| 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.
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