4.1. Gas Emission Craters Formation Dates
Landforms under discussion are extremely dynamic. Time span between GEC formation and the closest time of acquisition matters a lot in terms of the crater size, degree of its flooding, and state of the frozen blocks ejected. The fact that this time span differs for various GECs make it complicated to conclude on relations between the parameters of the mound-predecessor and the resulting crater.
The GEC-1 formation timeframe was estimated to be between 9 October and 1 November 2013 (Table 2
) based on the analysis of Landsat 8 imagery [16
]. Our interpretation of imagery was guided by the following considerations. According to the measurements taken during the field survey on 16 July 2014 [31
], as well as interpretation of WorldView-1 images dated 15 June 2014, the rim of GEC-1 was 25–29 m in diameter. As far as this GEC formed in the fall of 2013, since then its rim had retreated from its original size due to the thaw and collapse of its frozen icy walls. Based on a series of multi-temporal images, we suggest that a small (1.5 × 1.5 pixel) dark patch (with low reflectivity) is a GEC that can be interpreted with confidence in images starting 1 November 2013 [3
]. Simultaneously, a light (high-reflectivity) patch appears on these images. Its spatial position coincides with the accumulative parapet clearly visible on later, very-high-resolution images from the summer of 2014. The expansion of the dark patch until 3 April 2014, can be explained by the fast melting of a thin layer of snow over the accumulative hillocks within the parapet that formed in the fall, 2013. These forms, clearly visible, were the first to expose from the snow.
A publication by Sizov [1
] offers another viewpoint concerning GEC-1 formation time using the same Landsat imagery. Sizov concluded that GEC-1 had formed between 21 February and 3 April 2014. Different readings of identical images arise due to small dimensions of the newly formed object relative to image resolution (15 m per pixel). It would be impossible to validate any viewpoint using more detailed imagery as none is found in global operators’ catalogs. Thus, the difference in GEC-1 formation dates remains within the range of 9 October 2013, to 3 April 2014, i.e., the GEC was formed between fall 2013 and spring 2014.
] pointed out that GEC-2 had formed no later than 2 May 2013, by highlighting a depression on a snow-covered Landsat image. Images listed above (Table 2
) have enabled to narrow considerably the GEC-2 formation timeframe down to 16 days—between 24 September and 15 October 2012.
The wide GEC-3 formation timeframe between 22 October 2012 and 10 June 2013 (Table 2
) is due to a lack of cloud-free images between identified dates with a resolution sufficient for confident interpretation of the newly formed GEC (no more than 15 m per pixel).
There are no discussions concerning AntGEC and SeYkhGEC formation dates in literature, the only source being reports from local citizens pointing particular dates of 27 September 2013, and 28 June 2017, respectively (Table 2
4.2. Geomorphic and Environmental Patterns Associated with Gas Emission Craters
GECs under study reveal an extremely wide range of geomorphic settings (Table 3
). All GECs in the Yamal Peninsula are associated with geodynamic accumulation zones, such as slope foot adjacent to valleys of permanent or temporary streams (GEC-1, GEC-2, GEC-3), or a river channel (SeYkhGEC). Slopes are rather gentle since they are never steeper than 7°. All these areas are low and wet, with increased snow accumulation in winter. In that respect, the only known GEC in the Gydan Peninsula (AntGEC) is different. It is located at the border between a denudation zone and a transit zone downslope, on the edge of a watershed surface. This terrain inflection is sloping at 7–15°, several times steeper than slopes of surfaces where Yamal GECs had formed. AntGEC is the only GEC to form on a well-drained surface with sandy deposits. However, none of the known craters is located in the central part of the watershed surfaces.
Differences noted in the GEC position regarding terrain contradicts the hypothesis proposed in [7
] stating that the GEC form by exploding of perennial frost mound (known as a bulgunniakh, or pingo, resulting from freezing of a drained lake talik) in a closed system. The existence of such a talik can be assumed under a lake in the area of GEC-1 (even though this GEC is located at the foot of a slope). However, the location of all other GECs (GEC-2, GEC-3, SeYkhGEC, AntGEC) on slopes or in the river channel is not appropriate to the formation of a closed talik that could yield a frost mound upon freezing over.
The height of the mound-predecessors correlates well with the diameter of their base. The exception is the mound-predecessor of SeYkhGEC. Perhaps the overestimated diameter of this mound is since it was determined by ArcticDEM Stripes, based on images dated 19 April 2016, and the foot of the mound was covered with snow.
GECs are actively expanding due to the thaw and collapse of frozen icy walls, and are flooded by water from melted ground ice, snow accumulating inside the craters in winter, and rainfalls. GEC diameters across their rims were determined after different times elapsed after their formation. All the studied GECs have closely matching diameter values within 25–37 m (Table 3
). Based on field observations, satellite imagery analysis, and the period between the formation of the GECs and the date the measurements were taken, we believe that all GECs were initially 20–25 m in diameter. It should also be noted that the diameter of the crater is not related to the size of the mound-predecessor. AntGEC had the smallest mound-predecessor with relative height and diameter two to three times less than in other GECs, but the diameter of the resulting crater was the same. Perhaps the similarity of the diameters of the GECs is explained not only by the pressure of gas that led to the formation of the mound-predecessors but also by the mechanical properties of the deformed frozen rocks when bending deformation occurs.
The SeYkhGEC is unique as its mound-predecessor was located immediately next to the Myudriyakha river channel. At SPOT5 images from 2011 and 2012, the river course in the study area is stable and virtually straight with a small indentation on the south-eastern bank (Figure 7
). SPOT7 images from 2015 and 2016 showed extensive dynamics of mound growing and river channel bending. After the GEC had formed on 28 June 2017, it was immediately flooded.
Analyzing multi-temporal imagery for SeYkhGEC we suppose that the life cycle of a GEC, including the growth and explosion of its mound-predecessor, can be as short as three–five years (and even shorter for SeYkhGEC; from 2015 to 2017). Similar estimates were made by other researchers who suggested that ground surface deformation started in this area in 2013 with most active motion having occurred between 2015 and 2017 [7
]. In comparison, the life cycle of GEC-1 was calculated using dendrochronology at around 65 years [32
]. The SeYkhGEC crater appearance supports our conclusion that the extremely warm summer of 2012 was a trigger of several GEC formations. This warming, probably, was insufficient for the blow out of SeYkhGEC mound-predecessor, but only for its growth. Probably, this mound-predecessor experienced “heat-struck” once again during an even warmer summer of 2016, which caused its explosion.
A significant diameter of the Myudriyakha river course expansion observed on a satellite image one month after SeYkhGEC formation can be explained, first, by a considerable force behind material ejection and formation of a huge GEC in the place of a mound-predecessor; and second, by intense destruction of SeYkhGEC walls due to river erosion. Bogoyavlensky et al. [7
] mentioned riverbank erosion rates as high as 1.47 m per day immediately after GEC formation.
It should be noted that we disagree with the mechanism of the formation of a mound-predecessor in a river channel proposed in the paper being a frost mound (pingo) [7
]. A classical pingo with an ice core is driven by cryostatic or hydraulic pressure. Such conditions do not exist in a river channel where there is a warming effect of water. Talik in the river bottom does not freeze back in winter; thus, cryostatic or hydraulic pressure is not produced. Talik beneath the riverbed would not allow the formation of an ice core through migration or intrusion mechanism. We believe that a positive landform (GEC mound-predecessor) results from surface deformation affected by gas migration and expansion. This process has no relation to classic pingo growth schemes [33
]. Misusing the term “frost mound” (“pingo”) in this case would cause terminology confusion and wrong understanding of the particular origin of the phenomenon under consideration. Better explaining our feature is the term “gas hydrate pingo” used in literature when discussing sea-floor landform-predecessors of pockmarks [36
The internal structure of GECs was observed in GEC-1, GEC-2, and AntGEC. However, we assume the nature of all GECs to be identical in mechanisms and triggers. Thus, as they were not observed before inundation, SeYkhGEC and GEC-3 most likely had the same structure of cylindrical lower part and funnel-shaped upper part.
The presence of tabular ground ice in GEC walls is a common feature of all GECs. Ice is an important control of GEC formation [17
]. The release of methane from permafrost (including gas hydrate decomposition) is likely caused by rising air and ground temperature over the past decades. The formation of all GECs was preceded by anomalously warm summers [40
]. Gas may accumulate in cryopegs below tabular ground ice layers serving as traps [16
]. Irregularities in the tabular ground ice body together with its rough base yield favorable conditions for gas concentration that later causes deformation of ice and overlaying deposits thus forming a mound-predecessor.
We revealed the effect of lowering the surrounding surface only in the vicinity of GEC-2 (Figure 6
). A comparison of the DSMs before and after the formation of GEC-2 for this area showed a negative change in height up to 2–3 m (Figure 6
b). Part of this area was flooded by lake water. This subsidence of the surface within a radius of 80 m from the crater occurred in the first six months after the formation of GEC-2. During the same half-year, a big lake appeared due to flooding of the crater, with a diameter six–seven times larger than the initial diameter of the crater [20
]. Since then, the lake has been increasing in size only due to the thawing and retreat of the steep lake shore in the northern part of the lake [7
]. In the southern part of the lake, the shore has not eroded and is represented by a flooded, gently sloping towards the center of the lake surface with grass vegetation emerging through the water (Figure 6
a). The position of the shoreline in this part of the lake remained stable as confirmed by comparison of field survey in September 2017 and the 2013 satellite image. Such surface flooding is possible only if a portion of a valley where lake is forming has a reverse sloping of valley profile. We assume that during the explosion of the mound-predecessor in the fall of 2012, gas released and a cavity left in which gas had accumulated beneath the ice layer serving as a trap [17
]. Probably, during the explosion, cracks formed in the upper horizons of the frozen deposits, and the blocks settled into the cavity freed from the gas and gas hydrates.
4.3. Applicability of Remote Sensing Data
We conducted raw image processing with DSM extraction. Since various products of satellite images post-processing are in open access, it became necessary to assess the applicability of various types of data to solve our tasks.
DSM relative height accuracy (0.35–1.01 m for GEC-3 and AntGEC, see Table 2
) makes up 12–34% for rather low mounds 23 m in height preceding formation of GEC-3 and AntGEC (Table 3
). For GEC-1 and GEC-2 mound-predecessors are 4 to 6 m high (Table 3
), while the relative height accuracy (0.35–0.45 m) of DSM comprises only 7–10% of their height. Thus, DSMs’ relative height accuracy is crucial for the estimation of lower mounds height, which is rather close to the height accuracy itself.
We also considered the usability of the ArcticDEM covering the Arctic region to identify features linked to GEC formation. ArcticDEM v3.0 with a 2-m spatial resolution is available currently. The comparison between our DSM compiled for GEC-1 (see Table 2
) and the ArcticDEM Stripes data, based on the same images, both allowed to identify the mound-predecessor and to determine its morphometric properties (Figure 8
). Both products render the mound in a very similar manner with differences only in representing microrelief due to higher spatial resolution of authors’ DSMs. Considering that an example in Figure 8
confirms the local accuracy of mound-predecessor identification, we believe that ArcticDEM Stripes can be used when searching for other mound-predecessors or predicting the occurrence of new GECs.
However, there are limitations concerning the use of this data. These limitations arise due to the automated process used for generating the ArcticDEM without a thorough spatial analysis for possible artifacts on the resulting surfaces. Such artifacts may take place from stereo matching errors in the presence of clouds, cloud shades, and errors associated with waterway surfaces (both open waters and icebound). It is furthermore important to consider the season when stereo pairs for the compilation of ArcticDEM Stripes were obtained as snow cover may camouflage the relief of the initial surface.
For instance, for the GEC-2 site, one may end up with an erroneous conclusion that a new mound had formed in summer 2016 (see Figure 9
c, red line) when analyzing multi-temporal data from ArcticDEM Stripes. Remote sensing data (Figure 9
) obtained during August 2016 field surveys indicated that there was no mound, while a lake that had formed in the GEC site in 2013 had continued to expand. A Sentinel-2 optical image obtained on the same date as the source images used for ArcticDEM Stripes (16 June 2016) provides a reliable indication of lake water surface with residual signs of snow cover on the western coast (Figure 9
a). The lake appears as well on a higher spatial resolution SPOT6 image dated 15 July 2016 (Figure 9
b). Thus, the “mound” in ArcticDEM does not match the reality. Though ArcticDEM can generally be used when searching for mound-predecessors, it should be taken into consideration that this DSM in some cases may comprise an artifact most likely caused by errors in the processing of a signal reflected from the water surface.
Multi-temporal ArcticDEM Stripes allows identifying areas with relief changes. If these changes are expressed in the form of appearing or disappearing mounds, then such sites require special attention and thorough analysis. One of the instruments for such an analysis is DSMs compiled from random stereo pairs. They can be used to refine the rate and magnitude of changes in elevation that are fixed in ArcticDEM.
We can state that surface settings, which are confidentially interpreted on satellite imagery, cannot be considered strong attributes characterizing an area of the probable mound and GEC formation. A joint analysis of remote sensing data and the study of the cryolithologic structure infield is required.