the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Mapping subsea permafrost around Tuktoyaktuk Island (NWT, Canada) using electrical resistivity tomography
Abstract. Along much of the Arctic coast, shoreline retreat and sea level rise combine to inundate permafrost. Once inundated by seawater permafrost usually begins to degrade. Tuktoyaktuk Island (Beaufort Sea, NWT, Canada) is an important natural barrier protecting the harbor of Tuktoyaktuk, but will likely be breached within the next three decades. The state of subsea permafrost and its depth distribution around the island are, however, still largely unknown. We collected marine electrical resistivity tomography (ERT) surveys (vertical electrical soundings) north and south of Tuktoyaktuk Island using a floating cable with 13 electrodes in a quasi-symmetric Wenner-Schlumberger array. We filtered the data with a new approach to eliminate potentially falsified measurements due to a curved cable and inverted the profiles with a variety of parameterizations to estimate the position of the top of the ice-bearing permafrost table (IBPT) below the sea floor. Our results indicate that north of Tuktoyaktuk Island, where coastal erosion is considerably faster, IBPT depths range from 5 m below sea level (120 m from the shoreline) to around 20 m bsl (up to 800 m from the shoreline). South of the island, the IBPT dips more steeply and lies at 10 m bsl a few meters from the shore to more than 30 m bsl 200 m from the shore. We discuss how marine ERT measurements can be improved by recording electrode position, but choices made in data inversion can be a more likely source of uncertainty in IBPT position than electrode positions. At Tuktoyaktuk Island, IBPT depths below the sea floor increase with distance from the shoreline; comparing the northern and southern sides of the island, its inclination is inversely proportional to coastline retreat rates. On the island’s north side, historical coastal retreat rate suggests a mean degradation rate of 5.3 ± 4.0 cm/yr.
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Status: open (until 28 May 2024)
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RC1: 'Comment on egusphere-2024-1044', Wojciech Dobiński, 26 Apr 2024
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Mapping of submarine permafrost remains an important scientific issue despite the increasing number of works published in this field. Undersea permafrost is much more difficult to access empirically and requires the use of a new, original methodological approach. Systematic research covers only the last few decades, so the recognition of undersea permafrost and especially the active layer is still imperfect and requires a lot of scientific effort. The presented article addresses both of these issues: substantive and methodological, contributing to this important topic with new, original research.
The authors try to present a number of scientific and methodological findings that may certainly be of great cognitive value to the growing number of undersea permafrost researchers. With this very general sentence I want to express my rather positive opinion about this work. I think it will be more interesting to present some comments and suggestions that would help improve this article. I will present them generally in this review, which is also significantly supplemented by detailed comments attached to the text of the article in the PDF version, in the form of notes
- In the title, the authors rightly bring to the fore the substantive issue, i.e. determining the extent of permafrost in the studied area. I think it would be good to maintain this order in the structure of the entire work. However, in some chapters methodological issues come to the fore, such as limitations in the application of the method, difficulties in field work, uncertainty of some results, etc. This is an understandable and honest approach to the issue, but it slightly weakens the scientific result of the article. I would rather reverse this order.
- An extremely important issue is to clearly define what is actually the subject of the authors' research. Just saying it's about permafrost degradation is not enough and not very original. Although the definition of permafrost applies to its terrestrial and undersea occurrence to the same extent, in the undersea environment the situation becomes much more complicated, for example because permafrost is not a purely climatic geological formation (subaerial). Therefore, in my opinion, a chapter is needed that characterizes the differences in the occurrence of terrestrial and submarine permafrost. This is even more important because a clear definition of the subject of research has important methodological consequences, i.e. in the interpretation of research results. This is not a difficult issue. In the published articles of the team of prof. Angelopoulos it is clearly presented.
- Ice-bearing permafrost (IBP) is not the same as permafrost (PF) in general. IBP is to PF as a part is to the whole. In this context, it is important to comment on the issue of cryotic permafrost in your work. This would be a very interesting and necessary statement.
- Regarding the method used. Of course, I agree that, just like on land, also in the case of undersea permafrost, its detection is associated with a sudden increase in resistance. It is worth noting, however, that this applies only to IBP but not to PF in general. A big problem arises when it comes to the issue of the cryotic state, especially present in sedimentary material saturated with salt water. This requires clarification and a broader discussion. In this respect, the conclusions chapter should also be clarified.
I also have a few minor comments:
- I think it is more important to present at what depth the IBPT is located, not only under the water surface, but also under the seabed.
- It seems to me necessary to present and comment on many more profiles analogous to the one shown in Fig. 4. The reader should be able to compare. It doesn't have to be an extensive description.
- It would be very interesting for substantive and methodological reasons to see several ERT profiles with DOI isolines shown on its interpretation.
- I think it would be very interesting to speak about the active layer that usually accompanies the occurrence of permafrost. This is a relatively difficult issue and there are few publications in this field, but that is why it is worth mentioning at least briefly on this occasion.
As I mentioned, the text also contains comments and some (not all) highlights that are the result of quite significant controversies. I hope they will be helpful in improving this interesting work.
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