be samples, so move the referring (SM Table 1) to the end of the sentence
  
  Thank you, we will move reference to SM Table 1 to the end of the sentence.
  
  The erratic boulder (17-VP-07), how would you sure that this is an erratic boulder? a rockfall from top?
  
  This is a fair point. We cannot definitively rule out that 17-VP-07 may be from a rockfall event (albeit an old one). The boulder is subangular and we did not observe any distinctive evidence of glacial transport. However, if this was a rockfall deposit, we would expect to find several other similar clasts in the area, but there was just one smaller boulder 10’s of meters away. We will provide additional detail on the geomorphic position of 17-VP-07 in our revised manuscript.
  
  L323: Most of the columns of this table (Table 1) has no units and last four columns of Table 1 have no explanations.
  
  We apologize for this formatting error (conversion of the Google Document into a PDF causes several lines to be dropped from the table).
  
  Table 3: It is not clear what are the numbers at the last 3 columns of Table 3?
  
  This is also due to the formatting error mentioned above and will be fixed in the revised manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2900-AC1
RC2:
'Comment on egusphere-2024-2900', Anonymous Referee #2, 20 Nov 2024

Overview:
The manuscript describes the utility and application of Terrestrial Cosmogenic Nuclides Beryllium-10 and in situ Carbon-14 (C-14) from boulder, bedrock surface, and bedrock depth profiles from cores to quantify and model glacier exposure/burial scenarios during the Holocene at Vintage Peak, Southern Coast Mountains, British Columbia. At the time of sample collection in 2017, very few, if any, published studies of portable backpack drilling for cosmogenic depth profile analysis had been done, so the application is a novel technique. Modelling for exposure/burial/erosion scenarios were leveraged to characterize glacier size in concert with modern observations of glacier extent and snow depth partial shielding. Results show uncovering from the Cordilleran Ice Sheet around 11.6-14.5 ka, and Vintage peak retaining local ice until 10-12 ka before retreating smaller than modern limits. The ice then began to regrow during the late Holocene/Little Ice Age, and then retreated to modern limits.
The technique of bedrock depth profile shows promise, and C-14 has applications for complex exposure/burial modelling due to the short half life and small mass of quartz necessary compared to Be-10. Difficulties observed during the study were limits of drill penetration due to bit type (this was a common problem with the early portable drill systems cutting bits designed for sedimentary rock) that prevented recovery of the deeper muon dominant depth section, and the relatively larger uncertainty with C-14 compared to Be-10.
The study is generally well written with occasional grammatical inconsistencies or regional word spelling variation. It is in my opinion that the geochronologic techniques applied, in particular the application of in situ C-14 in bedrock depth profiles, is a novel and powerful technique that can and should be applied in more studies. This study tested the method and experienced some difficulties but exhibits the utility and need for additional efforts in furthering in situ C-14 and depth profiles. There are additional analyses that could have been done regarding C-14 that are currently in the literature but were not discussed as part of this study and may provide merit, but the authors state there is future work that could be applied to better understand the data. The scientific quality is good, and additional analyses would improve the study, but the results and techniques presented are appropriate for the conclusions made. There is a concern about inheritance and incomplete nuclide resetting that may be addressable through additional evaluation of C-14 modelling from Secular equilibrium (ex Schweinsberg et al,2018; Graham et al, 2019; Sbarra et al, 2022) or consideration of deep muon long term production (Briner et al, 2016; Balter-Kennedy et al, 2021). The presentation quality is good, and minor revision would improve the quality of the manuscript.
General notes:
There are some parts of the manuscript that could be improved. In general, the application mindset of the utility of cosmogenic nuclide analysis appears to be strictly focusing on the method as a chronology technique, however, studies referenced in the paper expand on using the method for more than just chronology (ex Balter-Kennedy et al, 2021, Graham et al, 2023). Part of the utility of depth profile analysis and modelling is the a priori knowledge of the glacial history to constrain the modelling parameters. When referring to the depth profile, it may be more accurate to refer to it as “cosmogenic nuclide analysis” or “analysis” rather than “dating” because the methods can infer more than an age. Alternatively, the authors could refer to the measured concentrations as “relative exposure ages” for comparison of shielded samples that are lower than the expected age if the sample was not affect by additional mass depth (example Graham et al, 2023). “Relative exposure ages” can be useful in relating a more audience understandable metric of difference.
Another general note is to uniformly use the term “modelling” vs “modeling”. I recommend using “modelling” throughout as it appears to be more universal amongst international audiences whereas “modeling” is more generalized to the United States.

Comments per line number:
31 – You reference the Little Ice Age for the first time in the body of the paper outside the abstract, but do not provide the shorthand of (LIA). Add (LIA) after you name Little Ice Age, and give the age range. (1300-1850 CE).
36 & 39 – “Cosmogenic surface exposure dating” - consider changing to “Cosmogenic exposure analysis” because the methods applied in the manuscript to evaluate more than just the surface and dating.
43 – Consider stating an approximate duration of burial required to reset or “forget” previous exposure (Hippe, 2017, Graham et al, 2019). This assumption that C-14 is completely reset during LGM burial may not be valid in some samples.
45 – Consider referencing previous field based applications to model burial (Goehring et al, 2011, 2013; Schweinsberg et al, 2018; Pendleton et al, 2019; Graham et al, 2019; Sondergard et al, 2020; Sbarra et al, 2022; etc.)
50-52 – Include references to those studies
Something not clearly addressed is the potential value-added information from collecting bedrock cores or subsurface samples for depth profile analysis. Why should you go to the trouble of bringing 30 kgs of equipment up the side of a mountain, scrounge for cooling water, and spend all day manually drilling to collect the core? What does the core give you that a bedrock surface sample doesn’t? Please explain. Consider referencing Schaefer et al, 2016 in addition to Goehring et al 2013, Balter-Kennedy et al, 2021 and Graham et al, 2023
110 – Is the lidar data shown in the manuscript? Please include the figure number.
116 – What type of drill was used? A portable electric hand drill or a gasoline powered system?
120 – Consider changing “dating” to “measurements”. The samples from the core are not being measured for ages, but concentrations that are correlated and represent the same exposure history with different shielding parameters. The concentrations from the core are what is used in the model, not the age. In general, expand on the utility of cores.
184 – Identify in the text which erratic boulder was used.
187 – State the Figure number for the two isotope plot.
Additional details about the Monte Carlo simulation would be helpful.
194 – 50 meter ice thickness is considered sufficiently thick to cease c-14 production from muons. The authors state below (Line 228) that ice was likely 35-40 m thick. Production likely did not cease, but was probably not significant (beyond the uncertainty of the measurements) at the maximum extent. As ice thinned, and if it maintained thickness below 10 meters, it can become significant (Pendleton et al, 2019).
220-221 – modelled and modeled used in the same sentence. Fix throughout.
245 Figure 4 caption – Consider removing in situ from the organic in situ 14C samples because it is more common to see in situ referring to in situ cosmogenic C-14 rather than organic. Stating organic 14C should be sufficient and likely more consistent with other literature.
260 Figure 5 – This is a great figure and it is very informative that this data was made available for this study.
264 – Is there a classic canonical definition for the LIA? State clearly when the beginning is.
266, 270, 271, 272 – The authors flip between ka (years before present) and CE (years after 0 CE). Please be consistent throughout the paper and reference events in ka or a as defined earlier in the paper. If CE is beneficial to link to other worldly events, accompany with ka and have the CE date in ().
276 – consider replacing “some” with “approximately”. Also, “returned” is very informal and colloquial, as a boulder did not deliver the age. Consider replacing with “has a calculated age of… for 10Be and …
285 – This is a great analysis and it supports the snow data over longer time periods.
Table 1 – It appears that a second row in the headers is missing.
If there isn’t a second row missing, what is the difference between Exposure and Exposure Age?
For the 10Be/9Be ratio, 1 sigma, Blank-corrected, Blank-corrected (assuming 1 sigma), consider using the same 10^x for each column. Cosmo tables should be formatted to easily copy and paste from the manuscript into a spreadsheet for uniform analysis. It also maintains significant figures between samples and allows the reader to quickly see which samples are orders of magnitude different. I recommend: 10^-14, 10^-15, 10^04, 10^02, respectively.
Figure 8 – Consider showing the measurement points as a box with depth to represent the integrated measurement thickness of each sample. Refer to Schaefer et al, 2016 as an example.
384 – Consider simplifying to insolation, as “solar insolation” is redundant as insolation refers to the input from the Sun.
401 – Consider replacing “outside” with “outboard”

An assumption made throughout the paper is minimal sub-glacial erosion, and no inheritance. If there is inheritance of Be-10 in bedrock samples, is the duration of burial during the LGM and depth of erosion during the LGM sufficient enough to reset the C-14 system to background levels? If it is not, can you model to account for that?
There are other assumptions made that may not be entirely valid, however, if those assumptions are not fully valid, the effect of the assumptions likely does not significantly impact the results presented. Consider reviewing all assumptions and state the effect if those assumptions are not valid.

Citation: https://doi.org/10.5194/egusphere-2024-2900-RC2
- AC2: 'Reply on RC2', Adam Hawkins, 22 Dec 2024
  
  Please see our replies to the reviewer's comments in bold below:
  Overview:
  The manuscript describes the utility and application of Terrestrial Cosmogenic Nuclides Beryllium-10 and in situ Carbon-14 (C-14) from boulder, bedrock surface, and bedrock depth profiles from cores to quantify and model glacier exposure/burial scenarios during the Holocene at Vintage Peak, Southern Coast Mountains, British Columbia. At the time of sample collection in 2017, very few, if any, published studies of portable backpack drilling for cosmogenic depth profile analysis had been done, so the application is a novel technique. Modelling for exposure/burial/erosion scenarios were leveraged to characterize glacier size in concert with modern observations of glacier extent and snow depth partial shielding. Results show uncovering from the Cordilleran Ice Sheet around 11.6-14.5 ka, and Vintage peak retaining local ice until 10-12 ka before retreating smaller than modern limits. The ice then began to regrow during the late Holocene/Little Ice Age, and then retreated to modern limits.
  The technique of bedrock depth profile shows promise, and C-14 has applications for complex exposure/burial modelling due to the short half life and small mass of quartz necessary compared to Be-10. Difficulties observed during the study were limits of drill penetration due to bit type (this was a common problem with the early portable drill systems cutting bits designed for sedimentary rock) that prevented recovery of the deeper muon dominant depth section, and the relatively larger uncertainty with C-14 compared to Be-10.
  The study is generally well written with occasional grammatical inconsistencies or regional word spelling variation. It is in my opinion that the geochronologic techniques applied, in particular the application of in situ C-14 in bedrock depth profiles, is a novel and powerful technique that can and should be applied in more studies. This study tested the method and experienced some difficulties but exhibits the utility and need for additional efforts in furthering in situ C-14 and depth profiles. There are additional analyses that could have been done regarding C-14 that are currently in the literature but were not discussed as part of this study and may provide merit, but the authors state there is future work that could be applied to better understand the data. The scientific quality is good, and additional analyses would improve the study, but the results and techniques presented are appropriate for the conclusions made. There is a concern about inheritance and incomplete nuclide resetting that may be addressable through additional evaluation of C-14 modelling from Secular equilibrium (ex Schweinsberg et al,2018; Graham et al, 2019; Sbarra et al, 2022) or consideration of deep muon long term production (Briner et al, 2016; Balter-Kennedy et al, 2021). The presentation quality is good, and minor revision would improve the quality of the manuscript.
  Thank you for your time and comments that will improve our manuscript.
  General notes:
  There are some parts of the manuscript that could be improved. In general, the application mindset of the utility of cosmogenic nuclide analysis appears to be strictly focusing on the method as a chronology technique, however, studies referenced in the paper expand on using the method for more than just chronology (ex Balter-Kennedy et al, 2021, Graham et al, 2023). Part of the utility of depth profile analysis and modelling is the a priori knowledge of the glacial history to constrain the modelling parameters. When referring to the depth profile, it may be more accurate to refer to it as “cosmogenic nuclide analysis” or “analysis” rather than “dating” because the methods can infer more than an age. Alternatively, the authors could refer to the measured concentrations as “relative exposure ages” for comparison of shielded samples that are lower than the expected age if the sample was not affect by additional mass depth (example Graham et al, 2023). “Relative exposure ages” can be useful in relating a more audience understandable metric of difference.
  We intentionally limit a priori knowledge of past glacier activity in our depth profile analysis. Unlike in the studies mentioned above, we have limited direct evidence of glacier cover through the Holocene at Vintage peak, and the relative paucity of bedrock samples from this site lead to us deciding to focus on determining relative glacier extent change. We agree that our use of ‘cosmogenic nuclide dating’ when referring to the bedrock core analysis is better changed to ‘cosmogenic nuclide analysis’.
  Another general note is to uniformly use the term “modelling” vs “modeling”. I recommend using “modelling” throughout as it appears to be more universal amongst international audiences whereas “modeling” is more generalized to the United States.
  Thank you for catching this inconsistency (due to Canada/US author team). We will change the text to ‘modelling’ throughout.
  Comments per line number:
  31 – You reference the Little Ice Age for the first time in the body of the paper outside the abstract, but do not provide the shorthand of (LIA). Add (LIA) after you name Little Ice Age, and give the age range. (1300-1850 CE).
  Will change in text as suggested.
  36 & 39 – “Cosmogenic surface exposure dating” - consider changing to “Cosmogenic exposure analysis” because the methods applied in the manuscript to evaluate more than just the surface and dating.
  We agree with this comment when referring to the bedrock samples. While our boulder samples do have calculated exposure dates for their surfaces, the cosmogenic nuclide measurements at the bedrock sites have a more nuanced analysis.
  43 – Consider stating an approximate duration of burial required to reset or “forget” previous exposure (Hippe, 2017, Graham et al, 2019). This assumption that C-14 is completely reset during LGM burial may not be valid in some samples.
  We will include a statement on both the burial duration and subglacial erosion rate during the Last Glacial Maximum needed to fully reset the 14C inventory in the bedrock. As mentioned in Hippe (2017), 10 ka of burial with 2.4 m of subglacial erosion would reset the 14C inventory in the bedrock. We will expand in text on the evidence of other studies that give us confidence that the bedrock on Vintage Peak did experience sufficient burial and erosion during the LGM to reset the 14C inventory.
  45 – Consider referencing previous field based applications to model burial (Goehring et al, 2011, 2013; Schweinsberg et al, 2018; Pendleton et al, 2019; Graham et al, 2019; Sondergard et al, 2020; Sbarra et al, 2022; etc.)
  Thank you for these suggested studies, some of which are applied in different contexts than our study, but still have merit for consideration and discussion in our paper.
  50-52 – Include references to those studies
  We agree that we should specifically reference those studies and will include in the revised manuscript.
  Something not clearly addressed is the potential value-added information from collecting bedrock cores or subsurface samples for depth profile analysis. Why should you go to the trouble of bringing 30 kgs of equipment up the side of a mountain, scrounge for cooling water, and spend all day manually drilling to collect the core? What does the core give you that a bedrock surface sample doesn’t? Please explain. Consider referencing Schaefer et al, 2016 in addition to Goehring et al 2013, Balter-Kennedy et al, 2021 and Graham et al, 2023
  This is a fair comment. We will provide additional detail and background on the potential benefit of including bedrock cores over taking just surficial samples. Namely, cores allow possible exposure and burial histories to be constrained more than surface samples alone.
  110 – Is the lidar data shown in the manuscript? Please include the figure number.
  We will include reference to Figure 5 in the manuscript text, which leverages the LiDAR data.
  116 – What type of drill was used? A portable electric hand drill or a gasoline powered system?
  We used a Shaw Tools Portable Backpack drill, which is a gasoline-powered drill. We will include this detail in the main text.
  120 – Consider changing “dating” to “measurements”. The samples from the core are not being measured for ages, but concentrations that are correlated and represent the same exposure history with different shielding parameters. The concentrations from the core are what is used in the model, not the age. In general, expand on the utility of cores.
  Will change in the manuscript, as suggested, and will expand on the benefit of measuring nuclide concentrations in cores rather than surface samples alone.
  184 – Identify in the text which erratic boulder was used.
  We will specify the boulder, 17-VP-07.
  187 – State the Figure number for the two isotope plot.
  We will directly reference this plot in text.
  Additional details about the Monte Carlo simulation would be helpful.
  We agree our detail on our Monte Carlo approach is lacking and will provide additional detail on our methodology.
  194 – 50 meter ice thickness is considered sufficiently thick to cease c-14 production from muons. The authors state below (Line 228) that ice was likely 35-40 m thick. Production likely did not cease, but was probably not significant (beyond the uncertainty of the measurements) at the maximum extent. As ice thinned, and if it maintained thickness below 10 meters, it can become significant (Pendleton et al, 2019).
  While glacier ice may have been thicker than 35-40 m at Lockie’s Glacier, we provide a likely minimum glacier thickness at the glacier’s maximum Holocene extent. We agree that there were likely times when ice was thin enough to allow muon-produced cosmogenic isotopes, but our data are unable to quantify the duration of thin mass shielding by ice. Given the apparent exposure and burial durations at cores 2 and 3 are equivalent (within errors) we interpret the response of Lockie’s Glacier to Holocene climate change to be roughly binary; the glacier was either in a advanced position close to its Holocene maximum position, or retreated to an extent similar to its modern position.
  220-221 – modelled and modeled used in the same sentence. Fix throughout.
  Will change all instances of “modeled” to “modelled” in text.
  245 Figure 4 caption – Consider removing in situ from the organic in situ 14C samples because it is more common to see in situ referring to in situ cosmogenic C-14 rather than organic. Stating organic 14C should be sufficient and likely more consistent with other literature.
  In this case, we used in situ to refer to the organic 14C samples, which were specifically organics that were directly killed by glacier advance and were in situ. While in situ 14C is commonly used to describe organic debris directly killed by glacier advance (e.g. Luckman, 2000), we will aim to reduce confusion and will change the text to read “... organic 14C samples from overridden wood in glacial forefields and composite moraines…”
  260 Figure 5 – This is a great figure and it is very informative that this data was made available for this study.
  Thank you!
  264 – Is there a classic canonical definition for the LIA? State clearly when the beginning is.
  While we define the LIA in the abstract (1300-1850 CE), we don’t include this definition in the Introduction (Luckman, 2000). We will add this timeframe to the first sentence of the Introduction.
  266, 270, 271, 272 – The authors flip between ka (years before present) and CE (years after 0 CE). Please be consistent throughout the paper and reference events in ka or a as defined earlier in the paper. If CE is beneficial to link to other worldly events, accompany with ka and have the CE date in ().
  This is a valid critique and we will change the text to be consistent with ages as recommended.
  276 – consider replacing “some” with “approximately”. Also, “returned” is very informal and colloquial, as a boulder did not deliver the age. Consider replacing with “has a calculated age of… for 10Be and …
  Agreed, will change in text.
  285 – This is a great analysis and it supports the snow data over longer time periods.
  Thank you
  Table 1 – It appears that a second row in the headers is missing.
  We apologize for this formatting error (conversion of the Google Document into a PDF causes several lines to be dropped from the table).
  
  If there isn’t a second row missing, what is the difference between Exposure and Exposure Age?
  This is also a formatting error as mentioned above, which cut off additional text in the column header. This will be resolved in the revised manuscript.
  For the 10Be/9Be ratio, 1 sigma, Blank-corrected, Blank-corrected (assuming 1 sigma), consider using the same 10^x for each column. Cosmo tables should be formatted to easily copy and paste from the manuscript into a spreadsheet for uniform analysis. It also maintains significant figures between samples and allows the reader to quickly see which samples are orders of magnitude different. I recommend: 10^-14, 10^-15, 10^04, 10^02, respectively.
  Same as above.
  Figure 8 – Consider showing the measurement points as a box with depth to represent the integrated measurement thickness of each sample. Refer to Schaefer et al, 2016 as an example.
  We agree with this point and will modify Figure 8 as suggested.
  384 – Consider simplifying to insolation, as “solar insolation” is redundant as insolation refers to the input from the Sun.
  Agreed, will change to ‘insolation’
  401 – Consider replacing “outside” with “outboard”
  Will change to “outboard”.
  An assumption made throughout the paper is minimal sub-glacial erosion, and no inheritance. If there is inheritance of Be-10 in bedrock samples, is the duration of burial during the LGM and depth of erosion during the LGM sufficient enough to reset the C-14 system to background levels? If it is not, can you model to account for that?
  Our model does iterate through a range of subglacial erosion scenarios, and our data from Cores 2 and 3 are best explained with a small (but consistent in the literature) subglacial erosion rate. We will include a section in our Background that describes the time to reset any inherited 10Be or 14C under various erosion rates. As discussed in Hippe (2017), 10 ka of burial accompanied by 2.4 m of subglacial erosion removes 95% of the 14C inventory. We do think it is highly likely that Core 4 experienced ~10 ka of burial and over 2.4 m of erosion during the LGM, thereby resetting the 14C inventory. We don’t have the data to evaluate the magnitude of possible 14C inheritance; however, we can modify our model to evaluate the impact of 14C inheritance on possible exposure/burial histories. We will test this effect and will include our results in either the main text or in the supplement.
  There are other assumptions made that may not be entirely valid, however, if those assumptions are not fully valid, the effect of the assumptions likely does not significantly impact the results presented. Consider reviewing all assumptions and state the effect if those assumptions are not valid.
  We aim in this paper to be explicit about each of our assumptions made and their potential impact on our conclusions. Our revised manuscript will better address how variables such as inheritance and subglacial erosion impact our results.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2900-AC2

Adam C. Hawkins, Brent M. Goehring, and Brian Menounos

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https://doi.org/10.5194/egusphere-2024-2900-supplement

Adam C. Hawkins, Brent M. Goehring, and Brian Menounos

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Short summary

We use a method called cosmogenic nuclide dating on bedrock surfaces and moraine boulders to determine the relative length of time an alpine glacier was larger or smaller than its current extent over the past 15 thousand years. We also discuss several important limitations to this method. This method gives information on the duration of past ice advances and is useful in areas without other materials that can be dated.


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