the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Collision with Seamount Triggers Breakup of Antarctic Iceberg
Abstract. Iceberg A68a calved from Larsen C ice shelf, experienced several major calving when drifting around the South Georgia Island in late 2020. Here, we show for the first time that the decisive factor for its calving was a collision with the surrounding seamount. By treating the iceberg as a deformable body in an established ice-flow model, we show how its collision with the seafloor created huge stresses within the iceberg that led to its disintegration. The drifting and rotating of the iceberg, while grounded, further enhanced its breakup. Moving over a grounded shoal increased the tensile stresses by a factor of almost one hundred more than immobile grounding alone, and rotational motion about the pinning point increased the stresses by another twenty percent. Modeling the fracture and breakup of a large tabular iceberg is an essential step toward better understanding the life cycle of an iceberg. The possible collapse of the marine-based sectors of the great ice sheets in a warming world may lead to a massive increase in the number of icebergs in the surrounding oceans. It will be crucial to be able to understand where such icebergs drift and how they ultimately disintegrate into the ocean.
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RC1: 'Comment on egusphere-2024-2790', Anonymous Referee #1, 22 Nov 2024
In this study, Wang and co-authors investigate how a piece broke off iceberg A68 when it collided with the seafloor near South Georgia in 2020. While our understanding of iceberg-ground collisions are limited and there is a need to find ways to represent these processes in models, I don't see a clear path for this manuscript to contribute a significant advancement. Two central concerns are
(1) The documentation of the collision and resulting breakup is not new. Huth et al (2022) presented on this rather clearly, including a representation of the breakup in their modeling account, which uses a bonded-particle iceberg [I am not a co-author of that paper]. Therefore, the contribution here would be to gain fundamental insight into the physical processes of how iceberg-ground collisions drive breakup, and I don't believe this is convincingly provided. This brings me to my 2nd comment.
(2) The authors use Ua to model this collision and breakup. This is a rather befuddling choice of model, since Ua is an idealized large-scale ice flow model using a shallow shelf approximation. My understanding is that it is designed to model viscous creep of ice sheets/shelves over long time periods and at large spatial scales, not the rapid and comparatively small-scale fracture of icebergs. The choice of this model is not justified in the text, nor are potential issues with temporal or spatial resolution discussed in any way.
One way forward I can see for this study to become a valuable contribution is through validating that the Ua model can indeed meaningfully represent such fracture events. This could be done for example by comparing it to a model that represents the iceberg as an elastic/brittle object (which is the relevant rheology on the fast time scales of fracture events). This would entail a considerable amount of extra work and lead to a fundamental reframing of the study.
Citation: https://doi.org/10.5194/egusphere-2024-2790-RC1 -
AC1: 'Reply on RC1', Xianwei wang, 07 Jan 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2790/egusphere-2024-2790-AC1-supplement.pdf
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AC1: 'Reply on RC1', Xianwei wang, 07 Jan 2025
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RC2: 'Comment on egusphere-2024-2790', Anonymous Referee #2, 27 Feb 2025
This study seeks to investigate a breakup event experienced by Iceberg A68 which occurred during a collision with a seamount. They use a combination of remote sensing to identify conditions of collision and modeling to investigate the stress regimes on the iceberg during such a collision. I believe there is valuable insight to be gleaned from modeling such a collision and understanding the physical process of iceberg breakup due to interaction with bathymetry. However, the current form of the manuscript makes it difficult to pull out such physical insight and I have concerns about overlap with existing work as well as the novelty of the modeling work. I detail this further below.
[1] Overlap with current work: I believe the authors are correct in that there is more work needed to understand the process of such a collision and its influence on fracture propagation in an iceberg. However, I think the focus of the current manuscript distracts from this central point. Much of the remote sensing work, to my understanding, seems to be similar to the investigations of Braakmann-Folgmann et al. 2022 (citation below), which used satellite observations to identify the regimes of calving in Iceberg A68A and, importantly, identified the seamount collision as a likely explanation for this first breakup, which is the focus of the current manuscript. This study was not cited in line 85 where the authors discuss the potential mechanism of the first breakup and I believe there should be much more justification of the need for further observations to understand the breakup beyond what is done in the Braakmann-Folgmann et al. 2022 paper in the introduction. Without this, it’s unclear to me what new physical insight can be drawn from the authors’ remote sensing investigations.
[2] Modeling work: I am interested in the modeling investigation of the iceberg breakup, as previous modeling work (e.g. Huth et al. 2022) has not focused primarily on this first breakup. That being said, I’m not sure I understand the use of Ua as the model to apply to such a problem. Firstly, it is a 2D model, and the processes at play here seem, very importantly, to be three-dimensional. The collision with the seamount will occur at depth and allow for the vertical propagation of fractures, all of which can’t be accounted for in Ua. Since fractures occur due to the concentrated localized stresses, I’m not necessarily convinced a 2D model can shed enough insight into this problem to understand the physics. At the very least, a much more thorough description of the assumptions underlying the use of 2D model is needed here.
My next concern is the lack of a fracture model applied here. I understand that the authors are interested primarily in evaluating the intensification of the stress field and identifying regions of enhanced tensile stresses. However, I think the lack of time-dependence limits our ability to understand the whole process of breakup by only looking at the instantaneous stress field upon collision. While this may be a novel experiment, I’m not sure it’s enough to fully answer the question posed by the authors, which is a full investigation of the collision process.
I believe a modeling framework that can account for 3D stresses and perhaps one that allows for an investigation of rift propagation would be useful here.
Other comments:
- The introduction could use some organization in order to identify the central goal of the manuscript. The first paragraph discusses processes that affect icebergs and the second paragraph discusses fracture mechanics, but these are not separate things – processes such as melt, shearing of ocean currents, etc. can (and certainly do) influence fracture and breakup events. Similarly, in lines 50-53, I wouldn’t necessarily say icebergs “survive” things like shearing of ocean currents, as the Huth et al. 2022 paper shows that this likely contributes to the breakup of icebergs
- Line 54 – I would say “speeding up” rather than “hurrying up”, for precision
- Lines 69-70 – studies have been done to understand the breakup of tabular icebergs (e.g. Braakmann-Folgmann et al. 2022, Huth et al. 2022, among others)
- Line 83 – the influence of iceberg melt on ecosystems need citations
- Line 125 – I would do a quick description of the Ua model here, for those unfamiliar with it
- I would strongly recommend avoiding bodily language in scientific publications, such as describing icebergs as “giving birth” (lines 72-73, 293, 313, perhaps others). There’s other language that is easy to swap this out with (icebergs producing smaller bergs, for example) and such language relating to birth and birthing can make people uncomfortable.
- In general, I’d recommend doing a quick edit of some parts of the paper – there are typos, and I caught a few (listed below) but perhaps others I didn’t
- Line 50 – icebergs shouldn’t’ be capitalized
- Lines 80-81 – “When A68a melting in the ocean” “When A68A melts in the ocean, it cools down and dilutes…”
- Line 366 – “Ua is a two-dimension glacier model” “Ua is a two-dimensional glacier model”
- Line 384 – “more observation data” either “more observations” or “more data”?
- Lines 437-438 – “How polar cyclone and ocean tide contributing to breakup…” “How polar cyclone and ocean tides contributes to breakup…”
Citations
- Braakmann-Folgmann, A. Shepherd, L. Gerrish, J. Izzard, A. Ridout, Observing the disintegration of the A68A iceberg from space. Remote Sens. Environ. 270, 112855 (2022).
Citation: https://doi.org/10.5194/egusphere-2024-2790-RC2
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