the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Jun 2024
Detecting Holocene retreat and readvance in the Amundsen Sea sector of Antarctica: assessing the suitability of sites near Pine Island Glacier for subglacial bedrock drilling
Abstract. Unambiguous identification of past episodes of ice sheet thinning below the modern surface and grounding line retreat inboard of present requires recovery and exposure dating of subglacial bedrock. Such efforts are needed to understand the significance and potential future reversibility of ongoing and projected change in Antarctica. Here we evaluate the suitability for subglacial bedrock recovery drilling of sites in the Hudson Mountains, in the Amundsen Sea sector of West Antarctica. We use an ice sheet model and field data − geological observations, glaciological observations and bedrock samples from nunataks, and ground-penetrating radar from subglacial ridges − to rate each site against four key criteria: i) presence of ridges extending below the ice sheet, ii) likelihood of increased exposure of those ridges if the grounding line was inboard of present, iii) suitability of bedrock for drilling and geochemical analysis, and iv) accessibility for aircraft and drilling operations. Our results demonstrate that although no site in the Hudson Mountains is perfect for this study when assessed against all criteria, Winkie Nunatak (74.86° S / 99.77° W) is suitable. The accessibility, N-S orientation and basaltic bedrock lithology of its southernmost ridge make the nunatak a feasible site both for drilling and subsequent cosmogenic nuclide analysis. Furthermore, it is strewn with erratic cobbles at all elevations, providing constraints on the earlier Holocene deglacial history and time at which the ice sheet surface reached its present elevation. Such information is necessary for determining the maximum duration over which any Holocene grounding line readvance could have occurred.
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RC1: 'Comment on egusphere-2024-1452', Jonathan Harbor, 15 Jul 2024
Review Criteria:
Does the paper address relevant scientific questions within the scope of TC? Yes
Does the paper present novel concepts, ideas, tools, or data? Yes, but…. This is largely a technical report aimed at selecting a specific drilling site, which they do very well. For the larger scientific audience, what would make this much more compelling is more insight into what they learned about site suitability assessment, and how this might be done in the future. That is the broader significance of this, in the absence of drilling results.
Are substantial conclusions reached? Yes, but see detailed comments.
Are the scientific methods and assumptions valid and clearly outlined? Yes
Are the results sufficient to support the interpretations and conclusions? Yes
Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? Yes
Do the authors give proper credit to related work and clearly indicate their own new/original contribution? Yes
Does the title clearly reflect the contents of the paper? Largely
Does the abstract provide a concise and complete summary? Yes, but see detail comments
Is the overall presentation well-structured and clear? Largely – see suggestion for a larger methodological overview.
Is the language fluent and precise? Yes, very well written
Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? Yes
Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? See detailed comments about potential use of supplementary data.
Are the number and quality of references appropriate? Yes
Is the amount and quality of supplementary material appropriate? See detailed comments about potential use of supplementary data for extensive field data.
Detailed Comments:
Abstract is clear, and presents the context, rationale, and key results well. For the larger audience, the particular site that is selected is less important than the method proposed and used to select suitable site(s). The abstract might be modified to say something about what was learned from the process that might inform changes or improvements in future site selection work by the authors or others.
Introduction provides a clear overview of the importance of subglacial drilling in determining if/when an ice sheet was ever smaller in the recent geological past than at present. It also makes a compelling case for the importance of this knowledge in Antarctic science. Relevant past work in subglacial drilling is well described, as is the broader significance of Antarctic ice sheet change on global sea level. This work is placed in context of other related studies, in particular the work at Mt. Murphy (Balco et al., 2023).
Although I appreciate the effort taken to make clear the significance of the larger scientific work to which this project intends to contribute, and the importance of the study region, the unique contribution of the paper (as indicated by the title and the abstract) is the approach used to assess site suitability. This is not reflected in the balance between describing the larger scientific context of the work and describing the background and past work around site selection. It’s almost as if the authors start writing the paper about the results that should have come from the analysis of the core samples, but then focus on the site selection (given that they were unfortunately unsuccessful in obtaining core samples due to due to factors unrelated to the choice of drill site). While I appreciate the importance of the detailed background for the intended results of the drilling, I suggest that the authors make this a shorter more focused discussion of what was learned from the site selection process. To guide future work by them or others in site selection.
Section 3 provides a clear explanation of the key criteria used in selecting a drilling site, based on prior work and a nice example (Figure 2) from Mt. Murphy, Balco et al., 2023. The discussion around acceptable lithologies for in situ 14C measurements is particularly helpful for those less familiar with recent advances for non-quartz- bearing bedrock lithologies. The discussion of technical factors is helpful, in particular that “the presence of permeable firn near the base of a shallow borehole or crevassing extending near the bed could potentially allow fluid loss and preclude coring”. However, I don’t recall seeing later in the paper how your approach evaluated the possible presence of permeable firn near the base of a shallow borehole.
4: I think we are missing a short paragraph that provides an overview of the entire process that is used to evaluate the potential sites – that introduces that there is initial work done (4.1) that then drives the information collected during the field survey (4.2), and that this then leads to the final selection (which the authors have in 4.2.4, but could be renumbered 4.3). This will give readers the big picture of what readers will then experience as they progress through the rest of the sections. Perhaps even a flow chart.
Section 4.1.1 makes clear the importance of prior work on Holocene exposure history, and section 4.1.2 is particularly important in showing how ice sheet modelling is key in helping us consider how potential drilling sites would have responded to past change by looking at potential future change. While the use of potential future change has several advantages, there should be some discussion of why the authors did not model the past periods of change that they are seeking to reconstruct and understand. I appreciate the candor in discussing the current spatial resolution limitations of modeling at the scale of a set of nunataks. What stood out as particularly important was the ability to look at whether the grounding line retreat directions changed relative to the ridge orientation, and the way in which sites might become “islands” rather than exposed.
4.1.3 might more logically come after 4.1.1. Rather than judging all sites against all criteria, it is clear from table 1 that once a site failed the ridge test, the rest of the criteria became N/A. So perhaps this can be described this as an initial filter. Then the rest of the discussion can focus on the sites that passed this initial filter (and exclude the others from table 1).
4.1.5 It is very helpful to have this summary to provide the logic for the focus of the fieldwork.
4.2. The authors provide a very complete description of the field data and results of the analyses on samples collected for the three candidate drill sites. This is important information that should be available to the wider scientific community. But I struggled with the level of detail - if the purpose of the paper is to advance understanding of a process for drilling site suitability assessment, then it is too much (a subset should be in the main paper, and if we want a comprehensive report out, put it in supplementary data). If the purpose is defined as also to provide a repository of all the data collected (which is valuable) then the current structure works fine.
4.2.4 This is more than a summary of the results; it is also then a selection of the best site – so I’d label this 4.3 and have the title also indicate “site selection”.
5. Conclusion. Although to the project team the selection of the best site for drilling is a key deliverable, for the larger audience reading this paper what is more important is what the team also learned about assessing site suitability. What lessons were learned, what would the authors suggest that others do the same or differently in future drilling site suitability assessments for this type of scientific work, what are areas for improvement (eg in the modeling)?
The postscript provides helpful context as the reader is likely to be keenly interested in knowing how the coring went and about any results. I can only imagine the frustration with this outcome – field work in challenging environments is tough, and we often don’t report the many cases where things did not work out.
Citation: https://doi.org/10.5194/egusphere-2024-1452-RC1 - AC1: 'Authors' response to RC1', Joanne Johnson, 04 Sep 2024
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RC2: 'Comment on egusphere-2024-1452', Anonymous Referee #2, 29 Jul 2024
This is a valuable contribution for anyone planning to access subglacial bedrock through shallow ice (≤100m); that is anyone who is interested in quantifying if an ice margin had previously been retreated beyond than the present extent. The manuscript presents a structured account of the steps taken to select a suitable drilling site in the Amundsen Sea sector of West Antarctica. As such this manuscript is more of a technical report than a scientific contribution with associated results that address the underlying aim of “unambiguous identification of past episodes of ice sheet thinning”. What is missing is what the authors learned from their process and how they would improve it for future site selection. The manuscript is well written, and the figures are excellent.
Detailed comments:
The two-part title “Detecting Holocene retreat and advance in the Amundsen Sea sector of Antarctica: assessing the suitability of sites near Pine Island Glacier for subglacial bedrock drilling” suggests that the manuscript will provide new data on detecting Holocene retreat and advance in the Amundsen Sea sector of Antarctica, but the manuscript is focused on the second part, “Assessing the suitability of sites near Pine Island Glacier for subglacial bedrock drilling”. This would be a more correct and informative title.
The introduction gives a thorough account of the need for subglacial bedrock drilling, contextualising this into the wider issue of improving our understanding of West Antarctic ice sheet change from the mid-Holocene to the onset of satellite observations.
Section 2 starts by reiterating the limited number of projects that have successfully retrieved subglacial bedrock and measured cosmogenic nuclides in those, before introducing the Hudson mountains field area.
Section 3 describes the approach and criteria used for choosing a suitable drill site. One paragraph is informatively spent on explaining the need for certain lithologies, and that new mineral systems are being explored by several research groups to increase the utility of measuring in-situ 14C and 10Be. The section on drilling and the potential problem of fluid leakage is interesting but I would have valued a section on how to avoid leaky permeable firn. Can it be identified in GPR data?
Section 4 begins by highlighting the importance of understanding the Holocene exposure history based on fieldwork and surface exposure ages. This could be followed by Section 4.1.3 which focuses on identifying potential suitable ridges using remote sensing. Table 1 (Section 4.5.1) shows that sites without ridges can be excluded as suitable sites. With respect to the manuscript, further mention of the non-suitable sites is unnecessary.
Section 4.2 provides a nice example of the value of using ice sheet modelling to estimate grounding line retreat and how this changes the orientation of bedrock ridges relative to ice flow, or that some nunataks become islands and are therefore unlikely to retain a Holocene record.
Section 4.2 is a detailed account of implementing the procedures developed throughout the manuscript at Winkie, Webber, and Maish nunataks. Each nunatak is treated in informative detail. The thin sections were revealing because they demonstrate how difficult it will be to retrieve sufficient material suitable for analyses.
The manuscript is all about site selection. It would have been valuable for the authors to reflect on their selection process and what they have learned that might improve future site selection.
Citation: https://doi.org/10.5194/egusphere-2024-1452-RC2 - AC2: 'Authors' response to RC2', Joanne Johnson, 04 Sep 2024
-
RC3: 'Comment on egusphere-2024-1452', Anonymous Referee #3, 30 Jul 2024
This is a very well written and well structured manuscript, and I found little to criticise beyond the few points already raised in the review by Jon Harbor. I generally agree with the points raised by JH, in particularly with regard to including a focus on more general lessons learned about site suitability assessment from this and previous campaigns, perhaps in a new section 5.
Minor comments
l. 108-110: For the field site presented in this study, grounding line retreat is the most likely cause of ice-sheet thinning, but perphaps mass-balance changes could drive thinning elsewhere in Antarctica? Since this section describes the problem in general terms, I am wondering if it should be presented more broadly. Also, are these criteria exhaustive? E.g. should criterias on subglacial topography/ice thickness (vs. feasible ice-drilling depths/realistic thinning depths), or surface stability/cover during subaerial exposure, be included? Maybe they are implicit in criteria i) and ii), if so, these criteria could be explained/expanded on as is the case for criteria iii) lithology and iv) accesibility.
l. 224-226: Elaborate what the problem with an island is. I assume that you mean that bedrock on flanks/ridges goes from subglacial to submarine rather than becoming subaerially exposed. But becoming an island isn’t in itself a problem as long as parts of the formerly subglacial section of a ridge line becomes subaerially exposed, i.e. if the sea level is lower than the ice level.
l. 271: In the text, Table 1 is presented as providing information from pre-field survey, but in reality it contains both pre, in-, and post field information. I found this confusing.
Section 4.2.3. Due to accessibility problems Maish Nunatak was not visited in the field to perform a site suitability assessment. I therefore wonder if this section could be cut from the manuscript, and replaced by a note of the accesibility issue in e.g., the beginning of section 4.2. Information on bedrock lithology and glaciology seems unessential once it becomes clear that the site isn’t feasible to access, but details could be moved to a supplement.
Figures
Figs. 9+13. Add approximate scale on photos or in captions
Fig. 7+11: Specify that the field of view in panels c (PPL) and d (XPL) is the same (i.e., not just the same rock).
Citation: https://doi.org/10.5194/egusphere-2024-1452-RC3 - AC3: 'Authors' response to RC3', Joanne Johnson, 04 Sep 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-1452', Jonathan Harbor, 15 Jul 2024
Review Criteria:
Does the paper address relevant scientific questions within the scope of TC? Yes
Does the paper present novel concepts, ideas, tools, or data? Yes, but…. This is largely a technical report aimed at selecting a specific drilling site, which they do very well. For the larger scientific audience, what would make this much more compelling is more insight into what they learned about site suitability assessment, and how this might be done in the future. That is the broader significance of this, in the absence of drilling results.
Are substantial conclusions reached? Yes, but see detailed comments.
Are the scientific methods and assumptions valid and clearly outlined? Yes
Are the results sufficient to support the interpretations and conclusions? Yes
Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? Yes
Do the authors give proper credit to related work and clearly indicate their own new/original contribution? Yes
Does the title clearly reflect the contents of the paper? Largely
Does the abstract provide a concise and complete summary? Yes, but see detail comments
Is the overall presentation well-structured and clear? Largely – see suggestion for a larger methodological overview.
Is the language fluent and precise? Yes, very well written
Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? Yes
Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? See detailed comments about potential use of supplementary data.
Are the number and quality of references appropriate? Yes
Is the amount and quality of supplementary material appropriate? See detailed comments about potential use of supplementary data for extensive field data.
Detailed Comments:
Abstract is clear, and presents the context, rationale, and key results well. For the larger audience, the particular site that is selected is less important than the method proposed and used to select suitable site(s). The abstract might be modified to say something about what was learned from the process that might inform changes or improvements in future site selection work by the authors or others.
Introduction provides a clear overview of the importance of subglacial drilling in determining if/when an ice sheet was ever smaller in the recent geological past than at present. It also makes a compelling case for the importance of this knowledge in Antarctic science. Relevant past work in subglacial drilling is well described, as is the broader significance of Antarctic ice sheet change on global sea level. This work is placed in context of other related studies, in particular the work at Mt. Murphy (Balco et al., 2023).
Although I appreciate the effort taken to make clear the significance of the larger scientific work to which this project intends to contribute, and the importance of the study region, the unique contribution of the paper (as indicated by the title and the abstract) is the approach used to assess site suitability. This is not reflected in the balance between describing the larger scientific context of the work and describing the background and past work around site selection. It’s almost as if the authors start writing the paper about the results that should have come from the analysis of the core samples, but then focus on the site selection (given that they were unfortunately unsuccessful in obtaining core samples due to due to factors unrelated to the choice of drill site). While I appreciate the importance of the detailed background for the intended results of the drilling, I suggest that the authors make this a shorter more focused discussion of what was learned from the site selection process. To guide future work by them or others in site selection.
Section 3 provides a clear explanation of the key criteria used in selecting a drilling site, based on prior work and a nice example (Figure 2) from Mt. Murphy, Balco et al., 2023. The discussion around acceptable lithologies for in situ 14C measurements is particularly helpful for those less familiar with recent advances for non-quartz- bearing bedrock lithologies. The discussion of technical factors is helpful, in particular that “the presence of permeable firn near the base of a shallow borehole or crevassing extending near the bed could potentially allow fluid loss and preclude coring”. However, I don’t recall seeing later in the paper how your approach evaluated the possible presence of permeable firn near the base of a shallow borehole.
4: I think we are missing a short paragraph that provides an overview of the entire process that is used to evaluate the potential sites – that introduces that there is initial work done (4.1) that then drives the information collected during the field survey (4.2), and that this then leads to the final selection (which the authors have in 4.2.4, but could be renumbered 4.3). This will give readers the big picture of what readers will then experience as they progress through the rest of the sections. Perhaps even a flow chart.
Section 4.1.1 makes clear the importance of prior work on Holocene exposure history, and section 4.1.2 is particularly important in showing how ice sheet modelling is key in helping us consider how potential drilling sites would have responded to past change by looking at potential future change. While the use of potential future change has several advantages, there should be some discussion of why the authors did not model the past periods of change that they are seeking to reconstruct and understand. I appreciate the candor in discussing the current spatial resolution limitations of modeling at the scale of a set of nunataks. What stood out as particularly important was the ability to look at whether the grounding line retreat directions changed relative to the ridge orientation, and the way in which sites might become “islands” rather than exposed.
4.1.3 might more logically come after 4.1.1. Rather than judging all sites against all criteria, it is clear from table 1 that once a site failed the ridge test, the rest of the criteria became N/A. So perhaps this can be described this as an initial filter. Then the rest of the discussion can focus on the sites that passed this initial filter (and exclude the others from table 1).
4.1.5 It is very helpful to have this summary to provide the logic for the focus of the fieldwork.
4.2. The authors provide a very complete description of the field data and results of the analyses on samples collected for the three candidate drill sites. This is important information that should be available to the wider scientific community. But I struggled with the level of detail - if the purpose of the paper is to advance understanding of a process for drilling site suitability assessment, then it is too much (a subset should be in the main paper, and if we want a comprehensive report out, put it in supplementary data). If the purpose is defined as also to provide a repository of all the data collected (which is valuable) then the current structure works fine.
4.2.4 This is more than a summary of the results; it is also then a selection of the best site – so I’d label this 4.3 and have the title also indicate “site selection”.
5. Conclusion. Although to the project team the selection of the best site for drilling is a key deliverable, for the larger audience reading this paper what is more important is what the team also learned about assessing site suitability. What lessons were learned, what would the authors suggest that others do the same or differently in future drilling site suitability assessments for this type of scientific work, what are areas for improvement (eg in the modeling)?
The postscript provides helpful context as the reader is likely to be keenly interested in knowing how the coring went and about any results. I can only imagine the frustration with this outcome – field work in challenging environments is tough, and we often don’t report the many cases where things did not work out.
Citation: https://doi.org/10.5194/egusphere-2024-1452-RC1 - AC1: 'Authors' response to RC1', Joanne Johnson, 04 Sep 2024
-
RC2: 'Comment on egusphere-2024-1452', Anonymous Referee #2, 29 Jul 2024
This is a valuable contribution for anyone planning to access subglacial bedrock through shallow ice (≤100m); that is anyone who is interested in quantifying if an ice margin had previously been retreated beyond than the present extent. The manuscript presents a structured account of the steps taken to select a suitable drilling site in the Amundsen Sea sector of West Antarctica. As such this manuscript is more of a technical report than a scientific contribution with associated results that address the underlying aim of “unambiguous identification of past episodes of ice sheet thinning”. What is missing is what the authors learned from their process and how they would improve it for future site selection. The manuscript is well written, and the figures are excellent.
Detailed comments:
The two-part title “Detecting Holocene retreat and advance in the Amundsen Sea sector of Antarctica: assessing the suitability of sites near Pine Island Glacier for subglacial bedrock drilling” suggests that the manuscript will provide new data on detecting Holocene retreat and advance in the Amundsen Sea sector of Antarctica, but the manuscript is focused on the second part, “Assessing the suitability of sites near Pine Island Glacier for subglacial bedrock drilling”. This would be a more correct and informative title.
The introduction gives a thorough account of the need for subglacial bedrock drilling, contextualising this into the wider issue of improving our understanding of West Antarctic ice sheet change from the mid-Holocene to the onset of satellite observations.
Section 2 starts by reiterating the limited number of projects that have successfully retrieved subglacial bedrock and measured cosmogenic nuclides in those, before introducing the Hudson mountains field area.
Section 3 describes the approach and criteria used for choosing a suitable drill site. One paragraph is informatively spent on explaining the need for certain lithologies, and that new mineral systems are being explored by several research groups to increase the utility of measuring in-situ 14C and 10Be. The section on drilling and the potential problem of fluid leakage is interesting but I would have valued a section on how to avoid leaky permeable firn. Can it be identified in GPR data?
Section 4 begins by highlighting the importance of understanding the Holocene exposure history based on fieldwork and surface exposure ages. This could be followed by Section 4.1.3 which focuses on identifying potential suitable ridges using remote sensing. Table 1 (Section 4.5.1) shows that sites without ridges can be excluded as suitable sites. With respect to the manuscript, further mention of the non-suitable sites is unnecessary.
Section 4.2 provides a nice example of the value of using ice sheet modelling to estimate grounding line retreat and how this changes the orientation of bedrock ridges relative to ice flow, or that some nunataks become islands and are therefore unlikely to retain a Holocene record.
Section 4.2 is a detailed account of implementing the procedures developed throughout the manuscript at Winkie, Webber, and Maish nunataks. Each nunatak is treated in informative detail. The thin sections were revealing because they demonstrate how difficult it will be to retrieve sufficient material suitable for analyses.
The manuscript is all about site selection. It would have been valuable for the authors to reflect on their selection process and what they have learned that might improve future site selection.
Citation: https://doi.org/10.5194/egusphere-2024-1452-RC2 - AC2: 'Authors' response to RC2', Joanne Johnson, 04 Sep 2024
-
RC3: 'Comment on egusphere-2024-1452', Anonymous Referee #3, 30 Jul 2024
This is a very well written and well structured manuscript, and I found little to criticise beyond the few points already raised in the review by Jon Harbor. I generally agree with the points raised by JH, in particularly with regard to including a focus on more general lessons learned about site suitability assessment from this and previous campaigns, perhaps in a new section 5.
Minor comments
l. 108-110: For the field site presented in this study, grounding line retreat is the most likely cause of ice-sheet thinning, but perphaps mass-balance changes could drive thinning elsewhere in Antarctica? Since this section describes the problem in general terms, I am wondering if it should be presented more broadly. Also, are these criteria exhaustive? E.g. should criterias on subglacial topography/ice thickness (vs. feasible ice-drilling depths/realistic thinning depths), or surface stability/cover during subaerial exposure, be included? Maybe they are implicit in criteria i) and ii), if so, these criteria could be explained/expanded on as is the case for criteria iii) lithology and iv) accesibility.
l. 224-226: Elaborate what the problem with an island is. I assume that you mean that bedrock on flanks/ridges goes from subglacial to submarine rather than becoming subaerially exposed. But becoming an island isn’t in itself a problem as long as parts of the formerly subglacial section of a ridge line becomes subaerially exposed, i.e. if the sea level is lower than the ice level.
l. 271: In the text, Table 1 is presented as providing information from pre-field survey, but in reality it contains both pre, in-, and post field information. I found this confusing.
Section 4.2.3. Due to accessibility problems Maish Nunatak was not visited in the field to perform a site suitability assessment. I therefore wonder if this section could be cut from the manuscript, and replaced by a note of the accesibility issue in e.g., the beginning of section 4.2. Information on bedrock lithology and glaciology seems unessential once it becomes clear that the site isn’t feasible to access, but details could be moved to a supplement.
Figures
Figs. 9+13. Add approximate scale on photos or in captions
Fig. 7+11: Specify that the field of view in panels c (PPL) and d (XPL) is the same (i.e., not just the same rock).
Citation: https://doi.org/10.5194/egusphere-2024-1452-RC3 - AC3: 'Authors' response to RC3', Joanne Johnson, 04 Sep 2024
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