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

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