the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Oct 2024
Glacial erosion and history of Inglefield Land, northwest Greenland
Abstract. We used mapping of bedrock lithology, bedrock fractures, and lake density in Inglefield Land, northwest Greenland, combined with cosmogenic nuclide (10Be and 26Al) measurements in bedrock surfaces, to investigate glacial erosion and the ice-sheet history of the northwestern Greenland Ice Sheet. The pattern of eroded versus weathered bedrock surfaces and other glacial erosion indicators reveal temporally and spatially varying erosion under cold- and warm-based ice. All of the bedrock surfaces that we measured in Inglefield Land contain cosmogenic nuclide inheritance with apparent 10Be ages ranging from 24.9 ± 0.5 to 215.8 ± 7.4 ka. The 26Al/10Be ratios require minimum surface histories of ~150 to 2000 kyr. Because our sample sites span a relatively small area that experienced a similar ice-sheet history, we attribute differences in nuclide concentrations and ratios to varying erosion during the Quaternary. We show that an ice sheet history with ~900 kyr of exposure and ~1800 kyr of ice cover throughout the Quaternary is consistent with the measured nuclide concentrations in most samples when sample-specific subaerial erosion rates are between 0 and 2 x 10-2 mm yr-1 and subglacial erosion rates are between 0 and 2 x 10-3 mm yr-1. These erosion rates help to characterize arctic landscape evolution in crystalline bedrock terrains in areas away from focused ice flow.
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RC1: 'Comment on egusphere-2024-2983', Anonymous Referee #1, 27 Nov 2024
General comments
This manuscript describes a field and GIS campaign conducted in Northwestern Greenland. Using cosmogenic nuclide dating, landfom/geological mapping, and modelling of possible exposure histories the authors describe exposure/ice-cover history at the margin of a relatively passive section of the Greenland Ice Sheet. The authors find a signal consistent with the onset of ice streams and periods of warm based ice amongst an otherwise predominantly cold-based regime. I found the manuscript to be clearly laid-out, well-written, and enjoyable to read.
Specific comments
Figure 1. Relating to my comment below (Line 100-102), I think adding an annual ice velocity composite (e.g, GrIMP) to the figure would be useful.
Figure 1. Could the approximate location of the images in Figure 3 be labelled here?
Figure 6. Does the size of the transparent arrow scale with the approximate magnitude of erosion?
Figure 7. As with figure 3, the location of these (and indeed all) sampling sites might be of interest and could be included as a supplementary figure.
Line 31-37: This might be a stronger opening paragraph if the formal definition of ice streams is moved elsewhere, and line 33 transitions straight into line 38. As it is here, it is not immediately clear what the relevance of ice streams are to this work.
Line 81: Given discussion of transitions between thermal state earlier in the intro, I’d suggest this reference to a key transition is unnecessarily unclear and the sentence could instead be simplified to something along the lines of: “Cosmogenic nuclide and luminescence analysis of sediment and bedrock samples collected from the glacier bed...”
Line 89--94: I would normally expect to see this text in the conclusions. A summary of the work along these lines is already given in the abstract and these may not be necessary.
Line 100-102: As far as I can see, this is the only comment on the current behaviour of ice bordering the study area. I wonder if there is a missed opportunity to further contextualise this study by commenting further on the current variability (or lack thereof) of the ice sheet along this margin. Annual ice velocity, though broadly fairly low (<~50 m yr-1) is variable and Hiawatha glacier in particular is relatively fast compared to surrounding ice.
Line 112: Is the Fig 1. On this line in reference to that of England 1999? Otherwise, the Figure 1 here does not show the LGM ice sheet configuration of the study area and I would suggest the removal of the reference here.
Line 117: “maintaining to a smaller position” is the to necessary?
Section 3.3.2: Could an approximate scale range for “landscape scale” be given here. Even a mapping scale (if fixed) or a minimum mapping scale (if variable) would be useful.
Line 187: Given this is a paper on variable erosion rates within a small area of an ice sheet, “erosive ice sheets” here feels a bit reductionist and could be more precise if it was "areas of high erosion rates beneath ice sheets" for example.
Line 200: What is meant by lakes “dammed by sediments”?
Line 274: Could you specify the threshold value here please?
Line 281: Is there precedent for this scaling? If not, could you include a brief comment on why you expect this to not affect your conclusions? It seems there might potentially be some uncertainty introduced if there was any lateral variation in retreat or any changes in the speed of retreat.
Line 355: could an example of glacial erosional features be given? I understand they are absent but it might be useful for a reader to know exactly what is absent that might otherwise be expected.
Line 381: I am missing the link here between the presence of ice-sculpting and the duration of any “wet-based ice”. As a sidenote, could a consistent terminology be used throughout (see also 573-575)? In the introduction it is warm and cold based, but here it is wet-based.
Line 438: Could a reference to recent advance and retreat patterns be added here?
Line 443: I am not sure how relevant the Petermann Glacier is to this exact setting. Although relatively close to the study site, the ice dynamic contexts are very different, with Petermann draining a large portion of the ice sheet relative to this peripheral area of the ice sheet.
Line 476: I am not suggesting this is carried out here, but bathymetric investigation in the Kane Basin may be a fruitful avenue for future work to look for geomorphic investigation of ice stream signatures.
Line 525: suggest adding “while our results suggest that Inglefield Land was covered…”
Lines 537-541: multiple uses of “these” make the subject of these sentences unclear.
Lines 540-541: It might also be worth adding the role of increased meltwater delivery to the bed in a warming climate
Technical comments
Line 44 (and elsewhere): glaciated is understood to mean “formerly occupied by glaciers”. The use of previously here and subsequent use of formerly are therefore redundant throughout.
Line 171: misspelling of terrains?
Line 162: these should be EM dashes.
Line 186: …generally topographically smooth on the landscape-scale…
Line 440: is there a missing the? E.g., …collapse over the Nares Strait
Citation: https://doi.org/10.5194/egusphere-2024-2983-RC1 -
RC2: 'Comment on egusphere-2024-2983', Pierre Valla, 20 Jan 2025
Dear Authors, dear Editors,
Please find below my evaluation concerning the manuscript by Walcott-George and co-authors entitled "Glacial erosion and history of Inglefield Land, northwest Greenland" (manuscript egusphere-2024-2983). First of all, I sincerely apologize for the delay in sending my evaluation.
This manuscript investigates the ice-sheet fluctuation history and subaerial/subglacial erosion over the Quaternary in the Inglefield Land (northwest Greenland). The authors combine mapping of glacial erosion features with in-situ cosmogenic nuclides (26Al/10Be) to investigate the glacial erosion dynamics in this area and propose a quantification of both Quaternary bedrock exposure/ice-sheet covering and subaerial/subglacial erosion rates. Their mapping allows to distinguish different spatial areas with intense (ice stream, coastal zones), minor (inland) and intermediate (ice margin) erosion from landscape features and field observations. Cosmogenic nuclide data confirm these differences and allow to propose subglacial/subaerial erosion rates together with a shared surface history (bedrock exposure/ice-sheet covering) during the Quaternary.
This is an interesting manuscript, overall well-written and nicely illustrated. I think that the approach combining landscape mapping and in-situ cosmogenic nuclide dating is highly valuable and allows to provide both a spatial pattern and a quantification of subglacial erosion. In addition, the combined use of 26Al/10Be nuclides shows contrasted glacial histories for the different sectors of the Inglefield Land. I think that their approach would be interesting to the community and readership of The Cryosphere, but at present the integration of literature evidence to further support or discuss the new findings is sometimes missing, as well as some methodological information (fracture mapping) which could further strengthen the present study and the author’s message.
I have outlined below my questions and suggestions in a set of general and specific comments below.
General comments:
1 – Literature data. I would suggest to present the literature 10Be and in-situ 14C data from Inglefield Land (Sondergaard et al., 2020) at the beginning of the manuscript (section 2), and mapped (Figure 1) since these datasets could be discussed in comparison with the new data). Furthermore, they are important to support the hypothesis of a shared ice-sheet fluctuation history between all studied samples (section 3.5) which are spanning 20-30km distance. At present the information are not missing but they appear at different places in the manuscript and the literature samples cannot be located in the study area.
2 – Mapping and figures. The figures 1 to 6 (excluding figure 3) are sometimes repeating the information and can be synthesized into fewer figures I think. Moreover, the figure 4 (fracture mapping) is somehow misleading since it already shows the areas with bedrock high-density fractures, but there is no figure to show the actual mapping of these areas (same remark for lake cover from LANDSAT images, a methodological zoom would be possibly interesting for readers). Please consider reworking the figures to provide more illustration of the mapped objects.
3 – Discussion of output erosion values. I enjoyed the model outcomes for subaerial and subglacial erosion rates (Figure 11), and their discussions in comparison to literature data. However, there is only little discussion about the spatial variability of these erosion rates between the different sectors (coast/inland/ice margin) and in comparison with the mapping outcomes (Figure 6). Are the output subglacial/suaerial erosion rates really correlated as proposed in the methods section? Can we hypothesize that subaerial erosion rates could be uniform over the study area, and if yes what would be the implications for subglacial erosion rates? Finally, are output subglacial erosion rates really variable between the different sectors to explain the differences in in-situ cosmogenic nuclides? I think the readers will benefit from more discussion around these points given the interesting model outcomes.
Specific comments, by line number:
- Line 21. “... requires minimum surface histories of ~150 to 2000 kyr”. Maybe precise what is meant by “surface histories” there for clarity.
- Lines 26-27. “between 0 and 2 x 10^-2 mm yr^-1”. Please consider rephrasing in “maximum 2 x 10^-2 mm yr^-1”. Same remark for line 27.
- Line 31. “the largest single contributor”. Why “single” there?
- Line 45. “...and basal processes”. Maybe specify which ones.
- Line 46. “The distribution of glacial erosional features”. Which ones, and at what spatial scale?
- Line 49. “ancient landscape”. How ancient, Quaternary or even pre-glacial?
- Line 54. “mapping of erosion imprints on a landscape”. I am wondering how these glacial surface features may be preserved or altered during post-glacial exposure (i.e. weathering and geomorphic processes). Maybe can the authors comment on this?
- Line 62. “from multiple periods of exposure”. Maybe specify “during interglacials”.
- Line 69. “shielded from the cosmic ray flux by ice...”. Please specify for non-experts the minimum ice thickness for complete shielding.
- Line 75. “and longer-term ice sheet history...”. I think this could be interesting for readers to have a brief summary of the main outcomes of these previous works.
- Line 77. “the bed of extant ice sheets”. Unclear, please rephrase.
- Line 81. “under ice sheets”. Maybe add “modern” for clarity.
- Line 94. “rates to vary for each sample”. I would suggest to recall there, as for the total surface history, the main outcome in terms of subglacial/subaerial erosion rates.
- Line 106. “Dallas Bay, and Marshall Bay”. From Figure 1, Marshall Bay appears more to the west than Dallas Bay, please correct.
- Line 115. “and in-situ 14C from erratic boulders”. Where are these samples in the study area? Would it be possible to show them on Figure 1? (see also my general comment).
- Line 121. “cosmogenic nuclide inheritance”. 10Be and/or 26Al data? Please specify.
- Line 121. “in boulders across Inglefield...”. Can this literature dataset be shown on figure 1 and compared to the new data from this study?
- Line 125. “bedrock at Camp Century and Summit”. Please provide locations of these zones for readers, maybe in Fig.1 inset, for clarity?
- Line 151. “variable erosion across the landscape”. Specify whether this is subglacial, subaerial or both.
- Line 167. “relative amount of weathering or ice sculpting”. How is this evaluated? What form(s) or evidence? Please provide more details about this approach.
- Line 180. “hillshade image with a three-times vertical exaggeration for our mapping (Fig. 4)”. It would be nice for readers to have an example of such hillshade images, to help evaluating how mapping has been performed to delimitate zones on figure 4.
- Line 182. “to avoid issues with differentiating fractures from other features”. Unclear, please rephrase.
- Line 192. “...shows larger lakes”. Are these the lakes shown on Figure 2? If yes, maybe specify it.
- Line 198. “a suitable threshold value”. Maybe indicate the chosen value for readers.
- Line 208. “along two SSE-NNW transects...”. Please refer to Table S1 for sample locations.
- Line 240. “and 26Al sample”. Is there a blank correction for 26Al (not provided in Table S2). Moreover 10Be/9Be and 26Al/27Al blank ratios are not provided in main text nor in Tables S1-S2, or maybe I missed them. Please correct or clarify for readers.
- Line 259. “in-situ 14C ages”. Are these ages also from boulder samples, or only those with 10Be ages? This reads unclear from the present sentence, please clarify.
- Line 264. “Given the relatively small distance from the modern ice margin to coast and the speed at which Inglefield Land deglaciated following the LGM”. This is a strong hypothesis, and poses question about its validity given that 14C ages are only described as Holocene ages. There is more information given on lines 281-284, but I would suggest to present further the 14C data (also show them on a map) since these are important evidence for the reasoning. See also my general comment on this point.
- Line 316. Would we expect similar ranges for subaerial and subglacial erosion rates? I see from the broad range that this would be possible, but maybe clarify for readers.
- Line 352. “of various minor rock units...”. Is this needed, since we do not see the minor units on figure 2?
- Lines 360-364. This was not entirely clear from the Methods text and Figure 4 that contoured zones are mapped fractured areas, not the investigated ones. Please clarify this, and potentially show a zoom on a fractured zone to show the approach to readers. See also my general comment.
- Line 379. “Apparent 10Be ages”. Maybe add “exposure” for clarity.
Can the authors specify the apparent 10Be exposure age for the boulder sample in this paragraph? This is not explicit from the text.
- Lines 418-419. Please refer to Figure 9 there. Would it be possible to evaluate a range/uncertainty for the threshold value, discarding one sample or another? I have the impression that the range 3.55-3.65‰ could be possible, would it be correct and what implications for the exposure/ice-cover durations? Please consider discussing this point.
- Line 421. “This best-fitting exposure history...”. Is this outcome in agreement with the Holocene final deglaciation as proposed by in-situ 14C ages? From Figure 10 I have the impression that a late post-LGM ice retreat is compatible with a 3.6‰ value, but this is difficult to see.
- Line 424. Please refer to Figure 11.
- Line 428. “likely has a different exposure history than the collocated bedrock.”. How about for output erosion rates, are they similar between bedrock surfaces and the boulder?
- Lines 438-442. These lines may already be presented in the settings, since they give important information for the rapid ice-sheet fluctuations in the study area and support the hypothesis given in section 3.5. See also my general comment.
Another secondary question: what is the Nares Strait, is it visible on figure 1?
- Line 453. “our other mapping”. What is “other”?
- Line 510. “0 mm yr^-1 and 2 mm yr^-1”. Maybe this can be rephrased as “0-2 mm yr^-1”. I am wondering whether the results can be discussed as spatially homogeneous or whether there is a spatial pattern for subaerial/subglacial erosion rates. See also my general comment.
- Lines 516-517. The output subaerial erosion rates appear much higher for some samples than for the literature value. What are the implications for subglacial erosion rates is a common subaerial erosion rate is taken (e.g. 2-3 x 10^-3 mm yr^-1)? And would this hypothesis of a regional similar subaerial erosion rate be plausible? Please discuss this point.
- Line 523. “0 and 2 x 10^-3 mm yr^-1” for subglacial erosion rates, no? Same remark for line 546.
- Line 538. Also the literature values for subglacial erosion rates are mostly integrated ofer short periods (maximum LGM), no? These then might be a temporal integration effect, as already shown by Spotila et al. (2004) or Koppes & Montgomery (2009). Maybe discuss this point.
- Lines 561-565. “if differential erosion is invoked...”; “Spatial variability in erosion is captured...”. Are subglacial erosion rates really different between the studied sites (coast/inland/ice margin)? This is not straightforward from Figure 11, and would require I think more discussion about the model outcomes. See also my general comment.
- Line 600. Can this part of the discussion be extended by comparing outcome results to those of previous studies using cosmogenic nuclides (in Greenland or Scandinavia) for estimating surface histories?
- Line 675 – Figure 1. Several questions/remarks for this figure:
- Maybe add the literature data (boulders with 10Be and in-situ 14C dating) for clarity.
- Is there a Dallas valley in front of the Dallas Bay, as for other locations?
- What is the resolution and source of the DEM used for topography?
- What are the text “100 m asl” and “255 m asl” for coastal sites?
- Line 703 – Figure 3: Can the authors provide the locations of pictures on Figure 1? And maybe a quick description of the pictures in the caption could help (or in the main text line167).
- Line 719 – Figure 4. This figure is not really explicit and a bit redundant to figures 1-2 and 6. Can this be replaced by a zoom on a fractured zone that was actually mapped?
- Line 752 – Figure 6. The label code “CR” is missing for SURF samples. Please also indicate the boulder sample for clarity. Finally, are the arrows (potential zone of increased erosion from mapping) discussed somewhere in the text with respect to subglacial erosion rates (Fig. 11)?
I hope these comments and suggestions may be useful for revising this interesting manuscript, and I look forward to seeing it published.
Sincerely,
Pierre Valla
(Grenoble, 20 January 2025)
Citation: https://doi.org/10.5194/egusphere-2024-2983-RC2 -
EC1: 'Comment on egusphere-2024-2983', Ian Delaney, 20 Jan 2025
Dear authors,
I have received two positive and constructive reviews for this manuscript. R1 largely has specific suggestions for improvement. However, R2 presents some discussion points and requests more detailed methodological information.
I invite the authors to prepare a response to the reviews.
Ian Delane - EditorCitation: https://doi.org/10.5194/egusphere-2024-2983-EC1
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