Improved Monitoring of Subglacial Lake Activity in Greenland
Abstract. Subglacial lakes form beneath ice sheets and ice caps if water is available, and if bedrock and surface topography are able to retain the water. On a regional scale, the lakes modulate the timing and rate of freshwater flow through the subglacial system to the ocean by acting as reservoirs. More than one hundred hydrologically active subglacial lakes, that drain and recharge periodically, have been documented under the Antarctic ice sheet, while only a handful of active lakes have been identified in Greenland. The small size of the Greenlandic subglacial lakes puts additional demands on mapping capabilitie aiming to resolve the evolving surface topography in sufficient detail to record their temporal behavior. Here, we explore the potential for combining data from CryoSat-2, TanDEM-X, and ArcticDEM to document the evolution of four active subglacial lake sites in Greenland. The inclusion of the new data sources provides important information on lake activity, documenting that the ice surface collapse basin on Flade Isblink ice cap was 50 % (30 meters) deeper than previously recorded. We also present evidence of a new active subglacial lake in Southwest Greenland, which shows signs of being hydrologically connected to another subglacial lake in that region. These findings show how improving the measurement capabilities of subglacial lakes, improves our current understanding and knowledge of the subglacial water system and its connection to surface hydrology.
Louise Sandberg Sørensen et al.
Status: final response (author comments only)
RC1: 'Comment on tc-2022-263', Anonymous Referee #1, 21 Feb 2023
- AC1: 'Reply on RC1', Louise Sandberg Sørensen, 24 Apr 2023
RC2: 'Comment on tc-2022-263', Anonymous Referee #2, 16 Mar 2023
- AC2: 'Reply on RC2', Louise Sandberg Sørensen, 24 Apr 2023
Louise Sandberg Sørensen et al.
Louise Sandberg Sørensen et al.
Viewed (geographical distribution)
This manuscript uses a combination of Cryosat-2 laser altimetry and DEMs from SAR and optical measurements to provide detailed measurements over four previously identified subglacial lakes in Greenland, and one prospective, but not previously identified lake. It provides some details of a set of techniques for combining measurements from these sensors, and offers a longer time series of elevation changes for the lakes than previous studies did, with somewhat more temporal detail. The use of Cryosat-2 data allows the authors to measure the depth of the lake under Flade Isblink immediately after its drainage, and finds a depth for the collapse feature that is significantly deeper than that measured in previous studies. I had trouble identifying the scientific questions that the study answered. Since four of the lakes had been identified in previous studies, the fact of their existence is not news, and the behavior documented in this study is not especially surprising. The fifth, potential lake identified here is extremely small and is close to one of the previously known lakes, so I am not sure what significance I should attach to its existence.
The study may be interesting to researchers with a deep knowledge of, and interest in, the particular subglacial lakes studied here, but I am not sure how wide this audience is likely to be. The authors suggest that measurements over subglacial lakes have the potential to inform our understanding of subglacial water flow, but I really didn’t see much development of this potential in this study. The abstract identifies the demonstration of techniques as a goal of the study, but the technical discussion of the techniques is brief and the presentation of the measurements is not very detailed. I would recommend reworking the study, either to focus on how each of the techniques performed at lake 4 (which had very large relief and elevation change) and at lakes 2 and 3 (which were small, and where the Cryosat-2 data didn’t work well), or to try to better understand the implications of the measurements for the subglacial hydrology of the ice sheet.
Line 34: Should note that this possibility was investigated in some detail by other studies (Stearns 2008, https://www.nature.com/articles/ngeo356) (Smith et al, 2017 (cited in the manuscript) And (Zwally and others, 2002, https://www.science.org/doi/10.1126/science.1072708), and that net dynamic changes after very large water inputs were negligible.
Line 88: “Classified” is not the right verb here. “Asserted” might be better
Section 3-1: Is there any way the selection of thresholds can be formalized? The thresholds selected here seem ad hoc, and it would be useful to discuss how they were chosen.
Line 140: “highly dynamic” should be “rough”
Line 141: Is the incoherent component in the processing, or in the radar reflections?
Line 145: should be “assumed to be representative”
Line 145: “were deemed as errors” should be “were assumed to be errors”
Line 146: “Across swath tracks close to the basin rim” should be “swath-processed data from tracks close to the basin rim”
Line 148: remove commas around “which is removed”
Line 183 “vertical alignment” should be “vertical offset”
Line 184: delete “found to be”
Line 197: “but we see” should be “and we see”
Line 201: “such as” -> “including”
Line 211: What is the basin shapefile?
Figure 1 (and all similar figures)
Line 224: Subtracting the median height does not make sense, as the offset subtracted is will depend on the height distribution of the rim. It would be better to subtract a median height anomaly relative to some reference DEM. Is this what the authors mean to say?
Line 226: “cubing the 2sigma”: What is this, and why does it give an error estimate? This needs much more detail to explain and/or justify what is done here.
Line 228: Need to specify which depths and volumes are used here, and need to connect these, using consistent terminology, with the depths derived from the DEMs and from CS2. Are “the depths” referenced here the depths of the deepest point from CS2?
Line 231 / equation 1. How does the derivation of R and V take the error bars into account? More detail is needed.
Line 236: It would be useful to demonstrate how R~(t) varies in time based on the available DEM data.
Lines 225-236: The methodology here does not seem to capture the true uncertainty in depth (and volume) estimates based on the CS2 data. When there is a large spatial variation in elevations in the DEM data, they are assessed a large error based on the slope and roughness within the relevant part of the lake, but CS2 data generally give a small number of elevation measurements at these times, and are assessed a smaller error. Would it not make sense to apply roughness information from the DEMs to the CS2 data to assess their errors?
Line 246: add comma after “coverage”
Line 264: It would be useful to explore why CS2 did not provide data over lakes 2 and 3. Were there no footprints that intersected the lake boundary? Was the coherence too low?
Line 265: Please show the power image from TanDEM-X for early 2011. It would be interesting to know if there are any reflectance features associated with the about-to-drain lake.
Line 279: “CS2 point data” :should this be “CS2 swath data”
Volume calculations: Except for Flade Isblink, these volumes are exceedingly small. Compared to lake discharges in Antarctica, they are miniscule, and those Antarctic discharges had almost no effect on ice dynamics. What is the justification for saying that the lakes studied here might be important for ice dynamics?
320: Should compare volume-change estimates against surface runoff estimates from (e.g.) RACMO.
358: “shortly” should be “briefly”
373: “off-nadir” should be “off nadir”
376-384: this repeats material found in the methods section.
378: delete “parameters”
387: is “highly active” all that can be determined here? This doesn’t seem like a lot has been learned.
To conclude that the activity of the new potential lake affected the drainage of lake 2, the authors would need to present evidence that it is unusual for water to reach the bed in volumes comparable to those discharged by the new lake. Looking at the images in appendix B, it appears that there is abundant water on the surface of the glacier, and it seems likely that this water often drains through moulins. Why, then, should we believe that the drainages of lakes 2 and 3 are anything but coincidental? Even if they were not coincidental, what specifically does this tell us about the hydrology of the glacier bed that we could not have inferred already?
Appendix A: Why would the basal melt rates be important in this area? Water fluxes from surface melt must dwarf these rates by orders of magnitude. Please consider surface melt first.
Figure B1: Indicate the location of this lake relative to lake 2. Also- what is being mapped here? The difference between panels a and b seems to mostly be that in panel B the surface is covered with snow, while in panel A it is mostly bare ice. The interpretation of the change in the collapse basin is not at all clear to me.
Figure B2: There is a lot of variability in surface conditions between these images. The interpretation in the text is not at all convincing.
Data availability: I didn’t see a statement about data availability for the CS2 swath-mode data.