| 29 Jul 2026
Evolution of basal terraces in the cold-cavity of Ekström Ice Shelf in East Antarctica
Abstract. Basal melting beneath Antarctic ice shelves drives ice-shelf thinning, impacts buttressing of inland ice streams, and contributes to grounding line retreat, thus affecting the rate of global sea-level rise. Yet, processes controlling the spatial variability of basal melt remain poorly constrained because observations of the evolving basal topography are sparse. One important but poorly understood feature are basal terraces. Here, we present a ground-penetrating radar (GPR) dataset imaging the evolution of basal terraces in the cold-water cavity beneath Ekström Ice Shelf. The quasi three-dimensional GPR data are complemented by an autonomous phase-sensitive radio echo sounder (ApRES) record and airborne radar data. No significant changes in the basal topography are observed between the two field seasons. Basal melt rates at the terrace roofs are less than a meter per year and lower than the regional average, ApRES-derived monthly variability ranges between 0.3 and 0.6 m a-1. A weak off-angle reflector suggests that melt rates at the terrace walls may be higher, but not higher than 4 m a-1. Overall, basal terracing occurs predominantly near the grounding zone, and the ice-ocean interface becomes smooth further offshore. We conclude that basal terraces occur beneath both warm- and cold-cavity ice shelves and that the low melt rates at terrace roofs are consistent with previous studies suggesting that a stratified ocean layer shields the ice base from oceanic heat transport. However, melt rates at the terrace walls are also low. Therefore, our results suggest that once basal terraces are created near the grounding zone, they may enter a stagnant mode and subsequently advect with the ice-shelf flow towards the ice edge where they eventually disappear.
Competing interests: At least one of the (co-)authors is a member of the editorial board of The Cryosphere.
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Review:
EGU-2026-1396: Evolution of basal terraces in the cold-cavity of Ekström Ice Shelf in East Antarctica
Overall Statement:
The submitted manuscript presents a nice analysis of some very high quality and exciting radar field observations that result from well-designed experiments. These findings are novel due to the presence of repeat observations, yielding insight into the temporal evolution of basal terraces. Overall, the text is well-written and clear, except for some minor areas that could use rewriting. We believe that there should be some changes regarding the interpretation of the results in the final discussion, given the limitations of the data. This manuscript will require minor revisions before it can be considered for publication in The Cryosphere. It was a pleasure to read and will certainly make for a significant contribution to the research fields concerned with ice shelves and ice-ocean interactions.
-Peter Washam and Veronica Hegelein
Larger Comments:
The approach to backing out the advection between the two radar grids in Li 144 – 159 is fairly convincing, but it would benefit from a comparison to a satellite-derived surface velocity field to increase confidence further. I realize that in Li 160 – 163 the authors state that they are unable to calculate velocity gradients due to the small domain area, but I would be surprised if velocity products down to ~30 m or so are not available. This is a relatively minor point, but I think it is necessary to strengthen the manuscript. See comment below on Fig.1 about adding velocity color map.
The Discussion Section can be strengthened by a comparison of the size of observed terraces at Ekström to those on Petermann and Pine Island (Dutrieux et al., 2014), Dotson (Wåhlin et al., 2024), Thwaites (Schmidt et al., 2023), and Roi Baudoin (Drews et al., 2017; Koch et al., 2023; Zhou et al., 2025). These sizes should then be discussed relative to the time spent in the ocean.
A discussion of the width of the terraces, as well as their height should be added to the text. This will help further contextualize how they evolve and the relative influence of vertical and lateral melting.
Section 4.1: Switching between ‘90 km downstream’ to ‘40 km from the ice-shelf front’ in the same paragraph is a bit confusing. How do these points compare to each other? Be consistent with where you’re measuring from or provide both options.
Section 5.3: I think there might be too much emphasis placed on the off-nadir melt rate based on the uncertainty provided for the source. A better way to frame this portion would be to present that there is a reflection, provide possible explanations for it, then to put forth the calculated melt rates as one of the several explanations. Finally, conclude with the discussion of how it compares to expectations or if there are potential flaws in the interpretation.
Specific Revisions:
Title:
This is exceedingly minor, so feel free to disregard this comment, but I suggest changing the title to the following, to increase readability: “Evolution of basal terraces in the cold ocean cavity of Ekström Ice Shelf, East Antarctica”
Abstract:
Li 1: Suggest to reword to increase readability: “Basal melting thins ice shelves”
Li 4: Suggest to add a general description of what a basal terrace is to the end of this sentence.
Li 5: Suggest to change to : “cold ocean cavity beneath Ekström Ice Shelf.”
Li 5: “quasi three-dimensional” is confusing here. I suggest to remove “quasi”
Li 13: I think that it is not surprising that the melt rates are low on this terrace wall, due to the cold ocean cavity, but they are potentially 6.5 – 13x (an order of magnitude) higher than the roof melt rate. I will continue to read on to properly formulate my comments on this, but I think that this potentially large discrepancy in melting deserves mentioning in the abstract.
Introduction:
Li 16: This is an awkwardly worded sentence. I suggest to rewrite it as follows to increase readability:
“Ice shelves are the floating extensions of grounded glaciers and ice streams that buttress the flow of inland ice towards the ocean.”
Li 16 – 20: Suggest to amend this sentence to increase its precision as follows:
“Ice shelf thinning…” or “grounding line retreat”
Li 19: Suggest to increase accuracy of text: “accelerating grounded ice discharge into the ocean”
Li 20 – 21: What other options are there to remove mass from an ice shelf? Surface melting and sublimation? I think this sentence should be reworded to make it more meaningful by stating what is the dominant means of ice shelf mass loss in Antarctica. This is presumably basal melting in the warm ocean cavities and equivalent between melting and calving in the cold ocean cavities.
Li 24 – 26: Please rewrite this sentence as follows to improve readability and accuracy:
“Basal melt rates are controlled by temperature and salinity gradients between seawater and the ice shelf base, as well as the balance of mixing and stratification that results from kinetic energy in the water column and freshwater released by melting, respectively (CITES)”
Li 27 – 29: I realize that this is a study focused on small-scale melt rate variability and ice shelf basal topography, but there needs to be some discussion of Circumpolar Deep Water or Warm Deep Water, shelf break transport, wind-driven gyres, and polynyas (or some combination of these factors) if there is going to be a discussion of what forms a warm or cold ocean cavity beneath an ice shelf.
Li 29 – 30: Provide a reason for this variability, e.g., seafloor topography, grounding line depth, etc…
Li 30 – 33: If there is going to be discussion of channels and terraces, then it also makes sense to discuss basal crevasses.
Li 34 – 36: Please provide a more detailed description of what a terrace is shaped like. I am looking for something along the lines of “stair case or step like, with flat roofs and steep sidewalls…”
Li 30 – 38: Please also include a description of basal crevasses and then mention that terraces can occupy the flanks of channels and crevasses.
Li 39 – 50: Dutrieux et al. (2014) provided estimates of melt rates on the terrace roof and sidewalls using repeat-pass GPR from Petermann Glacier. This needs to be summarized in this paragraph. There should also be a discussion of the current physical hypothesis for why terraces melt differentially, which is the stabilizing effect of buoyant meltwater beneath a flat ice base versus the destabilizing effect of a rising meltwater plume along sloping ice.
Li 49 – 50: Basal terraces were shown on Petermann Glacier in Dutrieux et al. (2014) before Rignot et al. (2025b). Please add this citation here.
Li 51 – 53: What about the radar measurements from Dutrieux et al. (2014)? Also, there is the following text nestled deep into the Methods of Schmidt et al. (2023):
“The best-fit melt-rate estimates are: region 1: 3.41 m year −1 versus 3 m year −1 observed; region 2: 4.80 m year −1 (T1)and 4.65 m year −1 (T2) versus 5 m year −1 observed; region 3: 2.37 m year −1 versus 2 m year −1 observed.”
“the ApRES at the borehole measured lateral melt rates of 70 m year −1 on the terrace wall (mean slope 79°) at 1,800 m along T1,”
Both of these observations show higher melt rates on terrace sidewalls than roofs
Study Area:
Li 60 – 62: Please rewrite this sentence following this structure to improve readability:
“The ice shelf extends XX km seaward of the grounding zone and experiences modern day velocities ranging between 100 and 230 m a-1. Which results in advective time scales for the ice of 800 a.”
Li 64 – 65: See above comment about including discussion of Circumpolar Deep Water and Warm Deep Water in the Introduction.
Li 69: Please provide a reason for the cold ocean. Is it because of a polynya? The -1.9°C temperature is the surface freezing point of seawater, which suggests that it is set by sea ice formation in a polynya.
Li 76 – 77: It is unclear to me what this sentence is attempting to communicate.
Data and methods:
Li 79 - 84: I recommend citing Fig. 1 in this opening paragraph.
Li 87: Suggest to change to increase readability: “We collected three types of profiles (Figure 1)”
Li 91: Awkward wording. Suggest to change to: “the initial 60 km were collected in 2021/22 and the final 40 km were completed in 2022/23”
Li 123: Figures must appear in the text in order of their presentation. Fig. 4 is referenced here before Figs. 2 or 3. This is incorrect and should be changed.
Li 128: Typo. Change “traveltime” to “travel time”
Li 127 – 128: Provide a reference that supports the decision of this radio wave velocity through ice.
Results:
Li 170 – 175: See comment above about adding basal crevasses to the Introduction Section.
Li 177: Typo. Change to “ice-ocean”
Li 178 – 179: Suggest to cite Fig. C1 in this sentence.
Li 179 – 181: Suggest to improve the accuracy of this sentence by adding in the distances from Fig. 2 for “near the grounding zone, mid shelf, and farther offshore.” Also, I suggest removing “uniformly smooth,” because there are still some bumps and wiggles in Fig. 2d seaward of 90 km. There are also smaller-scale roughness features that the radar will not be able to observe, down to the morphological scale.
Li 182 – 191: Suggest to add a description of the terraces near the grounding zone to this description. See comment below on adding a subplot to Fig. 3, as well.
Li 182 – 191: This section would benefit from a proper description of the shape of a terrace in the Introduction Section. See comment above.
Discussion:
Li 227 – 228: Typo. Change to: “provide a consistent picture at Ekström Ice Shelf, basal terraces…”
Li 236: Typo. Change to “affects”
Li 236 – 237: I do not believe this is the mechanism put forward for the formation of terraces in Wåhlin et al. (2024). Please go back and double check this.
Li 237: settings should be setting
Li 237 – 239: The wording of this sentence is confusing, as it is hard for the reader to understand whether the Dotson data or the Ekström data suggest a localized formation process for terraces. Also, there is no oceanographic data used in this manuscript, so I suggest to be careful when making inferences about the oceanographic setting without having access to data from the ocean.
Li 240 – 254: This is a nice description of how terraces might evolve at Ekström Ice Shelf. I think there should be some description of the uncertainty in melt rates along the central flow line for Moss et al. (2025). I find it surprising that there is minimal melt rate variation along the flow line, given the steeper slope around distance 10 – 30 km in Figure 3a, relative to seaward from there.
Li 301 – 311: This interpretation of the results does not take into context the coarse spacing of radar lines (100 m) relative to the possible lateral melt rate of a terrace sidewall over one year (3.95 – 4.35 m). Furthermore, it does not comment on how the processing of the radar data to 2.5 or 5 m might influence the interpretation. Essentially, I think that the authors need to take into further consideration the limitations of their data and rewrite this paragraph accordingly.
Conclusions:
Li 317: Same as the above comment. I suggest to not use the phrase “uniformly smooth”
Li 319: The phrase “not controlled primarily by far-field ocean thermal forcing” is vague. Please rewrite this to state simply that it “does not relate directly to the degree of ocean thermal forcing”, or something similar.
Figures and Tables:
Fig. 1: A color map of velocity over the ice shelf from ITS LIVE or something similar would increase the value of this figure, especially since there is some discussion of ice velocities as part of the Introduction and Methods Sections.
Fig. 3: Suggest to add a subplot that focuses on the near-grounding zone region as a line plot. This will help the reader better interpret the 3-d geometry in Fig. 4. Subplot b would benefit from a trendline to help the reader visualize the decrease in roughness.
Fig. 3c-e: This is a minor comment, but I think these panels should be filled in with the same color for the ice and ocean as Fig. 3a.
Fig. 4: This figure would benefit from a northward arrow, if the text is going to refer to east and west, as there is currently no geographic reference
Fig. 4c-f: The color map for these subplots should be reversed, where red is high change and blue is low change. The melt rate scale should be flipped, so that positive melt rates are on the top and negative melt rates are on the bottom.
Fig. 5: This figure would benefit from a northward arrow, if the text is going to refer to east and west, as there is currently no geographic reference.
References:
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