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the Creative Commons Attribution 4.0 License.
| 16 Dec 2024
Impact of glacial isostatic adjustment on zones of potential grounding line stability in the Ross Sea Embayment (Antarctica) since the Last Glacial Maximum
Abstract. Ice streams in the Ross Sea Embayment (West Antarctica) retreated up to 1,000 kilometers since the Last Glacial Maximum, constituting one of the largest changes in deglacial Antarctic ice sheet volume and extent. One way that bathymetry influenced this retreat was through the presence of local bathymetric highs, or “pinning points”, which decreased ice flux through the grounding line and slowed grounding line retreat. During this time, glacial isostatic adjustment vertically shifted the underlying bathymetry, altering the grounding line flux. Continental scale modeling efforts have demonstrated the impact of solid Earth-ice sheet interactions on the deglacial retreat of marine ice sheets, however, these models are too coarse to resolve small scale bathymetric features. We pair a high-resolution bathymetry model with a simple model of grounding line stability in an ensemble approach to predict zones of potential grounding line stability in the Ross Sea Embayment for given combinations of surface mass balance rate, degree of ice shelf buttressing, basal friction coefficient, and bathymetry (corrected for glacial isostatic adjustment using three different ice sheet histories). We find that isostatic depression within the interior of the Ross Sea Embayment during the Last Glacial Maximum restricts zones of potential grounding line stability to near the edge of the continental shelf. Zones of potential grounding line stability do not appear near the present-day grounding line until sufficient uplift has occurred (mid-Holocene; ~5 ka), resulting in a net upstream migration of zones of potential grounding line stability across the deglaciation. Additionally, our results show that coarse resolution bathymetry underpredicts possible stable grounding line positions, particularly near the present-day grounding line, highlighting the importance of bathymetric resolution in capturing the impact of glacial isostatic adjustment on ice stream stability.
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RC1: 'Comment on egusphere-2024-3465', Matt King, 07 Jan 2025
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The authors present an analysis of the impacts of (predominantly) GIA on the grounding line stability of Antarctica's Ross Ice Shelf region when combined with high-resolution bathymetry. They do this by combining a range of GIA predictions with a computationally fast (and simplified) ice model. The work shows that specific regions have a greater likelihood of producing stable grounding lines at both 20ka and present-day and these are largely invariant to the exact GIA model employed. They also show that high-resolution (500m) bathymetry produces substantially different regions of grounding line stability than those computed after downsampling the bathymetry to 20km resolution as is typical in coupled ice-GIA models. The paper therefore reaches some important conclusions. The work is well described and the figures are nicely prepared. While I am not an expert on ice modelling, it appears appropriate and suitable for the task. A range of tests in the supplementary material confirm the results to be robust to various choices.
I have just a few comments where further clarification or discussion is required.
More substantial remarks
L110 - this section, do note that Nield et al 2016 GJI put some constraint on upper mantle viscosity in this region as >10^20 Pa s. That said, their work considered late Holocene flow switching but not the more recent finding of large-scale retreat and readvance, which may affect their conclusions. On that general topic, how does the absence of these large mid to late Holocene signals from the ice models change anything? Perhaps not at all, but maybe worth noting when introducing the ice models.
In general, I thought the importance of far-field sea level was emphasised more than the impact of the forebulge collapse (on the shelf break region). I didn't see evidence to suggest one was more important than the other. Please review all mentions or add some extra tests if the distinction matters.
Minor remarks:
L75 please clarify the method for shifting the bathymetry to 20ka. Do you take model(PD)-model(20ka) and apply that to bedmachine? I guess that is the only approach one could use to adjust bedmachine but please clarify regardless
L85 sedimentation is mentioned along with sea level (and later far field sea level effects) but maybe it is worth adding other things that could contribute to sea level over LGM timescales such as changes in ocean dynamic topography and thermosteric effects.
L114 check the wording of this sentence as I did not understand it
L129 the use of 'geologic record' was confusing to me given the context is present day. That raised the question as to the meaning of 'present day' in the paper more generally. is it within the last few hundred years? Is there a definition you wish to use?
L150 should flow come before law here in L151?
Equation 5. I looked at the right side of Eq 1 and could not see how taking the derivative with respect to L would arrive at this equation. Please check but given the authorship's mathematical expertise, it is likely I was missing something. I note that h is defined only in table 1 and not in the text (cf hg).
Fig 2. a) Please add some distance markers so one can understand the profile in b). In b) xaxis distance from where? L246 some comment on the forebulge change on the right side of 2b would be appropriate.
L339 T-test to t-test
L358 the methods used to resample are missing. I presume this is some sort of mean. I wonder if using something other than the mean (like first quartile or max) may produce more realistic subsampling and useful advice to those running lower-resolution models out of computational necessity. maybe that messes with ice-ocean melt in those models.
Figure 5 The definition of misfit is in the caption but missing from the text. Including it would help the reader avoid confusion
L390 add cross reference to Fig 6b
L389 is 'logarithmic' strictly or is this by eye?
L396 there's already a median misfit of 25% at 1km so is it robust to say 1km? There does not appear to be convergence evident in Fig 6b and so I think you could argue that 500m may not be a high enough resolution. Correct? You could test that with some simulated higher-resolution topography. I guess.
L415 'offshore Victoria Land' could be Ross Sea or Southern Ocean. please be more specific
L444-447 I found this sentence unclear as to its meaning.
supp material
S2 'marine ice sheet evolution' does not make sense to me
Supp Table I think should be labelled Table S1 rather than STable 1. Relevant here and in the main paper, where do these values come from and does it matter if they are not realistic? I think so. I presume for instance that the SMB present-day is actually about 0.1m/yr (ice? water?)
Fig S4 define ZPS on yaxis label
Matt King, Jan 8, 2025.
Citation: https://doi.org/10.5194/egusphere-2024-3465-RC1 -
RC2: 'Comment on egusphere-2024-3465', Anonymous Referee #2, 03 Mar 2025
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The authors investigate how glacial isostatic adjustment (GIA) has influenced zones of potential grounding line stability in the Ross Sea Embayment since the Last Glacial Maximum (LGM). They use a high-resolution bathymetry model combined with a simple grounding line stability framework to assess how GIA-induced changes in bathymetry affect ice sheet retreat and stabilization. Their analysis incorporates three different ice sheet histories and several Earth models to account for uncertainties in past ice loading and mantle viscosity.
The results show that during the LGM, grounding line stability was concentrated near the continental shelf edge, but as isostatic rebound progressed, stability zones migrated upstream, aligning with the present-day grounding line. They also find that coarse-resolution bathymetry underestimates grounding line stability, emphasizing the importance of high-resolution models. The authors conclude that GIA plays a crucial role in stabilizing ice sheets over long timescales and that bathymetric resolution significantly impacts the accuracy of grounding line predictions.
General remarks
This study presents an insightful analysis of the role of glacial isostatic adjustment (GIA) in grounding line stability within the Ross Sea Embayment. The authors effectively apply a high-resolution bathymetry model and an ensemble of simple grounding line stability calculations to assess how GIA-induced bathymetric changes influence grounding line migration. The results contribute valuable knowledge on the spatial and temporal evolution of stability zones, and the study convincingly demonstrates the importance of bathymetric resolution in capturing these effects. The paper is well-structured, with clear research objectives and a logical progression of results. However, the introduction is quite brief and lacks sufficient discussion on how this study builds upon and differentiates itself from prior research on GIA and ice sheet stability. Providing more context on the relevance of grounding line stability zones and linking this work more explicitly to previous studies would strengthen the framing. The methods section, while detailed, could benefit from additional clarification in several places, particularly regarding the ice sheet histories, Earth models, and parameter choices. Please see the line-by-line comments for the details.
Line-by-line comments
L53-58: The connection between the paragraphs can be more informative. Lines 53-56 explain that the retreat have been found to be reduced due to GIA and that the models cannot explore a large parameter space at high resolution, but it is not stated explicitly why one needs to explore the full parameter space and which remaining research questions are still open.
L58: How large is the ensemble?
L59-61: Place your research in context here. Discuss other research that have been conducted in Antarctica to study how GIA modulates bathymetric features. Why do you not need coupled ice sheet – GIA to study the effect of bathymetry on grounding line migration? What is the method to obtain your ensemble of simple grounding line stability calculations? Have this method been applied in literature, maybe to other regions? Have the ensemble of simple grounding line stability calculations been used in other studies? Are there other regions, or other studies for the Ross Sea Embayment, where the effect of GIA on bathymetry have been studied using high resolution models?
L63-65: Explain why you made the choice to predict zones of potential grounding line stability and not reconstruct the exact history.
L65-68: Explain in more detail the relevance of the identification of locations.
L68: In the introduction, there is no mention of the contribution of GIA to grounding line stability at present-day. Please include a section in the introduction about ongoing GIA in the Ross Sea Embayment, how is it measured, how strong is the signal.
L75: Which version of Bedmachine?
L94-104: Please provide more information on the ice thickness histories. How have these three histories been selected? Is the output of the Gol14 and Gom18 models constrained by observations? For example, how well does bedrock change at present-day from Gom18 match with GPS observations? What is the uncertainty of the benthic d18O records? Furthermore, why do the models vary in Antarctic ice sheet volume change? What is the spatial and temporal resolution of the output of all three models? How do the ice sheet histories exactly differ from each other and is one more realistic than the other? Why have W12 and Gom18 a sharp transition in ice thickness upstream and downstream of the grounding line, respectively. Might this effect your results?
L109-110: Explain briefly how Whitehouse et al. (2012) determined the best fit 1D model.
L111-114: To improve readability, move this sentence to line 110 to discuss it right after you mention the 1D model. Also, explain the VM5a Earth model and include a reference for the representative Earth model for West Antarctica.
L114-115: The sensitivity analysis compares the 3D model to a 1D model, but a more comprehensive exploration of uncertainty would require varying the 3D Earth model itself, as it inherently contains uncertainties. Since mantle viscosity and lithospheric thickness cannot be measured directly, the 3D model is subject to assumptions and potential biases. Currently, the discussion does not acknowledge the uncertainties within the 3D model, which may affect the results. It would be valuable to include a discussion on this limitation and its potential impact on the findings.
L119-120: Explain in more detail how LGM ice stream flowlines are defined based on both the reconstructions and the present-day ice flow.
L121: Explain interglacial endmembers.
L130-131: Explain why the understanding of solid Earth-ice sheet interactions would not be possible with traditional ice-sheet modelling methods.
L136: Why is it spaced at 1 km if the resolution of the ice thickness is lower. How does it improve results to use a 1 km resolution along the flow line?
L136-138: Could you clarify the sample size used for each parameter? Are the values evenly spaced within the given range? Additionally, for clarity, it may be helpful to remove variables from Table 1 that do not have specified values.
L145: Variable b is not defined.
L149: Insert “and” instead of comma before “ice-shelf buttressing”.
L174-176: It is not entirely clear how a zone itself is defined and how the zones are defined as stable or unstable.
L179: The parameter space has been explored by ice sheet models as well, for example Albrecht et al. (2020) performed hundreds of simulations for which they systematically varied the parameters using full-factorial parameter sampling. Do you mean here that sampling a wide range of parameter space is not feasible with ice-sheet models on a relatively high resolution? Please clarify.
Albrecht, T., Winkelmann, R., and Levermann, A.: Glacial-cycle simulations of the Antarctic Ice Sheet with the Parallel Ice Sheet Model (PISM) – Part 2: Parameter ensemble analysis, The Cryosphere, 14, 633–656, https://doi.org/10.5194/tc-14-633-2020, 2020.
L204-206: This is not clear from figure S2, please be more specific on which location this can be seen.
L300-307: Also discuss how this effect the results of the Gom18 GIA output, since this sea level change does not include the effect of the northern hemisphere.
L312: Not clear how this is analyzed, since grounding zones were only analyzed at 20 ka and present day.
L330: Please define “grounding line depth” explicitly.
Figures
Fig. 2: Concerning panel b, please clarify which present day topography is shown, is it observed present day topography? Is the modelled present day topography by Gomez et al. (2018) equal to the observed present day topography? Please indicate in the text how the difference in present day topography between modelled and observed topography might effect your results.
Fig. S1: Include in the caption which flow lines are shown and where they are taken from. Also include that the present day grounding line is shown and explain where it is taken from or how it is computed.
Fig. S2: Label of the x axis is missing.
Citation: https://doi.org/10.5194/egusphere-2024-3465-RC2
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