the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Permafrost saline water and Early to Mid-Holocene permafrost aggradation in Svalbard
Dotan Rotem
Vladimir Lyakhovsky
Hanne Hvidtfeldt Christiansen
Yehudit Harlavan
Yishai Weinstein
Abstract. Deglaciation in Svalbard was followed-up by seawater ingression and the deposition of marine (deltaic) sediments in fjord valleys, while elastic rebound resulted in fast land uplift and the exposure of these sediment to the atmosphere, therefore the formation of epigenetic permafrost. This was then followed by the accumulation of aeolian sediments, which froze syngenetically. The permafrost was drilled in the east Adventdalen valley, Svalbard, 3–4 km from the maximum up-valley reach of post-deglaciation seawater ingression, and its ground ice was measured for chemistry. While ground ice in the syngenetic part is basically fresh the epigenetic part reveals a frozen fresh-saline water interface (FSI), with chloride concentrations increasing from the top of the epigenetic part (depth of 5.5 m) to about 15 % that of seawater at 11 m. We applied a one-dimensional freezing model in order to examine the rate of top-down permafrost aggradation, which could accommodate with the observed frozen FSI. The model examined permafrost development under different scenarios of mean average air temperature, water-freezing temperature and the degree of pore-water freezing. We found that even at the relatively high temperatures of the Early to mid-Holocene, permafrost could aggrade quite fast, e.g. down to 15 to 33 m in 200 years, therefore allowing freezing of the fresh-saline water interface despite of the relatively fast rebound rate and the resultant increase in topographic gradients toward the sea. This could be aided by non-complete pore water freezing, which possibly lead to slightly faster aggradation, resulting in the freezing of the entire marine section at that location (23 m) within less than 200 years. We conclude that freezing should have occurred immediately after the exposure of the marine sediment to atmospheric conditions.
Dotan Rotem et al.
Status: final response (author comments only)
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RC1: 'Comment on tc-2022-134', Anonymous Referee #1, 24 Nov 2022
Dear authors,
I have reviewed your paper entitled “Permafrost saline water and Early to Mid-Holocene permafrost aggradation in Svalbard”. It provides a novel approach to the understanding of permafrost evolution in one of the best studied areas in the polar regions with regards to past environments and permafrost dynamics. Even if I am not expert on the modelling part, I found it well-written and easy to follow. Figures and tables are of good quality.
The introduction constitutes a very complete assessment of the research background on the studied topics and objectives of the work. The study area does not provide present-day climate data, which would help to frame to understand (past) simulated conditions. The methodological part is very well described, and the different steps of the research approach clearly exposed. Discussion is concise and summarizes the main findings, comparing it with other areas where Holocene permafrost dynamics has been also examined. Conclusions capture also the main findings of the paper. It would be interesting to add what are the implications of these results for recently exposed (Late Holocene) areas, and how these data can be used to assess on the future evolution of permafrost in Svalbard (aggradation. vs degradation).
Specific comments
Page 3, l. 50 – include also snow cover dynamics.
Page 3, l. 52 –warm-based glaciers
Page 3, l. 54 –better to refer to Last Glacial Cycle than to the LGM
Page 4, l. 86 – last glacial cycle
Page 4, l. 88 – are all ages calibrated (cal BP)?
Page 4, l. 102 – to better understand and frame the study cases presented later in the paper, present-day MAATs should be given here. Similarly, as also described in the Discussion, precipitation values should be included here.
Figures
Consider adding a picture of the study site (and maybe also of the cores) to help the reader better understand the environmental/sedimentological setting.
Citation: https://doi.org/10.5194/tc-2022-134-RC1 -
AC1: 'Reply on RC1', Dotan Rotem, 17 Dec 2022
Dear referee #1
Thank you very much for your comments and support. Our comments are in bold.
I have reviewed your paper entitled “Permafrost saline water and Early to Mid-Holocene permafrost aggradation in Svalbard”. It provides a novel approach to the understanding of permafrost evolution in one of the best studied areas in the polar regions with regards to past environments and permafrost dynamics. Even if I am not expert on the modelling part, I found it well-written and easy to follow. Figures and tables are of good quality. The introduction constitutes a very complete assessment of the research background on the studied topics and objectives of the work. The study area does not provide present day climate data, which would help to frame to understand (past) simulated conditions.
In our work we relied on data collected at the Longyearbyen airport weather station, located on the fjord coast. There is no data for our inland drilling site, but there is clearly a significant temperature gradient inland, as described e.g. in Christiansen (2005) or Christiansen et al., (2013). As suggested, in the revised manuscript, we will dedicate a paragraph to describe the climate in the area.
The methodological part is very well described, and the different steps of the research approach clearly exposed. Discussion is concise and summarizes the main findings, comparing it with other areas where Holocene permafrost dynamics has been also examined. Conclusions capture also the main findings of the paper. It would be interesting to add what are the implications of these results for recently exposed (Late Holocene) areas, and how these data can be used to assess on the future evolution of permafrost in Svalbard (aggradation. vs degradation).
Although our paper is dedicated to paleo-freezing conditions during early to mid-Holocene, we believe there is indeed some implications to current and future trends in permafrost evolution. Basically, our results may suggest that even a short (years to decades) cooling period can slow down permafrost thawing and as pointed out by the reviewer, that recently exposed areas may go through permafrost aggradation even under the current global warming. It is also can point out that although the above ground climate changes fast, remnants of permafrost can persist long time because of local conditions. Actually, it can point out on a spatial process of an area turning from continuous permafrost to discontinues one.
Specific comments
We will correct all specific comments as proposed.
We will add pictures as proposed.
Many thanks
Dotan
Citation: https://doi.org/10.5194/tc-2022-134-AC1
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AC1: 'Reply on RC1', Dotan Rotem, 17 Dec 2022
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RC2: 'Comment on tc-2022-134', Anonymous Referee #2, 29 Nov 2022
Dear Editor and authors,
The manuscript by Rotem et al. investigates permafrost aggradation in the Adventdalen East (ADE) valley in High Arctic Svalbard. Following the last glaciation, the ADE was inundated by a seawater ingression, allowing for the deposition of saline marine sediments. Due to post-glacial rebound, these saline sediments were ultimately exposed to cold sub-aerial conditions, resulting in permafrost formation. Aeolian processes deposited more material on top of the marine sediments, resulting in syngenetic permafrost aggradation. The latter sediments contained fresh porewater. This interesting and useful paper uses a numerical model to quantify frozen permafrost formation rates under varying freezing conditions at the ground surface and sediment properties. For the sediment properties, the authors fine-tune the sediment porosity and freezing point (based on the porewater salinity). To compliment (but not necessarily validate) their thermal models, the authors show ground ice chemistry data from two boreholes down to a depth of ~13 m. The boreholes clearly show a fresh-saline transition in the epigenetic part of the permafrost, as well as evidence for “mixing” in the upper part of the epigenetic permafrost. This is important, because it demonstrates that the groundwater infiltration of freshwater was inhibited by ground freezing very soon after sub-aerial exposure. This point in particularly key to justify that the thermal models could neglect groundwater flow and the flushing of salts. Although I enjoyed reading this paper, I have some concerns about the numerical model. My comments are arranged into “major” and “minor” categories. While I point out a few typos, please note that I did not perform an exhaustive spelling and grammar check. For the next version of this manuscript, I recommend that more attention be given to proofreading. Overall, I classify this revision as “major” and that the paper can be considered for publication after the next iteration of the paper is reviewed.Major:
1. The time step of 32,600 seconds is not 0.5 days as written in the text. Half a day is 43,200 seconds. Please check the simulations.
2. I think the thermal conductivity is not correctly calculated and this can have a major impact on the results. In equation 2, the dry soil conductivity is used for the mineral fraction of the soil. However, the dry soil thermal conductivity is a bulk value. In this equation for saturated conditions, the “mineral thermal conductivity” should be used and this is typically around 3.0 W/(mK). Therefore, a value of 0.35 W/ (mK) is excessively low. Some nice examples of mineral conductivities are given in Overduin et al. (2019):
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JC014675
The authors should check if similar errors were made for density and heat capacity. Further, there is an extra bracket on the right side of all three equations.
3. Because of the thermal conductivity error, I am skeptical of the permafrost aggradation rates. I am a little bit surprised that changing the porosity has such a small effect on the results, especially because the latent heat associated with such a change is significant. If the authors re-run the simulations with the correct thermal properties, I hypothesize that, the permafrost aggradation rates would be more divergent when considering different porosities.
4. The numerical model does not consider salt diffusion and therefore salts cannot migrate during the advance of the freezing front. While I do not expect the authors to incorporate salt diffusion into their model, I would appreciate some more discussion on this process. As the authors point out, ground freezing results in ionic exclusion, thereby increasing the porewater salt concentration. Consequently, this creates a porewater salt concentration gradient. Since the advance of the freezing front slows with time, the porewater salt concentration can be sufficiently strong at a particular depth to increase the porewater salinity and create a cryopeg or partially frozen conditions. How would the permafrost aggradation rates change if salt transport were included in the model? For coupled heat and salt diffusion models, consider the following paper: https://doi.org/10.1029/2018JF0048235. Please add a conceptual diagram of the Ghyben-Hertzberg approximation and include two panels (1 with permafrost and 1 without permafrost). This would really help the reader visualize how the fresh-saline interface is expected to look in unfrozen and frozen environments.
6. If available, could you please include ground temperature data with the geochemical ground ice data in Table 1? At the very least, were in-situ frozen and unfrozen conditions recorded during drilling? Please add this information.
Minor:
Line 13: Should “valley” be capitalized?
Line 45: Consider rephrasing to “below 0 °C”
Line 51: Consider pointing out that permafrost can form in taliks beneath lagoons, as well as beneath bottom-fast ice conditions in shallow water. Consider the following paper: Solomon, S. M., Taylor, A. E., & Stevens, C. W. (2008, June). Nearshore ground temperatures, seasonal ice bonding, and permafrost formation within the bottom-fast ice zone, Mackenzie Delta, NWT. In Proceedings of the Ninth International Conference on Permafrost, Fairbanks, Alaska (Vol. 29, pp. 1675-1680). Fairbanks: Institute of Northern Engineering, University of Alaska Fairbanks.
Line 51: Replace “permafrost usually” with “permafrost is usually”
Line 54: Replace “Barents Sea” with “the Barents Sea”
Line 68: You mention that groundwater flow is practically impossible in continuous permafrost areas. Can you make a few comments about groundwater flow in cryopegs in continuous permafrost and if this is relevant to Svalbard?
Line 93: Replace “Exposed surface” with “The exposed surface.”
Line 95: Replace “Active layer thickness” with “The active layer thickness.”
Line 103: The units for “km” should not be capitalized.
Line 104: Replace “Permafrost section” with “The permafrost section.”
Line 105: Replace “1 to 5.5” with “1.0 to 5.5”
Figure 1: Please improve the resolution and include a higher quality figure.
Line 118: Replace “with serial” with “with a serial.”
Line 140: Replace “afresh” with “a fresh.”
Line 149: Should “Pingo” be capitalized?
Line 153: Please comment on why the high ratio of Ca/Cl and SO4/Cl at a depth of 5.45 m is enigmatic
Table 1: Please add a row for “standard seawater composition” to help put the results in context.
Line 225: Please be consistent. In the text, water freezing temperature (WFT) is used and in some of the figures (e.g., Figure 3) Tf (freezing point) is used.
Table 2: Careful with the units of thermal conductivity. The units should be W/ (mK).
Table 2: Please use appropriate notations for multipliers and exponents. The table looks a little messy.
Line 320: Please define “winter inflection point.”
Line 320: Replace “freezing front” with “the freezing front.”
Figure 4: Please add labels for panels “a”, “b”, and “c.”
Line 380: Replace “When freezing” with “When the freezing”
Line 383: What do you mean by “low water activity?”
Line 389: For simplicity, why not state the eutectic point of the H2O-NaCl system (-21 °C)?
Line 427: Replace “Less saline” with “The less saline”
Line 428: I suggest replacing “exposure” with “sub-aerial exposure.”
Line 429: Replace “when rebound” with “when the rebound.”
Line 463: Replace “Assuming groundwater” with “Assuming the groundwater.”Citation: https://doi.org/10.5194/tc-2022-134-RC2 -
AC2: 'Reply on RC2', Dotan Rotem, 17 Dec 2022
Dear referee #2
Thank you very much for your helpful comments.
Please find below our replies to the comments (in bold)
Major Comments
- 1. The time step of 32,600 seconds is not 0.5 days as written in the text. Half a day is 43,200 seconds. Please check the simulations.
This is correct. The time steps were originally taken as 0.5 days but later on reduced to 32,600 sec (~9 hr) as to keep simulation stability.
We will correct, accordingly, in the revised text.
- I think the thermal conductivity is not correctly calculated and this can have a major impact on the results. In equation 2, the dry soil conductivity is used for the mineral fraction of the soil. However, the dry soil thermal conductivity is a bulk value. In this equation for saturated conditions, the “mineral thermal conductivity” should be used and this is typically around 3.0 W/(mK). Therefore, a value of 0.35 W/ (mK) is excessively low. Some nice examples of mineral conductivities are given in Overduin et al. (2019):
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JC014675 The authors should check if similar errors were made for density and heat capacity. Further, there is an extra bracket on the right side of all three equations.
We used a thermal conductivity of 0.35 W/ (mK) for the dry sediment, following Wolfe, L. H., & Thieme, J. O. (1964) and Slusarchuk, W. A., & Watson, G. H. (1975), referring to silt or fine grain dry soils. Following the reviewer’s comment, we have re-run the model with 3.0 W/ (mK). In general, the obtained results predict faster freezing, which is compatible with the main conclusion of this paper. Our values of heat capacity and density are similar to those used in other works, including in the above reference (Overduin et al. (2019). In the revised version of the paper, we will also present diagrams with thermal conductivity of 3 W/ (mK).
- Because of the thermal conductivity error, I am skeptical of the permafrost aggradation rates. I am a little bit surprised that changing the porosity has such a small effect on the results, especially because the latent heat associated with such a change is significant. If the authors re-run the simulations with the correct thermal properties, I hypothesize that, the permafrost aggradation rates would be more divergent when considering different porosities.
We re-run the model with different porosities and with the suggested high conductivity values. As suggested by the reviewer the differences are notable. The results of the sensitivity tests will be presented and discussed in the revised version.
- The numerical model does not consider salt diffusion and therefore salts cannot migrate during the advance of the freezing front. While I do not expect the authors to incorporate salt diffusion into their model, I would appreciate some more discussion on this process. As the authors point out, ground freezing results in ionic exclusion, thereby increasing the porewater salt concentration. Consequently, this creates a porewater salt concentration gradient. Since the advance of the freezing front slows with time, the porewater salt concentration can be sufficiently strong at a particular depth to increase the porewater salinity and create a cryopeg or partially frozen conditions. How would the permafrost aggradation rates change if salt transport were included in the model? For coupled heat and salt diffusion models, consider the following paper: https://doi.org/10.1029/2018JF004823
Thanks for this comment. Basically, there is no indication of any cryopegs in this site, and the existence of an FSI (i.e. increase in salinity down core) suggest that salts did not migrate much away from the freezing front, at least within the depth we covered (12m). This is probably due to the fact that salt diffusion is 3 orders of magnitude lower than heat diffusion. Accordingly. at the beginning of Discussion chapter, we suggested that “permafrost pore space should hold a small fraction of residual brine solution, which contains most of the solutes originally dissolved in the bulk pore-space water”. We will expand on this in the revised version. We note that we cannot exclude the existence of cryopegs in deeper permafrost, which could indeed affect our simulations for this part. We will note about this in the revised version.
- Please add a conceptual diagram of the Ghyben-Hertzberg approximation and include two panels (1 with permafrost and 1 without permafrost). This would really help the reader visualize how the fresh-saline interface is expected to look in unfrozen and frozen environments.
The Ghyben-Hertzberg approximation is partly presented in Figure 7, but is probably not clear enough. We will dedicate a figure to this in the revised version
- If available, could you please include ground temperature data with the geochemical ground ice data in Table 1? At the very least, were in-situ frozen and unfrozen conditions recorded during drilling? Please add this information.
Such data is not available, since Temperature cannot be reliably determined during drilling. We refer to Hanne’s Christiansen data, which was taken down the valley, therefore not necessarily representing the site. We did record cores condition during drilling and it appears in the discussion. We will add it to table 1 in the revised version.
Minor Comments
We will follow these comments and correct accordingly in the revised version
Thank you again for your time and very instructive notes.
Dotan
Citation: https://doi.org/10.5194/tc-2022-134-AC2
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AC2: 'Reply on RC2', Dotan Rotem, 17 Dec 2022
Dotan Rotem et al.
Dotan Rotem et al.
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