Reorganisation of subglacial drainage processes during rapid melting of the Fennoscandian Ice Sheet

Hepburn, Adam Jake; Dow, Christine F.; Ojala, Antti; Mäkinen, Joni; Ahokangas, Elina; Hovikoski, Jussi; Palmu, Jukka-Pekka; Kajuutti, Kari

doi:https://doi.org/10.5194/egusphere-2023-2141

Preprints

https://doi.org/10.5194/egusphere-2023-2141

Preprints

16 Oct 2023

| 16 Oct 2023

Reorganisation of subglacial drainage processes during rapid melting of the Fennoscandian Ice Sheet

Adam Jake Hepburn, Christine F. Dow, Antti Ojala, Joni Mäkinen, Elina Ahokangas, Jussi Hovikoski, Jukka-Pekka Palmu, and Kari Kajuutti

Abstract. Unknown basal characteristics limit our ability to simulate the subglacial hydrology of rapidly thinning contemporary ice sheets. Subglacial water is typically conceptualised as being routed through either distributed, inefficient, and high pressure systems, or channelised, efficient, and low-pressure systems, transitioning between the two as a function of discharge. Sediment-based landforms generated beneath Pleistocene ice sheets, together with detailed digital elevation models, offer a valuable means of parameterising and testing models of subglacial hydrology. However, previous work using geomorphology to inform modelling has concentrated on landforms relating to channelised drainage (e.g., eskers) while using inherently channelised models unable to capture transitions in drainage state. Landscapes relating to the distributed drainage system, and the hypothesised transitional zone of drainage between distributed and channelised drainage modes have therefore been largely ignored. To address this, we use the Glacier Drainage System model (GlaDS), a 2D finite element model capable of capturing the transition between distributed and channelised drainage, to explore the genesis of ‘murtoos’, a distinctive triangular landform found in murtoo fields throughout Finland and Sweden. Murtoos are hypothesised to form 40–60 km from the former Fennoscandian ice margin at the onset of channelised drainage where water pressure is at or exceeds ice overburden pressure. Concentrating within a specific ice lobe of the former Fennoscandian Ice Sheet and parameterised using digital elevation models with a simulated former ice surface geometry, we carried out a range of sensitivity testing to explore murtoo genesis and drainage transitions beneath the palaeo ice sheet. Our modelling supports many of the predictions for murtoo origin, including the location of water pressures equal to ice overburden, the onset of channelised drainage, and the predicted water depths in terrain surrounding murtoo fields. Modelled channels also closely match the general spacing, direction and complexity of eskers and mapped meltwater routes. Our results demonstrate that examining palaeo basal topography alongside subglacial hydrology model outputs holds promise for mutually beneficial analyses of palaeo and contemporary ice sheets to assess the controls of hydrology on ice dynamics and subglacial landform evolution.

Received: 18 Sep 2023 – Discussion started: 16 Oct 2023

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Journal article(s) based on this preprint

29 Oct 2024

The organization of subglacial drainage during the demise of the Finnish Lake District Ice Lobe

Adam J. Hepburn, Christine F. Dow, Antti Ojala, Joni Mäkinen, Elina Ahokangas, Jussi Hovikoski, Jukka-Pekka Palmu, and Kari Kajuutti

The Cryosphere, 18, 4873–4916, https://doi.org/10.5194/tc-18-4873-2024,https://doi.org/10.5194/tc-18-4873-2024, 2024

Short summary

Adam Jake Hepburn, Christine F. Dow, Antti Ojala, Joni Mäkinen, Elina Ahokangas, Jussi Hovikoski, Jukka-Pekka Palmu, and Kari Kajuutti

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-2141', Anonymous Referee #1, 01 Nov 2023

See attached .pdf.

Citation: https://doi.org/10.5194/egusphere-2023-2141-RC1
- AC2: 'Reply on RC1', Adam Jake Hepburn, 02 Feb 2024
  
  We thank reviewer 1 for their encouraging and helpful comments. Please find attached a document in which we detail our response to each and in turn the changes we have made to our manuscript. We believe that these changes have improved the manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2141-AC2
RC2:
'Comment on egusphere-2023-2141', Sarah Greenwood, 24 Nov 2023

In principle, this paper puts forward an exciting approach to compare conceptual ideas (ie geologically-driven) of landform (murtoo) formation with model predictions of hydrological settings. I see a lot of potential in such data-model exploration, particularly in relation to murtoos, since they are hypothesised to represent a transition in drainage style. However, I found this manuscript hard work due to “heavy” text, to the extent it is difficult to evaluate the significance of what has been found.
The manuscript is suitably constructed in its overall structure, but I struggled with a lack of direction and clarity in the writing. I wasn’t sure by the end of the Introduction, or the Methods, what the study was designed to explore, specifically, or what question was being asked; I had to read the Results to try and work out what the authors were actually trying to achieve (beyond a generality). Parts of the Discussion are more a summary of results than an exploration of their significance, and important insights are hidden or implied rather than explicitly stated. The same is true of the Conclusions, which ultimately don’t say anything concrete about what has been learned.
Besides the presentation...
1. The study area for modelling – and the distribution of murtoos – encompasses the whole Lake District lobe but the entire focus of the results is in the distal few 10s km. I realise the duration of the model effectively only considers one ice time-slice in the development of the whole lobe’s landform system, i.e. when the ice margin sits at Salpausselkä II. However, I think this needs to be stated explicitly, and I think the paper needs to discuss what the modelling findings imply for the formation of the other (most of the) murtoos in the domain. Do your results imply (or reject) that the upstream murtoos could form at the same time as those you consider in the near-marginal zone? Do your results imply that they must form time-transgressively (headward) during margin retreat?
Related, considering the distribution of murtoos over your model domain and the 40-60km band you focus your analysis on, one might wonder why (if) this particular time-slice is well-suited to the investigation – murtoo fields are actually rather few/sparse in this band, compared to elsewhere up the trunk of the Lake District lobe. Could you offer some justification for your approach?
2. Some recent work is suggesting that the YD may have experienced extreme seasonality, with relatively warm summers but extreme winters (e.g. Schenk et al. 2018 Nat Comms, 2020 QSR; Amon et al. 2022 Clim. Past). You replicate a Younger Dryas climate by lowering present monthly temperatures uniformly by 15°C. I wonder how extreme (or simply, different) YD seasonality would impact your results? How important is this choice of climate forcing for your hydrology conclusions?
3. Section 3.2 implies that only the zone 40-60km upstream of the ice margin is analysed, on the basis that this zone is favourable to murtoo formation. This seems a little circular to me. By focussing only here, do you not exclude the possibility of identifying conditions that would suit murtoo formation elsewhere? And exclude possible comparable hydrological/glaciological conditions elsewhere that may or may not support murtoos?
4. One of your results is rather surprising, which calls into question the appropriateness of comparing model output with specific mapped geomorphology: if channel discharge in late summer is significantly higher outside mapped meltwater routes (line 395) than where meltwater routes have been recorded, then this suggests the spatial distribution of channels predicted by the model is offset from where channels are known to have existed. This mismatch therefore also questions the validity of comparing hydrological parameters with where murtoos have been mapped or not mapped.
A further uncertainty in this regard concerns what is meant by a meltwater route? Section 3.2 suggests these are based on Ahokangas et al. As far as I understand, those authors keep eskers separate from their meltwater route classes – how did you treat eskers in your classification of “meltwater routes”?
At the very least, I think the presentation of results relating to geomorphological classifications needs to be preceded by a justification that the model performance is adequate in terms of what we know of the geomorphology: does the model do a good job of replicating channels? If it does, then it’s a valid tool to explore other landforms, and if not, then it’s not. This partly is presented in the Discussion (524-540) (though the earlier result I’ve queried here is not addressed) and I think would better serve the Results section if it were brought earlier. I also wonder, if the exact location of channels is sensitive to mesh geometry (536) then does this also not suggest that specific site-to-site data-model comparisons may not be appropriate?
5. Overall, the manuscript would have benefitted from a thorough proof-read – there are numerous typos, left-over words from earlier constructions, unit errors, muddled variables.
I note these in a rather lengthy list of line-by-line technical comments in the attached pdf, where I also identify issues with writing clarity, section by section, and suggest how the direction and framing of the work could be improved.

Citation: https://doi.org/10.5194/egusphere-2023-2141-RC2
- AC1: 'Reply on RC2', Adam Jake Hepburn, 02 Feb 2024
  
  We thank reviewer 2 for their detailed and helpful comments. In the attached document, we list each comment, our reply and the changes we have made in response. We have made significant changes throughout to our writing and we thank reviewer 2 for their diligence.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2141-AC1

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-2141', Anonymous Referee #1, 01 Nov 2023

See attached .pdf.

Citation: https://doi.org/10.5194/egusphere-2023-2141-RC1
- AC2: 'Reply on RC1', Adam Jake Hepburn, 02 Feb 2024
  
  We thank reviewer 1 for their encouraging and helpful comments. Please find attached a document in which we detail our response to each and in turn the changes we have made to our manuscript. We believe that these changes have improved the manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2141-AC2
RC2:
'Comment on egusphere-2023-2141', Sarah Greenwood, 24 Nov 2023

In principle, this paper puts forward an exciting approach to compare conceptual ideas (ie geologically-driven) of landform (murtoo) formation with model predictions of hydrological settings. I see a lot of potential in such data-model exploration, particularly in relation to murtoos, since they are hypothesised to represent a transition in drainage style. However, I found this manuscript hard work due to “heavy” text, to the extent it is difficult to evaluate the significance of what has been found.
The manuscript is suitably constructed in its overall structure, but I struggled with a lack of direction and clarity in the writing. I wasn’t sure by the end of the Introduction, or the Methods, what the study was designed to explore, specifically, or what question was being asked; I had to read the Results to try and work out what the authors were actually trying to achieve (beyond a generality). Parts of the Discussion are more a summary of results than an exploration of their significance, and important insights are hidden or implied rather than explicitly stated. The same is true of the Conclusions, which ultimately don’t say anything concrete about what has been learned.
Besides the presentation...
1. The study area for modelling – and the distribution of murtoos – encompasses the whole Lake District lobe but the entire focus of the results is in the distal few 10s km. I realise the duration of the model effectively only considers one ice time-slice in the development of the whole lobe’s landform system, i.e. when the ice margin sits at Salpausselkä II. However, I think this needs to be stated explicitly, and I think the paper needs to discuss what the modelling findings imply for the formation of the other (most of the) murtoos in the domain. Do your results imply (or reject) that the upstream murtoos could form at the same time as those you consider in the near-marginal zone? Do your results imply that they must form time-transgressively (headward) during margin retreat?
Related, considering the distribution of murtoos over your model domain and the 40-60km band you focus your analysis on, one might wonder why (if) this particular time-slice is well-suited to the investigation – murtoo fields are actually rather few/sparse in this band, compared to elsewhere up the trunk of the Lake District lobe. Could you offer some justification for your approach?
2. Some recent work is suggesting that the YD may have experienced extreme seasonality, with relatively warm summers but extreme winters (e.g. Schenk et al. 2018 Nat Comms, 2020 QSR; Amon et al. 2022 Clim. Past). You replicate a Younger Dryas climate by lowering present monthly temperatures uniformly by 15°C. I wonder how extreme (or simply, different) YD seasonality would impact your results? How important is this choice of climate forcing for your hydrology conclusions?
3. Section 3.2 implies that only the zone 40-60km upstream of the ice margin is analysed, on the basis that this zone is favourable to murtoo formation. This seems a little circular to me. By focussing only here, do you not exclude the possibility of identifying conditions that would suit murtoo formation elsewhere? And exclude possible comparable hydrological/glaciological conditions elsewhere that may or may not support murtoos?
4. One of your results is rather surprising, which calls into question the appropriateness of comparing model output with specific mapped geomorphology: if channel discharge in late summer is significantly higher outside mapped meltwater routes (line 395) than where meltwater routes have been recorded, then this suggests the spatial distribution of channels predicted by the model is offset from where channels are known to have existed. This mismatch therefore also questions the validity of comparing hydrological parameters with where murtoos have been mapped or not mapped.
A further uncertainty in this regard concerns what is meant by a meltwater route? Section 3.2 suggests these are based on Ahokangas et al. As far as I understand, those authors keep eskers separate from their meltwater route classes – how did you treat eskers in your classification of “meltwater routes”?
At the very least, I think the presentation of results relating to geomorphological classifications needs to be preceded by a justification that the model performance is adequate in terms of what we know of the geomorphology: does the model do a good job of replicating channels? If it does, then it’s a valid tool to explore other landforms, and if not, then it’s not. This partly is presented in the Discussion (524-540) (though the earlier result I’ve queried here is not addressed) and I think would better serve the Results section if it were brought earlier. I also wonder, if the exact location of channels is sensitive to mesh geometry (536) then does this also not suggest that specific site-to-site data-model comparisons may not be appropriate?
5. Overall, the manuscript would have benefitted from a thorough proof-read – there are numerous typos, left-over words from earlier constructions, unit errors, muddled variables.
I note these in a rather lengthy list of line-by-line technical comments in the attached pdf, where I also identify issues with writing clarity, section by section, and suggest how the direction and framing of the work could be improved.

Citation: https://doi.org/10.5194/egusphere-2023-2141-RC2
- AC1: 'Reply on RC2', Adam Jake Hepburn, 02 Feb 2024
  
  We thank reviewer 2 for their detailed and helpful comments. In the attached document, we list each comment, our reply and the changes we have made in response. We have made significant changes throughout to our writing and we thank reviewer 2 for their diligence.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2141-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (further review by editor and referees) (08 Feb 2024) by Lev Tarasov

AR by Adam Jake Hepburn on behalf of the Authors (15 Feb 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (22 Feb 2024) by Lev Tarasov

RR by Anonymous Referee #1 (08 Mar 2024)

Suggestions for revision or reasons for rejection

General Comments:

While the authors have worked to address the comments, in my opinion, some work remains in making the manuscript more readable and addressing the underlying scientific questions presented in the manuscript's introduction.

The authors did not seem to discuss the role of greater seasonal variability or diurnal variability in water discharge in the revised manuscript, despite in being mentioned by both reviewers. I believe that this could be important in the sediment dynamics controlling the formation of murtoos below the glacier.

Despite the uncertainties of the variability of the hydraulic forcing, I believe that model application is generally rigorous and that method is appropriate to address the research objectives. However, the writing remains hard to follow and does not fully address the research objectives. Reviewer 2 spoke to this in the last review, and with the benefit of hindsight, I should have pushed this matter further as well in my original review.

In the first section of the results, for example, it seems that much of the presentation is about the sensitivities of the model and description of the base case as opposed to addressing the questions posed in the introduction. Likewise, much of the first part of the paper summarizes the state of the research, as opposed to creating an imperative to address the research questions. For example, the link between the murtoo formation stages in section 2 and the model output that could represent these processes is not clear to me.

In my opinion, for this to be an impactful and unique work the authors must show how their results fit into the two research objectives at the end of the introduction. I think that both aims, particularly the second, can be accomplished. However, the paper will need some substantial rewriting and streamlining.

Below I try to point out some of these issues, although the list is not extensive.

Specific comments:
-Velocity. Throughout the manuscript please clarify if velocity refers to water velocity or sliding velocity of the glacier.

Ln 1. glacier thinning-> increased glacier melt? Melt, in addition to thinning, impacts basal characteristics.

Ln 9-10. This sentence is strangely worded. I had the thought: if it is "hypothesised" then how can it be "ignored."

Introduction- Put e.g. in front of some citations. van den Broeke et al., 2023 were not the first to establish widespread melt in Greenland and Antarctica.

Ln 55. detailed treatment-> This challenge is also in part due to the computational resources, number of parameters, etc.

Ln 71. model ability -> model ability of what? to represent water pressure? Location of channelized vs pressurized flow?

Section 2. Title, I do not see how these two matters are connected, thus deserving their section together. The murtoo formation descriptions are pretty dense and I did not follow them. Maybe just include citations about the formation? Also, I was looking for a concise description of the model conditions that would be indicative of murtoo development (this is in Table1, which I appreciate and wish was addressed more directly in the results.)

Table 1. Reference labels. i.e. qs, Vw...

Ln 277. The GIA discussion could fit in the discussion at the end if it is important. To me, it distracts from other messages here.

Ln 316. I do not follow what was done with the PDD model here. Were Braitwaite and Olsen used or not? Please streamline and clarify.

Section 3.1.2- Maybe a sentence or two about the purpose of the sensitivity tests and how this will interact with the murtoo observations and paleo conditions.

Ln 364- Basal velocities -> Are these prescribed or output of ISSM? Please adjust the sentence to reflect this.

Section 3.2 - Much of this section could fit in the discussion if it impacts the results. Some matters, such as the model representing a single time slice, might fit in other sections as well.

Section 4.1.1 - This section should be reworked to link back to the research objectives at the beginning of the paper. While these results somehow seem reasonable, it is uncertain to me what the aim is. Given the content in Figure 2, it seems that channel distributions would be a reasonable place to begin.

Section 4.1.2 - Consider moving to the first section- Could this be used to show the viability of the model?

Figure 3. Can these variables be linked back to conditions in Table 1 and discussed? This is what I was hoping for when I looked at Table 1.

Paragraph 464-470 - This is a hard paragraph to follow.

Ln. 490 - This sentence is almost exactly like the one at the top of 3.1.1.

Paragraph 553-572 - The findings of the model results are not discussed here. I am curious about the interaction of the result with this existing knowledge.

Ln 580. For reasons regarding channel shape discussed in my last review, I believe that these findings about the channel radius are highly speculative.

Section 5.3 Isn't this topic introduced in the paragraph starting at 573?

Ln 690. Thin sediment, low sediment supply -> this needs a citation.

Ln 703. GIA is discussed here. Streamline the manuscript by removing the above.

Hide

ED: Reconsider after major revisions (further review by editor and referees) (14 Mar 2024) by Lev Tarasov

One of the original referees is away until the end of this month and
the other referee in their re-review has raised significant concerns
about poor presentation quality and insufficient extent of addressing
some of the referee concerns from both original reviews. Part of the
latter is due to eg zero carry-over of your response to referee 1 concerning
the lack of diurnal variations to the revised submission. Given that you
are comparing to data, and not just doing idealized process studies, you
need to give the reader guidance on which key sources of uncertainty may
invalidate your results and which are unlikely to and why.

I have also found an error in your experimental design:

"The number of positive degree days per month, PDD_M was taken as PDD_M
≥ −5◦ C. We used −5◦ C as our threshold (rather than the more commonly
used 0◦ C threshold) to account for melt which may occur even for days
with an average temperature of 0◦ C"

The Wake and Marshall 2015 (WM15) paper used mean hourly data to determine
their sigma_PDD not mean daily as you implicity assume in your reasoning above.
As such, there is no justification to use -5C as your reference (0 point) value
for computing PDDs.

Your justification of your PDD formula is also misleading. WM15 found
significant variation in skewness and kurtosis and their derived
distribution ("M2") for PDD accounts for this and is therefore not
Gaussian, but you make no mention of this when you cite them for
your choice of sigma(Tmonth) that you apply to a Gaussian.

Please also correct one other source of confusion due to your use of
"output parameter" to describe an output statistic (parameters are
generally model inputs).

Given the above, and to expedite this process, please submit a revised
manuscript that fully addresses the re-review by referee 1 and my
comments above. Please also update, if/as needed the response to
referee 2 (Sarah Greenwood). Note, you may have to append the latter
to the response to referee one, if the manuscript interface restricts
your options.

Hide

AR by Adam Jake Hepburn on behalf of the Authors (17 May 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (05 Jun 2024) by Lev Tarasov

RR by Sarah Greenwood (25 Jun 2024)

ED: Publish subject to minor revisions (review by editor) (07 Jul 2024) by Lev Tarasov

AR by Adam Jake Hepburn on behalf of the Authors (28 Jul 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (13 Sep 2024) by Lev Tarasov

AR by Adam Jake Hepburn on behalf of the Authors (13 Sep 2024) Author's response Manuscript

Journal article(s) based on this preprint

29 Oct 2024

The organization of subglacial drainage during the demise of the Finnish Lake District Ice Lobe

Adam J. Hepburn, Christine F. Dow, Antti Ojala, Joni Mäkinen, Elina Ahokangas, Jussi Hovikoski, Jukka-Pekka Palmu, and Kari Kajuutti

The Cryosphere, 18, 4873–4916, https://doi.org/10.5194/tc-18-4873-2024,https://doi.org/10.5194/tc-18-4873-2024, 2024

Short summary

Adam Jake Hepburn, Christine F. Dow, Antti Ojala, Joni Mäkinen, Elina Ahokangas, Jussi Hovikoski, Jukka-Pekka Palmu, and Kari Kajuutti

Data sets

Supplementary material for 'Reorganisation of subglacial drainage processes during rapid melting of the Fennoscandian Ice Sheet' Adam Hepburn, Christine F. Dow, Antti Ojala, Joni Mäkinen, Elina Ahokangas, Jussi Hovikoski, Jukka-Pekka Palmu, and Kari Kajuutti https://doi.org/10.5281/zenodo.8344208

Video supplement

Movie A1 Adam Hepburn, Christine F. Dow, Antti Ojala, Joni Mäkinen, Elina Ahokangas, Jussi Hovikoski, Jukka-Pekka Palmu, and Kari Kajuutti https://doi.org/10.5281/zenodo.8344208

Adam Jake Hepburn, Christine F. Dow, Antti Ojala, Joni Mäkinen, Elina Ahokangas, Jussi Hovikoski, Jukka-Pekka Palmu, and Kari Kajuutti

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Short summary

Terrain formerly occupied by ice sheets in the last ice age allows us to parameterise models of basal water flow using terrain and data unavailable beneath current ice sheets. Using GlaDS, a 2D basal hydrology model, we explore the origin of murtoos, a specific landform found throughout Finland that is thought to mark the upper limit of channels beneath the ice. Our results validate many of the predictions for murtoo origin and demonstrate that such models can be used to explore past ice sheets.


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