the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Nov 2025
Radiostratigraphy and surface accumulation history of the Amundsen-Weddell Ice Divide, West Antarctica
Abstract. Recent ground-based radio-echo sounding (RES) surveys across the Ellsworth Subglacial Highlands (ESH), a topographically complex region near the Amundsen–Weddell ice divide, reveal new insights into Holocene accumulation and ice dynamics in West Antarctica. We traced seven internal reflection horizons (IRHs) across approximately 2000 km of RES data spanning a 13,000 km2 area in the upper catchments of Pine Island Glacier, and the Rutford and Institute Ice Streams. Two of these IRHs intersect dated airborne radar lines tied to the WAIS Divide 2014 ice-core chronology. Applying the Dansgaard–Johnsen model with local accumulation rates from stake measurements and satellite-derived value we estimated ages of up to ~ 17.6 kyr for the deepest horizon (IRH7). Internal stratigraphy is well preserved in the slow-flowing alpine terrain of the ESH but becomes disrupted in areas of relatively fast flow and tributary convergence, such as the southern Ellsworth and CECs troughs. Despite these localised disturbances, IRHs remain traceable across most of the region, highlighting the potential for radiostratigraphic continuity in complex settings. Modern and Holocene accumulation patterns reveal a persistent asymmetry across the AWID, with consistently higher accumulation in the CECs Trough, supporting long-term ice divide stability since at least the mid-Holocene. Our study extends the spatial coverage of dated radiostratigraphy in West Antarctica and provides new linkage between the Weddell and Amundsen Sea Embayments. These results support several core goals of the AntArchitecture initiative of the Scientific Committee on Antarctic Research, including the expansion of a continent-wide IRH framework, improved Holocene accumulation reconstructions, identification of palaeoclimate archive targets, and enhanced boundary conditions for numerical ice-sheet models.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4670', Ella Wood & Tun Jan Young (co-review team), 10 Dec 2025
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RC2: 'Comment on egusphere-2025-4670', Joseph MacGregor, 16 Jan 2026
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.
Citation: https://doi.org/10.5194/egusphere-2025-4670-RC2
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- 1
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
Specific comments
L7
Satellite derived values
L13
New linkages or a new linkage
L19
Revealed significant and ongoing mass loss - no a?
L22
“a-1” à a-1”
L23
Geological records indicating
Figure 1
Suggest visualising the locations of the three sites of study (potentially here or in Figure 5): CECs Trough, (Ellsworth?) Subglacial Highlands and Alpine Terrain, and Ellsworth Trough. These sites are included in Figure 2 but it is difficult to georeference these locations alongside physical features, in particular the Ellsworth Trough which is not actually pinpointed.
L129
“internal layer continuity index”
L132
“as a consequence of—for example—topography…”
L136-7
Quantitatively constrain what defines the “upper part” and the “lower end” of the ice column—from Eq. 1 this is defined as the surface to 200 m and 400m to the bed, but then Figure 3 suggests the lower bounds to be much deeper. Regardless—these definitions need to be much more explicit and clear.
L184
Suggestion to elaborate on the constraints in which Nye-style modelling was found to yield comparable results to D-J modelling. Such conditions are implicitly given in L192-193 but could be made much more explicit.
L192-202
This paragraph seems to repeat several points multiple times—it could be condensed to be much more precise and succinct.
L248-251
A remnant clause still exists in this paragraph that will need to be removed.
L263
Equation reference to be pointed to Eq. 7 (I think, but am not sure). Potentially some additional explanation needs to be provided as to exactly how this specific uncertainty component has been defined and propagated.
L303
Suggest also providing additional offsets between IRH4 and H2, or if H2 and R2 are equivalent, then make this explicit.
Figure 7
Suggest to mark IRHs on this figure. Figure caption mentioned a red arrow in panel f that doesn’t appear in the figure.
Table 3
Could include headings for much better signposting to indicate the upper and lower bounds for each IRH and area. Are the alpha values representing snow-equivalent or (as referenced in L402) ice-equivalent depth-rate estimates? For L402, is this then the midpoint between the bracketed alpha values shown in Table 3?
L403-4
I am unsure as to how matching IRH4 to the equivalent age-depth reflector in Bodart et al. (2021) will produce accumulation rates—would this not be determined independently through the D-J model?
L427
“At least two” As far as I am aware, only two reflectors (IRH4 and IRH6) were discussed, so I am not sure there were more than two that were actually of note.
L473
Typo Arthem et al. (2006)
L476
Statement is quite vague “rates vary over short distances”. Can you quantify magnitude and distance in some way to make this statement more explicit.
L486
Consider including reference for broader WA climate trends.
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.