the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Brief communication: The hidden labyrinth: Deep groundwater in Wright Valley, Antarctica
- 1Center for Limnology, University of Wisconsin-Madison, Madison, WI, USA
- 2Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, USA
- 3Department of Geoscience, Aarhus University, Aarhus, Denmark
- 4Department of Earth and Planetary Sciences, University of California, Santa Cruz, Santa Cruz, CA, USA
- 5Department of Microbiology, University of Tennessee, Knoxville, TN, USA
- 6Department of Environmental Studies, Dartmouth College, Hanover, NH, USA
- 1Center for Limnology, University of Wisconsin-Madison, Madison, WI, USA
- 2Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, USA
- 3Department of Geoscience, Aarhus University, Aarhus, Denmark
- 4Department of Earth and Planetary Sciences, University of California, Santa Cruz, Santa Cruz, CA, USA
- 5Department of Microbiology, University of Tennessee, Knoxville, TN, USA
- 6Department of Environmental Studies, Dartmouth College, Hanover, NH, USA
Abstract. Since the 1960s, a deep groundwater system in Wright Valley, Antarctica, has been the hypothesized source of brines to hypersaline Don Juan Pond and Lake Vanda, both of which are rich in calcium and chloride. Modeling studies do not support other possible mechanisms, such as evaporative processes, that could have led to the current suite of ions present in both waterbodies. In 2011 and 2018, an airborne electromagnetic survey was flown over the Wright valley to map subsurface resistivity (down to 600 m) in exploration of liquid water. The surveys revealed widespread unfrozen brine in the subsurface near Lake Vanda, Don Juan Pond, and in the North Fork of Wright Valley. While our geophysical survey can neither confirm nor deny deep groundwater connectivity between Lake Vanda and Don Juan Pond, it does point to the potential for deep valley-wide brine conduits.
Hilary A. Dugan et al.
Status: closed
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RC1: 'Comment on tc-2022-91', Joseph Levy, 18 Jul 2022
General Comments:
This manuscript presents a fascinating first look at the electrical conductivity structure beneath Wright Valley, Antarctica, in order to evaluate long-standing predictions that subsurface brine conduits may link several water bodies in the valley. The work is well-argued, clearly presented, and nuanced in its analysis. The manuscript presents a compelling case that subsurface electrical conductivity anomalies are present east and west of Don Juan Pond, but that continuity between DJP and Lake Vanda could not be directly detected. High conductivity regions in the subsurface are clearly demonstrated in the North Fork of Wright Valley, which is strong evidence of conductive porewater solutions in the subsurface on that side of the Dias.
One question raised by the paper is the role of sampling geometry in the interpretation of the SkyTEM results. Line 1 in Fig. 1 seems to show highly conductive material extending from Don Quixote pond in the west, nearly all the way to Lake Vanda in the east. The high conductivity horizon in the subsurface is interrupted by the data dropout and by a highly resistive block of material shortly after 5000 m in the along-track direction. Is there any morphological or topographic evidence that could suggest that the high conductivity region could extend continuously from DQP to LV, but that the region of continuity was simply not imaged in the footprint of Line 1? Line 1 seems to have been targeted to intersect with DVDP14 and 4, but is it possible that in doing so, subsurface, high conductivity materials to the north could have been missed? If so, it seems possible that a subsurface connection between salty solutions in pore spaces in North Fork do extend downslope all the way from DQP to LV. Likewise, is it possible that the sampling geometry of Line 3 is what causes the pinch out of the conductive zone in the subsurface west of LV? Some of this could be addressed by mentioning the cross-track sampling width of the SkyTEM.
Recognizing that the SkyTEM instrument is insensitive to shallow subsurface processes (i.e., one pixel for the upper 4 m of the soil/water column), the introductory text provides a somwhat facile or dismissive treatment of the role of surface and near-surface waters in affecting DJP chemistry and hydrology. For example, the text suggests that the variability in DJP extent and salinity indicates a hydrological driver beyond surface conditions. But surface conditions strongly control DJP lake level and extent as shown by (Dickson et al., 2013), who found a strong correlation between insolation (hence, snowmelt) and DJP spatial extent. Likewise, (Dickson et al., 2013) show input of water track solutions from the east, which also are associated with high insolation days which drive snowmelt and expansion of the active layer. (Hassinger and Mayewski, 1983) and (Dickson et al., 2013) both report that these near-surface water track solutions have high Ca, low Na, and excess Ca (Ca exceeding that which can be derived from dissolution of gypsum or calcite), which together, represent a potential contributing near-surface source for Ca-rich waters in DJP. The (Toner et al., 2017) modeling work is an important contribution to the understanding of potential subsurface processes in the DJP/Vanda region, but should not be considered an exhaustive analysis of hydrological contributors in the region because it largely considers only regional freshwater systems over the near-surface brines.
Specific Comments:
Title: The (real) Labyrinth is a network of bedrock channels. And so, while I love the title, it seems like a network of bedrock and sedimentary fractures or pores in the subsurface really isn’t what the manuscript suggests is occurring around DJP, DQP, and LV. In some ways this gets at my general comment above—there may very well be a hidden labyrinth of subsurface brine conduits—but can single TEM lines identify that geometry?
Line 8. Are brine conduits implied by the observations or brine presence? I’d interpret “conduits” to mean localized zones of high permeability, which does not seem to be implied by the observations.
Line 49. Suggest removing the editorial tone of “convincing arguments.” It is a really excellent and intriguing paper, but a more neutral introduction might help readers weigh the different arguments about water sources for DJP.
Line 97. How do you interpret the abrupt stop to the high conductivity zone at depth between DQP and Vanda? Is it a bedrock spur? A cold/dry permafrost pocket? Or evidence of brine diverging off the sensor path (in which case, there really is evidence for a subsurface labyrinth!).
Line 100. It’s really interesting that the low resistivity regions east and west of DJP extent up higher than the modern lake level. That could provide evidence of a perched saline aquifer that provides the hydraulic head observed in the brief artesian discharge episodes from the DJP boreholes, and would suggest that the low-resistivity zones east and west of the pond are at least partially connected to the brine in the ponds. This would be a really important finding because it differs from the classic groundwater interpretation for DJP (which is also invoked in the Toner et al, 2017 paper), which invokes cyclic deep groundwater upwelling. Line 2 seems to show that there is brine adjacent to and higher than the lake, suggesting that DJP solutions may not be exlusively upwelling from deeper sources.
References.
Dickson, J.L., Head, J.W., Levy, J.S., Marchant, D.R., 2013. Don Juan Pond, Antarctica: Near-surface CaCl2-brine feeding Earth’s most saline lake and implications for Mars. Scientific Reports 3. https://doi.org/10.1038/srep01166
Hassinger, J.M., Mayewski, P.A., 1983. Morphology and Dynamics of the Rock Glaciers in Southern Victoria Land, Antarctica. Arctic and Alpine Research 15, 351. https://doi.org/10.2307/1550831
Toner, J.D., Catling, D.C., Sletten, R.S., 2017. The geochemistry of Don Juan Pond: Evidence for a deep groundwater flow system in Wright Valley, Antarctica. Earth and Planetary Science Letters 474, 190–197. https://doi.org/10.1016/j.epsl.2017.06.039
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RC3: 'Reply on RC1', Jonathan Toner, 29 Jul 2022
There are a few points I'd like to respond to.
"...should not be considered an exhaustive analysis of hydrological contributors in the region because it largely considers only regional freshwater systems over the near-surface brines": In the Toner et al. 2017 paper we did model the surface brine evolution, and furthermore we considered all surface waters in Wright Valley as candidates. None of these surface waters can evaporatively evolve to form a DJP brine. Our recent paper Toner et al. 2022 provides an even more comprehensive look at deep, near surface, and surface water compositions in the South Fork of Wright Valley. This more recent paper supports the unique chemistry of DJP.
Regarding the comments on surface discharges into DJP, we observed groundwater discharging east of DJP in the field, just as in Dickson et al. 2013. See the timelapse of discharge events over a month in the supplementary part of Toner et al. 2022. However, we also sampled many of these groundwater outflows, even during active outflow events, and analyzed the chemistry (unpublished unfortunately). There is no hint of any surface water contribution; the samples are pure DJP groundwater. Furthermore, these outflows have no observed connectivity to water tracks east of DJP, they simply upwell at the eastern edge the DJP playa. In my opinion, these are just groundwater outflows.
Finally, regarding the discharge events and their correlation with insolation and snowfall, the most direct correlation with DJP groundwater levels appears to be air pressure, which is the expected behavior for a confined aquifer. There is data from the DVDP 13 borehole (sorry, again unpublished) that measures air pressure and water levels in the borehole, showing a strong correlation. Harris and Cartwright presented an analysis of the same, although there are many transient features that remain a mystery. We know that surface waters are contributing to DJP from streams on the western end of DJP from the rock glacier, but their influence on the chemistry is very slight (possibly, this might explain the small nitrate component of DJP).
All this is to say that a deep groundwater interpretation for DJP presented in this paper is well supported by the evidence.
-
AC2: 'Reply on RC1 & RC3', Hilary Dugan, 22 Sep 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-91/tc-2022-91-AC2-supplement.pdf
-
AC2: 'Reply on RC1 & RC3', Hilary Dugan, 22 Sep 2022
-
RC3: 'Reply on RC1', Jonathan Toner, 29 Jul 2022
-
RC2: 'Comment on tc-2022-91', Jonathan Toner, 28 Jul 2022
I read the paper with interest and have just a few suggestions that the authors may or may not want to include depending on scope. I don’t have expertise in the resistivity analysis, so I have no comments on that.
-The article talks a lot about the connectivity between DJP and Vanda, but an equally intriguing question is the ultimate origin of the DJP brine. This is mentioned in passing, but worth emphasizing more. Harris and Cartwright hypothesized that the brine is ultimately sourced from beneath the East Antarctic Ice Sheet i.e. west of DJP and through the Labyrinth. Is there any evidence for this from the resistivity data? A related question is if the DJP brine extends beneath the ‘rock glacier’. It looks like west of DJP there is a low resistivity band at depth extending under the ‘rock glacier’, and very low values at the extreme that look like a numerical artifact.
-The DJP transect also shows an interesting, vertical low conductivity feature to the E of DJP. E of DJP the elevation along the valley floor rises and then plateaus along a series of small basins. The first basin you encounter holds VXE-6 pond, which is typically dry at the surface but shallow groundwater occurs. This pond has a high CaCl2 content like Lake Vanda, but also high nitrate indicating considerable surface inputs. None of the other ponds have CaCl2. Cartwright and Harris analyzed this pond, and we recently analyzed it in Toner et al. 2022 (also discusses the mixing between NO3-rich and CaCl2-rich endmembers). I suspect that wind alone can’t explain the CaCl2 in this pond; otherwise, why aren’t other ponds similarly enriched? The resistivity data seems to suggest a connection between DJP and VXE-6, which would make sense. This would also put the DJP brine on the right path to connecting Lake Vanda, although the data can’t show this. Too bad the flight line didn’t extend to Lake Vanda!
-We recently published a paper on DJP and surrounding soils and groundwaters (https://www.sciencedirect.com/science/article/abs/pii/S0012821X22002187). One of the findings of the paper was that CaCl2 brine/salts like DJP infuse the Dolerite bedrock up to 200 m above the pond surface. The argument is that salt composition of the dolerite bedrock is so DJP like and different from surrounding soils, that inputs from wind alone can’t explain the chemistry (you’d get mixing from nitrate-rich soils if deposited from wind), it must be primary. This supports a much stronger association between the DJP brine and the Ferrar Dolerite than previously thought. This suggests that you might "follow the Dolerite" to understand where the DJP groundwater is going. Might be interesting to include discussion about where the Dolerite is going, perhaps infered from the strike/dip of the unit.
-Line 90: The conductivity of salt solutions depends on concentration and composition, and the conductivity decreases at very high concentrations for CaCl2. Could the low conductivity be explained in this way? Also, is the conductivity of CaCl2 different from equivalent ionic strenth NaCl solutions. Would the porosity of the sediments and groundwater affect the result? Just wondering if the relatively low conductivity in DJP could be explained more easily.
~Jonathan Toner
-
AC1: 'Reply on RC2', Hilary Dugan, 22 Sep 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-91/tc-2022-91-AC1-supplement.pdf
-
AC1: 'Reply on RC2', Hilary Dugan, 22 Sep 2022
Status: closed
-
RC1: 'Comment on tc-2022-91', Joseph Levy, 18 Jul 2022
General Comments:
This manuscript presents a fascinating first look at the electrical conductivity structure beneath Wright Valley, Antarctica, in order to evaluate long-standing predictions that subsurface brine conduits may link several water bodies in the valley. The work is well-argued, clearly presented, and nuanced in its analysis. The manuscript presents a compelling case that subsurface electrical conductivity anomalies are present east and west of Don Juan Pond, but that continuity between DJP and Lake Vanda could not be directly detected. High conductivity regions in the subsurface are clearly demonstrated in the North Fork of Wright Valley, which is strong evidence of conductive porewater solutions in the subsurface on that side of the Dias.
One question raised by the paper is the role of sampling geometry in the interpretation of the SkyTEM results. Line 1 in Fig. 1 seems to show highly conductive material extending from Don Quixote pond in the west, nearly all the way to Lake Vanda in the east. The high conductivity horizon in the subsurface is interrupted by the data dropout and by a highly resistive block of material shortly after 5000 m in the along-track direction. Is there any morphological or topographic evidence that could suggest that the high conductivity region could extend continuously from DQP to LV, but that the region of continuity was simply not imaged in the footprint of Line 1? Line 1 seems to have been targeted to intersect with DVDP14 and 4, but is it possible that in doing so, subsurface, high conductivity materials to the north could have been missed? If so, it seems possible that a subsurface connection between salty solutions in pore spaces in North Fork do extend downslope all the way from DQP to LV. Likewise, is it possible that the sampling geometry of Line 3 is what causes the pinch out of the conductive zone in the subsurface west of LV? Some of this could be addressed by mentioning the cross-track sampling width of the SkyTEM.
Recognizing that the SkyTEM instrument is insensitive to shallow subsurface processes (i.e., one pixel for the upper 4 m of the soil/water column), the introductory text provides a somwhat facile or dismissive treatment of the role of surface and near-surface waters in affecting DJP chemistry and hydrology. For example, the text suggests that the variability in DJP extent and salinity indicates a hydrological driver beyond surface conditions. But surface conditions strongly control DJP lake level and extent as shown by (Dickson et al., 2013), who found a strong correlation between insolation (hence, snowmelt) and DJP spatial extent. Likewise, (Dickson et al., 2013) show input of water track solutions from the east, which also are associated with high insolation days which drive snowmelt and expansion of the active layer. (Hassinger and Mayewski, 1983) and (Dickson et al., 2013) both report that these near-surface water track solutions have high Ca, low Na, and excess Ca (Ca exceeding that which can be derived from dissolution of gypsum or calcite), which together, represent a potential contributing near-surface source for Ca-rich waters in DJP. The (Toner et al., 2017) modeling work is an important contribution to the understanding of potential subsurface processes in the DJP/Vanda region, but should not be considered an exhaustive analysis of hydrological contributors in the region because it largely considers only regional freshwater systems over the near-surface brines.
Specific Comments:
Title: The (real) Labyrinth is a network of bedrock channels. And so, while I love the title, it seems like a network of bedrock and sedimentary fractures or pores in the subsurface really isn’t what the manuscript suggests is occurring around DJP, DQP, and LV. In some ways this gets at my general comment above—there may very well be a hidden labyrinth of subsurface brine conduits—but can single TEM lines identify that geometry?
Line 8. Are brine conduits implied by the observations or brine presence? I’d interpret “conduits” to mean localized zones of high permeability, which does not seem to be implied by the observations.
Line 49. Suggest removing the editorial tone of “convincing arguments.” It is a really excellent and intriguing paper, but a more neutral introduction might help readers weigh the different arguments about water sources for DJP.
Line 97. How do you interpret the abrupt stop to the high conductivity zone at depth between DQP and Vanda? Is it a bedrock spur? A cold/dry permafrost pocket? Or evidence of brine diverging off the sensor path (in which case, there really is evidence for a subsurface labyrinth!).
Line 100. It’s really interesting that the low resistivity regions east and west of DJP extent up higher than the modern lake level. That could provide evidence of a perched saline aquifer that provides the hydraulic head observed in the brief artesian discharge episodes from the DJP boreholes, and would suggest that the low-resistivity zones east and west of the pond are at least partially connected to the brine in the ponds. This would be a really important finding because it differs from the classic groundwater interpretation for DJP (which is also invoked in the Toner et al, 2017 paper), which invokes cyclic deep groundwater upwelling. Line 2 seems to show that there is brine adjacent to and higher than the lake, suggesting that DJP solutions may not be exlusively upwelling from deeper sources.
References.
Dickson, J.L., Head, J.W., Levy, J.S., Marchant, D.R., 2013. Don Juan Pond, Antarctica: Near-surface CaCl2-brine feeding Earth’s most saline lake and implications for Mars. Scientific Reports 3. https://doi.org/10.1038/srep01166
Hassinger, J.M., Mayewski, P.A., 1983. Morphology and Dynamics of the Rock Glaciers in Southern Victoria Land, Antarctica. Arctic and Alpine Research 15, 351. https://doi.org/10.2307/1550831
Toner, J.D., Catling, D.C., Sletten, R.S., 2017. The geochemistry of Don Juan Pond: Evidence for a deep groundwater flow system in Wright Valley, Antarctica. Earth and Planetary Science Letters 474, 190–197. https://doi.org/10.1016/j.epsl.2017.06.039
-
RC3: 'Reply on RC1', Jonathan Toner, 29 Jul 2022
There are a few points I'd like to respond to.
"...should not be considered an exhaustive analysis of hydrological contributors in the region because it largely considers only regional freshwater systems over the near-surface brines": In the Toner et al. 2017 paper we did model the surface brine evolution, and furthermore we considered all surface waters in Wright Valley as candidates. None of these surface waters can evaporatively evolve to form a DJP brine. Our recent paper Toner et al. 2022 provides an even more comprehensive look at deep, near surface, and surface water compositions in the South Fork of Wright Valley. This more recent paper supports the unique chemistry of DJP.
Regarding the comments on surface discharges into DJP, we observed groundwater discharging east of DJP in the field, just as in Dickson et al. 2013. See the timelapse of discharge events over a month in the supplementary part of Toner et al. 2022. However, we also sampled many of these groundwater outflows, even during active outflow events, and analyzed the chemistry (unpublished unfortunately). There is no hint of any surface water contribution; the samples are pure DJP groundwater. Furthermore, these outflows have no observed connectivity to water tracks east of DJP, they simply upwell at the eastern edge the DJP playa. In my opinion, these are just groundwater outflows.
Finally, regarding the discharge events and their correlation with insolation and snowfall, the most direct correlation with DJP groundwater levels appears to be air pressure, which is the expected behavior for a confined aquifer. There is data from the DVDP 13 borehole (sorry, again unpublished) that measures air pressure and water levels in the borehole, showing a strong correlation. Harris and Cartwright presented an analysis of the same, although there are many transient features that remain a mystery. We know that surface waters are contributing to DJP from streams on the western end of DJP from the rock glacier, but their influence on the chemistry is very slight (possibly, this might explain the small nitrate component of DJP).
All this is to say that a deep groundwater interpretation for DJP presented in this paper is well supported by the evidence.
-
AC2: 'Reply on RC1 & RC3', Hilary Dugan, 22 Sep 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-91/tc-2022-91-AC2-supplement.pdf
-
AC2: 'Reply on RC1 & RC3', Hilary Dugan, 22 Sep 2022
-
RC3: 'Reply on RC1', Jonathan Toner, 29 Jul 2022
-
RC2: 'Comment on tc-2022-91', Jonathan Toner, 28 Jul 2022
I read the paper with interest and have just a few suggestions that the authors may or may not want to include depending on scope. I don’t have expertise in the resistivity analysis, so I have no comments on that.
-The article talks a lot about the connectivity between DJP and Vanda, but an equally intriguing question is the ultimate origin of the DJP brine. This is mentioned in passing, but worth emphasizing more. Harris and Cartwright hypothesized that the brine is ultimately sourced from beneath the East Antarctic Ice Sheet i.e. west of DJP and through the Labyrinth. Is there any evidence for this from the resistivity data? A related question is if the DJP brine extends beneath the ‘rock glacier’. It looks like west of DJP there is a low resistivity band at depth extending under the ‘rock glacier’, and very low values at the extreme that look like a numerical artifact.
-The DJP transect also shows an interesting, vertical low conductivity feature to the E of DJP. E of DJP the elevation along the valley floor rises and then plateaus along a series of small basins. The first basin you encounter holds VXE-6 pond, which is typically dry at the surface but shallow groundwater occurs. This pond has a high CaCl2 content like Lake Vanda, but also high nitrate indicating considerable surface inputs. None of the other ponds have CaCl2. Cartwright and Harris analyzed this pond, and we recently analyzed it in Toner et al. 2022 (also discusses the mixing between NO3-rich and CaCl2-rich endmembers). I suspect that wind alone can’t explain the CaCl2 in this pond; otherwise, why aren’t other ponds similarly enriched? The resistivity data seems to suggest a connection between DJP and VXE-6, which would make sense. This would also put the DJP brine on the right path to connecting Lake Vanda, although the data can’t show this. Too bad the flight line didn’t extend to Lake Vanda!
-We recently published a paper on DJP and surrounding soils and groundwaters (https://www.sciencedirect.com/science/article/abs/pii/S0012821X22002187). One of the findings of the paper was that CaCl2 brine/salts like DJP infuse the Dolerite bedrock up to 200 m above the pond surface. The argument is that salt composition of the dolerite bedrock is so DJP like and different from surrounding soils, that inputs from wind alone can’t explain the chemistry (you’d get mixing from nitrate-rich soils if deposited from wind), it must be primary. This supports a much stronger association between the DJP brine and the Ferrar Dolerite than previously thought. This suggests that you might "follow the Dolerite" to understand where the DJP groundwater is going. Might be interesting to include discussion about where the Dolerite is going, perhaps infered from the strike/dip of the unit.
-Line 90: The conductivity of salt solutions depends on concentration and composition, and the conductivity decreases at very high concentrations for CaCl2. Could the low conductivity be explained in this way? Also, is the conductivity of CaCl2 different from equivalent ionic strenth NaCl solutions. Would the porosity of the sediments and groundwater affect the result? Just wondering if the relatively low conductivity in DJP could be explained more easily.
~Jonathan Toner
-
AC1: 'Reply on RC2', Hilary Dugan, 22 Sep 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-91/tc-2022-91-AC1-supplement.pdf
-
AC1: 'Reply on RC2', Hilary Dugan, 22 Sep 2022
Hilary A. Dugan et al.
Hilary A. Dugan et al.
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