Review: Napoleoni et al. Radiostratigraphy and surface accumulation history of the Amundsen-Weddell Ice Divide, West Antarctica

Manuscript #: egusphere-2025-4670

Ella Wood and TJ Young

This manuscript extends the age-depth radiostratigraphy of the Amundsen-Weddell Ice Divide through the incorporation of 2000 km long of Centro de Estudios Cientificos (CECs) ground-based RES survey. In this dataset, seven internal reflection horizons dating to the last ~17.6 kyr were traced of which two were linked to wider mapped radiostratigraphy datasets in this region.

Overall, this paper is well written and clearly structured with nicely presented figures showing interesting findings on the accumulation history of the Amundsen-Weddell Ice Divide. We have minor comments to improve the clarity of certain points and emphasise and contextualise the significance of the findings.

General comments

• In general, the contextual scope of the manuscript is currently positioned between a paper describing a dataset and an analysis of the larger flow stability of the region—these were defined in the study aims (L74-75). Though the former aim was clearly accomplished and detailed, the latter aim was only discussed in a broad scope in Section 4.2, with regards to the overall lack of disturbance of ice flow in the wider region of study. We suggest redefining the secondary aim to be more focused and reframing Section 4.2 in terms of tying in where this study extends in interpretation from the previous work done in the region.
• Though the statement of a stable ice divide is indeed evidenced across multiple past studies (an addition to the list would be Conway and Rasmussen 2009 for WAIS Divide), there is little mention of how this study extends / expands beyond the understanding of complex flow dynamics present in certain sections of the regions, particularly at the boundaries of the Institute-Moller ice streams. Here, there will be benefits in contextualising the study and positing interpretations where appropriate in relation to previous work done in the region (in particular Siegert et al. 2019, Ross et al. 2020). Potentially, addressing this point will also in part address comment (a).
• For someone less familiar with the region, the use of the CECs abbreviation was at times confusing in that it was sometimes referring to the region and sometimes to the source of your radar data. It would be beneficial to add a sentence early on to clarify these differences. Occasionally the final S in CECS is also capitalised (line 84, figure 1 caption), I assume this is a typo. The reference to the Subglacial Lake CECs (SLC) was slightly confusing, consider adding a short statement to clarify that the lake has been previously identified and also why it is referred to as CECs.
• Figure 8 is only very briefly mentioned in the text, perhaps this figure doesn’t need to be in the main text body or could be discussed more detail.
• Paragraph (L385) here you talk about high and low accumulation scenarios. This is not previously mentioned, please provide further information to clarify what is meant by this and why contrasting high / low scenarios are relevant here.
• Table 3 showcases significantly larger minimum / maximum bounds for Area 3, especially in comparison with bounds of corresponding IRHs in the other two Areas. Although it is understandable that the lowest IRH (7) will have substantially large bounds, there is scope to contextualise why these errors are so much larger in the discussion.

Specific comments

References

Conway H and Rasmussen LA (2009) Recent thinning and migration of the Western Divide, central West Antarctica. Geophysical Research Letters, 36(L12502). doi: https://doi.org/10.1029/2009GL038072

Siegert, M.J., Kingslake, J., Ross, N., Whitehouse, P.L., Woodward, J., Jamieson, S.S.R., Bentley, M.J., Winter, K., Wearing, M., Hein, A.S., Jeofry, H. and Sugden, D.E. (2019). Major Ice Sheet Change in the Weddell Sea Sector of West Antarctica Over the Last 5,000 Years. Reviews of Geophysics, 57(4), pp.1197–1223. doi: https://doi.org/10.1029/2019rg000651.

Ross, N., Corr, H. and Siegert, M. (2020). Large-scale englacial folding and deep-ice stratigraphy within the West Antarctic Ice Sheet. The Cryosphere, 14(6), pp.2103–2114. doi: https://doi.org/10.5194/tc-14-2103-2020.

Review of “Radiostratigraphy and surface accumulation history of the Amundsen-Weddell Ice Divide, West Antarctica” by F. Napoleoni et al.

Joe MacGregor, NASA/GSFC

16 January 2026

Summary and general comments

This MS describes the mapping and interpretation of newly traced radiostratigraphy from two CECs ground-based surveys across an unusually important and complex portion of the Amundsen-Weddell ice divide. I wasn’t familiar with this survey but based on its location and breadth it’s arguably one of the most important Antarctic ground-based radar surveys performed to date. The methods to map and interpret the 7 traced layers are close to the state-of-the-art. The interpretation in terms of accumulation rates is straightforward but very well contextualized and its relevance is fairly described. This MS could be published virtually as is, but I think there are a couple of substantive opportunities for improvement.

I am satisfied with how the layers were traced, the QC thereof given the crossovers (Figure 8), and the matching to the already dated layers (IRH 4/6). What it took me a little while to understand in the MS, and what concerns me slightly, is the use of the Dansgaard-Johnsen model and accumulation rate measurements/models to both loosely verify the dating of those two layers and to then also date the others.

First, the range of basal shear layer thicknesses seems too narrow, given the range of ice flow conditions covered by the survey. Compare, for example, the 20-30% range used to that considered by Waddington et al. (2005, 10.1130/G21165.1) for the Siple Dome ice divide region: 25–70%, and h/H is known to vary a lot across Greenland (MacGregor et al., 2016, 10.1002/2015jf003803). I understand that Bodart 2021 used that narrow range (20–30%), but that precedent alone does not seem sufficient to justify its use here.

Second, this seems to be a real missed opportunity to use an existing method, quasi-Nye dating (MacGregor et al., 2015), to potentially better date the undated layers in a manner independent of the somewhat stronger assumptions imposed by the Dansgaard-Johnsen model. In quasi-Nye dating, the core assumption is that the vertical strain rate in the sandwich between or near a layer pair is uniform, i.e., it is not tied to the bed directly or indirectly. Given the layer depths under consideration relative to the assumed relative basal shear layer thicknesses (Figure 4), this seems like a satisfactory assumption. I would have started from the two independently dated layers and made my way through the other five that way. The code is out there; while set up for a more complex situation, it could be readily simplified to this interesting survey: https://github.com/joemacgregor/pickgui/tree/master/dating. The substantial benefit of this proposed approach is that the (hopefully) independently dated set of seven layers could then themselves be used to estimate past accumulation rates across the survey region, using e.g. the Dansgaard-Johnsen model, rather than only being able to “calibrate” accumulation rates using Dansgaard-Johnsen using the two independently dated layers.

Specific comments:

14-17: This last sentence of the abstract is a concise statement about the potential value of AntArchitecture, but it doesn’t connect effectively to the work done in this study. I suggest revising to better focus on relevant future outcomes from this study.

Figure 1: 1. In panel A, the red box and ice divides should be bolder. 2. In panel B, add a legend to define the white star rather than resorting to the end of the caption. 3. Label all panels with a summary of what the main field they are showing is, e.g., A. Regional map; B. Surface elevation, etc.

206-9: The rationale for using the accumulation-rate field from Arthern et al. (2006) needs to be improved substantially to justify its use in this study. That field is coarser than RACMO and nearly two decades old in a field of study that has evolved substantially since then. I don’t think comparison with Arthern is adding much to the MS.

232-4: A lot of work has now been done by the altimetry community to improve estimates of firn layer variability across both ice sheets. It would be good to take advantage of this to improve the estimate of the firn correction and its spatial variation, given that the rest of the methods used to estimate layer depths are at or close to state-of-the-art. For Greenland radiostratigraphy v2 (MacGregor et al., 2025, 10.5194/essd-17-2911-2025), I used the firn air thickness estimates from Medley et al. (2022, 10.5194/tc-16-3971-2022). Something similar can be done for Antarctica layer depths. This probably won’t change the layer depths and their uncertainties much, but it is a process improvement that is relatively straightforward to implement.

Figure 6: 1. In panel B, are the blue fills the *observed* depths of those subglacial lakes, or simply illustrated? If the latter, only a blue line seems appropriate. 2. The caption statements about layer drawdown over the lakes require more support to be convincing (they also don’t really belong there anyway), because layers often dip down in steep troughs, independent of the nature of the bed there.

Minor issues:

22: a-1 -> yr^{–1}

48-50: This sentence perhaps inadvertently implies that radiostratigraphic calibration of ice sheet models had *already* reduced uncertainty in future projections, which I don’t believe that it has. Suggest rephrasing.

326: What are the “subglacial gradients” referred to here? A confusing turn of phrase.

467-470: I suggest reversing the order of this sentence, as it seems to unintentionally imply that the model is more reliable than the observations.