the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Misidentified subglacial lake beneath the Devon Ice Cap, Canadian Arctic: A new interpretation from seismic and electromagnetic data
Abstract. In 2018 the first subglacial lake in the Canadian Artic was proposed to exist beneath Devon Ice cap, based on the analysis of airborne radar data. Here, we report a new interpretation of the subglacial material beneath Devon Ice Cap, supported by data acquired from multiple surface-based geophysical methods in 2022. The geophysical data recorded included 9 km of active source seismic reflection profiles, 7 transient electromagnetic soundings and 17 magnetotellurics stations. These surface-based geophysical datasets were collected above the inferred locations of the subglacial lakes and show no evidence for the presence of subglacial water. The acoustic impedance of the subglacial material, estimated from the seismic data, is 9.49 ± 1.92 x 106 kg m-2 s-1, comparable to consolidated or frozen sediment. The resistivity models obtained by inversion of both the transient electromagnetic and magnetotelluric measurements show the presence of highly resistive rock layers (1000 – 100000 Ω.m) directly beneath the ice. Re-evaluation of the airborne reflectivity data show that the radar attenuation rates were likely overestimated, leading to an overestimation of the basal reflectivity in the original radar studies. Here, we derive new radar attenuation rates using the temperature- and chemistry-dependent Arrhenius equation, and when applied to correct the returned bed power, the bed power does not meet the basal reflectivity threshold expected over subglacial water. Thus, the radar interpretation is now consistent with the seismic and electromagnetic observations of dry or frozen, non-conductive basal material.
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Status: open (until 21 May 2024)
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RC1: 'Comment on egusphere-2024-279', Anonymous Referee #1, 25 Mar 2024
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This paper further explores a region of the Canadian Arctic where a previous radio echo sounding mission had "discovered" a putative hypersaline lake beneath a thick layer of glacial ice. The authors have followed up on the purported discovery with further geophysical investigations including seismic, magnetotellurics and time-domain electromagnetics. All of the new data indicate that the radio sounding detection of a hypersaline lake may have been a misidentification. The new data, including seismic reflectivity and MT/TEM inversions, do not support the interpretation of a hypersaline layer. A refined analysis of radar attenuation rates involving a couple of methods now point toward the absence of a hyper saline layer; such that the authors now conclude that the provocative feature is not likely to exist beneath the glacial ice. The conclusion of the present study highlights the importance of using multiple geophysical methods to follow up on provocative findings based on analysis of a single method. Generally, the work in this paper has been carefully performed and checked.
I do not see any major flaws in the analysis. However, the main finding is simply that a previous study was in error and has now been corrected. As such, despite the obvious large amount of effort the authors have undertaken, the paper does not stand alone as a significant new piece of research. The geophysical methodology is standard and there is not a new discovery being reported herein. As such, I strongly recommend the paper be published as a "comment" on the original manuscript. This does not diminish its importance to the scientific community compared to how a stand-alone article would be received; in fact, I think publishing it as a correction enhances its visibility and moreover it is the appropriate course of action.Citation: https://doi.org/10.5194/egusphere-2024-279-RC1 -
RC2: 'Comment on egusphere-2024-279', Anonymous Referee #2, 28 Mar 2024
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This work investigates a subglacial lake that was proposed to exist in 2018 beneath Devon Ice Cap, Canada. The existence of this lake was interpreted from airborne radio echo sounding (RES) data. The authors conducted an extensive multi-methodological, ground based geophysical survey with the expectation to map the geometry and electrical properties of this subglacial lake using transient electromagnetic, magneto telluric and seismic reflection data. Yet, the results from the three ground-based geophysical methods show that there exists no lake beneath Devon Ice Cap and indicate that the previously published RES data have been misinterpreted. Hence, this study mainly highlights three things: 1) that the detection of subglacial lakes from RES data is highly sensitive to the selected attenuation rate , which demands a sensitivity analysis of the attenuation-rate to determine the uncertainty of RES datasets and 2) that the collection of complimentary geophysical data sets is highly valuable in solving possible ambiguity in RES data and 3) There is no subglacial lake beneath the Devon Ice Cap.
Overall, the literature review is appropriate and the objective of this study is clearly explained. The paper is well-written and the majority of the plots are of high-quality and support the results and conclusions. Additionally, I highly appreciate that the authors did numerical experiments for each geophysical method to demonstrate the absence of the subglacial lake, because it helps the reader to understand how different each geophysical data set would look like in presence of a subglacial lake.
Please find specific comments and technical corrections (suggestions) in the accompanying pdf.
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