the Creative Commons Attribution 4.0 License.
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
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