Modelling the effects of ice transport and sediment sources on the form of detrital thermochronological age probability distributions from glacial settings

Bernard, Maxime; Steer, Philippe; Gallagher, Kerry; Lundbek Egholm, David

doi:https://doi.org/10.5194/esurf-8-931-2020

Articles | Volume 8, issue 4

https://doi.org/10.5194/esurf-8-931-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/esurf-8-931-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 8, issue 4

Research article

|

10 Nov 2020

Research article |

| 10 Nov 2020

Modelling the effects of ice transport and sediment sources on the form of detrital thermochronological age probability distributions from glacial settings

Maxime Bernard, Philippe Steer, Kerry Gallagher, and David Lundbek Egholm

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Final revised paper (published on 10 Nov 2020)
Supplement to the final revised paper
Preprint (discussion started on 11 Feb 2020)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review of Bernard et al. by Todd Ehlers', Todd A. Ehlers, 16 Mar 2020
- AC1: 'Response to Pr. Ehlers', Maxime Bernard, 03 Jun 2020
RC2: 'review', Anonymous Referee #2, 15 Apr 2020
- AC2: 'Response to the anonymous reviewer', Maxime Bernard, 03 Jun 2020
EC1: 'Recommendation', Jean Braun, 20 Apr 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Maxime Bernard on behalf of the Authors (04 Jun 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (19 Jun 2020) by Jean Braun

RR by Anonymous Referee #2 (28 Jul 2020)

Suggestions for revision or reasons for rejection

Here is my second review of “Modelling the effects of ice transport and sediment sources on the form of detrital thermochronological age probability distributions
from glacial settings” by Maxime Bernard and co-authors.

The authors have done a good job in addressing the comments from the first round of review. My main concern was that sediment transport by the subglacial hydrology system was ignored. I still think it is a very strong limitation of this study, but the authors clearly acknowledge it now. Much work is still required on how moraines form and how long it takes for sediments to be exported from a glacier and/or glaciated catchment. The only study (that I am aware of) that tried to date the age of sediments in moraines is Cogez et al. (2018) (https://esurf.copernicus.org/articles/6/121/2018/). Using U and Th series and cosmogenic nuclide dating, they found transit times that can be as long as 100-200 kyr. This is substantially longer than the 1500 years argued here, which implies that sediments can reside for a very long time in a glaciated catchment and reflect several glacial cycles. This point may be included in section 4.2, even though it is clear that this number is likely to change from one glacier to another.

Minor comments:
1/24 (abstract): Transport times of sediment must be highly affected by subglacial hydrology too.
2/20: I guess the authors wish to cite Willenbring and Jerolmack (2016) and not Willenbring et al. (2016).
6/8: Please, give orders of magnitude of what is meant by “reasonable sliding velocities”.
8/18: How reliable is the ice-thickness estimate from the ITMIX experiment? It is a global analysis, and many observe large variations for each individual glacier.
10/1: What type of noise is added?
18/30: Erosion rates of 31 mm/yr sound high at first, but it seems that it scales reasonably well with the prescribed velocity. (The ratio between erosion rates and sliding velocity is about 10^-4 for most glaciers where people have attempted to quantify erosion rate and sliding simultaneously.)

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RR by Todd A. Ehlers (01 Sep 2020)

Suggestions for revision or reasons for rejection

My apologies for the delay in submitting this review. When I sat down to review this about a month ago I couldn't access it on Copernicus, and only in the last week did Copernicus respond to my request to have access. There appears to have been some technical issues.

I have read the revised manuscript, response to reviews, and supplemental information. Overall the authors have done a very thorough and diligent job responding to both reviewers (Thank you!). My previous comments largely focused on the need for (1) improved clarity in the methods so I can actually understand what they've done, (2) inclusion of some model sensitivity tests to broaden the utility of the manuscript, and (3) better comparisons to relevant data that were previously published. The authors have addressed all of these concerns well, and the manuscript reads much better now. Minor grammatical issues have also been fixed.

There are still some vagaries in my opinion concerning the conclusion that there is bias towards mid-altitude ages showing up. It would be nice if the authors were a little more specific about this in the conclusions (i.e. expanded the sentence some) and also clarified why this occurs and if it is a problem that will be systematic of all detrital sampling studies, or is specific to this glacier. This makes a difference for those of us also actively collecting new samples. This is a very minor point - and should not impede publication.

Finally, I also read the 2nd (anonymous) reviewer's comments and the authors response to them. The anonymous reviewer raised some good points, but many of the more serious criticisms this reviewer raised are (in my opinion) the result of the lack in clarify of the first version of the manuscript and many of the points I raised. Thus, while the anonymous reviewer's opinion should be valued, I can add here that in my opinion many of the comments raised by this reviewer are addressed in the now clarified revised manuscript.

This is a useful study and I think I think it advances our knowledge on this topic. I recommend publication in its current form, or with the small technical correction to the conclusions specified above.
-Todd Ehlers.

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ED: Publish subject to technical corrections (04 Sep 2020) by Jean Braun

ED: Publish subject to technical corrections (06 Sep 2020) by Andreas Lang (Editor)

AR by Maxime Bernard on behalf of the Authors (09 Sep 2020) Author's response Manuscript

Short summary

Detrital thermochronometric age distributions of frontal moraines have the potential to retrieve ice erosion patterns. However, modelling erosion and sediment transport by the Tiedemann Glacier ice shows that ice velocity, the source of sediment, and ice flow patterns affect age distribution shape by delaying sediment transfer. Local sampling of frontal moraine can represent only a limited part of the catchment area and thus lead to a biased estimation of the spatial distribution of erosion.