the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
New 10Be exposure ages improve Holocene ice sheet thinning history near the grounding line of Pope Glacier, Antarctica
- 1British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
- 2Department of Earth Science & Engineering, Imperial College London, London SW7 2AZ, UK
- 3Department of Earth & Environmental Sciences, Tulane University, New Orleans, LA 70118, USA
- 4Australia’s Nuclear Science and Technology Organisation (ANSTO), New Illawarra Road, Lucas Heights, NSW 2234, Locked Bag 2001, Kirrawee DC 2232, Australia
- 5Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA
- 6School of Earth and Climate Sciences and the Climate Change Institute, University of Maine, Orono, ME 04469 USA
- 7Department of Geography and Environmental Sciences, Northumbria University, Newcastle-upon-Tyne, NE1 8ST, UK
- 1British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
- 2Department of Earth Science & Engineering, Imperial College London, London SW7 2AZ, UK
- 3Department of Earth & Environmental Sciences, Tulane University, New Orleans, LA 70118, USA
- 4Australia’s Nuclear Science and Technology Organisation (ANSTO), New Illawarra Road, Lucas Heights, NSW 2234, Locked Bag 2001, Kirrawee DC 2232, Australia
- 5Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA
- 6School of Earth and Climate Sciences and the Climate Change Institute, University of Maine, Orono, ME 04469 USA
- 7Department of Geography and Environmental Sciences, Northumbria University, Newcastle-upon-Tyne, NE1 8ST, UK
Abstract. Evidence for the timing and pace of past grounding line retreat of the Thwaites Glacier system in the Amundsen Sea embayment (ASE) of Antarctica provides constraints for models that are used to predict the future trajectory of the West Antarctic Ice Sheet (WAIS). Existing cosmogenic nuclide surface exposure ages suggest that Pope Glacier, a former tributary of Thwaites Glacier, experienced rapid thinning in the early to mid-Holocene. There are relatively few exposure ages from the lower ice-free sections of Mount Murphy (< 300 m asl) that are uncomplicated by either nuclide inheritance or scattering due to localised topographic complexities; this makes the trajectory for the latter stages of deglaciation uncertain. This paper presents 12 new 10Be exposure ages from erratic cobbles collected from the western flank of Mt Murphy, within 160 m of the modern ice surface and 1 km from the present grounding line. The ages comprise two tightly clustered populations with mean deglaciation ages of 7.1 ± 0.1 ka and 6.4 ± 0.1 ka (1SE). Linear regression analysis applied to the age-elevation array of all available exposure ages from Mt Murphy indicates that the median rate of thinning of Pope Glacier was 0.27 m yr-1 between 8.1–6.3 ka, occurring 1.5 times faster than previously thought. Furthermore, this analysis better constrains the uncertainty (95 % confidence interval) in the timing of deglaciation at the base of the Mt Murphy vertical profile (~80 m above the modern ice surface), shifting it to earlier in the Holocene (from 5.2 ± 0.7 ka to 6.3 ± 0.4 ka). Taken together, the results presented here suggest that early–mid Holocene thinning of Pope Glacier occurred over a shorter interval than previously assumed and permit a longer duration over which subsequent late Holocene rethickening could have occurred.
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Jonathan Richard Adams et al.
Status: final response (author comments only)
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RC1: 'Comment on tc-2022-82', Anonymous Referee #1, 27 May 2022
ice sheet history of Pope Glacier in Amundsen Sea embayment (ASE). Based on newly obtained Be-10 surface exposure ages and evaluation of the existing data set from Johnson et al. (2020), the authors refined the ice thinning rate and timing of deglaciation at the lowest site currently exposed. Because constraining the past ice behavior will provide insight into the drivers and mechanisms of the rapid ice mass loss and for model validation and refinement, this research is of international scientific interest.
Although this paper makes an excellent addition to our knowledge about the Holocene ice thinning history in West Antarctic Ice Sheet, I found some points need to be clear before publication.
The issues authors should better address are summarized as follows.
- Effect of the geometry of the ice sheet. The authors use the same value (80 masl) as the modern ice surface elevation. However, the curvature of the ice surface around the scoria cone looks not simple and may affect the timing of the exposure of samples. Topographic profiles of the scoria cone (including outcrop A to B) and ice surface nearby should be presented. The relative height of each sample site from the contemporary ice sheet surface may be better for the thinning rate calculation. Another point to note is the measurement of sample altitudes. I do not see any description of how the authors obtained the altitudes of the samples. If these are based on GPS measurements, the altitude data should be corrected to Geoid highest. The difference will not be large, but it is thought to be crucial for the interpretation with this high resolution.
- Origin of the faster ice thinning. I think the refined ice sheet history probably requires some revisions for the interpretation done by Johnson et al. (2020). Could you address this by adding a discussion about the paleoclimatic context for Holocene thinning in ASE?
Minor issues
- The geological background of the scoria cone should be mentioned. What are their age and origin? And also, “bedrock surface at a scoria cone” (in the caption of Fig.3) sounds a little bit awkward for me.
- Discerption about the arcuate ridge landform is preferable. What is the origin of this? It looks like a moraine ridge might be formed by readvance. Could you discuss the origin of this?
- Line 315: Delete pace between “7.” and “5”
- Figure 6: Please make clear the origin of samples (which ones are from the Scoria cone?)
- Table 1: uniform the number of digits for the site coordinates. I think the number of digits exceeds the precision of the measurement (Needs more info about this).
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AC1: 'Reply on RC1', Jonathan Adams, 09 Sep 2022
We would like to thank Anonymous Reviewer 1 for a thorough, constructive, and thought-provoking review of our manuscript. We are happy with the positive nature of this review and pleased that the Reviewer deems the manuscript a good addition to the existing knowledge of the Holocene ice thinning history of West Antarctica. The reviewer has provided many helpful comments and suggestions, which we will fully address if we are invited to submit a revised manuscript.
Firstly, the reviewer is correct that the local topography could affect the linear regression to some unknown extent. In our original manuscript, we selected the most representative nearest point on Pope Glacier a few hundred metres away that is at a consistent elevation (80 m asl) along the ice stream adjacent to scoria cone. In the revised manuscript, we would address the reviewer’s comment and include topographic profiles, including across outcrop A and outcrop B as well as the position of scoria cone relative to our reference modern ice surface height of 80 m asl. The topographic profiles would then be utilised to inform the selection of the proximal ice heights used for Outcrop A and Outcrop B. We would then perform a sensitivity analysis using these distinct ice height for Outcrop A and another for Outcrop B, respectively, to determine what effect the choice of reference ice heights as inputs to the linear regression analysis has on our interpretation of the rate and timing of the thinning history. We would then quantify if the results of the sensitivity analysis fall within the uncertainty boundaries of our original preferred thinning history, and directly address whether our choice of ice height changes our primary conclusions or not.
Secondly, we acknowledge the reviewer’s comment that there is insufficient information of how we obtained the altitude of the samples, these elevations are corrected to the EGM08 Geoid, and we would provide a more detailed description in the revised paper methods.
Thirdly, the reviewer raised the question of what caused the rapid Holocene ice sheet thinning in this region. The answer to this question is presently unknown, although it was discussed in association with the wider Holocene paleoclimatic context of the region by Johnson et al. (2020). Since that paper provides a detailed discussion of the topic, we have decided not to repeat that work here, but to instead include specific reference to Johnson et al. (2020) in our revised discussion section.
Finally, we also thank the reviewer for their attention to minor issues regarding the manuscript and will also address these within the author’s response and manuscript revisions.
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RC2: 'Comment on tc-2022-82', Derek Fabel, 26 Aug 2022
The authors present 12 new 10Be surface exposure ages from glacial erratics collected on scoria cones at the northern extent of Mt Murphy, close to the grounding line of Pope Glacier which drains into Crosson Ice Shelf in the Amundsen Sea Embayment. The new ages allow the authors to improve the previously published Holocene ice sheet lowering rates from cosmogenic nuclide data obtained from the area, concluding that lowering was more rapid by a factor of about 1.5 and occurred about 1100 years earlier than previously established.
Overall this is a very good paper. It is clearly written. It presents new data that fill a gap in the existing vertical profile data at Mt Murphy. The figures are clear and necessary, although the figure numbering does not match the numbers in the text and Supplementary Material.
In Figure 5 caption at line 242, (Fig. A3) should be (Fig. B1), and at line 243 (Fig. A4) should be (Fig. B2).
There is a full stop missing in line 251.
In the text at line 288 Fig. 5a should be Fig. 4a.
At line 380 the word “is” after Mt. murphy should be deleted.
At line 471 the ITGC Contribution number should be added.Other than these very minor editorial issues, I have no further comments, except that the paper was a pleasure to read.
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AC2: 'Reply on RC2', Jonathan Adams, 09 Sep 2022
We thank Derek Fabel for sharing with us that he enjoyed reading through the manuscript, we really appreciate his considerate and encouraging comments as well as his thoroughness detecting mistakes that had been overlooked by us, particularly regarding figure numbering. We would incorporate Derek’s suggestions and corrections into our revised manuscript and outline in greater detail how we have dealt with these minor issues in the Author’s response.
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AC2: 'Reply on RC2', Jonathan Adams, 09 Sep 2022
Jonathan Richard Adams et al.
Jonathan Richard Adams et al.
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