the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Dec 2025
Modeling the Subglacial Sediment System of the Finnish Lake District Ice Lobe During Deglaciation
Abstract. The systematic connection of glacial conditions in ice models with subglacial geomorphological observations has been limited by an inability to model subglacial sediment processes. The Finnish Lake District Ice Lobe (FLDIL) presents an opportunity to apply new model approaches in a situation of relative simplicity, with a well-preserved sedimentary record of its past subglacial hydrology and ice flow. With a recent model of the FLDIL subglacial hydrology as driver, we derive a sediment system model ensemble using the Graphical Subglacial Sediment Transport model (GraphSSeT). Model scenarios analyse the impact of varying sedimentary conditions, and resolve spatial and temporal variations in basal sediment thickness, sediment flux rate, grain size and detrital provenance. Our results show the development of a supply-limited system within 10 years characterised by strong seasonal cycles of winter gains from bed erosion, spring and summer losses from the mobilisation of basal sediment and autumn gains from deposition. Modelled at-outlet grain size also varies seasonally and would yield clastic varves, if deposited in a proglacial lake. The results define a submarginal zone of basal sediment depletion extending 40–60 km back from the terminus, in line with the modern-day sediment thickness.The mobilisation of an extensive blanket of sediment from this submarginal zone is proposed to form the Salpausselkä II ice marginal complex. Our model approach provides a template for the validation of subglacial hydrology models against sedimentary observables, opening a path to employ such constraints to study hard-to-observe modern and past subglacial hydrology, and ice conditions.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-5074', Anders Damsgaard, 20 Jan 2026
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RC2: 'Comment on egusphere-2025-5074', Anonymous Referee #2, 04 Mar 2026
General comments:
This manuscript applies a coupled ice-sheet, hydrology, and sediment transport model to reproduce the geomorphological features of the Finnish Lake District Ice Lobe (FLDIL) utilizing the Graphical Subglacial Sediment Transport model (GraphSSeT). The GraphSSET model evolves based on subglacial hydrology inputs from the hydrology model GLaDS coupled with ISSM. The manuscript highlights the strong seasonality of the sediment system across multiple model ensembles under initial sediment/bed conditions. In particular, coarse grains are transported through the system in the early spring and then fine sediment dominates the summer melt season. These results show that coupling sediment transport with subglacial hydrology produces sediment deposition patterns that resemble geomorphological features consistent with the ice-marginal deposits of Salpausselkä II.
The authors address a central challenge in glaciology: reproducing observed glacial landforms within a process-based modeling framework. By linking sediment transport dynamics informed by subglacial hydrology with preserved geomorphological features, this study provides insight into how subglacial hydrology can influence the formation of ice-marginal features. This manuscript demonstrates how intertwined processes, such as seasonality of subglacial hydrology, ice dynamics, and topography can inform landscape evolution. The research presented here would be of interest to those who study sediment in subglacial systems, glacial geomorphology, and those seeking to use numerical models to interpret or reconstruct past ice sheet behavior. Given this broad interest, I believe the manuscript is well suited for publication in The Cryosphere, with the minor revisions suggested below.
I have a few suggestions on how to improve this manuscript, in particular, where to expand on certain modeling choices.
- As a reader, I would have appreciated a short summary or schematic of equations/variables that show the sediment and hydrology coupling. This manuscript follows up the idealized GraphSSeT model publication (Aitken et al. 2024) by applying the GraphSSeT model to the FLDIL. Readers are referred to the original manuscript that does include a schematic, but I suggest the coupling between GraphSSeT and GlaDS (named FLDIL-GLaDs simulation in line 133) can be briefly described with a new schematic that shows the application of these models to this system. In the appendices, important parameters discussed in the results (e.g., volume, mean flux, mean grain size, and proportion of basal sediment) vary within the model time. A schematic that shows how these variables are derived within the coupled sediment-hydrology model would be appreciated.
- I suggest the authors discuss seasonality in the Younger Dryas. The FLDIL-GLaDs simulation assumes a modern precipitation record, a depressed modern temperature record, and an elevation dependent lapse rate of 7.5 C/km. This climatology is then used to force a hydrology model with a timestep of four days, and a sediment model at an even finer timescale. Thus, I imagine that uncertainties in the starting climatology might influence the results. How sensitive are the results to the prescribed climatic conditions? What is known about seasonality in this region during the Younger Dryas? I encourage the authors here to engage with previous work suggesting that seasonality was different from modern during the Younger Dryas (e.g. Denton et al., 2005; Hall et al., 2008).
- I suggest the authors discuss the time dissonance with 10 years of model time aiming to replicate geomorphic features that take much longer to form. The manuscript presents spatial agreement between the modeled sediment distribution and the observed geomorphic features of Salpausselkä II. However, the authors note (lines 321–331) that the rates of discharge in most of their models would take much longer to form these features than implied by the geology. I encourage the authors to expand on this discussion of the temporal framework of the simulations. The experiments are conducted over 10 years of model time yet the geomorphic features being replicated potentially require substantially longer timescales to form (1000-10000 years). This creates a potential time-scale dissonance that can be expanded on in the discussion. Specifically, what does a 10-year simulation meaningfully inform about features that develop over millennial timescales? This connects to another question that I had in the context of seasonality - even on short timescales the ice margin might advance and retreat, and during the younger dryas we know there were phases of ice sheet growth and decay. It’s fair enough that for this study the authors use a fixed ice sheet geometry, but I would appreciate a deeper discussion of how they think the results might change in the future were a transient geometry to be considered. For example, the discussion outlined in L305-319 regarding implications of the modeled sediment distribution against observations is expanded in a way that discusses such limitations.
- I suggest the authors replot the spatial results in Figures 4-6 (b-c) with a focus closer to the ice margin. The changes occurring in the modeled interior do not appear to be the primary focus of the results. A zoomed-in view of the model domain near the ice margin would likely improve visualization of the spatial patterns and make the color scale easier to interpret. The model extent shown in Figure 7 may provide a useful domain for the subplots in Figures 4-6.
Specific comments:
L29: A reference to Sommers et al., 2022 (ISSM-SHAKTI application at Helheim Glacier, Greenland) is also appropriate here when referring to subglacial hydrology models coupled with ISSM.
L57-L62 and Figure 1: It may be beneficial to add and label Salpausselka I and II on Figure 1a, whether as an average line or arrow.
Figure 1: A label for the model domain in black would be useful to add either to the legend or figure caption.
L110-114: A figure or schematic showing how GLaDs and GraphSSeT are coupled would be useful here (see point 1 above).
Figure 3d - a gradient color bar would be suitable rather than a divergent color bar. Additionally, is the percentage calculated here = basal sediment divided by basement sediment? I think a clarification in the figure legend would be helpful.
Table 1 - include a column in Default 2 parameters. This seems important in Figure 3, but is not mentioned in this table.
L183: Are you referring to Figure 3b or 3d for the variable volumetric discharge? A clarification on which sub-figure is useful here.
Figure 9: For the sediment classifications outlined in the legend, I would appreciate it if they can connect back to the legend in Figure 1b. Is metasedimentary all sedimentary classifications in the Figure 1b legend? A connection back to the region can be helpful here.
Citation: https://doi.org/10.5194/egusphere-2025-5074-RC2
Data sets
Supplementary material for ’Reorganisation of subglacial drainage processes during rapid melting of the Fennoscandian Ice Sheet’ A. Hepburn et al. https://zenodo.org/records/8344208
Geological and Geomorphological data from GTK Finland GTK Finland https://hakku.gtk.fi/en
Model code and software
GraphSSeT A. Aitken et al. https://github.com/al8ken/GraphSSeT
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Overall Comments
This manuscript presents an application of the Graphical Subglacial Sediment Transport Model (GraphSSeT) to the Finnish Lake District Ice Lobe (FLDIL), using GlaDS simulations from ISSM as hydrological forcing. The study advances beyond the synthetic test cases of Aitken et al. (2024) by applying the model to a realistic glacial system with well-preserved geomorphological constraints. The analysis of sediment conditions, flux rates, grain size dynamics, and provenance yields interesting results, particularly the strong seasonal cyclicity characterized by fluvial erosion in spring/summer and deposition in autumn, and the extensive sediment mobilization near the glacier terminus.
I have some small(ish) concerns regarding presentation, model coupling and its implications that could be a useful addition, if addressed:
One-way coupling limitations. The sediment model is forced by GlaDS but does not feed back to the hydrological system as channel and conduit geometry remain unaffected by sediment presence. Given the presence of eskers in the FLDIL, which provide evidence of channel infill, this limitation warrants discussion. Sediment accumulation would reduce the hydraulic radius and effectively choke the drainage system (see Hewitt and Creyts, 2019; doi:10.1029/2019GL082304). I recommend the authors expand the discussion to address how the hydraulic system is likely to respond to the modelled sediment dynamics, and how such feedbacks might influence the spatial zonation between erosion, mobilization, and transport. While the authors discuss time-transgressive margin behaviour, the likely implications for hydraulic system evolution would strengthen the manuscript.
Temporal resolution. GraphSSeT receives input from GlaDS at four-day intervals, and ice geometry is held fixed. I recommend the authors consider in the discussion what higher-frequency diurnal dynamics in meltwater transport might imply for sediment transport patterns.
Sensitivity to transport initiation (possibly out of scope for this paper). The inverse relationship between peak sediment flux and grain size is intriguing, explained as a consequence of the upper drainage system mobilizing high volumes of finer sediment. In this context, the results are likely sensitive to parameterization at the onset of sediment transport under relatively low flow velocities. Some discussion of this sensitivity would be valuable.
Specific Comments
- L2: I suggest adding that the FLDIL is interpreted as a re-advancing lobe in the Fennoscandian Ice Sheet during the Younger Dryas.
- L26-27: Integration of subglacial hydrology in ice-sheet models is not recent. However, recent developments include more sophisticated hydrology modeling, like GlaDS with a two-component drainage system and discrete channels.
- L160-169: From this description, I do not fully comprehend how the initialization run is set up. Obviously, the model needs sediment to be present or added somewhere in order to mobilise it in the drainage system during the initialization. Hepburn et al. 2024 ran FLDIL-GlaDS for a hard-bed geometry, where topography was constructed by removing sediment (L134-135). For the initialization of the three scenarios with GraphSSeT, did you apply a uniform initial sediment thickness (low and high-H) for the entire area, before letting GraphSSeT evolve it under a winter-state from GlaDS, or something else entirely? If so, how do the two scenarios compare to the volume removed in Hepburn et al., 2024 (i.e. the volume in the mixed-bed case)?
- Fig. 3: I think it would be an aid to the reader if the labels of panel a are also included in panels b-d, i.e. default 1, default 2, detrital, high-sigma, as well as Mixed-bed, High-H and Low-H. Currently, the eyes have to find the corresponding label for a given run by running from each panel with the physical result and the ensemble tree structure, and it is easy to get lost during the travel.
- Fig. 3: I'm a bit lost in the explanation between parameterization between fig. 3, L.174-181. I do not understand why there are two runs for each configuration, i.e. there are two "Low-H + detrital" nodes, etc. Also, what is the difference between "default 1" and "default 2"? L180-181 mention a cascade of runs for each leg, does this imply some kind of perturbation since the results are not equal?
- Fig. 3d: Is the proportion of basal sediment referring to the proportion of sediment in active transport?
- L176: I suggest adding the explaining text (high-sigma) to mode b, so it is easier to understand the equivalent modes explained in text and table.
- Fig. 4: Consider making the "Grain size" label text on the secondary y axis red, as a visual aid. Would it be meaningful to include the channel water flux also, to show sediment starvation?
- Fig. 4 (cont.): Is it correctly understood that the basal sediment flux is the locally eroded and mobilized sediment along the channel (edge), and the total sediment flux is the local contribution and upstream flux combined? I am a bit confused about the "basal" terminology, as I understand the Engelund and Hansen 1967 to account for both bed-load and suspended sediment transport.
- Fig. 4b and c: Could you please consider inverting one of the colorbars? In panel b, black is low and yellow is high, and the opposite is the case in panel c.
- L190-205: These findings are very interesting, in particular the drop in grain size during peak discharge. Did you try and visualize the spatial patterns in grain size in these figures (4 and 5) or in the transient maps (fig. 7)?
Thank you for considering my comments.
Anders Damsgaard