the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Aug 2025
The response of permafrost to inundation below a rapidly eroding Arctic island
Abstract. Tuktoyaktuk Island acts as a natural breakwater, protecting the harbour and townsite of Tuktoyaktuk — an Arctic community that has faced coastal retreat and its consequences for decades. Increasing storm activity, coupled with a longer open-water season, is rapidly eroding the island's shoreline and inundating the underlying permafrost. Once inundated, permafrost warms and degrades, further undermining coastal stability. This study investigates both short and long-term permafrost changes during the transition from terrestrial to subsea. We used Electrical Resistivity Tomography (ERT) to estimate the depth of the ice-bearing subsea permafrost table (IBPT), capturing the short-term response. By integrating subsurface resistivity data with historical shoreline positions and thermal modelling, we also gain insights into long-term degradation patterns. Our results reveal a distinct contrast in IBPT shape between the ocean-facing and harbour-facing nearshore zones, indicating the influence of coastal erosion rates and corresponding inundation times. Additionally, small-scale variations appear linked to local geological differences. In the long term, changes in subsurface composition point to more rapid ice loss within the permafrost than can be explained by the temperature gradient caused by inundation alone. We suggest that subsea permafrost north of the island is more degraded than previously thought, potentially accelerating the projected breach, which was last estimated to occur by 2044. These findings enhance our understanding of subsurface processes driven by coastal retreat and offer valuable insights that can inform engineering strategies to fortify the island.
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Status: closed
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RC1: 'Comment on egusphere-2025-2675', Anonymous Referee #1, 17 Aug 2025
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AC1: 'Reply on RC1', Mehriban Aliyeva, 05 Dec 2025
1 Response to Review No 1 ”The response of permafrost to inundation below a rapidly eroding Arctic island”
1.1 General response
We would like to thank the reviewer for reviewing our manuscript and for highlighting important aspects of petrophysical theory. We acknowledge the reviewer’s concern regarding the limitations of applying simplified Archie-type relations and that some terminology (e.g. “tortuosity”, ”saturation-dependent m”) was imprecisely used. We have revised the manuscript to modify these points. Please see the detailed points below. At the same time, we respectfully disagree that these issues constitute a “major flaw that prevents publication”. We applied Archie’s Law to permafrost in a similar fashion to other recently published papers, (e.g Oldenborger and LeBlanc (2018); Offer et al. (2025)). We are convinced that even without the additional implementation of Archie’s Law our study would still stand out as a unique contribution to the highly relevant topic of Arctic coastal dynamics. To clarify, our study does not aim to develop a new petrophysical model, but to link subsea permafrost degradation beneath Tuktoyaktuk Island to observed coastal retreat rates. Our paper builds on a recent marine ERT study (Erkens et al., 2025) and reveals new insights into permafrost processes operating on the north and south side of Tuktoyaktuk Island. Thus, our findings can contribute to the design of coastal protection measures. The key results of the paper – the contrasting IBPT geometries between the harbour and ocean-facing coastline, the spatial link between inundation age and IBPT depth, and the inference that subsea permafrost north of the island is more degraded than previously assumed – rely primarily on relative variations in resistivity, on geometry, and on the integration with shoreline change and thermal modelling. None of these key conclusions of our study were addressed in this review.
1.2 Specific comments
Reviewer comment (Summary): ”ERT alone cannot separate pore-water and surface conduction; the manuscript uses an unchecked assumption instead of confronting this limitation.”
Response: We acknowledge that, without induced polarization (IP), the electrical problem is under-determined and that porewater and surface conductivity in clay-dominated sediments cannot be uniquely separated. However, our substrate is silt-dominated (as stated in the manuscript), based on the cited seafloor grab samples and borehole data (Angelopoulos et al., 2025; Lapham et al., 2020; Whalen et al., 2022).
Reviewer comment (summary): ”Equation 1 is presented as Archie’s law, but it ignores surface conductivity and decades of petrophysical work (Winsauer & McCardell 1953; Waxman & Smits 1968; Vinegar & Waxman 1984, etc.).”
Response: We do not agree with the reviewer’s assessment. The functional form we use in Eq. (1) is the standard Archie-type relationship between bulk conductivity, porosity and porewater conductivity, which has been widely applied in permafrost and nearshore settings (including by Oldenborger et al. (2018)). In line with this literature, we used Archie’s law as a first-order approximation, under the assumption that surface conduction is of secondary importance for our silt-dominated substrate. However, to address the reviewer’s over-arching criticism of our use of Archie’s Law, we remove any use of Archie’s Law in the revised manuscript.
Reviewer comment (summary): ”The factor a in the Archie-type relation was called tortuosity”; this is incorrect because tortuosity is related to porosity, not simply to a. ”
Response: Since alpha(a) was set to 1, this does not affect our results.
Reviewer comment (summary): ”Allowing m to change with saturation is unphysical, because m characterizes the topology of the pore network.”
Response: We disagree: freezing and melting of the pore ice changes pore-water network topology.
Reviewer comment (summary): ”No petrophysical measurements were performed; established models and laboratory data from the literature are not used.”
Response: In contrast to many reservoir or onshore studies, acquiring dedicated petrophysical core material from offshore permafrost settings is extremely challenging. Drilling boreholes in the nearshore zone off Tuktoyaktuk would require marine operations, specialized equipment, complex permitting and licensing procedures, in addition to very substantial financial resources that are far beyond the scope of this project. However, we have constrained sediment properties using available field data, including seafloor grab samples and borehole data as stated above.
Reviewer comment (summary): ”Temperature effects on pore-water conductivity and the redistribution of salinity during freezing/thawing are not discussed.”
Response: We argue that the reviewer has misunderstood our method. We used borehole temperatures to model warming permafrost under changing upper boundary conditions at 20 m depth in the sediment column. We calculate bulk resistivities based on water contents from this heat flow model. Our model explicitly accounts for uncertainties via sensitivity to porosity and porewater conductivity. We acknowledge that brine rejection from sea ice formation in shallow waters or bottom-fast ice zones may increase the salinity in the near-surface sediment (e.g., Overduin et al. (2012)), but our measurements indicate that these seasonal processes are not relevant at 20 m depth.
Reviewer comment (Summary): ”Modern geophysics can easily circumvent surface conductivity problems by using induced polarization; this should have been done.”
Response: We acknowledge that frequency-dependent IP can, in principle, provide additional constraints on pore-water versus surface conductivity, as demonstrated by Revil et al. (2025). However, the term ”easily” is a mischaracterization of the intense and ongoing academic discussion of the past two decades. There are no published examples of the operational use of IP to determine ice content with commercially available ERT devices. Beyond the logistical constraints of Arctic field campaigns, IP in frozen and saline environments is itself non-trivial to interpret (Fereydooni et al., 2025), depends on a case-by-case quantification of the importance of contributing mechanisms (Kemna et al., 2012) and would require dedicated survey design and laboratory calibration that are beyond the scope of the present work. While future work incorporating IP would improve the study, its omission is not sufficient grounds for rejection.
References
Angelopoulos, M., Aliyeva, M., Cable, W. L., and Overduin, P. P.: Van Veen grab samples acquired in the region of Inuvik, Tuktoyaktuk and the Mackenzie Delta, Northwest Territories, Canada, https://doi.pangaea.de/10.1594/PANGAEA.981226, 2025.
Erkens, E., Angelopoulos, M., Tronicke, J., Dallimore, S. R., Whalen, D., Boike, J., and Overduin, P. P.: Mapping subsea permafrost around Tuktoyaktuk Island (Northwest Territories, Canada) using electrical resistivity tomography, The Cryosphere, 19, 997–1012, https://doi.org/10.5194/tc-19-997-2025, 2025.
Fereydooni, H., Gruber, S., Stillman, D., and Cronmiller, D.: Detecting ground ice in warm permafrost with the dielectric relaxation time from SIP observations, EGUsphere, pp. 1–24, https://doi.org/10.5194/egusphere-2025-1801, 2025.
Kemna, A., Binley, A., Cassiani, G., Niederleithinger, E., Revil, A., Slater, L., Williams, K. H., Orozco, A. F., Haegel, F.-H., H¨ordt, A., Kruschwitz, S., Leroux, V., Titov, K., and Zimmermann, E.: An overview of the spectral induced polarization method for near-surface applications, Near Surface Geophysics, 10, 453–468, https://doi.org/10.3997/1873-0604.2012027, 2012.
Lapham, L. L., Dallimore, S. R., Magen, C., Henderson, L. C., Powers, L. C., Gonsior, M., Clark, B., Cˆot´e, M., Fraser, P., and Orcutt, B. N.: Microbial Greenhouse Gas Dynamics Associated With Warming Coastal Permafrost, Western Canadian Arctic, Front. Earth Sci., 8, https://doi.org/10.3389/feart.2020.582103, 2020.
Offer, M., Weber, S., Krautblatter, M., Hartmeyer, I., and Keuschnig, M.: Pressurised water flow in fractured permafrost rocks revealed by borehole temperature, electrical resistivity tomography, and piezometric pressure, The Cryosphere, 19, 485–506, https://doi.org/10.5194/tc-19-485-2025, 2025.
Oldenborger, G. A. and LeBlanc, A.-M.: Monitoring changes in unfrozen water content with electrical resistivity surveys in cold continuous permafrost, Geophysical Journal International, 215, 965–977, https://doi.org/ 10.1093/gji/ggy321, 2018.
Overduin, P. P., Westermann, S., Yoshikawa, K., Haberlau, T., Romanovsky, V., and Wetterich, S.: Geoelectric observations of the degradation of nearshore submarine permafrost at Barrow (Alaskan Beaufort Sea), Journal of Geophysical Research: Earth Surface, 117, https://doi.org/10.1029/2011JF002088, 2012.
Revil, A., Richard, J., Ghorbani, A., Magnin, F., Duvillard, P. A., Marcer, M., Abdulsamad, F., Ingeman-Nielsen, T., Ravanel, L., Lambiel, C., Bodin, X., Cai, H., Hu, X., and Vaudelet, P.: Induced polarization as a tool to characterize permafrost 1. Theory and laboratory experiments, Geophys J Int, p. ggaf443, https://doi.org/10.1093/gji/ggaf443, 2025.
Whalen, D., Forbes, D., Kostylev, V., Lim, M., Fraser, P., Nedimovi´c, M., and Stuckey, S.: Mechanisms, volumetric assessment, and prognosis for rapid coastal erosion of Tuktoyaktuk Island, an important natural barrier for the harbour and community, Can. J. Earth Sci., 59, 945–960, https://doi.org/10.1139/cjes-2021-0101, 2022.
Citation: https://doi.org/10.5194/egusphere-2025-2675-AC1
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AC1: 'Reply on RC1', Mehriban Aliyeva, 05 Dec 2025
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RC2: 'Comment on egusphere-2025-2675', Anonymous Referee #2, 28 Sep 2025
Review "The response of permafrost to inundation below a rapidly eroding Arctic island"
The paper attempts to assess how permafrost in a coastal setting transforms when an island is inundated. To assess permafrost distribution, a range of methods are used, including ERT, permafrost probing, and borehole temperature measurements.
The study appears to have two objectives: (1) identifying different permafrost regimes under current conditions, and (2) understanding how inundation length changes permafrost conditions. While I believe objective (1) can be achieved with additional work, I think the assumptions required to address objective (2) are inherently flawed, which the authors themselves state.
In general, I found the paper rather confusing, and a clear "red line" linking the observations with the objectives is missing. Consequently, it remains unclear what the paper's aim is and what novel insights it provides. Since ERT is the main data source, I would also have expected a more thorough processing and interpretation of the data and results.
I provide a range of comments in the attached PDF, which I hope will help improve the current manuscript. Nevertheless, I think the authors may want to reconsider their objectives, keeping in mind the available data and its limitations.
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AC2: 'Reply on RC2', Mehriban Aliyeva, 05 Dec 2025
1 Response to Review No 2 ”The response of permafrost to inundation below a rapidly eroding Arctic island”
1.1 General Response
We would like to thank the reviewer for the time taken to read our manuscript and for providing detailed feedback. We split our response in two sections: addressing the reviewer’s general feedback as well as the more detailed comments from the pdf.
In this manuscript we demonstrate that subsea permafrost north of Tuktoyaktuk Island is more degraded – and breaching more imminently – than previously thought. This is critical because the island acts as a natural barrier protecting the harbour and community, i.e. extending the study by Erkens et al. (2025). Our amphibious use of ERT in a permafrost environment represents a novel application of the ERT method providing unique measurements across an ice-rich coastline undergoing thermoerosion.
We would like to point out that ERT is only one part of the study; we also quantified rapid coastal erosion using historical shoreline positions and combined these with thermal modelling to interpret long-term changes of the ice-bearing permafrost table and bulk resistivity at depth. This integrated approach allows us to assess how inundation alters subsea permafrost beyond conductive warming alone and to reassess the state and stability of permafrost north of Tuktoyaktuk Island.
1.2 Specific Comments
Reviewer comment (Lines 121-122): I find this statement misleading, as a wenner array has a comparably low spatial resolution and depth penetration, yet it provides strong voltage signals.
Response: We agree and have removed this text from the paper.
Reviewer comment (Line 122): How would the system adjust for poor contact resistances?
Response: Thank you for pointing this out. This statement was incorrectly phrased. The system automatically reduces its transmitter voltage if the selected maximum voltage cannot be applied. We have removed this statement.
Reviewer comment (Lines 137-138): Does this mean that you assume that the bathymetry hasn’t changed since 2018? Is this realistic?
Response: Please see our response to the next bathymetry comment (lines 173-176) for details on this.
Reviewer comment (Lines 148-149): By excluding data points with a stacking error of 5% you are not reducing complexity, but you reduce data of poor quality.
Response: We have reworded this statement in line with the reviewer’s comment.
Reviewer comment (Lines 163-164): This is not correct. You want to apply anisotropic constraints when prior knowledge of the geology or the imaged structures suggest that. But this is not a feature of surface measurements.
Response: We have removed this sentence.
Reviewer comment (Lines 173-176): It is unclear from the description above, but how well do you know the water level above the sediments? Is there strong wave action?
Response: We attached an echo-sounder to our boat in 2023, so we know the water level to an accuracy of 10 cm at specific points and specific times. Data was collected during calm conditions, so that wave action during data collection was small. Overall, we collected thousands of data points for our interpolation. To fill spatial gaps, we also used water depth data from prior expeditions. There are, however, uncertainties: the water depth changes over time due to tides, air pressure and storm surges. Of these, only the tidal range affected our measurement period, and only over a small range. Subsidence from subsea permafrost thaw could affect the bathymetries, but this has not been demonstrated or quantified. Our interpolated bathymetry map is the best available data for interpreting our ERT data with respect to water depth and seafloor structures, and it reflects the shoreface morphology at the site.
Reviewer comment (Lines 184-186): Do I understand this correctly that you define the location of the IBPT by looking at when the resistivity has increased by one order of magnitude from the surface value? Could you validate this based on your ALT measurements?
Response: Correct, we define the permafrost table as the point of resistivity increase by an order of magnitude in the logarithmic space beyond a threshold cut-off value of 10 ohm-m. The validation of this method with the terrestrial parts of the survey is impractical because the electrode spacing (5 m) was too large to resolve differences in the active layer depth on land. We will clarify our description to make it clearer.
Reviewer comment (Line 248): I may not fully understand this, but why do you get high geometric factors on the bluffs. I agree that the topography will indeed have an impact on the geometric factor, but you use Wenner-Schlumberger measurements where geometric factors are small, compared to, e.g., dipole-dipole surveys.
Response: We were referring to deviations in the geometric factor compared to a flat surface. We removed this statement from the paper.
Reviewer comment (Figure 4): Wouldn’t it be better to show the data in 3D?
Response: We originally had this figure in 3D, which made it very difficult to see differences between profiles.
Reviewer comment (Lines 231-234): It might be worth discussing the impact of surface conduction here as well.
Response: Please find our responses to reviewer 1 where we address this in detail.
Reviewer comment (Line 244): This is quite a bold statement. A chi² of almost 5 is indicative of a poor data fit, while perhaps still acceptable for bad quality data. But generally, you should aim for a chi² of 1, which would indicate that your model explains your data within their error bounds.
Response: We are directly citing this statement from Günther et al. (2006).
Reviewer comment (Lines 249-250): I don’t agree with that. Particularly for the perpendicular profiles you get strong spatial patterns.
Response: We have removed this statement.
Reviewer comment (Line 279): Then don’t these results indicate that you get some 3D effects in your data, and perhaps it would be better to invert the data in a 3D domain?
Response: 3D effects likely affect our terrestrial profiles, as demonstrated in the Figure A2. These profiles are not used in further analyses. The spatial data density is not high enough to justify 3D inversion, which would require extended measurements.
Reviewer comment (Heading 3.4 Resistivity changes over time): I find this heading misleading. You are not looking at resistivity changes over time. You are looking at the change in resistivity across the length of your profile, that you relate to the change in shoreline over time, and hence inundation. But this inherently assumes 1D thermo–hydraulic flow, which is most likely not the case.
Response: We are using a space-for-time substitution approach, which is justified by the fact that lateral temperature gradients over distances of hundreds of meters are negligible compared to vertical gradients over distances of meters and tens of meters induced by inundation. The purpose of the simplified modelling is to evaluate the plausibility of hypothesis that the observed lateral change in resistivity is largely controlled by pace of inundation/erosion.
Reviewer comment (Lines 352-360): I’m not sure how this relates to what you are showing above.
Response: We have removed this paragraph.
Reviewer comment (Lines 369-372): Doesn’t this very broad range indicate that your model has too many unknowns to provide some reliable estimates?
Response: The broad range results from parameter uncertainties inherent to any thermo-hydrological simulation without laboratory determined sediment properties.
Reviewer comment (Lines 396-397): Exactly. Then wouldn’t a statistical approach be useful here to assess the uncertainties?
Response: We assume the reviewer is referring to a statistical analysis of multiple inversions here. A full 2D global inversion is computationally demanding (Arboleda-Zapata et al., 2022) and beyond the scope of the present study. Our objective here is to obtain a geologically consistent IBPT estimate rather than to perform an ensemble-based uncertainty quantification. Importantly, the IBPT location inferred from the inversion is independently constrained by borehole observations, which provides empirical validation of our interpretation.
Reviewer comment (Lines 397-398): chi² values of 4 and RRMS values of 15% do not indicate good data fits.
Response: We removed this statement. However, we must note that higher RMS values are expected for complex surveys such as ours which involve submerged and terrestrial electrodes and sections collected on different days.
Reviewer comment (Lines 417-420): If this is important here, then maybe it would be worth adding a figure that shows this transition. But what impact has this permafrost transition on the system?
Response: We agree and will add a schematic representing these processes and their effects.
Reviewer comment (Lines 493-495): What about changes in grain sorting in the unfrozen sediments and hence changes in porosity? What is your petrophysical relationship to link unfrozen water content to resistivity?
Response: While frost-related processes may alter grain sorting and porosity, such changes cannot be resolved with ERT. We test a plausible porosity range in our ensemble modelling, and show that within the observed predominantly silt-rich sediments our main conclusions are far more sensitive to porewater resistivity than to reasonable variations in porosity and grain sorting.
References
Arboleda-Zapata, M., Angelopoulos, M., Overduin, P. P., Grosse, G., Jones, B. M., and Tronicke, J.: Exploring the capabilities of electrical resistivity tomography to study subsea permafrost, The Cryosphere, 16, 4423–4445, https://doi.org/10.5194/tc-16-4423-2022, 2022.
Erkens, E., Angelopoulos, M., Tronicke, J., Dallimore, S. R., Whalen, D., Boike, J., and Overduin, P. P.: Mapping subsea permafrost around Tuktoyaktuk Island (Northwest Territories, Canada) using electrical resistivity tomography, The Cryosphere, 19, 997–1012, https://doi.org/10.5194/tc-19-997-2025, 2025.
Günther, T., Rücker, C., and Spitzer, K.: Three-dimensional modelling and inversion of dc resistivity data incorporating topography — II. Inversion, Geophysical Journal International, 166, 506–517, https://doi.org/10.1111/j.1365246X.2006.03011.x, 2006.
Citation: https://doi.org/10.5194/egusphere-2025-2675-AC2
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AC2: 'Reply on RC2', Mehriban Aliyeva, 05 Dec 2025
Status: closed
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RC1: 'Comment on egusphere-2025-2675', Anonymous Referee #1, 17 Aug 2025
The manuscript deals with a very interesting and relevant scientific subject in the context of climate change. That said, it suffers from a major flaw that prevent its publication. The authors used electrical resistivity alone without induced polarization and because they cannot solve the issue of two resistivity contributions and one observation, they resort to an assumption that is unchecked and even not discussed. More precisely equation 1 is NOT Archie (first or second laws). In modern physics and geophysics, Archie’s law is the relationship between the intrinsic formation factor (which expression can be obtained by upscaling the local dissipation of Joule energy) and the (connected porosity). It can be also obtained by plotting for a collection of rocks from the same formation their formation factors versus their connected porosity. The second Archie’s law is reefing to the resistivity index. We know since the seminal works by Winsauer and McCardell 1953 that equation 1 is incomplete because of the effect of the surface conductivity (which was already understood than to Bikerman a century ago!) occurring at the surface of the grains in their electrical double layer (see Waxman and Smits, 1968, and Vinegar and Waxman, 1984). Even for sea water saturated sediments, surface conductivity cannot be neglected as shown by over 60 years of borehole logging data in the realm of the oil and gas industry. I am ready to accept that surface conductivity could be negligible or neglected based on experimental evidence but… the present work suffers from a total absence of petrophysical work. Furthermore the manuscript is plgued with misconceptions that are unfortunately more and more present in the literature and associated with a poor knowledge of the underlying physics of the problem. For instance the factor a in equation 1 is called tortuosity. This is totally wrong. The tortuosity of the bulk pore space is the product of the formation factor by the connected porosity. Sadly, the problem of surface conductivity can be easily overcome in modern geophysics by using induced polarization data that can be performed with the same equipment as used for ERT and in the same time frame. I have to conclude that the authors should pay more attention to the literature on this subject. I am very surprised that equation 1 is presented as the only equation representing the conductivity of a rock putting in more 60 years of literature. Such a position is a bit scary. Another issue among many is the change of m with saturation. This is a non sense since the exponent m characterize the topology of the pore network. Such type of mistakes arises when the authors are not cautious in taking the appropriate models for the conductivity of rocks. This is not a letter of choice but underlying physics. I found most of the modeling very speculative in terms of petrophysics while relevant petrophysical models exist and have been checked/proven through serious laboratory measurements. Many other effects should have been discussed as well. 2 examples, the effect of temperature on the pore (liquid) water conductivity itself, the effect of the fate of salinity in freeze and thaw, etc. At the point, this manuscript is not mature enough to be published.
Citation: https://doi.org/10.5194/egusphere-2025-2675-RC1 -
AC1: 'Reply on RC1', Mehriban Aliyeva, 05 Dec 2025
1 Response to Review No 1 ”The response of permafrost to inundation below a rapidly eroding Arctic island”
1.1 General response
We would like to thank the reviewer for reviewing our manuscript and for highlighting important aspects of petrophysical theory. We acknowledge the reviewer’s concern regarding the limitations of applying simplified Archie-type relations and that some terminology (e.g. “tortuosity”, ”saturation-dependent m”) was imprecisely used. We have revised the manuscript to modify these points. Please see the detailed points below. At the same time, we respectfully disagree that these issues constitute a “major flaw that prevents publication”. We applied Archie’s Law to permafrost in a similar fashion to other recently published papers, (e.g Oldenborger and LeBlanc (2018); Offer et al. (2025)). We are convinced that even without the additional implementation of Archie’s Law our study would still stand out as a unique contribution to the highly relevant topic of Arctic coastal dynamics. To clarify, our study does not aim to develop a new petrophysical model, but to link subsea permafrost degradation beneath Tuktoyaktuk Island to observed coastal retreat rates. Our paper builds on a recent marine ERT study (Erkens et al., 2025) and reveals new insights into permafrost processes operating on the north and south side of Tuktoyaktuk Island. Thus, our findings can contribute to the design of coastal protection measures. The key results of the paper – the contrasting IBPT geometries between the harbour and ocean-facing coastline, the spatial link between inundation age and IBPT depth, and the inference that subsea permafrost north of the island is more degraded than previously assumed – rely primarily on relative variations in resistivity, on geometry, and on the integration with shoreline change and thermal modelling. None of these key conclusions of our study were addressed in this review.
1.2 Specific comments
Reviewer comment (Summary): ”ERT alone cannot separate pore-water and surface conduction; the manuscript uses an unchecked assumption instead of confronting this limitation.”
Response: We acknowledge that, without induced polarization (IP), the electrical problem is under-determined and that porewater and surface conductivity in clay-dominated sediments cannot be uniquely separated. However, our substrate is silt-dominated (as stated in the manuscript), based on the cited seafloor grab samples and borehole data (Angelopoulos et al., 2025; Lapham et al., 2020; Whalen et al., 2022).
Reviewer comment (summary): ”Equation 1 is presented as Archie’s law, but it ignores surface conductivity and decades of petrophysical work (Winsauer & McCardell 1953; Waxman & Smits 1968; Vinegar & Waxman 1984, etc.).”
Response: We do not agree with the reviewer’s assessment. The functional form we use in Eq. (1) is the standard Archie-type relationship between bulk conductivity, porosity and porewater conductivity, which has been widely applied in permafrost and nearshore settings (including by Oldenborger et al. (2018)). In line with this literature, we used Archie’s law as a first-order approximation, under the assumption that surface conduction is of secondary importance for our silt-dominated substrate. However, to address the reviewer’s over-arching criticism of our use of Archie’s Law, we remove any use of Archie’s Law in the revised manuscript.
Reviewer comment (summary): ”The factor a in the Archie-type relation was called tortuosity”; this is incorrect because tortuosity is related to porosity, not simply to a. ”
Response: Since alpha(a) was set to 1, this does not affect our results.
Reviewer comment (summary): ”Allowing m to change with saturation is unphysical, because m characterizes the topology of the pore network.”
Response: We disagree: freezing and melting of the pore ice changes pore-water network topology.
Reviewer comment (summary): ”No petrophysical measurements were performed; established models and laboratory data from the literature are not used.”
Response: In contrast to many reservoir or onshore studies, acquiring dedicated petrophysical core material from offshore permafrost settings is extremely challenging. Drilling boreholes in the nearshore zone off Tuktoyaktuk would require marine operations, specialized equipment, complex permitting and licensing procedures, in addition to very substantial financial resources that are far beyond the scope of this project. However, we have constrained sediment properties using available field data, including seafloor grab samples and borehole data as stated above.
Reviewer comment (summary): ”Temperature effects on pore-water conductivity and the redistribution of salinity during freezing/thawing are not discussed.”
Response: We argue that the reviewer has misunderstood our method. We used borehole temperatures to model warming permafrost under changing upper boundary conditions at 20 m depth in the sediment column. We calculate bulk resistivities based on water contents from this heat flow model. Our model explicitly accounts for uncertainties via sensitivity to porosity and porewater conductivity. We acknowledge that brine rejection from sea ice formation in shallow waters or bottom-fast ice zones may increase the salinity in the near-surface sediment (e.g., Overduin et al. (2012)), but our measurements indicate that these seasonal processes are not relevant at 20 m depth.
Reviewer comment (Summary): ”Modern geophysics can easily circumvent surface conductivity problems by using induced polarization; this should have been done.”
Response: We acknowledge that frequency-dependent IP can, in principle, provide additional constraints on pore-water versus surface conductivity, as demonstrated by Revil et al. (2025). However, the term ”easily” is a mischaracterization of the intense and ongoing academic discussion of the past two decades. There are no published examples of the operational use of IP to determine ice content with commercially available ERT devices. Beyond the logistical constraints of Arctic field campaigns, IP in frozen and saline environments is itself non-trivial to interpret (Fereydooni et al., 2025), depends on a case-by-case quantification of the importance of contributing mechanisms (Kemna et al., 2012) and would require dedicated survey design and laboratory calibration that are beyond the scope of the present work. While future work incorporating IP would improve the study, its omission is not sufficient grounds for rejection.
References
Angelopoulos, M., Aliyeva, M., Cable, W. L., and Overduin, P. P.: Van Veen grab samples acquired in the region of Inuvik, Tuktoyaktuk and the Mackenzie Delta, Northwest Territories, Canada, https://doi.pangaea.de/10.1594/PANGAEA.981226, 2025.
Erkens, E., Angelopoulos, M., Tronicke, J., Dallimore, S. R., Whalen, D., Boike, J., and Overduin, P. P.: Mapping subsea permafrost around Tuktoyaktuk Island (Northwest Territories, Canada) using electrical resistivity tomography, The Cryosphere, 19, 997–1012, https://doi.org/10.5194/tc-19-997-2025, 2025.
Fereydooni, H., Gruber, S., Stillman, D., and Cronmiller, D.: Detecting ground ice in warm permafrost with the dielectric relaxation time from SIP observations, EGUsphere, pp. 1–24, https://doi.org/10.5194/egusphere-2025-1801, 2025.
Kemna, A., Binley, A., Cassiani, G., Niederleithinger, E., Revil, A., Slater, L., Williams, K. H., Orozco, A. F., Haegel, F.-H., H¨ordt, A., Kruschwitz, S., Leroux, V., Titov, K., and Zimmermann, E.: An overview of the spectral induced polarization method for near-surface applications, Near Surface Geophysics, 10, 453–468, https://doi.org/10.3997/1873-0604.2012027, 2012.
Lapham, L. L., Dallimore, S. R., Magen, C., Henderson, L. C., Powers, L. C., Gonsior, M., Clark, B., Cˆot´e, M., Fraser, P., and Orcutt, B. N.: Microbial Greenhouse Gas Dynamics Associated With Warming Coastal Permafrost, Western Canadian Arctic, Front. Earth Sci., 8, https://doi.org/10.3389/feart.2020.582103, 2020.
Offer, M., Weber, S., Krautblatter, M., Hartmeyer, I., and Keuschnig, M.: Pressurised water flow in fractured permafrost rocks revealed by borehole temperature, electrical resistivity tomography, and piezometric pressure, The Cryosphere, 19, 485–506, https://doi.org/10.5194/tc-19-485-2025, 2025.
Oldenborger, G. A. and LeBlanc, A.-M.: Monitoring changes in unfrozen water content with electrical resistivity surveys in cold continuous permafrost, Geophysical Journal International, 215, 965–977, https://doi.org/ 10.1093/gji/ggy321, 2018.
Overduin, P. P., Westermann, S., Yoshikawa, K., Haberlau, T., Romanovsky, V., and Wetterich, S.: Geoelectric observations of the degradation of nearshore submarine permafrost at Barrow (Alaskan Beaufort Sea), Journal of Geophysical Research: Earth Surface, 117, https://doi.org/10.1029/2011JF002088, 2012.
Revil, A., Richard, J., Ghorbani, A., Magnin, F., Duvillard, P. A., Marcer, M., Abdulsamad, F., Ingeman-Nielsen, T., Ravanel, L., Lambiel, C., Bodin, X., Cai, H., Hu, X., and Vaudelet, P.: Induced polarization as a tool to characterize permafrost 1. Theory and laboratory experiments, Geophys J Int, p. ggaf443, https://doi.org/10.1093/gji/ggaf443, 2025.
Whalen, D., Forbes, D., Kostylev, V., Lim, M., Fraser, P., Nedimovi´c, M., and Stuckey, S.: Mechanisms, volumetric assessment, and prognosis for rapid coastal erosion of Tuktoyaktuk Island, an important natural barrier for the harbour and community, Can. J. Earth Sci., 59, 945–960, https://doi.org/10.1139/cjes-2021-0101, 2022.
Citation: https://doi.org/10.5194/egusphere-2025-2675-AC1
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AC1: 'Reply on RC1', Mehriban Aliyeva, 05 Dec 2025
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RC2: 'Comment on egusphere-2025-2675', Anonymous Referee #2, 28 Sep 2025
Review "The response of permafrost to inundation below a rapidly eroding Arctic island"
The paper attempts to assess how permafrost in a coastal setting transforms when an island is inundated. To assess permafrost distribution, a range of methods are used, including ERT, permafrost probing, and borehole temperature measurements.
The study appears to have two objectives: (1) identifying different permafrost regimes under current conditions, and (2) understanding how inundation length changes permafrost conditions. While I believe objective (1) can be achieved with additional work, I think the assumptions required to address objective (2) are inherently flawed, which the authors themselves state.
In general, I found the paper rather confusing, and a clear "red line" linking the observations with the objectives is missing. Consequently, it remains unclear what the paper's aim is and what novel insights it provides. Since ERT is the main data source, I would also have expected a more thorough processing and interpretation of the data and results.
I provide a range of comments in the attached PDF, which I hope will help improve the current manuscript. Nevertheless, I think the authors may want to reconsider their objectives, keeping in mind the available data and its limitations.
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AC2: 'Reply on RC2', Mehriban Aliyeva, 05 Dec 2025
1 Response to Review No 2 ”The response of permafrost to inundation below a rapidly eroding Arctic island”
1.1 General Response
We would like to thank the reviewer for the time taken to read our manuscript and for providing detailed feedback. We split our response in two sections: addressing the reviewer’s general feedback as well as the more detailed comments from the pdf.
In this manuscript we demonstrate that subsea permafrost north of Tuktoyaktuk Island is more degraded – and breaching more imminently – than previously thought. This is critical because the island acts as a natural barrier protecting the harbour and community, i.e. extending the study by Erkens et al. (2025). Our amphibious use of ERT in a permafrost environment represents a novel application of the ERT method providing unique measurements across an ice-rich coastline undergoing thermoerosion.
We would like to point out that ERT is only one part of the study; we also quantified rapid coastal erosion using historical shoreline positions and combined these with thermal modelling to interpret long-term changes of the ice-bearing permafrost table and bulk resistivity at depth. This integrated approach allows us to assess how inundation alters subsea permafrost beyond conductive warming alone and to reassess the state and stability of permafrost north of Tuktoyaktuk Island.
1.2 Specific Comments
Reviewer comment (Lines 121-122): I find this statement misleading, as a wenner array has a comparably low spatial resolution and depth penetration, yet it provides strong voltage signals.
Response: We agree and have removed this text from the paper.
Reviewer comment (Line 122): How would the system adjust for poor contact resistances?
Response: Thank you for pointing this out. This statement was incorrectly phrased. The system automatically reduces its transmitter voltage if the selected maximum voltage cannot be applied. We have removed this statement.
Reviewer comment (Lines 137-138): Does this mean that you assume that the bathymetry hasn’t changed since 2018? Is this realistic?
Response: Please see our response to the next bathymetry comment (lines 173-176) for details on this.
Reviewer comment (Lines 148-149): By excluding data points with a stacking error of 5% you are not reducing complexity, but you reduce data of poor quality.
Response: We have reworded this statement in line with the reviewer’s comment.
Reviewer comment (Lines 163-164): This is not correct. You want to apply anisotropic constraints when prior knowledge of the geology or the imaged structures suggest that. But this is not a feature of surface measurements.
Response: We have removed this sentence.
Reviewer comment (Lines 173-176): It is unclear from the description above, but how well do you know the water level above the sediments? Is there strong wave action?
Response: We attached an echo-sounder to our boat in 2023, so we know the water level to an accuracy of 10 cm at specific points and specific times. Data was collected during calm conditions, so that wave action during data collection was small. Overall, we collected thousands of data points for our interpolation. To fill spatial gaps, we also used water depth data from prior expeditions. There are, however, uncertainties: the water depth changes over time due to tides, air pressure and storm surges. Of these, only the tidal range affected our measurement period, and only over a small range. Subsidence from subsea permafrost thaw could affect the bathymetries, but this has not been demonstrated or quantified. Our interpolated bathymetry map is the best available data for interpreting our ERT data with respect to water depth and seafloor structures, and it reflects the shoreface morphology at the site.
Reviewer comment (Lines 184-186): Do I understand this correctly that you define the location of the IBPT by looking at when the resistivity has increased by one order of magnitude from the surface value? Could you validate this based on your ALT measurements?
Response: Correct, we define the permafrost table as the point of resistivity increase by an order of magnitude in the logarithmic space beyond a threshold cut-off value of 10 ohm-m. The validation of this method with the terrestrial parts of the survey is impractical because the electrode spacing (5 m) was too large to resolve differences in the active layer depth on land. We will clarify our description to make it clearer.
Reviewer comment (Line 248): I may not fully understand this, but why do you get high geometric factors on the bluffs. I agree that the topography will indeed have an impact on the geometric factor, but you use Wenner-Schlumberger measurements where geometric factors are small, compared to, e.g., dipole-dipole surveys.
Response: We were referring to deviations in the geometric factor compared to a flat surface. We removed this statement from the paper.
Reviewer comment (Figure 4): Wouldn’t it be better to show the data in 3D?
Response: We originally had this figure in 3D, which made it very difficult to see differences between profiles.
Reviewer comment (Lines 231-234): It might be worth discussing the impact of surface conduction here as well.
Response: Please find our responses to reviewer 1 where we address this in detail.
Reviewer comment (Line 244): This is quite a bold statement. A chi² of almost 5 is indicative of a poor data fit, while perhaps still acceptable for bad quality data. But generally, you should aim for a chi² of 1, which would indicate that your model explains your data within their error bounds.
Response: We are directly citing this statement from Günther et al. (2006).
Reviewer comment (Lines 249-250): I don’t agree with that. Particularly for the perpendicular profiles you get strong spatial patterns.
Response: We have removed this statement.
Reviewer comment (Line 279): Then don’t these results indicate that you get some 3D effects in your data, and perhaps it would be better to invert the data in a 3D domain?
Response: 3D effects likely affect our terrestrial profiles, as demonstrated in the Figure A2. These profiles are not used in further analyses. The spatial data density is not high enough to justify 3D inversion, which would require extended measurements.
Reviewer comment (Heading 3.4 Resistivity changes over time): I find this heading misleading. You are not looking at resistivity changes over time. You are looking at the change in resistivity across the length of your profile, that you relate to the change in shoreline over time, and hence inundation. But this inherently assumes 1D thermo–hydraulic flow, which is most likely not the case.
Response: We are using a space-for-time substitution approach, which is justified by the fact that lateral temperature gradients over distances of hundreds of meters are negligible compared to vertical gradients over distances of meters and tens of meters induced by inundation. The purpose of the simplified modelling is to evaluate the plausibility of hypothesis that the observed lateral change in resistivity is largely controlled by pace of inundation/erosion.
Reviewer comment (Lines 352-360): I’m not sure how this relates to what you are showing above.
Response: We have removed this paragraph.
Reviewer comment (Lines 369-372): Doesn’t this very broad range indicate that your model has too many unknowns to provide some reliable estimates?
Response: The broad range results from parameter uncertainties inherent to any thermo-hydrological simulation without laboratory determined sediment properties.
Reviewer comment (Lines 396-397): Exactly. Then wouldn’t a statistical approach be useful here to assess the uncertainties?
Response: We assume the reviewer is referring to a statistical analysis of multiple inversions here. A full 2D global inversion is computationally demanding (Arboleda-Zapata et al., 2022) and beyond the scope of the present study. Our objective here is to obtain a geologically consistent IBPT estimate rather than to perform an ensemble-based uncertainty quantification. Importantly, the IBPT location inferred from the inversion is independently constrained by borehole observations, which provides empirical validation of our interpretation.
Reviewer comment (Lines 397-398): chi² values of 4 and RRMS values of 15% do not indicate good data fits.
Response: We removed this statement. However, we must note that higher RMS values are expected for complex surveys such as ours which involve submerged and terrestrial electrodes and sections collected on different days.
Reviewer comment (Lines 417-420): If this is important here, then maybe it would be worth adding a figure that shows this transition. But what impact has this permafrost transition on the system?
Response: We agree and will add a schematic representing these processes and their effects.
Reviewer comment (Lines 493-495): What about changes in grain sorting in the unfrozen sediments and hence changes in porosity? What is your petrophysical relationship to link unfrozen water content to resistivity?
Response: While frost-related processes may alter grain sorting and porosity, such changes cannot be resolved with ERT. We test a plausible porosity range in our ensemble modelling, and show that within the observed predominantly silt-rich sediments our main conclusions are far more sensitive to porewater resistivity than to reasonable variations in porosity and grain sorting.
References
Arboleda-Zapata, M., Angelopoulos, M., Overduin, P. P., Grosse, G., Jones, B. M., and Tronicke, J.: Exploring the capabilities of electrical resistivity tomography to study subsea permafrost, The Cryosphere, 16, 4423–4445, https://doi.org/10.5194/tc-16-4423-2022, 2022.
Erkens, E., Angelopoulos, M., Tronicke, J., Dallimore, S. R., Whalen, D., Boike, J., and Overduin, P. P.: Mapping subsea permafrost around Tuktoyaktuk Island (Northwest Territories, Canada) using electrical resistivity tomography, The Cryosphere, 19, 997–1012, https://doi.org/10.5194/tc-19-997-2025, 2025.
Günther, T., Rücker, C., and Spitzer, K.: Three-dimensional modelling and inversion of dc resistivity data incorporating topography — II. Inversion, Geophysical Journal International, 166, 506–517, https://doi.org/10.1111/j.1365246X.2006.03011.x, 2006.
Citation: https://doi.org/10.5194/egusphere-2025-2675-AC2
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AC2: 'Reply on RC2', Mehriban Aliyeva, 05 Dec 2025
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The manuscript deals with a very interesting and relevant scientific subject in the context of climate change. That said, it suffers from a major flaw that prevent its publication. The authors used electrical resistivity alone without induced polarization and because they cannot solve the issue of two resistivity contributions and one observation, they resort to an assumption that is unchecked and even not discussed. More precisely equation 1 is NOT Archie (first or second laws). In modern physics and geophysics, Archie’s law is the relationship between the intrinsic formation factor (which expression can be obtained by upscaling the local dissipation of Joule energy) and the (connected porosity). It can be also obtained by plotting for a collection of rocks from the same formation their formation factors versus their connected porosity. The second Archie’s law is reefing to the resistivity index. We know since the seminal works by Winsauer and McCardell 1953 that equation 1 is incomplete because of the effect of the surface conductivity (which was already understood than to Bikerman a century ago!) occurring at the surface of the grains in their electrical double layer (see Waxman and Smits, 1968, and Vinegar and Waxman, 1984). Even for sea water saturated sediments, surface conductivity cannot be neglected as shown by over 60 years of borehole logging data in the realm of the oil and gas industry. I am ready to accept that surface conductivity could be negligible or neglected based on experimental evidence but… the present work suffers from a total absence of petrophysical work. Furthermore the manuscript is plgued with misconceptions that are unfortunately more and more present in the literature and associated with a poor knowledge of the underlying physics of the problem. For instance the factor a in equation 1 is called tortuosity. This is totally wrong. The tortuosity of the bulk pore space is the product of the formation factor by the connected porosity. Sadly, the problem of surface conductivity can be easily overcome in modern geophysics by using induced polarization data that can be performed with the same equipment as used for ERT and in the same time frame. I have to conclude that the authors should pay more attention to the literature on this subject. I am very surprised that equation 1 is presented as the only equation representing the conductivity of a rock putting in more 60 years of literature. Such a position is a bit scary. Another issue among many is the change of m with saturation. This is a non sense since the exponent m characterize the topology of the pore network. Such type of mistakes arises when the authors are not cautious in taking the appropriate models for the conductivity of rocks. This is not a letter of choice but underlying physics. I found most of the modeling very speculative in terms of petrophysics while relevant petrophysical models exist and have been checked/proven through serious laboratory measurements. Many other effects should have been discussed as well. 2 examples, the effect of temperature on the pore (liquid) water conductivity itself, the effect of the fate of salinity in freeze and thaw, etc. At the point, this manuscript is not mature enough to be published.