the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Aug 2024
History and dynamics of Fennoscandian Ice Sheet retreat, contemporary ice-dammed lake evolution, and faulting in the Torneträsk area, northwestern Sweden
Abstract. The prospect of alarming levels of future sea level rise in response to the melting of the Antarctic and Greenland ice sheets affirms an urgency to better understand the dynamics of these retreating ice sheets. The history and dynamics of the ephemeral ice sheets of the Northern Hemisphere, such as the Fennoscandian Ice Sheet, reconstructed from glacial geomorphology, can thus serve as a useful analogue. The recent release of a 1 m LiDAR-derived national elevation model reveals an unprecedented record of the glacial geomorphology in Sweden. This study aims to offer new insights and precision regarding ice retreat in the Torneträsk region of northwestern Sweden, and the influence of ice-dammed lakes and faulting on the dynamics of the ice sheet margin during deglaciation. Using an inversion model, mapped glacial landforms are ordered in swarms representing spatially and temporally coherent ice sheet flow systems. Ice-dammed lake traces such as raised shorelines, perched deltas, spillways, and outlet channels, are particularly useful for pin-pointing precise locations of ice margins. A strong topographic control on retreat patterns is evident, from ice sheet disintegration into separate lobes in the mountains to orderly retreat in low-relief areas. Eight ice-dammed lake stages are outlined for the Torneträsk Basin, the lowest of which yield lake extents more extensive than previously identified. The three youngest stages released a total of 26 km3 of glacial lake outburst floods (GLOFs) through Tornedalen, changing the valley morphology and depositing thick deltaic sequences in Ancylus Lake at its highest postglacial shoreline at around 10 cal ka BP. The Pärvie Fault, the longest-known glacially-induced fault in the Sweden, offsets the six oldest lake stages in the Torneträsk Basin. Cross-cutting relationships between glacial landforms and fault scarp segments are indicative of the Pärvie Fault rupturing multiple times during the last deglaciation and, indeed, before deglaciation. Precise dating of the two bracketing raised shorelines or the ages of the corresponding GLOF sediments would pinpoint the age of this rupture of the Pärvie Fault. Collectively, this study provides data for better understanding the history and dynamics of the Fennoscandian Ice Sheet during final retreat, such as interactions with ice-dammed lakes and rupture or re-activation of faults through glacial isostatic adjustment.
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RC1: 'Comment on egusphere-2024-2486', Colby A. Smith, 30 Aug 2024
The manuscript is well organized around the geomorphic map, and draws conclusions related to the timing and dynamics of both the ice sheet retreat and the associated glacially induced faulting in northwestern Sweden. I recommend that the manuscript be published after minor revisions.
I have made some minor comments related to language, grammar, or general readability. These are included in the attached pdf of the manuscript. Here, I will focus on the single larger issue that I believe needs to be addressed prior to publication.
The authors make a case that different segments of the Pärvie fault ruptured at different times during or shortly after the late Weichselian deglaciation. Along Torneträsk, the authors present a strong case for fault rupture shortly after deglaciation based on the fact that older shorelines are displaced by the fault but not younger shorelines. Farther south, the other examples of where faulting either precedes or follows deglaciation are not as well presented. I do not question your conclusions, but that is because I have spent a lot of time looking at these faults in LiDAR. Other readers may need further convincing.
The crosscutting relationships are not obvious in figure 10 and they are not adequately described in the text. Thus, I suggest revising figure 10 to be more like figure 3 and clearly show the crosscutting relationships between the fault and glacial landforms, and the fault and the shorelines. Additionally, include text that explains the crosscutting relationships and the conclusions drawn from them.
This is a nice contribution to the deglacial history of northern Sweden, and I look forward to seeing it published.
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RC2: 'Comment on egusphere-2024-2486', Benjamin Boyes, 16 Sep 2024
General remarks
This article presents a new framework for understanding late glacial landscape evolution in northern Sweden. The study uses original geomorphological data and previously published chronometric data to reconstruct Fennoscandian Ice Sheet retreat patterns, ice dammed lake development, and the evolution of post-glacial faults. This publication is suitable for publication in The Cryosphere after minor revisions, and I look forward to seeing it published.
Academic rigour and accuracy
The study’s methodology is comprehensive, and the results are clearly laid out. However, it would be useful if the following points are clarified:
- The mapping methods could be more clearly laid out. The manuscript suggests you mapped a wide array of features, but details (e.g. how polylines are drawn to map landforms) are only provided for ice dammed lakes (and associated features).
- The fault lines and rock slope failure deposits are not presented in the results. These data are from previous work (as suggested by Figure 1b) and the source of these data need to be more obviously discussed in the text. If you checked these against the LiDAR data, this needs to be discussed.
- The mapping is good and has added considerable detail to the region, and I like how clear the supplementary map is. However, from personal experience mapping landforms in this region from similar LiDAR data and in the field, I think some features have not been mapped. This is entirely subjective, but it would be good to know why you chose to map certain features and if you chose to omit any?
- The relative timing (e.g. last glacial vs previous glacial) of some of the features needs clarification. Why have you decided which landforms are pre-last glacial, and maybe show these features on a map of their own? You mention that this is a thing, but don’t provide any evidence of pre-last glacial landforms.
- As I understand it, mkm-1 is a unit referring to slope? A short sentence clarifying what this means would be helpful.
- Some discussion on how your geomorphological mapping compares with existing geomorphological maps could be interesting. You do this comprehensively for the lakes, but not the other landforms.
Style and structure
The style and structure of the manuscript is currently in keeping with the The Cryosphere and is well organised around the geomorphological mapping and interpretations. The grammar is generally sound, but the manuscript would benefit from checking before publication.
Specific comments
Table 1: This is a nice comprehensive table – I particularly like the “possible identification error” column. It would be helpful to have in this table a column that explains the mapping approach, as an example figure in e.g. Boyes et al., 2021 (https://doi.org/10.1080/17445647.2021.1970036) and/or as text.
Figure 1: In panel b, consider thinning out the isochrons or making the panel bigger. At present, it’s a little difficult to see all of the components in the figure.
Figure 9: If you are unsure whether the ice sheet also retreated into the Kebnekaise/Sarek Mountains, consider leaving a ? symbol over these locations in your retreat pattern to acknowledge this.
Figure 10: The cross-cutting is really difficult to see. Make the panels bigger with nice and clear LiDAR hillshade images.
Line 119: Chandler et al., 2018 don’t suggest these values for hillshade images, other authors do (specifically Chandler cite Smith and Clark, 2005 and Hughes et al., 2010). Change (or add) the citation to other sources.
Lines 119-121: Either remove the statement or define which azimuths were used.
Lines 164-167: Here you provide numbers of how many features you have mapped. However, because you have not detailed the mapping approach for each landform type, it is not clear whether the quoted 6,633 mapped features are individual features or groups of features. For example, you say you have mapped 38 vieki moraines – is that 38 areas of veiki moraine, or 38 individual veiki moraine plateau?
Later on in the results section, you go on to say landforms are found “relatively often” in x locations. It would be better to put a number (i.e. %) on this.Lines 257-259: Please point to this cross-cutting relationship on the figure.
Lines 419-423: Topographic controls on ice sheet geometry during retreat of a thinning ice sheet have also been highlighted in northwest Russia (https://doi.org/10.1002/jqs.1130; https://doi.org/10.1016/j.quascirev.2022.107872; https://doi.org/10.1111/bor.12653).
Lines 436-439: You briefly mention timing of lakes here and have more detail on faulting chronology in Section 5.4. Could you have a single chronology section that deals with the chronologies of each component (ice sheet retreat, ice dammed lake formation/drainage, and faulting) as they are interlinked. It would be better to use the point chronometric data presented by Stroeven et al., 2016 and in the DATED-1 database rather than comparing to the isochrons as this may provide more relevant information for your reconstruction.
Line 515: You’ve suggested that the rock slope failure deposits are a result of post-glacial earthquakes. Such landslides can also be triggered by glacial de-buttressing during glacier retreat. You should include some discussion on this point, and if you still consider these landslides to be earthquake induced, then you need to clearly provide evidence for this.
Citation: https://doi.org/10.5194/egusphere-2024-2486-RC2
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