Evidence for Pathways of Concentrated Submarine Groundwater Discharge in East Antarctica from Helicopter-Borne Electrical Resistivity Measurements
, Jill Mikucki 4, Krista F. Myers 3, Nikolaj Foged 2, Hilary Dugan 5, Esben Auken 2 and Ross Virginia 6Round 1
Reviewer 1 Report
In my opinion this manuscript has a lot of hand waving without direct prove or quantitative calculations.
To start with, the abstract gives a "flux" in the m/s unit which is speed, not flux.
More importantly, I am not convinced that groundwater is detected. Granted that high resistivity layer is detected but is it water? If it is indeed water is it flowing and why is the glacier sitting on top not sliding down? Also, if it is not flowing there is no flux.
Further, I don't see how flux is estimated. Page 255 gives a very small flux of 60m3/yr. Page 292 gives a larger, albeit still very small flux of 3000m3/yr. But, page 305 gives a very large flux of 1 km3/yr without showing how this is arrived at. Similarly, the large flux of 170 Gg/yr of Fe is totally a mystery.
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Comments and Suggestions for Authors
In my opinion this manuscript has a lot of hand waving without direct prove or quantitative calculations.
Direct measurements of the subsurface are very hard in Antarctica, part of the reason why our EM survey is the first such done there. Undoubtedly future work will greatly improve our understanding of SGD and groundwater in Antarctica. As we note in our introduction, there has been only one direct measurement of SGD on the entire continent. This paper adds to that by demonstrating a complementary technique, presenting evidence of a previously little-considered and only once-measured process, and gives an estimate of the potential scale of operation across the larger continent.
We would love to have more data to strengthen our estimates and create more nuanced models, but this is the best anyone has and we try to get the most out of it. Even with simple estimates we narrow the range of fluxes estimated previously by Wadham and others (2013).
To start with, the abstract gives a "flux" in the m/s unit which is speed, not flux.
Changed!
More importantly, I am not convinced that groundwater is detected. Granted that high resistivity layer is detected but is it water?
In general, low resistivity is caused by electrolytic conduction or metals/sulfides. The latter is not geologically plausible here. As shown in previous papers published by authors on this paper, and cited in our introduction, there is very strong evidence for subglacial and sub-permafrost water in the region, and that it is best detected by EM resistivity. Foley et al (2015) goes to some length to show that sub-zero liquid brines correspond to changes in measured resistivity. Further, this subsurface aquifer is expressed at the surface and can be directly sampled at Blood Falls and to a lesser extent, the bottom waters of some of the lakes in the region.
If it is indeed water is it flowing and why is the glacier sitting on top not sliding down? Also, if it is not flowing there is no flux.
Glacier sliding is a determined by the balance of driving and resisting stresses. The reviewer is correct to note that basal water tends to enhance glacier sliding by reducing the basal stress. But the total state of stress is a very complicated thing to determine. We’d note that the shear stress from the confining valley walls is probably great. It’s hard to say much because hardly anything has been published on the Koettlitz Glacier, but we’d direct the reviewer to 3 papers by Kanvanaugh and Cuffey (below) and one by Hubbard and other (below) that explore the stresses, velocity, and presence of water underneath nearby Taylor Glacier.
The fact that the water is flowing is does not matter for glacier sliding; only the pressure of the water influences sliding. Whatever the total stress state of the glacier, numerous lines of evidence (radar, EM, direct sampling, modeling) exist to demonstrate that Taylor Glacier (and with fewer measurements) Koettlitz Glacier do indeed have water at their bases.
Kavanaugh, J.L., Cuffey, K.M., Morse, D.L., Conway, H. and Rignot, E., 2009. Dynamics and mass balance of Taylor Glacier, Antarctica: 1. Geometry and surface velocities. Journal of Geophysical Research: Earth Surface, 114(F4).
Kavanaugh, J.L. and Cuffey, K.M., 2009. Dynamics and mass balance of Taylor Glacier, Antarctica: 2. Force balance and longitudinal coupling. Journal of Geophysical Research: Earth Surface, 114(F4).Kavanaugh, J.L., Cuffey, K.M., Morse, D.L., Bliss, A.K. and Aciego, S.M., 2009. Dynamics and mass balance of Taylor Glacier, Antarctica: 3. State of mass balance. Journal of Geophysical Research: Earth Surface, 114(F4).
Hubbard, A., Lawson, W., Anderson, B., Hubbard, B. and Blatter, H., 2004. Evidence for subglacial ponding across Taylor Glacier, Dry Valleys, Antarctica. Annals of Glaciology, 39, pp.79-84.
Further, I don't see how flux is estimated. Page 255 gives a very small flux of 60m3/yr. Page 292 gives a larger, albeit still very small flux of 3000m3/yr. But, page 305 gives a very large flux of 1 km3/yr without showing how this is arrived at. Similarly, the large flux of 170 Gg/yr of Fe is totally a mystery.
This was an oversight on our part as we rushed to make the deadline. We’ve greatly expanded the text to show our reasoning and sources, plus included a table to present our results.
In short, we follow the methodology of Wadham et al (2013) of multiplying a water flux by a concentration of dissolved ions. We attempt to narrow the window of possible ion fluxes by providing a more realistic water flux for the high concentration of ions we detect in the MDV groundwater. We show that Wadham’s upper bound is unrealistically high, but we determine estimates of ions carried by SGD that are in line with her lower bound estimate for total subglacial flux.
Submission Date
12 March 2019
Date of this review
18 Mar 2019 07:45:45
Reviewer 2 Report
Foley et al present electrical resistivity measurements to identify submarine groundwater discharge pathways in East Antarctica. The paper is well-written and the data are well-presented. I find the title of the paper to be slightly misleading in that the authors have found potential submarine groundwater discharge pathways; they do not present evidence of direct measurement of submarine groundwater discharge at the ocean terminus of the pathways. I recommend adding "pathways" to the title of the manuscript. The introduction provides relevant background information and is sufficiently referenced with current literature. The methods are also well described. I appreciate the comparison between DC and AEM data collection. The results are generally well presented but it seems like the authors lost a little momentum going into the discussion piece of the manuscript based on numerous grammatical issues and the need for more references/citations. The designations presented in Figure 3 need much more justification in the text. As presented, the designations seem like speculation. The conclusions are adequate. The references are inconsistently formatted and many are incomplete or inaccurate. Overall, my three biggest concerns with the manuscript are: 1) The misleading title (I don't think it was intentional) 2) The nutrient piece - nutrients were not directly measured in the paper, so the authors need to justify the nutrient aspect of the paper with more references and actual published values while acknowledging that the nutrient piece is not based on direct measurements 3) Figure 3 - the figure was adapted from previously published research. The authors need to justify the simplification and if the authors want to keep the figure, the figure needs to be redrafted to have a more professional look. The authors also need to more thoroughly justify the three area designations in the text, and place the designations within context of the AEM data that were collected and presented within the paper. Text Comments: *Line 25 - The word "likely" is speculation as SGD at the terminus of the pathways was not directly measured. Perhaps say "may" contain and preface the statement with based on previously published research. *Line 33 - the definition of SGD as being water from the sea floor to the ocean is odd for SGD literature. SGD is water of any origin that flows from land to sea. Suggest rephrase. *Line 55 - Space between [11].As *Line 71 - Too much space between is reduced? *Lines 81-84 - This statement seems like speculation with "may" and no reference. If speculation, it is better served in the discussion section. If previously documented, statement needs a citation. *Line 89 - Introduce (MDV) here, not in line 100, which coincidentally does not have the acronym defined. *Line 101 - Too much space between suggests cryoconcentrated? *Line 116 - moment, the *Line 117 - moment, the *Line 120 - Too much space between to ~600? *Results and Discussion general comment - perhaps show AEM data where there is no SGD pathway for comparison in figure 2D? Perhaps from an area that others have suggested a pathway might exist? *Line 201 - Capitalize "Sea" *Line 215 - recharge on a continent-wide *Line 232 - potentially providing *Line 233 - It is not good practice to start a paragraph with "Therefore." Is this paragraph part of the previous paragraph? *Line 249 - Please provide an estimate of the uncertainty or a range of values on the hydraulic conductivity analysis. *Line 258 - I know what LGM is but acronym is not defined and should be. *Line 264 - extra space, delete *Lines 266-268 - It seems like a reference is needed here *Line 269 - detectable, this results *Line 273 - margins; it is *Line 276 - delete the *Line 278 - from the origin *Line 278 - sea with prolonged *Line 283 - Koettlitz Glacier is mentioned in the text but is not identified in Fig. 2C *Line 285 - space between 30m *Line 291 space between 10km and 3km *Line 300 - Why is there overall lower basal melting in East Antarctica? Concept is not adequately described or referenced. *Line 301 - of as trapping *Line 302 - inside the continent? (inside of what?) *Line 302 - escape the coast via what process? *Line 305 - on the order of 1 cubic kilometer per year can be replaced with ~1 km3/yr *Line 339 - define acronym DVDP *Line 340 - (2015) is unnecessary VERY brief check of references shoes that at least the following are incomplete, inaccurate, or inconsistently formatted: 3, 10, 11, 12, 18, 21, 23, 24, 26, 29, 33, 34, 35, 37, 40, 41, 46, 56, 58, 59. Figure 1 - Would be nice to have latitude and longitude markers along the edge of the map. It would also be helpful to show where study sites 2A-2C are specifically located on the big map. Figures 2A-2C - All figures need a source for the visible light imagery, a north arrow, a scale bar, and latitude and longitude markers. Figure 2C - Label the Ross Sea, Lake Miers, the crossed rivers, and Koettlitz Glacier on the visible light imagery. It would also be helpful to show where the no data area is on the visible light imagery either by breaking the solid red line or by dashing the area of the line where no data were collected. Also, the scale bar in the AEM data looks like it is covering up important data. Figure 3 - Honestly, this figure doesn't add anything to the paper especially since it is adapted from a different publication. If the authors insist on keeping it, please have an illustrations specialist do a major redraw of the figure as the figure looks unprofessional (made with paint?) and is lacking latitude/longitude, a scale bar, a north arrow, and Antarctica is flipped relative to Figure 1. I think the authors can convey the information from the figure in the text. Furthermore, the three ares of the figure need to be more thoroughly described and justified in the text. Much of the presentation in the figure seems like speculation given the current accompanying text. The explanation of orange and green should go into the text, regardless of whether the authors keep the figure or remove it. I enjoyed reviewing your manuscript and am looking forward to seeing the revised version. Thank you for helping me learn more about SGD in Antarctica.
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Comments and Suggestions for Authors
Foley et al present electrical resistivity measurements to identify submarine groundwater discharge pathways in East Antarctica. The paper is well-written and the data are well-presented. I find the title of the paper to be slightly misleading in that the authors have found potential submarine groundwater discharge pathways; they do not present evidence of direct measurement of submarine groundwater discharge at the ocean terminus of the pathways. I recommend adding "pathways" to the title of the manuscript. The introduction provides relevant background information and is sufficiently referenced with current literature. The methods are also well described. I appreciate the comparison between DC and AEM data collection. The results are generally well presented but it seems like the authors lost a little momentum going into the discussion piece of the manuscript based on numerous grammatical issues and the need for more references/citations. The designations presented in Figure 3 need much more justification in the text. As presented, the designations seem like speculation. The conclusions are adequate. The references are inconsistently formatted and many are incomplete or inaccurate. Overall, my three biggest concerns with the manuscript are: 1) The misleading title (I don't think it was intentional) 2) The nutrient piece - nutrients were not directly measured in the paper, so the authors need to justify the nutrient aspect of the paper with more references and actual published values while acknowledging that the nutrient piece is not based on direct measurements 3) Figure 3 - the figure was adapted from previously published research. The authors need to justify the simplification and if the authors want to keep the figure, the figure needs to be redrafted to have a more professional look. The authors also need to more thoroughly justify the three area designations in the text, and place the designations within context of the AEM data that were collected and presented within the paper.
Greatly expanded and fixed in response to the main criticisms. Thanks! Detailed responses to detailed issues below.
Text Comments: *Line 25 - The word "likely" is speculation as SGD at the terminus of the pathways was not directly measured. Perhaps say "may" contain and preface the statement with based on previously published research.
Changed to likely and expanded our discussion of ions later in the text
*Line 33 - the definition of SGD as being water from the sea floor to the ocean is odd for SGD literature. SGD is water of any origin that flows from land to sea. Suggest rephrase.
I not sure I’m understanding the distinction here, but as a newcomer to the SGD I am happy to be corrected if I’m missing something. The source we cite defines SGD as “a discharging flow out across the sea floor” (pg 8). I replaced ‘exchange’ in our text with ‘flow’ to more accurately reflect that SGD need not be balanced by ocean water.
*Line 55 - Space between [11].
Added a space
As *Line 71 - Too much space between is reduced?
Removed the extra space
*Lines 81-84 - This statement seems like speculation with "may" and no reference. If speculation, it is better served in the discussion section. If previously documented, statement needs a citation.
Rephrased to make none-speculative. Cryoconcentration is of course well documents from glaciers to sea ice, to the food industry. We do like to include this in our introduction because it is important in framing how and why we approached this unusual example of SGD,
*Line 89 - Introduce (MDV) here, not in line 100, which coincidentally does not have the acronym defined.
Fixed
*Line 101 - Too much space between suggests cryoconcentrated?
Yep. Fixed.
*Line 116 - moment, the
Added the comma
*Line 117 - moment, the
Added the comma
*Line 120 - Too much space between to ~600?
Yep. Deleted
*Results and Discussion general comment - perhaps show AEM data where there is no SGD pathway for comparison in figure 2D? Perhaps from an area that others have suggested a pathway might exist?
This is an interesting idea, but it’s actually pretty hard to represent. Two issues:
(1) To our knowledge, no one has previously suggested a specific area where SGD might be found, besides us at the end of Taylor Valley, where it is found. So demonstrating invalidation of a previous hypothesis is not possible.
(2) More fundamentally, a measurement over an area lacking in conductive material above the DOI just returns a system response signal, like when the system is flown to high altitude to eliminate the earth response. These data are culled and no models are inverted – the signal does not exceed the instrument noise and the only valid conclusion is that the system is very resistive down to the DOI, but nothing further can be said.
We added text to our methods section to clarify point #2.
*Line 201 - Capitalize "Sea"
Done!
*Line 215 - recharge on a continent-wide
Good catch, added ‘a’
*Line 232 - potentially providing
Added ‘ly’
*Line 233 - It is not good practice to start a paragraph with "Therefore." Is this paragraph part of the previous paragraph?
Good point. I made it part of the previous paragraph
*Line 249 - Please provide an estimate of the uncertainty or a range of values on the hydraulic conductivity analysis.
Added a chunk of text about this. Unfortunately, there is a dearth of these fundamental measurements in the area, but it’s heartening that using grain size data puts us in a reasonable range.
*Line 258 - I know what LGM is but acronym is not defined and should be.
Yep. Changed.
*Line 264 - extra space, delete
Done!
*Lines 266-268 - It seems like a reference is needed here
Line numbers are drifting apart here…Is this is reference to the bit about flotation and 90% of the ice height? I added a reference.
*Line 269 - detectable, this results
Changed to a more clear phrasing that avoids the previous clumsy phrasing
*Line 273 - margins; it is
Reworked the sentence
*Line 276 - delete the
My copy’s line numbers no longer line up with the reviewer’s so I’m not sure what this is referring to. I read through the vicinity a few times and think everything is okay.
*Line 278 - from the origin
Rewrote and included a ‘the’
*Line 278 - sea with prolonged
I like that change.
*Line 283 - Koettlitz Glacier is mentioned in the text but is not identified in Fig. 2C
Technically, Koettlitz is not displayed on 2C, but it is on Figure 1, where I added a label (also there is much more space available on Figure 1).
*Line 285 - space between 30m
added
*Line 291 space between 10km and 3km
added
*Line 300 - Why is there overall lower basal melting in East Antarctica? Concept is not adequately described or referenced.
I expanded the text to explain the main reasons why (thin ice and cold surface temperatures). The referenced paper goes into lots more detail.
*Line 301 - of as trapping
Added ‘as’
*Line 302 - inside the continent? (inside of what?)
Added ‘inside the ring’
*Line 302 - escape the coast via what process?
Added ‘via surface or groundwater discharge’ *Line 305 - on the order of 1 cubic kilometer per year can be replaced with ~1 km3/yr
*Line 339 - define acronym DVDP
Added this to the results section
*Line 340 - (2015) is unnecessary
deleted
VERY brief check of references shoes that at least the following are incomplete, inaccurate, or inconsistently formatted: 3, 10, 11, 12, 18, 21, 23, 24, 26, 29, 33, 34, 35, 37, 40, 41, 46, 56, 58, 59.
Fixed the errors
Figure 1 - Would be nice to have latitude and longitude markers along the edge of the map. It would also be helpful to show where study sites 2A-2C are specifically located on the big map.
Good call. Added them.
Figures 2A-2C - All figures need a source for the visible light imagery, a north arrow, a scale bar, and latitude and longitude markers.
Fixed.
Figure 2C - Label the Ross Sea, Lake Miers, the crossed rivers, and Koettlitz Glacier on the visible light imagery. It would also be helpful to show where the no data area is on the visible light imagery either by breaking the solid red line or by dashing the area of the line where no data were collected. Also, the scale bar in the AEM data looks like it is covering up important data. Figure 3 - Honestly, this figure doesn't add anything to the paper especially since it is adapted from a different publication. If the authors insist on keeping it, please have an illustrations specialist do a major redraw of the figure as the figure looks unprofessional (made with paint?) and is lacking latitude/longitude, a scale bar, a north arrow, and Antarctica is flipped relative to Figure 1. I think the authors can convey the information from the figure in the text. Furthermore, the three ares of the figure need to be more thoroughly described and justified in the text. Much of the presentation in the figure seems like speculation given the current accompanying text. The explanation of orange and green should go into the text, regardless of whether the authors keep the figure or remove it. I enjoyed reviewing your manuscript and am looking forward to seeing the revised version. Thank you for helping me learn more about SGD in Antarctica.
Thanks! Greatly revised figure 3 to conform with the reviewer’s suggestions, and updated accompanying text. I’m embarrassed to admit the original figure took way too long to make in inkscape given the end product. New version should be much more satisfactory.
Round 2
Reviewer 1 Report
The authors did not answer my question whether the groundwater is flowing. If it is not, then there is no SGD.
My major concern about the flux remains. The manuscript gives very small fluxes in the study area but the authors apply a water balance to the entire Antarctic then come up with, and report, an extremely large flux. I don't see any connection of their measurements with the large flux they report at the end. If the authors do not apply the results of their measurements the manuscript can be shortened by two third at least, but surely the authors know that the water balance has a very large uncertainty.
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Comments and Suggestions for Authors
The authors did not answer my question whether the groundwater is flowing. If it is not, then there is no SGD.
Clearly, short of making direct measurements of groundwater discharge in the coastal zone of the study area, we cannot prove beyond any doubt that groundwater is flowing and discharging as SGD. However, here is a list of related evidence that we feel supports the assumption that groundwater is flowing from MDVs to the coastal zone:
1. Spigel, R.H., Priscu, J.C., Obryk, M.K., Stone, W. and Doran, P.T., 2018. The physical limnology of a permanently ice‐covered and chemically stratified Antarctic lake using high resolution spatial data from an autonomous underwater vehicle. Limnology and Oceanography, 63(3), pp.1234-1252. = This study reports evidence of discharge of subglacial brines identified using AEM directly into Lake Bonney (at the snout of Taylor Glacier in Taylor Valley).
2. Warrier, R.B., Castro, M.C., Hall, C.M., Kenig, F. and Doran, P.T., 2015. Reconstructing the evolution of Lake Bonney, Antarctica using dissolved noble gases. Applied Geochemistry, 58, pp.46-61. = isotopic evidence for discharge of subglacial brines identified using AEM directly into Lake Bonney (at the snout of Taylor Glacier in Taylor Valley).
3. Harris, H.J.H., Cartwright, K. and Torii, T., 1979. Dynamic chemical equilibrium in a polar desert pond: a sensitive index of meteorological cycles. Science, 204(4390), pp.301-303. = Reports on artesian outflow of groundwater triggered by drilling into Don Juan Pond in Wright Valley. Also reports on geochemical evidence interpreted as evidence for deep groundwater discharge into this pond.
4. Cartwright, K., Harris, H. and Follmer, L.R., 1975. DVDP Hydrogeological studies. Dry Valley Drilling Project Bulletin, (5), pp.134-8. = Reports on rapid rise in borehole water levels upon connection of the borehole to sub-permafrost liquid water in DVDP 10, which was drilled right at the coastline of Taylor Valley.
5. Uemura, T., Taniguchi, M. and Shibuya, K., 2011. Submarine groundwater discharge in Lützow‐Holm Bay, Antarctica. Geophysical Research Letters, 38(8). = Reports on fast submarine seepage from Antarctic continent.
6. Null, K.A., Corbett, D.R., Crenshaw, J., Peterson, R.N., Peterson, L.E. and Lyons, W.B., 2019. Groundwater discharge to the western Antarctic coastal ocean. Polar Research. = Report on submarine groundwater discharge near Palmer Station, Antarctica
We are proposing a reasonable interpretation based on our observation. Proving that groundwater is flowing (and if so, at what rates) into the coastal zone of MDV will require a separate project.
My major concern about the flux remains. The manuscript gives very small fluxes in the study area but the authors apply a water balance to the entire Antarctic then come up with, and report, an extremely large flux. I don't see any connection of their measurements with the large flux they report at the end. If the authors do not apply the results of their measurements the manuscript can be shortened by two third at least, but surely the authors know that the water balance has a very large uncertainty.
We consistently emphasize that our groundwater flux estimates are good to an order of magnitude. Our estimates of solute fluxes to the ocean from the continent are much lower than the previous flux estimates published by Wadham et al. So, we are somewhat surprised that the reviewer considers our estimates to be large. Both of the published measurements (Null et al. 2019 and Uemura et al. 2011) support large local/regional fluxes of groundwater into the coastal ocean that, if extrapolated around the 50,000km long perimeter of Antarctica could be used to justify SGD of dozens of cubic kilometers of groundwater per year into the coastal zone of this continent. The flux rate we are assuming off the Antarctic continent is several orders of magnitude lower than the average SGD flux rate from other continents.
We’ve added a calculation that shows that our estimated continent-wide residence times are consistent with residence times calculated from our study area fluxes. Shrinking our calculation to just the Ross Sea does not necessarily improve accuracy; instead we think it increases the contributions of unrepresentative heterogeneities (e.g., any one of our three examples describes a potential RS setting, and none represents the western RS, and our aquifer thickness estimates are very conservatively measured as the detectable (high porosity and shallow) lowest resistivity zone). Further, we believe it is important to tie this work in context of a continental budget: the slow flow, high concentration scenario is to this point barely studied but potentially present on a continental scale, and it refines continent-wide estimates presented in Wadham et al. Even if further study shows our calculation to be an overestimate, it enhances our understanding of what the endmember solute flux scenario presented in Wadham et al would have to look like to be realistic. Finally, it is much easier to make estimates for a naturally closed system (e.g., an entire continent) than for a part of such a system, for which we cannot, for instance assume a hydrologic budget based on Pattyn’s (2011) published numbers. If we were to focus on just the Ross Sea region, as suggested, an additional large uncertainty would be introduced by the fact that it is unknown what part of the Antarctic continental groundwater system is contributing discharge to the Ross Sea region, as opposed to any other part of Antarctic coastline.
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