However, excavation in June confirmed the continuity of the two wires across the trough, but the wires broke just outside the logger box above the ground surface. Accordingly, the wire disconnection is considered to have originated from tension above the ground surface. These results imply that this ice wedge is active.
Due to the uncertainties, however, the activity of TR2 is excluded from further analysis.
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The occurrences, however, are subjected to significant interannual variations. The wire at 0. Excavation in the following summer confirmed that the upper wire broke near the center of the trough for all events. Small and sporadic events were recorded also in the other winters and occasionally in summers. The monitoring systems indicated significant cracking activity in at least eight out of the 12 winters. For comparison between the monitored troughs, the temperature near the permafrost table is represented by the value at 1.
To derive the opening at the ground surface, the data were corrected with reference to tilting of the frame and upheaval of the frame due to frost heave and settlement Supporting information Appendix S2.
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Such estimations and corrections were conducted for the winters accompanied by rapid expansion events. All three sensors detected signals indicative of thermal contraction cracking within this period. Next, the upper extensometer 0.
This initial pulse was followed by gradual expansion and warming. The second, less drastic expansion started on March 17 with recovery of cooling, which continued until the end of March.
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The upper extensometer recorded total expansion W U of 5. The lower extensometer also gradually extended from February 28 and recorded total expansion W L of 3. The shallower copper wire at 0. No acceleration was recorded on March The sudden termination of the cold period on April 1 probably prevented further propagation of the crack into the permafrost. Both extensometers recorded contraction in early April synchronized with warming of the ground, indicating closure of the thermal contraction crack.
This event suggests that a major crack opening took place at the ground surface, probably on February Expansion continued until the end of March with two significant pulses during cooling and finally amounted to 7. Disconnection of the copper wire at 0. The disconnection may have been associated with the expansion event, but its timing was not resolved by the data.
The field visit on February 28 confirmed the extensive occurrence of new cracks on the ice cover. Only the shallower copper wire at TR3 broke on February 16 , which was confirmed by excavation in August The first step C12a recorded at TR3 on February 13 comprised rapid expansion 2. Local snow trapped in the relatively deep trough may have caused the delayed response of the subsurface thermal probes to cooling in the air and surrounding ground.
Between the two steps, all ground cooling phases had acceleration events indicative of microcracking but lacked significant expansion. The second step C12b occurred at both troughs from late March to early April The copper wire at 0. The total expansion at the surface W S including the two steps reached 5.
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During the winter, TR3 experienced wire disconnection on March 19 at 0. The disconnection was confirmed by excavation in August The shock logger recorded a single acceleration event on March 17, which could be the precursor of the disconnection, but otherwise no event in March. Whether thermal contraction or another mechanism induced this event is unclear. The total expansion at the surface W S was 6. The breakage at 0. Excavation in August confirmed the disconnections of both wires. The shallower loop at 0. In contrast, the deeper loop at 0.
Ice‐wedge polygon dynamics in Svalbard: Lessons from a decade of automated multi‐sensor monitoring
Total expansion, represented by W L because of the malfunction of the upper extensometer, amounted to 9. These expansion pulses corresponded to rapid ground cooling. The pattern of ground acceleration was very similar between TR1 and TR3 during this event. TR3 lost data due to submergence of the logger during snowmelt. TR1 showed significant cracking. The resulting expansion of the ridges causes the trough to shrink slightly in early winter.
Solifluction from the adjacent ridges may enhance contraction of the trough where thaw settlement occurs in late summer. The contrasting movements between the ridge and trough indicate that the seasonal soil deformation within the active layer below a pair of ridges basically constrains the deformation of the intervening trough. Progressive cooling increases horizontal tensile stress, which eventually produces rapid expansion of the trough that probably indicates opening of a crack a few millimeters in width. Thermal contraction cracking can propagate in two ways: downwards from the ground surface and upwards from the top of the ice wedge.
In addition, the shallower wire is more frequently broken than the deeper one and, when both wires are broken, the shallower one breaks earlier. These situations suggest the predominance of downward crack propagation. The depth of the thermal contraction crack is uncertain, because copper wires can only detect the first significant crack. The copper wires do not always respond to crack opening immediately, but tend to break with some delay or even escape failure. These results do not exclude the possibility of crack penetration into permafrost and consequent increment of an ice vein, despite survival of the wire at 0.
Such a possibility is indeed supported by the presence of deep cracks 1. The model is in general agreement with observations from northern Alaska, 12 Svalbard 36 and Martian high latitudes, 34 and is supported by our field observations showing that colder more brittle frozen ground subject to more rapid cooling favors thermal contraction cracking.
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Consequently, based on frozen ground rheology, the threshold for thermal contraction cracking can be expressed in terms of two basic parameters, T TOP or T and the thermal gradient of the frozen active layer G AL. We mainly use average values of G AL in the whole active layer instead of the cooling rate, because the latter is variable on both temporal and spatial depth scales and thus difficult to define the representative values.
The marginal values around the threshold, however, may represent shallow cracking confined within the frozen active layer. Because of much colder winters, these Canadian Arctic sites experience earlier cracking 19 and longer and wider expansion 39 than the Adventdalen sites. Kokelj et al. This situation is further intensified when rainfall turns a shallow snow cover into an ice cover eg, C10 event , allowing more effective ground cooling. In addition, the warming trend in the second half of the monitoring period has not yet effectively degraded cracking activity, supported by short, but rapid cooling events recurring almost every winter.
Cracking occurs even in a period with relatively high seasonal air and ground temperatures, although the frequency of cracking may be decreasing. Thermal contraction cracking occurred in at least eight out of the 12 winters, although the occurrences were spatially variable. We thank a number of colleagues, in particular O. Humlum, A. Avian, A. Ikeda and M. Ishikawa for assistance and suggestions during fieldwork.
Detailed and constructive comments along with English editing received from B. Hallet, H. O'Neill and an anonymous reviewer were most helpful in improving our manuscript. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries other than missing content should be directed to the corresponding author for the article.
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Norikazu Matsuoka Corresponding Author E-mail address: matsuoka geoenv. Hanne H. Tools Request permission Export citation Add to favorites Track citation. Share Give access Share full text access. Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. Location a and air photograph b of the research site. Figure 2 Open in figure viewer PowerPoint. Geometry of polygons and monitored troughs TR1—TR3. I started to laugh along with the audience when I mentioned that the staunch anti-war Senator Joseph S.
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