the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The impact of regional-scale upper mantle heterogeneity on glacial isostatic adjustment in West Antarctica
Abstract. West Antarctica is underlain by a laterally heterogenous upper mantle, with localized regions of mantle viscosity reaching several orders of magnitude below the global average. Accounting for 3-D viscosity variability in glacial isostatic adjustment (GIA) simulations has been shown to impact the predicted spatial rates and patterns of crustal deformation, geoid, and sea level changes in response to surface ice loading changes. Uncertainty in the viscoelastic structure of the solid Earth remains a major limitation in GIA modeling. To date, investigations of the impact of 3-D Earth structure on GIA have adopted solid Earth viscoelastic models based on global- and continental-scale seismic imaging, with variability at spatial length scales > 150 km. However, regional body-wave tomography shows mantle structure variability at smaller length scales (~50–100 km) in central West Antarctica. Here, we investigate the effects of incorporating smaller-scale lateral variability in upper mantle viscosity into 3-D GIA simulations. Lateral variability in upper mantle structure at the glacial drainage basin scale is found to impact GIA model predictions for modern and projected ice mass changes, especially in coastal regions that undergo rapid ice mass loss. Differences between simulations adopting upper mantle viscosity structure inferred from regional- versus coarser continental-scale seismic imaging are large enough to impact the interpretation of crustal motion observations and reach up to ~15 % of the total predicted sea level change during the instrumental record. Incorporating a strong transition from lower viscosities at the mouth of the Thwaites and Pine Island glaciers to higher viscosities in the interior of the glacier basins results in a ~10–20 % difference in predicted sea level change in the vicinity of the grounding line over the next ~300 years. These findings have a range of implications for the interpretation of geophysical observables and improving constraints on feedbacks between the West Antarctic Ice Sheet and the solid Earth.
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Status: open (until 22 Dec 2024)
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RC1: 'Comment on egusphere-2024-2957', Anonymous Referee #1, 06 Dec 2024
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“The impact of regional-scale upper mantle heterogeneity on glacial isostatic adjustment in West Antarctica” by Erica M. Lucas, Natalya Gomez, and Terry Wilson
In this study the authors investigate lateral heterogeneity in upper mantle viscosity below West Antarctica. They compare three different viscosity profiles based on three different seismic models, two that are able to distinguish higher resolution features. They use the viscosity profiles to investigate the effect on GIA in the region using three ice histories: extended modern, modern and future. The results are compared to GPS vertical and horizontal rates. The main findings are that there is up to 20% difference in sea-level change near the grounding line of Thwaites and Pine Island glaciers over the next 300 years. Showing that including higher resolution 3D variations in upper mantle viscosity is important for these crucial and highly dynamic areas.
General Comment:
Overall, the paper is interesting, and the science is solid. However, my main concern is that the impact is lost in the sheer length of the paper, with already a lot of figures in the supplementary material. My comments are geared towards simplifying this paper to allow the science to be more impactful.
Specific Comments:
- Scales: Abstract line ~18 and introduction. It becomes a bit confusing with the mention of scales here. You mention continental, regional, local scale, and then >150km, 50-100km, and 50-200km separately. It would be useful to clearly define the spatial scale in km of each of the continental, regional and local scale – perhaps lines on 78 and 80. Furthermore, “local scale” is then not mentioned in the results – they are either CONT or REG. Is the regional scale actually representative of regional and local combined? Please clarify and simplify. Line 86, again clarify the scale in km.
- Section 2.1: I think this section is too long.
- 1D model – This is introduced as if there will be lots of comparisons between the 1D and 3D, but I can only see it in Figure 6 of the main paper. This is fine, and I’m more interested in the differences between the 3D models anyway, but perhaps state on line 132 that most 1D results are in supp info.
- Line 162: why is this 1D viscosity profile different to that mentioned earlier? It is a global average? If so, I would suggest to delete the end of the sentence “consistent with previous GIA modelling…. In west Antarctica” as this makes it sound like previous modelling shows UM viscosity in West Ant to be 5x10^20
- Line 175 to 181: Suggestion - I think you can simplify this to: “…47degS (Fig 1a). [delete the next bit] Global viscosity variations are estimated from GLAD because it offers improved structure compared to S326ANI that was used in Gomez 2024”
- Paragraphs starting line 217 and 234: I think most of this detail should be moved to supp info, particularly (all of?) the second paragraph.
- I’m interested to know how you join the regional models into the ANT20 model – in terms of the edge effects. Do you have to do smoothing where these two datasets are joined together? Or are they referenced to the same background model so its not an issue? Perhaps include in the supp information with (d).
- Line 294: “error” feels a bit harsh. Change to limitations? Maxwell is still valid! Could you return to this in the discussion – any idea what might the impact be if the experiments were repeated with a transient viscosity?
- Section 2.2:
- just on terminology here – modern/extended modern ICE-25/ICE-125. Later on ICE-125 is also referred to as “modern ice mass change” e.g. section 3.2 title. Either change “modern” to “extended modern” where referring to ICE-125, or state that there are two version of modern in this section, and then be clear which modern you’re talking about.
- Figure 2 caption – some of this description is repeating what is in the main text on how the ice histories are constructed. Not needed in the caption. Panel b – over what area or drainage basic is this calculated?
- Section 4.1: comparison to horizontal rates. I question the inclusion of comparing to horizontal GPS rates (line 656 to 685; Figure 7b and c). As the authors state, the horizontal GPS are not corrected for any kind of background GIA rate, and so already the comparison between the model and GPS observed rates in 7b is not going to match. I think this whole section could be removed or put into supplementary information.
Minor Comments:
Line 51: Change to “Accurate modelled predictions” since empirical estimates of GIA do not rely on Earth properties.
Line 111: remove “the” > according to Wan
Line 175: put the transition zone depth in brackets here to help the reader, and also for the Moho on line 216
Line 194: something has happened with the end of this sentence – “how to correct”.
Figure S1: the scale of 200km looks like a label, a bit confusing as I thought I was looking at two slices at 200km depth
Line 388: modern or extended modern?
Line 416: Super picky – but can you change the reference to Fig 3a where PSK is also labelled? To save a lot of going backwards and forwards to different figures.
Lines 541-551 and the next paragraph: I’m not sure I follow here. Does “higher relative sea level” mean “less sea level fall”? So the CONT model predicts more sea level fall than the REG models? Might be worth just clarifying here and relating to the colours on the graph.
Figure 6: Is this figure really needed?
Figure 7: The symbols are hard to see because they are so small. In fact, the statement on line ~650 “introducing regional variations in viscosity reduces differences between predicted and observed crustal rates at a number of sites” is really hard to see in this figure. It looks like the opposite is true. I wonder if REG_P ICE-25 should be removed, REG_S colour could be changed to group it in with REG_P, leaving CONT and 1D in blue/green.
Section 4.1: Is there also an elastic correction made to the GPS rates for SMB?
Line 716: What is this “O” in O(100m)? Please define
Line 720: alter the amount of uplift by 20% impacting the strength of the feedback…. Which way does this go – does it increase or decrease the strength of the feedback, i.e., is it stabilising or destabilising/less stabilising by including regional scale 3D variations in viscosity?
Code availability: It’s great to see that this code is on its way to being made publicly available.
Citation: https://doi.org/10.5194/egusphere-2024-2957-RC1
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