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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- RC1: 'Comment on egusphere-2025-2675', Anonymous Referee #1, 17 Aug 2025 reply
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- 1
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.