the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Towards modelling of corrugation ridges at ice-sheet grounding lines
Katarzyna L. P. Warburton
Alastair G. C. Graham
Jerome A. Neufeld
Duncan R. Hewitt
Julian A. Dowdeswell
Robert D. Larter
Abstract. Improvements in the resolution of seafloor mapping techniques have revealed extremely regular, sub-meter scale ridge landforms produced by the tidal flexure of ice-shelf grounding lines as they retreated very rapidly (i.e., at rates of several kilometres per year). Guided by such novel seafloor observations from Thwaites Glacier, West Antarctica, we present three mathematical models for the formation of these corrugation ridges at a tidally migrating grounding line (that is retreating at a constant rate) where each ridge is formed by either constant till flux to the grounding line, till extrusion from the grounding line, or by the resuspension and transport of grains from the tidal cavity. We find that both till extrusion (squeezing out till like toothpaste as the ice sheet re-settles on the seafloor), and resuspension and transport of material from the grounding-zone bed can qualitatively reproduce regular, delicate ridges at a retreating grounding line as described from seafloor observations. By considering the known properties of subglacial sediments we agree with existing schematic models that the most likely mechanism for ridge formation is till extrusion at each low-tide position, essentially preserving an imprint of the ice-sheet grounding line as it retreated. However, when realistic (shallow) bedslopes are used in the simulations ridges start to overprint one another suggesting that, to preserve the regular ridges that have been observed, grounding line retreat rates (driven by dynamic thinning?) may be even higher than previously thought.
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Kelly A. Hogan et al.
Status: final response (author comments only)
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RC1: 'Comment on tc-2022-222', Anonymous Referee #1, 14 Dec 2022
General Comments
Hogan et al present a first modelling study aimed at understanding periodic seafloor features (corrugation ridges) that have been imaged proximal to rapidly retreating grounding lines. These features are intriguing and perhaps significant if they can be used as a diagnostic of the speed of retreat. This study provides a context within which these features can now be interpreted. The manuscript is well presented and referenced with ample discussion placing the work in a wider context. An example of the thoroughness of this contribution is quantification of the uncertainty in effective slope introduced by uncertainty in ice thickness gradient close to the grounding zone. This is helpful, as the region of enhanced surface gradient just upstream of grounding zones is often overlooked. In general, the modelling approach is presented as a first effort, and limitations are openly discussed in a useful way. The supplementary material is useful, and the animations provide additional insight.
Specific Comments
These specific comments are minor and intended to help link this study to what we observe at grounding zones.
An omission I noted, which is not a major concern, was the role of englacial sediment delivery, which is then delivered to the grounding zone ocean cavity by subglacial melt, over a length scale partly determined by the debris-rich ice thickness and the ice velocity. I would suggest this mechanism is acknowledged in the introduction, and then need not be addressed further. As it is I believe this mechanism is first mentioned in the Results and Discussion on Line 344. Some explicit statement on the influence that additional accommodation space generated by melt in the grounding zone ocean cavity would also help connect this modelling study to the real system.
I also think an upfront statement on the impact of the assumption of the constant 6 m retreat rate is inserted in Materials and Methods (around Line 76, 101) to allay any fears of circularity in the results.
Technical Comments
The manuscript is very well presented and requires very few technical corrections. As a reviewer I appreciate this.
L71 ‘...due to basal melt..’ suggest change to ‘due to an increase in basal melt..’ as basal melt could be in steady state.
L107 expected some mention of debris delivery by basal melt not being addressed around here.
L295 ...which perhaps supports dynamic thinning... This is an interesting point that might be lost on first reading. Suggest ‘...which perhaps supports widespread dynamic thinning...’
L363 ‘therefor’
L407 ‘lift-off off’
In closing, I thank the authors for their interesting and well-presented study.
Citation: https://doi.org/10.5194/tc-2022-222-RC1 - RC2: 'Comment on tc-2022-222', Sarah Greenwood, 04 Jan 2023
Kelly A. Hogan et al.
Data sets
Gridded multibeam bathymetry data from 'The Bump' at Thwaites Glacier, collected by 'Ran' Kongsberg Hugin AUV (Kongsberg EM2040), on the RV Nathaniel B. Palmer during cruise NBP19-02 (2019) Graham, A. G. C., Wåhlin, A. K., Hogan, K. A., Nitsche, F. O., Heywood, K. J., Minzoni, R., Smith, J. A., Hillenbrand, C.-D., Simkins, L., Wellner, J. S., and R. D. Larter https://doi.org/10.6084/m9.figshare.20359920.v1
Kelly A. Hogan et al.
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