the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 May 2025
Formation of mega-scale glacial lineations far inland beneath the onset of the Northeast Greenland Ice Stream
Abstract. Rapidly-flowing ice streams drain the interior of the Greenland Ice Sheet, currently accounting for around half of its annual mass loss. The Northeast Greenland Ice Stream (NEGIS) is one of the largest, recognisable almost 600 km inland, and extends close to the central ice divide. Numerical ice sheet models are unable to accurately reproduce the configuration of the NEGIS, but understanding its bed properties and spatial and temporal evolution is critical to predicting its future contribution to sea-level change. Here, we use swath radar imaging to create a high-resolution Digital Elevation Model of the bed close to where the NEGIS initiates. Surprisingly, this reveals a landscape interpreted to include mega-scale glacial lineations (MSGLs) that are often assumed to be indicative of rapid ice stream flow (100s m yr-1), under present-day flow velocities of only ~60 m yr-1. Given that MSGLs are thought to form under much higher flow velocities, their presence so far inland at an onset zone raises important questions about their formation and preservation under ice streams, as well as past configurations of the NEGIS. Elongate bedrock landforms outside the current shear margins also suggest that the NEGIS was wider than its present configuration at some point in the past.
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RC1: 'Comment on egusphere-2025-1743', Edouard Ravier, 26 May 2025
Review of Carter et al. – High-resolution DEM of the NEGIS Onset
In this study, Carter et al. present the first high-resolution (25 m) digital elevation model (DEM) of subglacial topography at the onset of the Northeast Greenland Ice Stream (NEGIS), derived from swath radar imaging. The data reveal the presence of mega-scale glacial lineations (MSGLs) beneath the ice stream onset zone—a truly relevant discovery given the relatively low present-day ice velocities (~60 m yr⁻¹), which are considerably lower than the velocities classically associated with the formation of MSGL formation.
The authors interpret a subglacial landscape composed of both soft-sediment features (e.g., MSGLs, sedimentary basins) and hard-bed landforms (e.g., crag-and-tails, drumlins), indicative of a complex, mixed-bed basal environment. The presence of these lineations far inland (~600 km from the coast) and near the ice divide (~200 km) challenges conventional associations between MSGLs and rapid ice flow, suggesting instead that relatively slow but sustained ice streaming may suffice for their development. This has important implications for the use of subglacial bedforms as indicators of past ice dynamics and flow velocities.
The manuscript is concise, clearly written, and well-illustrated. The unexpected discovery of MSGLs in an area with modest ice flow velocity is particularly significant, and the study will be of broad interest to the community, especially those reconstructing past ice-sheet dynamics from the morpho-sedimentary record. However, I have several comments and suggestions for strengthening the discussion and interpretations. Especially the discussion would benefit from greater clarity around MSGLs genesis and evolution, including mechanisms for MSGLs formation under varying basal conditions.
Major Comments
- Interpretation of Sediment vs. Bedrock Features
The interpretation of landforms as either sedimentary or bedrock-based is critical to the study's conclusions. However, this distinction is primarily made using morphologies depicted in DEM. I recommend that the authors more clearly justify how these distinctions are made (give a guideline for identifying sediments or bedrock) and discussing the associated uncertainties.
- Timescales of MSGL Formation
The authors suggest that 2000 years is a short period for MSGL formation. Please clarify: short compared to what (other studies, modelling) ? You could include comparisons with MSGLs associated with rough dating of ice stream duration in other settings (e.g., Margold et al., 2018; Laurentide ice sheet) to contextualize this assertion. Some of the ice streams are suggested to be short-lived in Margold studies while elongated streamlined bedforms are also described in some of them.
- Organization of Section 2.2
Much of the content in Section 2.2 introduces and describes landform characteristics from the swath radar images and should be part of the results section rather than the Methods. This reorganization would also reduce some redundancies between Sections 2.2 and 3.1.
- Discussion of MSGL Formation Scenarios
The authors could expand the discussion by considering various combination of factors that could control the evolution of bedform metrics, i.e. ice flow speed, duration, sediment availability and deformability that could occur at the NGEIS:- Slow flow / long duration
- Slow flow / short duration + highly deformable sediments (low cohesion, high dilatancy, high porewater pressure
- Episodic fast flow (e.g., surging phase of ice streams, maybe in relation with changes in subglacial hydrology) during short duration
- Lateral variation in till thickness ?
- …
- Role of Meltwater
Could meltwater have contributed to the rapid formation of MSGLs, (erosion ? Increasing ice flow velocity) ? Is there any evidence for upstream subglacial lakes, as seen in Antarctica?Minor Comments
- The shear margin should be depicted as a band rather than a line; its width and evolution could influence the interpretation of some nearby landforms (being along the shear band rather than outside the ice stream).
- Terminology should be standardized early (e.g., "elongated ridges" vs. MSGLs vs. drumlins vs. crag-and-tails).
- Consider including a rose diagram showing the orientation of MSGLs relative to present-day ice flow vectors and maybe relative to the axis of shear margins.
- If you can mark locations of sediment cores or previous seismic data from existing literature on Figure 1 to highlight how this survey area extends previous knowledge.
- Maybe consider the possibility that oblique ridges seen are deformed ribbed bedforms (long axis 45° oblique to the MSGLs and shear margins (becoming oblique, or due to lateral strain variations in the shear zone, cf. lines and circles on Figure 7 on the annotated pdf ).
- A brief overview of competing models for bedform formation (e.g., bed deformation vs. accretion/erosion) as a preamble for MSGLs explanation would strengthen discussion and interpretation
- AC1: 'Reply on RC1', Charlotte Carter, 11 Aug 2025
- Interpretation of Sediment vs. Bedrock Features
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CC1: 'Comment on egusphere-2025-1743', Jessey Rice, 30 Jun 2025
Formation of mega-scale glacial lineations far inland beneath the onset of the Northeast Greenland Ice Stream
Charlotte M. Carter, Steven Franke, Daniela Jansen, Chris R. Stokes, Veit Helm, John Paden, Olaf Eisen
General comments:
The manuscript provides significant new insights into the subglacial geomorphology and subglacial evolution of the NEGIS. The identification of MSGLs under current low velocities and within relatively close proximity to an ice divide provides an important contribution to our understanding of subglacial dynamics. While this data provides invaluable details on subglacial landform geology and basal conditions, we feel this argument could be strengthened with a brief discussion of paleo-reconstruction from the LIS that has also highlighted warm-based, moderately fast-flowing conditions in relatively close proximity to ice divides (e.g., Hodder et al., 2019; Rice et al. 2020; McMartin et al. 2021). Specifically, Hodder et al. (2019) identified a complex till stratigraphy that suggested considerable till production in the vicinity of the last position of the Keewatin Ice Divide, possibly related to deposition as part of a large tributary ice stream flowing north toward the Arctic Ocean. McMartin et al. (2021) identified that the Dubawnt Lake Ice Stream propagated back to an area where the Keewatin ice divide migrated over and is presumed to have been located. Paulen et al. (2017, 2025) documented mineral dispersal from a rare earth element deposit occurring in the trunk of the Kogaluk River ice stream at Strange Lake, Labrador, where the onset zone of that ice stream (IS #187 of Margold, 2015) was a mere 30 km from the mapped location of the Ancestral Labrador Ice Divide in the George River region (Dubé-Loubert et al., 2021). Finally, Rice et al. (2024) provided evidence of ice divide migration in close proximity to both the Ungava Bay Ice Streams, the Cabot Lake Ice Stream, and the Smallwood Reservoir Ice Stream. We believe these LIS comparative studies, especially their documentation of “mixed bed landform assemblages,” provide particularly insightful examples for your manuscript, as they may have had similar glacial lineation formation to what you are observing in the NEGIS. Again, this manuscript provides important insights into the formation of these subglacial conditions, which offer unique insights into the broader subglacial mechanics at play, especially regarding ice flow velocities. However, we believe the examples from the LIS will also provide important insights into their formation and preservation. Please see specific examples below. If you have any questions or require any clarification on these comments, please feel free to reach out to me: jessey.rice@nrcan-rncan.gc.ca.
Jessey Rice, Geological Survey of Canada, Ottawa
Specific examples:
Line 220 (Dowdeswell et al., 2014), the Amundsen Sea Embayment (Graham et al., 2009) and the North Sea (Roberts et al., 2019) (Fig. 7). These types of subglacial landscapes are characterised by the presence of crag and tails, drumlins, and highly elongate lineations, and thought to be formed under ice streams (Roberts et al., 2019; Dowdeswell et al., 2014; Graham et al., 2009)
Additional North American studies that could strengthen this argument: Margold et al. (2015), Eyles et al. (2018), Sookhan et al. (2021), Rice et al. (2024).
Line 266: To our knowledge, the only other high-resolution survey of an ice stream onset zone (King et al., 2007) revealed classic drumlin forms (elongation ratios 1:1.5 to 1:4) and a potential ribbed moraine under Rutford Ice Stream, West Antarctica, where velocities accelerate from 72 to >200 m yr-1. Furthermore, the location of the MSGLs in our study, 600 km from the grounding line and only ~200 km from the main ice divide, is also the furthest inland that MSGLs have been identified
Depending on the definition of high resolution, air photo interpretation (see Geological Survey of Canada Canadian Geoscience Maps (CGM) 410 and 429 (1:100 000 scale) produced from 1:60 000 scale air photos) indicates the onset zone of an ice stream in even closer proximity to the divide (summarized by Rice et al., 2024). Similar findings using higher satellite imagery were made by Dubé-Loubert et al. (2021).
Line 270: Furthermore, the location of the MSGLs in our study, 600 km from the grounding line and only ~200 km from the main ice divide, is also the furthest inland that MSGLs have been identified.
Assuming this is just for the NEGIS, as MSGLs have been identified further inland in other regions of former continental ice sheets (i.e., the Laurentide Ice Sheet), can you please clarify?
Line 308: Whilst we cannot rule out an episode of enhanced flow at this location in a previous glaciation, the sedimentary basins outside of the northwestern shear margin (Fig. 6) show little evidence of MSGL formation. This would mean that the ice stream would have to have formed in the same configuration as observed today in a prior glaciation, potentially with a higher velocity, to produce the observed MSGLs. Even so, if this had occurred, this would then suggest that MSGLs can be preserved for 100s to 1000s of years under relatively slow ice velocities.
We agree with this assumption, but postulate, is it potentially also possible that the lower velocity of the ice stream is due to ice divide migration closer to the study area? (i.e., was the divide previously further upstream and slowly propagated toward the study area, lowering velocity as it did so?
Cited references:
Dubé-Loubert, H., Roy, M., Veillette, J.J., Brouard, E., Schaefer, J.M., and Wittmann, H. 2021. The role of glacial dynamics in the development of ice divides and the Horseshoe Intersection Zone on the northeast- ern Labrador Sector of the Laurentide Ice sheet. Geomorphology, 387: 107777. https://doi.org/10.1016/j.geomorph.2021.107777
Eyles, N., Arbelaez-Moreno, L., and Sookhan, S., 2018. Ice streams within the last Cordilleran Ice Sheet of western North America; Quaternary Science Reviews, v. 179, p. 87–122. https://doi.org/ 10.1016/j.quascirev.2017.10.027
Hodder, T.J., Ross, M., and Menzies, J. 2016. Sedimentary record of ice divide migration and ice streams in the Keewatin core region of the Laurentide Ice Sheet. Sedimentary Geology 338(1): 97–114. https://doi.org/10.1016/j.sedgeo.2016.01.001
Margold, M., Stokes, C.R., Clark, C.D., Kleman, J., 2015. Ice streams in the Laurentide Ice Sheet: a new mapping inventory. J. Maps 180–395. https://doi.org/10.1080/ 17445647.2014.912036
McMartin, I., Godbout, P.-M., Campbell, J.E., Tremblay, T., and Behnai, P. 2021. A new map of glacigenic features and glacial landsystems in central mainland Nunavut, Canada. Boreas, 50(1)
https://doi.org/10.1111/bor.12479
Paulen R.C., Stokes C.R., Fortin R., Rice J.M., Dubé-Loubert H., and McClenaghan, M.B., 2017. Dispersal trains produced by ice streams: An example from strange Lake, Labrador, Canada. In: Tschirhart V and Thomas MD (eds.) , Proceedings of Exploration 17, Sixth Decennial International Conference on Mineral Exploration, pp. 871–875.
Paulen, R.C., McClenaghan, M.B., & Evans, D.J.A., 2025. Glacial erratics and till dispersal indicators. In S. Elias (Ed.), Encyclopedia of Quaternary Science (3rd ed., Vol. 2, pp. 370–379). Elsevier. https://doi.org/10.1016/B978-0-323-99931-1.00277-4
Paulen, R.C., Rice, J.M., and Ross, M., 2020. Surficial geology, Lac Laporte, Quebec, NTS 23-P southwest; Geological Survey of Canada, Canadian Geoscience Map 410, scale 1:100 000. https://doi.org/10.4095/314756
Paulen, R.C., Rice, J.M., Ross, M., 2022. Surficial geology, lac aux Goèlands, Quebec, NTS 23-P southeast. Geological Survey of Canada, Canadian Geoscience Map 429, scale 1:100 000 https://doi.org/10.4095/328291
Rice, J.M., Ross, M., Paulen, R.C., Kelley, S.E., and Briner, J.P., 2020. A GIS-based multi-proxy analysis of the evolution of subglacial dynamics of the Quebec–Labrador ice dome, northeastern Quebec, Canada; Earth Surface Processes and Landforms. https://doi.org/10.1002/esp.4957
Rice, J.M., Ross, M., Campbell, H.E., Paulen, R.C., and McClenaghan, M.B., 2024. Net evolution of subglacial sediment transport in the Quebec–Labrador sector of the Laurentide Ice Sheet; Canadian Journal of Earth Sciences, v. 61, p. 524–542. https://doi.org/10.1139/cjes-2023-0050
Sookhan, S., Eyles, N., Bukhari, S., and Paulen, R.C., 2021. LiDAR-based quantitative assessment of drumlin to mega- scale glacial lineation continuums and flow of the paleo Seneca-Cayuga paleo-ice stream; Quaternary Science Reviews, v. 263. https://doi.org/10.1016/j.quascirev.2021.107003
Citation: https://doi.org/10.5194/egusphere-2025-1743-CC1 -
CC2: 'Reply on CC1', Jessey Rice, 01 Jul 2025
Apologies, I should have stated that these comments were a product of discussions between Roger Paulen (GSC-Ottawa), Martin Ross (University of Waterloo), and myself (Jessey Rice).
Citation: https://doi.org/10.5194/egusphere-2025-1743-CC2 - AC3: 'Reply on CC1', Charlotte Carter, 11 Aug 2025
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CC2: 'Reply on CC1', Jessey Rice, 01 Jul 2025
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RC2: 'Comment on egusphere-2025-1743', Kiya Riverman, 02 Jul 2025
This paper presents a novel and compelling dataset describing bedforms of the initiation of the NE Greenland Ice Stream (NEGIS). Studies of this ice stream are impactful because of the unique geometry and flow style of the ice stream: it extends deep within the interior of the Greenland Ice Sheet, and rapid flow initiates from a singular point, with flow widening downglacier. Interestingly, NEGIS does not flow within a bed trough — the locations of its shear margins are set by some process other than topographic forcing. This means that there is the potential for the ice stream to widen or shrink on rapid timescales. There has been a history of publications discussing the role of the bed in controlling the location and geometry of this ice stream using seismic and radar surveying. However, since those publications were released, radar surveying techniques have improved to now allow for much a much higher resolution look at the shape of the bed. What this enables is now a very compelling test of many of the hypotheses laid by prior work. With a more complete view of the bed of NEGIS, we can better understand how its subglacial geology and hydrology control its flow. The new radar dataset shows a streamlined bed with a variety of interesting features worthy of discussion. The dataset itself is well worthy of publications in TC, though I have some concerns about how the interpretation of the features is placed within the context of existing literature from the same field site.
As it stands, the paper draws strong conclusions from only one geophysical dataset instead of positioning itself as an excellent hypothesis test (our validation) of theories about controls on NEGIS that were developed with other geophysical data sets. The manuscript would be improved by incorporating the results of prior geophysical surveying (radar, common offset and common midpoint active seismic surveying) to strengthen the interpretations made here. With the incorporation of prior surveying into the analysis presented here, the paper will be a robust contribution to the literature on this important ice stream.
The radar survey of Christianson et al (2014) includes processed basal reflection strength within the survey area suggesting regions of wetter and drier bed, which could be used to support your hypothesis. The same radar survey was processed to focus on internal stratigraphy in Keisling et al (2014) which drew the conclusion that NEGIS has been a persistent feature across the Holocene and that modern flow is accommodated by a slippery bed. The active-source common offset survey of Riverman et al (2019a) identifies bed material across the ice stream (as seen in their Figure 2). These results alone, if incorporated into the existing manuscript, would strengthen the interpretation of bed materials across the ice stream. The seismic AVO work of Christianson et al (2014) could also be better incorporated.
199 What other evidence exists that these are large channels? These are LARGE channels — it would be somewhat of a surprise to find the sufficient volume of water necessary to carve them this far into the accumulation zone. Do the channels follow hydropotential lows? Or topographic lows? Whether or not the reflect subglacial water drainage or proglacial water drainage might be able to to be determined from their relative positioning across the wider landscape.
201 Prior radar and seismic work across the shear margins at this location has identified these as shear margin moraines and discussed formation hypotheses — that work should probably be discussed here (Riverman et al 2019a). How does this new dataset either support or reject the formation hypotheses put forward in that paper (that ice ‘drops’ its subglacial sediments as it enters the ice stream and effective pressures drop).
222-230 This section oversimplifies the conclusions of Christianson et al (2014). That paper finds a drape of subglacial till broadly across the entire region. Those sediments become dilatant within the fastest flowing section of the ice stream - but this is a result of fast ice flow (not the cause for fast ice flow). Christianson argues that the positioning of NEGIS is a more complicated hydrologic feedback — effectively, water is broadly present across the region, but routing of water to the shear margins (because of the hydropotential low set by the ice surface troughs) causes water to collect in the shear margins. This dries the region adjacent to the shear margins, slowing ice incorporation into the shear margins. Those ideas were then further supported by Riverman et al 2019a and 2019b, which performed detailed hydropotential analysis and meltwater routing modeling across the region and found that water indeed should be routed to the shear margins. Does this new dataset support this hypothesis for what sets the shear margin location of NEGIS?
245 Some of the main conclusions presented in this manuscript are that MSGLs can form at slower flowspeeds than previously thought — is it possible that they could even form at flow speeds down to 10-25 m/yr?
246 If the shear margin moraine forms through sediment rain-out during ice incorporation into the ice stream, then perhaps this could occur quickly, and this margin too could be a more transient feature.
275-280 I found it difficult to track the logic through this section. I would expand on these arguments so that they are more clearly made. Specifically, how would we have observed higher shear strain rates within the margins at some point in the past? In the paleo record in some way?
Figure 5 this figure reflects the sum of so much work — I could spend hour staring at it! No notes, just impressed.
Again, this work is impressive for its generation of a truly novel dataset that has the potential to really change the way we think about NEGIS. I apologize for being so ‘you should better incorporate my work into this work’ in this review — I usually try to avoid that! I also see that the seismic works I’m suggesting be incorporated here are not readily available online, which likely limited any efforts you would have made in that space. Dang! I am happy to provide any/all of the Penn State seismic surveying effort and processed results. Please do not hesitate to be in touch, riverman@up.edu
Citations for works mentioned above
Christianson 2014: Dilatant till facilitates ice-stream flow… EPSL
Riverman 2019a: Wet subglacial bedrooms of the NE Greenland Ice stream shear margins.. Annals of Glaciology
Riverman 2019b: Enhanced firn densification in high-accumulation shear margins of the NE Greenland Ice Stream… JGR Earth Surface
Keisling et al 2014: Basal conditions and ice dynamics inferred from radar stratigraphy… Annals of Glaciology--Kiya Riverman
Citation: https://doi.org/10.5194/egusphere-2025-1743-RC2 - AC2: 'Reply on RC2', Charlotte Carter, 11 Aug 2025
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