the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Apr 2026
Seasonal icings reveal subsurface water drainage of a Greenland Ice Sheet outlet glacier
Abstract. In many glacial settings, winter outflows of proglacial water create stratified domed ice structures on glacial forefields. These structures, called naledi, provide an opportunity to characterise the solute fluxes of hydrological systems in winter which are poorly constrained compared to summer outflows which are dominated by supraglacial meltwater input. To characterise the different hydrological systems feeding naledi and provide conceptual models of their formation, we sampled 12 overwinter naledi and 4 supercooled summer ice accumulations (accreted ice) from the forefield of Isunnguata Sermia, a western outlet glacier of the Greenland Ice Sheet, during four field campaigns. Major ions and stable water isotopes reveal complex geochemical signatures, where the composition of successive naled layers reveal fluctuations in water source and transport throughout winter. In comparison, core analysis of summer accreted ice shows uniform geochemistry throughout, suggesting freeze-on of water from one single source in quick discrete events. These findings are supported by Electrical Resistivity Tomography (ERT) geophysical surveys, which reveal shallow hydrological pathways (1–5 m below the surface) in the glacial foreland (surveyed 400 m from the glacier terminus). Here, meltwater is transported through a saturated sediment zone feeding proglacial upwellings. Our results inform the first conceptual models of summer vs. wintertime subglacial water routing and naled formation in the proglacial zone of the Greenland Ice Sheet. We show that wintertime naledi form from a combination of top-down (unconfined) and bottom-up (confined) freezing processes which incorporates subglacial discharge, overland flow and precipitation, whereas accreted ice forms from upwellings of high velocity, highly pressurised water which freezes upon release. These seasonal changes drive different geochemical and nutrient outputs, which are critical for evaluating glacier meltwater contributions to proglacial groundwater systems and the impact of glacial discharge on downstream ecosystems.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2131', Marek Stibal, 01 Jun 2026
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RC2: 'Comment on egusphere-2026-2131', Anonymous Referee #2, 19 Aug 2026
There is much to like in this paper. First and foremost, there is not a sufficient literature on the types of physical, chemical and, perhaps, biological information that becomes stored in aufeis or naledi, and how this compares with that stored in icings. Second, there is precious little of this information on Greenlandic naledi/icings. Third, the interpretation of these data is far from straightforward, and it is great to see photogrammetry and water isotopes being used to help identify the processes controlling the accumulation of the ice and the chemicals stored in them. Overall, I am very supportive of the paper, but you know that the gripes always follow statements of reviewer’s support.
- Meltwater solute fluxes……. You simply cannot say you are contributing to studies of meltwater fluxes without quantifying the amount of solute stored in these types of ice relative to that exported in the meltwater over the rest of the melt season. A FLUX has fundamental units of mass per unit time, and often in meltwater solute studies, it becomes equivalent to concentration x discharge, summed over a period that is often a year. This work presents concentrations, period, and does not contribute to solute flux studies unless more quantitative analysis is included.
- Nutrient fluxes…….. The same argument, but much worse. Only nitrate is measured, and although the figures purport to show nitrate concentrations, these are super difficult to make out. I’m comfortable that you say that this work could contribute to such studies, but not given the analysis you have provided. We’ve measured some big concentrations is not enough. Big concentrations in a small puddle of ice, relative to the small fjord that discharges from the ice sheet each year, does not necessarily sum up to a major contribution of solute and nutrient overall. My hunch is that if you can find a contribution of above a percent you will have done very well.
- Overwinter fluxes of solutes and nutrients…… but you suggest that the solute is all retained in your frozen water. How does this get to the oceans?
- Flash freezing in icings shouldn’t give you much of a chemical gradient in the ice overall (or does it?), in terms of vertical stratification in the ice and along the freezing water sheet. In principle, the di-valent carbonates will precipitate out first, then CaSO4 and then most soluble sulphates that contain monovalent ions and the nitrates and chlorides. Have to check the spatial variation of the icing chemistry to check for this patterning that is found in Svalbard? In principle, the waters should become more concentrated and have a very different chemistry as they flow away from their source, so complicating your interpretation of subglacial/supraglacial chemistries.
- This pattern too should be found in your aufeis. You claim top down and bottom up freezing and it is unclear how much lateral water is occurring when this occurs. In principle, the first formed ice should be dilute (and so may not be supraglacial water), think of the purification of water by freezing, and you should get successive chemical layers building up that might be thick enough to measure these differences if there was an initial water column containing sufficient solute and sufficiently thick. Lateral flow of water during freezing can also give rise to an initial carbonate rich chemistry that evolves into a more sulphate rich chemistry. I think you need to acknowledge these types of caveats when you are claiming that the ice chemistry translates simply to subglacial water chemistry. It is very unlikely to unless there was a static water body that completely froze and through which you have sampled all the ice, from the initial dilute ice through to the last concentrated ice, without bias.
I remain a big fan of the paper. Someone has to go out there and get the data, come up with the ideas and form a case for doing more work. I think you’ve done a good job overall.
Citation: https://doi.org/10.5194/egusphere-2026-2131-RC2
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(Disclaimer: I am a biogeochemist and not an expert on glacial hydrology and geophysical surveys.)
In this paper McCerery et many al sampled proglacial icings and accreted ice in the forefield of Isunnguata Sermia in SW Greenland to get an insight into their formations and the dynamics of the glacier’s hydrological system. Using geophysical surveys they show water flow saturated sediment in the subsurface, feeding proglacial upwellings. Geochemical analysis revealed differences between the icings and accreted ice, suggesting different origins. Based on their results the authors contend that the icings are formed from winter discharge through both top-down and bottom-up processes , while the accreted ice features are the result of summertime upwelling of pressurised water.
Given the importance of this site as a representative large catchment of the W GrIS these data and their interpretation are extremely interesting and useful for understanding the hydrological system of the ice sheet and can (and will) inform future research in the area and on the GrIS in general.
Comments and questions:
208-220 Please give your limits of detection and precisions and accuracies for your chemical analyses. Is there a way to quantify the uncertainty of your charge balance estimations of HCO3-? Do you know the concentration of phosphate? Did you measure TSS (particulates) at all?
263-271 Any idea what this yellow water was?
Figures 7-9 Would it be possible to integrate the two bars (total ions and ion proportions) into one to make the differences between samples/layers clearer?
Figure 13 Is the subsurface water flow constrained by permafrost? I.e., is the depth of it determined by the permafrost table and/or distribution?
555-557 If the accreted ice features were formed quickly during outburst events, could the samples be used as representatives of those outbursts? I would then expect them to contain more sediment than ‘normal’ accreted ice that forms during the melt season around the upwelling. Also, as per Livingstone et al 2026 JGlac I understand the October 2023 outburst was linked to lake drainage (primarily of an ice-marginal lake but potentially also secondarily of subglacial lakes)? Could the sediment in these features then be expected to come from these lakes?
607 the correct year for this reference is 2026