Ocean-Induced Weakening of George VI Ice Shelf

Zinck, Ann-Sofie P.; Wouters, Bert; Jesse, Franka; Lhermitte, Stef

doi:10.5194/egusphere-2025-573

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https://doi.org/10.5194/egusphere-2025-573

Preprints

19 Mar 2025

| 19 Mar 2025

Ocean-Induced Weakening of George VI Ice Shelf

Ann-Sofie P. Zinck, Bert Wouters, Franka Jesse, and Stef Lhermitte

Abstract. Channelized basal melting is a critical process influencing ice shelf weakening, as basal channels create zones of thinning and vulnerability that can potentially lead to ice shelf destabilization. In this study, we reveal and examine the rapid development of a channel within the George VI Ice Shelf's extensive channelized network, characterized by a 23 m surface lowering over a nine-year period. We study changes in ice flow, ocean circulation and heat potential as possible drivers behind the channel, under the hypotheses that it is either a fracture, a basal melt channel, or a combination of the two. Our findings show that the onset of this channel coincides with significant changes in ocean forcing, including increased ocean temperatures and salinity, that occurred during the 2015 El Niño Southern Oscillation event. Modelling of basal melting further suggests that channel re-routing has taken place over this nine-year period, with the channel serving as a basal melt channel in the latest years. We further observe subtle shifts in ice flow indicative of fracturing. Our findings thus indicate that this channel likely contributes to the weakening of an already thin ice shelf through a combination of basal melting and fracturing. These findings offer insight into how similar potentially destabilizing processes could unfold on other Antarctic ice shelves. Monitoring the evolution of this channel and its impact on ice shelf integrity will be critical for understanding the mechanisms of ice shelf retreat, especially on heavily channelized ice shelves.

Received: 07 Feb 2025 – Discussion started: 19 Mar 2025

Competing interests: At least one of the (co-)authors is a member of the editorial board of The Cryosphere. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Journal article(s) based on this preprint

10 Nov 2025

Ocean-induced weakening of George VI Ice Shelf, West Antarctica

Ann-Sofie P. Zinck, Bert Wouters, Franka Jesse, and Stef Lhermitte

The Cryosphere, 19, 5509–5529, https://doi.org/10.5194/tc-19-5509-2025,https://doi.org/10.5194/tc-19-5509-2025, 2025

Short summary

Ann-Sofie P. Zinck, Bert Wouters, Franka Jesse, and Stef Lhermitte

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-573', Anonymous Referee #1, 31 Mar 2025

Please refer to my review report as the attached.

Citation: https://doi.org/10.5194/egusphere-2025-573-RC1
- AC1: 'Reply on RC1', Ann-Sofie Priergaard Zinck, 04 Apr 2025
  
  Please see the attached PDF
  
  Citation: https://doi.org/10.5194/egusphere-2025-573-AC1
RC2:
'Comment on egusphere-2025-573', Anonymous Referee #2, 05 May 2025

Please see attached file

Citation: https://doi.org/10.5194/egusphere-2025-573-RC2
- AC2: 'Reply on RC2', Ann-Sofie Priergaard Zinck, 16 Jun 2025
  
  Please see the attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2025-573-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-573', Anonymous Referee #1, 31 Mar 2025

Please refer to my review report as the attached.

Citation: https://doi.org/10.5194/egusphere-2025-573-RC1
- AC1: 'Reply on RC1', Ann-Sofie Priergaard Zinck, 04 Apr 2025
  
  Please see the attached PDF
  
  Citation: https://doi.org/10.5194/egusphere-2025-573-AC1
RC2:
'Comment on egusphere-2025-573', Anonymous Referee #2, 05 May 2025

Please see attached file

Citation: https://doi.org/10.5194/egusphere-2025-573-RC2
- AC2: 'Reply on RC2', Ann-Sofie Priergaard Zinck, 16 Jun 2025
  
  Please see the attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2025-573-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (further review by editor and referees) (17 Jul 2025) by Jan De Rydt

Dear Ann-Sofie and co-authors,

Thank you for submitting your replies to the reviewer reports. Reviewer 1 (R1) has raised a number of important concerns, and has not recommended your work for publication in its current form. Reviewer 2 (R2) has requested minor revisions, although some of their comments echo the concerns from R1. Based on my own reading and your replies, I would like to provide you with the opportunity to submit a revised manuscript. To help you target your efforts, I suggest focussing on the following aspects:

1. Timeseries

You aim to explore the causal relationship between channel emergence and various external factors, such as ocean forcing. However, without a fully diagnostic model that resolves the relevant processes at the appropriate spatial and temporal scales (as noted by R1), it will not be possible to definitively demonstrate causality. To avoid unrealistic expectations for the reader, I suggest to remove or add nuance to claims in this regard. That said, I think you can provide a more robust analysis of possible correlations. I feel that the concerns raised by both reviewers likely stem from a lack of robust analysis of these correlations. This can be addressed by making more effective use of the time series data you already have. In particular, to assess the relationship between channel emergence and ENSO variability, consider plotting the ENSO index (as you allude to in your response to R2) against MITgcm melt rates averaged over a region encompassing the channel. If ENSO—and its associated impacts on ocean circulation and heat/salt transport—plays a role, this should be evident in the melt rate variations, which are more directly relevant to ice-ocean interactions than heat/salt or flow fields. It is then important to demonstrate confidence in the MITgcm-derived melt rates and their variability, which you can do by comparing them with the independent BURGEE (and potentially, LADDIE) melt estimates. Do the latter datasets exhibit similar variability linked to ENSO? While you correctly note the difficulty in directly comparing 2D melt rate patterns due to topographic advection, you can still examine whether broader-scale averages show consistent changes tied to large-scale climate variability. The BURGEE and LADDIE time series will be relatively short, but the ENSO index and MITgcm data span a much longer period (e.g., from 1979 onward). This extended record allows you to evaluate whether the 2015 ENSO event stands out against the background of long-term variability. Doing so may strengthen your argument for why the channel emerged in 2015 rather than, for example, during earlier ENSO events. This would strengthen the message of the paper.

2. Spatial maps

Some aspects of the story are best illustrated using spatial maps. However, in line with R1's comments, I am not convinced that maps of MITgcm velocities at fixed depths for two different years provide meaningful insights. Simulated ocean currents at multi-kilometer scale exhibit substantial variability across a wide range of timescales, and comparing two yearly-averaged snapshots does not offer information about the significance or persistence of the simulated differences between both. What is more relevant for ice–ocean interactions are the melt rates. This is why I recommend focusing on time series of MITgcm-derived melt rates rather than on heat, salinity, or current speed. Additionally, it is important to remember that the observed channel is not stationary; it advects downstream over time and may encounter varying oceanic conditions along its path. These changing conditions could influence plume–ice interactions and potentially lead to re-routing of the channel, without the need to invoke changes in large-scale ocean conditions linked to climate variability.

The other key aspect of the study (fracture or not?) is the correlation between channel formation and changes in ice surface velocity. Again following R1, I am not convinced that your current method of calculating velocity divergence across the channel captures the full complexity of the evolving flow field. To better analyse this, I suggest presenting 2D maps of effective strain rates—or principal strain rates—derived from the surface velocity fields at different times before and after the channel’s formation. In fact, the BURGEE method likely performs a similar analysis internally via the divergence term ∇⋅u in the conservation equations, as R2 noted. These strain rate maps will allow you to separately analyse across-channel (tensile) and along-channel (shear) deformation patterns. To reduce noise and obtain more reliable derivatives, it is common to apply a local quadratic (or cubic) regression to the velocity field over a fixed-size spatial window (see, for example, Fig. 7b in De Rydt et al. (2018) and many other references in the literature).

3. Disparity of scales

One possible way to address R1’s concerns about scales is to use MITgcm output in a aggregated, large-scale manner—specifically, to assess the influence of climate variables on melt rates using time series, as suggested above, rather than presenting spatially detailed maps. In this context, LADDIE serves as a complementary tool to MITgcm, as it enables investigation of smaller-scale processes. You make a convincing case that melt rates could be elevated within the channel. However, it is important to acknowledge that ocean dynamics and melt physics in narrow channels with steeply sloping sidewalls (exhibiting ageostrophic flow—shown by e.g. Millgate et al.; https://doi.org/10.1002/2013JC009402) are fundamentally different from those in broad channels with gentle slopes (geostropich flow—where LADDIE has previously been shown to perform well). Apart from the limited resolution, LADDIE may not capture key processes in the steeper, more complex geometries. It would strengthen the manuscript if you could clarify these limitations of more directly. Similarly, it would be helpful to reiterate the limitations of the BURGEE approach for calculating basal melt rates at spatial scales smaller than a few ice thicknesses, where the hydrostatic assumption may not be valid.

Taking into account the reviewer comments and my additional notes above, I invite you to submit a revised version of your manuscript. If you need more time than the standard revision period then please don’t hesitate to contact the editorial office to request an extension.

All the best,
Jan De Rydt – topical editor

Hide

AR by Ann-Sofie Priergaard Zinck on behalf of the Authors (05 Sep 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (09 Sep 2025) by Jan De Rydt

RR by Anonymous Referee #1 (19 Sep 2025)

ED: Publish subject to minor revisions (review by editor) (22 Sep 2025) by Jan De Rydt

AR by Ann-Sofie Priergaard Zinck on behalf of the Authors (22 Sep 2025) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (23 Sep 2025) by Jan De Rydt

AR by Ann-Sofie Priergaard Zinck on behalf of the Authors (26 Sep 2025) Manuscript

Journal article(s) based on this preprint

10 Nov 2025

Ocean-induced weakening of George VI Ice Shelf, West Antarctica

Ann-Sofie P. Zinck, Bert Wouters, Franka Jesse, and Stef Lhermitte

The Cryosphere, 19, 5509–5529, https://doi.org/10.5194/tc-19-5509-2025,https://doi.org/10.5194/tc-19-5509-2025, 2025

Short summary

Ann-Sofie P. Zinck, Bert Wouters, Franka Jesse, and Stef Lhermitte

Data sets

Dataset belonging to the article: Ocean-Induced Weakening of George VI Ice Shelf A. P. Zinck, B. Wouters, F. Jesse, and S. Lhermitte https://doi.org/10.4121/dbf9ade9-9f85-49f4-89ba-3d8d8310c9e4.v1

Ann-Sofie P. Zinck, Bert Wouters, Franka Jesse, and Stef Lhermitte

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Nov 2025	12	0	12

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Short summary

Ocean-driven basal melting of ice shelves can carve channels into the ice shelf base. These channels represent potential weak areas of the ice shelf. On George VI Ice shelf we discover a new channel which onset coincides with the 2015 El-Nino Southern Oscillation event. Since the channel has developed rapidly and is located within a highly channelized area close to the ice shelf front it poses a potential thread of ice shelf retreat.


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Ocean-Induced Weakening of George VI Ice Shelf

Journal article(s) based on this preprint

Interactive discussion

Interactive discussion

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Journal article(s) based on this preprint

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