the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Oct 2024
Groundwater dynamics beneath a marine ice sheet
Abstract. Sedimentary basins beneath many Antarctic ice streams host substantial volumes of groundwater, which can be exchanged with a “shallow” subglacial hydrological system of till and channelised water. This exchange contributes substantially to basal water budgets, which in turn modulate the flow of ice streams. The geometry of these sedimentary basins is known to be complex, and the groundwater therein has been observed to vary in salinity due to historic seawater intrusion. However, little is known about the hydraulic properties of subglacial sedimentary basins, and the factors controlling groundwater exfiltration and infiltration. We develop a mathematical model for two-dimensional groundwater flow beneath a marine-terminating ice stream on geological timescales, taking into account the effect of seawater intrusion. We find that seawater may become “trapped” in subglacial sedimentary basins, through cycles of grounding line advance and retreat or through “pockets” arising from basin geometry. In addition, we estimate the sedimentary basin permeability which reproduces field observations of groundwater salinity profiles from beneath Whillans Ice Stream in West Antarctica. Exchange of groundwater with the shallow hydrological system is primarily controlled by basin geometry, with groundwater being exfiltrated where the basin becomes shallower and re-infiltrating where it becomes deeper. However, seawater intrusion also has non-negligible effects on this exchange.
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CC1: 'Comment on egusphere-2024-2880', Giacomo Medici, 18 Nov 2024
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General comments
Very interesting mathematical model on a challenging hydrological topic and region of the world. Please, follow my suggestions to improve the manuscript.
Specific comments
Line 6. “Two-dimensional groundwater flow”. Add text in the discussion section on assumptions and limitations underneath the choice of a 2D model.
Lines 30-34. Add relevant and recent literature on tracer and hydraulic tests in sedimentary deposits of glacial origin made by clay, sand, breccias and conglomerates:
- Tracking flowpaths in a complex karst system through tracer test and hydrogeochemical monitoring: Implications for groundwater protection (Gran Sasso, Italy). Heliyon, 10(2).
- Forms of hydraulic fractures created during a field test in overconsolidated glacial drift. Quarterly Journal of Engineering Geology and Hydrogeology, 28(1), 23-35.
Line 496. “Complex model” to develop in the future. Do you mean a model with multiple units to account for the heterogeneities of the system?
Line 496. “Complex model” do you also mean more attention on the anisotropies? You mention heterogeneities in the manuscript, but not anisotropies
Lines 620-720. Add the recent literature suggested above on the glacial environment.
Figures and tables
Figure 1. Do you need an approximate spatial scale for your conceptual model?
Figure 3. Very busy figure, consider to split it in two parts.
Figure 6. There is room to make the figure larger.
Figure 11. Same here, there is room to make the figure larger. The figure would benefit from that.
Citation: https://doi.org/10.5194/egusphere-2024-2880-CC1 -
AC1: 'Reply on CC1', Gabriel Cairns, 19 Nov 2024
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Thank you for these helpful comments on additional literature, assumptions and figures. We will certainly take account of these suggestions if invited to revise the manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-2880-AC1
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AC1: 'Reply on CC1', Gabriel Cairns, 19 Nov 2024
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RC1: 'Comment on egusphere-2024-2880', Anonymous Referee #1, 18 Dec 2024
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The manuscript by GJ Cairns, GP Benham and IJ Hewitt presents results from mathematical modelling of porewater flows in an aquifer beneath a marine-based ice sheet. The authors use the model to investigate seawater intrusions and exfiltration rates under a range of hydraulic conductivities and porewater flows driven by ice sheet geometry, topographic setting and cycles of grounding line advance and retreat.
There is growing interest in subglacial groundwater and its role in heat transfer and lubrication of the basal motion of ice sheets. In this manuscript, the authors show how sea water may become trapped when grounding lines move or when the subglacial sedimentary basin has a certain structure.
This study omits many of the processes that are known to drive the vertical exchange of water across the ice-sediment interface. However, it provides a useful long-term analysis of re-charge and exfiltration driven by horizontal pressure gradients. E.g., in this model subglacial groundwater exfiltration occurs where the basin becomes shallower while re-charge occurs where the basin deepens. The model outputs explain how the sea water intrusions may influence subglacial aquifers. In a case study focussing on the Siple Coast of West Antarctica, the model suggests a high permeability is needed to reproduce freshwater lenses as observed.
The strength of the work is a simple and elegant mathematical design. There are, however some limitations, notably the use of the shallow ice approximation, which is a poor choice in the Siple Coast test case because the fast motion of glaciers there is almost exclusively caused by basal slip. The authors offer a discussion of other model limitations, but not this one. I doubt the model reproduces the actual geometry of the Siple Coast, but this is perhaps not so important, given the “first-order” nature of the study more generally.
The main goal is to provide a long-term perspective of freshwater lenses and trapped subglacial seawater. However, the exclusion of vertical pressure gradients is a significant limitation because past work have shown groundwater flows in Antarctica to be quite sensitive to those. The manuscript includes a discussion with references to the inferred hydrological budget of ice streams at the Siple Coast, but previous work has also modelled the vertical exchange. To give an example, Christoffersen and Tulaczyk (Annals of Glaciology, 2003) included glacial-interglacial simulations of groundwater exchange at the Siple Coast. There may be a relevant discussion in that thermally driven exfiltration is shallow compared to the horizontally driven exchange presented in this manuscript.
A final couple of questions. Why not use reconstructed air temperature and precipitation records from Antarctic ice cores instead of a periodic function? Presumably, this would provide more direct evaluation of glacial-interglacial changes. Also, how sensitive is the exchange of water at the top of the aquifer to the assumed impermeable basement? What if the basement wasn't impermeable?
Last but not least, thank you for advancing our field with a well-written and well-illustated manuscript. The animated supplementary information is neat too.
Citation: https://doi.org/10.5194/egusphere-2024-2880-RC1 -
CC2: 'Comment on egusphere-2024-2880', Matthew Tankersley, 20 Dec 2024
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Hi, I saw this preprint as it cited my 2022 GRL paper; Basement topography and sediment thickness beneath Antarctica's Ross ice shelf. I noticed a few small errors in the citation of my work which I thought I would inform you of. The sedimentary basins we discussed were modeled with airborne magnetics data, not imaged with radar data as mentioned in the text. I think this is an important distinction as the modeling aspect, as opposed to direct imaging, introduces a lot more uncertainty which your readers should be aware of, and magnetic and radar techniques are quite different. No worries, but if you're able to change your mentions of radar to magnetics that would be great. Nice work on the paper, I enjoyed reading it!
Citation: https://doi.org/10.5194/egusphere-2024-2880-CC2
Model code and software
Groundwater dynamics beneath a marine ice sheet – code Gabriel Cairns https://doi.org/10.5281/zenodo.13759411
Video supplement
Groundwater dynamics beneath a marine ice sheet – supplementary animations Gabriel Cairns https://doi.org/10.5281/zenodo.13759494
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