4.1. General Cryostratigraphy of Permafrost Exposed in the Batagay Megaslump
The Batagay megaslump is the largest one in the Northern Hemisphere occupying an area of 81 hectares, and with a length of more than 1000 m and a width of 800 m in 2019 (see Figure 1
). The slump expands at high rates along the perimeter of up to 0.026 km2
(between 1991 and 2018) [3
]. The exposed depth along the slump perimeter varies depending on the ice content of the permafrost deposits and the activity of thermo-erosion. It reaches up to about a 60 m depth in the central part of the headwall. The slope inclination is pre-defined by exogenous geological processes that formed the slopes of the adjacent Kirgilyakh and Khatyngnakh mounts, subsequent long-term aggradation of partly very ice-rich permafrost, i.e., the Ice Complex and thermo-erosion along with valley structures on the slopes by ground ice melt. The Batagay megaslump morphodynamics is characterized by active thermo-erosion at the walls and the fluvial erosion by temporary streams on the slump bottom. The latter transports eroded fine material with meltwater along the central slump channel into the floodplain of the Batagay River. However, substantial amounts of the eroded material, as well as permafrost remnants, remain on the slump bottom, forming a high relief of ridges and hills with different heights.
The stratigraphy in the southern exposure of the slump—close to the central headwall (sampling points A1 and B1)—was characterized by the lower and the upper Ice Complex units (about 3–7 m and 20–25 m thick, respectively), separated by the lower sand unit (up to about 20 m thick) and a distinct woody bed (up to about 3 m thick) at the base of the upper Ice Complex (Figure 2
and Figure 3
). The upper Ice Complex was covered by the uppermost sediment layer with a thickness of 1.5 m.
Ice wedges of the upper Ice Complex were distributed to a depth of 14–24 m from the top of the slump. The upper Ice Complex contained many thin plant roots as well as large fragments of tree trunks. Below the upper Ice Complex at a depth of about 23 m, a sandy loam layer with an admixture of reddish-orange sand with a thickness of 1 m was striking. Underneath this layer with a thickness of 1.5–2.0 m, there was an organic-rich layer with numerous plant and wood fragments of different degrees of decomposition (woody bed; Figure 3
). From a depth of 26 m, there was a sandy loam of gray and light brown color with narrow veins of sand of orange color with narrow composite wedges and many thin roots (lower sand unit). There were brown organic residues with a pungent odor of rotting material, as well as visible individual inclusions of charcoal.
The lower sand unit ended at a depth of 51 m and had a thickness of 25 m. Between the depths of 51 and 52 m, there was a layer of reddish-orange sand with a thickness of 0.4–0.5 m. From the depth of 51.5 m, the lower Ice Complex began with a thickness of up to 6–7 m. The upper boundary (visible part) of the ice wedges had a height of 0.4–1.2 m. There were ice schlieres with angled directions from the sides of the ice wedges. Above the ice wedges, there was a layer consisting of icy sandy loam of dark gray color which was penetrated by roots, and below was a layer of silty sandy loam of brown color alternating with crushed stones, inclusions of charcoal and narrow strips of peat.
The wedge ice of the upper Ice Complex was pillar-shaped, resembling a solid wall of ice from a distance. Table 1
summarizes descriptions of the cover deposits (samples in profiles B1, C1 and D1) of the upper Ice Complex, mainly composed of sandy and dusty particles with a localized separate newly formed structure (ironing) and individual mechanical inclusions of pebbles and gravel, which formed one soil pedon for the Batagay megaslump. The unit sampled at A1 represents a separate feature of the lower Ice Complex. The formation of the lower Ice Complex and its preservation remains uncertain. Perhaps, the lower Ice Complex survived at least partly due to the thick and rapidly accumulated lower sand unit that is up to about 25 m thick and is relatively ice-poor. But in this case, what contributed to such an active accumulation of sedimentary cover over the lower Ice Complex? The lower Ice Complex is surrounded by a heterogeneous composition of materials such as sandy loam and loess. The origin and existence of loess to a depth of 58 m suggests the accumulation of loess formations occurred as a result of frost weathering and solifluction processes due to periglacial phenomena in the Batagay area.
4.2. Carbon Stocks of the Lower and the Upper Ice Complex Units, and the Uppermost Cover
It is necessary to take into account that microorganisms begin to multiply actively in thawing permafrost [20
]. When organic matter is oxidized, they remove carbon from the bound state into the atmosphere. The degree of decomposition of soil organic matter depends on the properties of the soil as well as the depth of the profile and community of bacteria, archaea and eukaryotes, which are mostly found in frozen soils. This leads to the intensification of microbial metabolic activity and the possible creation of positive feedback to the conditions of anticipating thawing of the permafrost [34
The main fraction of OC was concentrated in the active layer topping the upper Ice Complex due to the productivity of this layer, both in terms of greater seasonal thawing and processing of soil organic matter by microbes (Table 2
The resulting determinants are soil warming up to positive temperatures and activation of microbiological processes; as a result, destruction and intensive mineralization of animals and plant residues occur. Variations in the OC content in the active layer varied widely from a minimum value of 0.3% to a maximum of 12.5%. Higher content of OC was prevalent on the slopes of the southern exposure. Consequently, for the northern exposure slope (C1), the average OC value was 2.1%, while for the southern exposure (B1) and the ravine (D1) it was higher with 3.5% and 2.8%, respectively. The deep penetration of heat on the southern exposure slopes causes permafrost thaw and collapse in the warm period of the year compared with the northern exposure slopes, as is noticeable in the picture (see Figure 1
). This increases the physical runoff of soil material into the Batagay River, as well as its subsidence on the bottom of the slump.
The properties of the shielding layer are closely related to the active (seasonally thawed) layer. The susceptibility of the seasonally thawed layer to the constant cycle of freezing and thawing determines the presence or absence of the frozen shielding layer at the uppermost part of permafrost. In case of destruction of the vegetation cover, the active layer degrades as a result of soil temperature increase, and the shielding layer loses its usual boundaries and merges with seasonally thawed soil, which leads to a lack of shielding properties leading to permafrost degradation and thaw subsidence. The average OC content in the shielding layer was low if compared to the active layer, reaching only 0.3%. This low OC content is most likely due to the buffer properties of the shielding layer, its low thickness, low moisture content and the paretic rate of chemical reactions. In the permafrost layer, the average OC content for the lower Ice Complex (A1) was 0.65% and for the upper Ice Complex (B1) is 1.04%. There was a concentration increase of 58.5% and 74.0% compared to the shielding layer. Perhaps this is the result of the genetic features of lower cryogenic horizon formation and the water presence in it with ongoing metabolic processes even at temperatures below 0 °C. It is worth noting that in the past, organic residues deposited in combination with heterotrophic microbial activity were capable of carbon remineralizing, despite the negative temperatures of the deposits.
The deposits of the Batagay megaslump had a small amount of carbonate in their composition. This is apparent in low inorganic carbon values, and there were no significant variations over depth. The inorganic carbon distribution was homogeneous both in the active layer and permafrost with systematic low values over the entire profile of sediments. Consequently, the oxide group of minerals took part in the formation of the deposits, for example, quaternary formations of the slump mainly consisted of quartz minerals in the form of sand and they are represented by the loess fraction.
TC content is formed from the two previous carbon fractions (OC and IC); therefore, it basically repeats the nature of their distribution in differentiated (elementary) layers.
shows the carbon stocks of the studied stratigraphic features of the Batagay megaslump. The enormous amount of organic carbon was stored in the active layer for sections B1 and D1, where 60% of organic carbon was concentrated. At the same thickness of the active layer, the OC in C1 amounted to 9.52 kg m−2
, while the OC of D1 reached 18.03 kg m−2
, which was almost twice as much. Even with a changing of slope exposure, the stocks for these sections in the active layer increased from the northern to the southern exposure by 42%–47% of organic carbon.
The lowest carbon stocks of all studied stratigraphic features were recorded in the shielding layer, reaching 1.3 kg OC m−2 and 0.5 kg IC m−2. IC stocks in the active layer of section B1 did not differ from the other two profiles (C1 and D1) and ranged from 2 to 2.2 kg m−2. Carbonates enriched the first meter from the surface in non-permafrost soils. Under permafrost conditions, rather high inorganic carbon data were observed, 1.8 kg m−2 close to the level of the active layer. Section A1 had a higher IC stock of 3.6 kg m−2. By moving to the lower Ice Complex, the IC reserves were in an an inactive state due to the influence of negative soil temperature, with substances trying to penetrate with soil solutions to the Yedoma horizons, and most likely the reserves remained unchanged for an extended period. Low estimates of inorganic carbon in the deposits of the Batagay slump are the result of low natural carbonate, as well as frosty weathering of rocks. Carbonate formation occurs in soil pores and frost cracks, indicating their predominant dissolution and leaching in cold climate conditions.
The upper Ice Complex (B1) shows the distribution of the carbon in the elementary layers and the conjugation of carbon from the thickness and the amount of carbon available for mobilization in them. The maximum reserves of total carbon are contained in the active layer and amount to 18.72 kg m−2, the minimum in the shielding layer is 1.81 kg m−2, and in permafrost rocks, it is 10.34 kg m−2. In total, 30.87 kg m−2 was deposited in a two-meter thickness. The ratio of organic and inorganic carbon in the section varies in scale for the active layer-15, the shielding layer-3 and permafrost deposits-5. The ratio indicators show that organic carbon is the dominant feature in the soil horizon, layer or sediment. The higher this value, the more organic part is contained in a particular element of the Ice Complex. Also, this indicates the ability of organic carbon to be a nutrient medium for microorganisms and a source of CO2. Inorganic carbon indicates the level of carbonate in a given horizon.
In the upper two meters of the Ice Complex, carbon stocks were 30.87 kg m−2
. In the study area, the dominating forest species is Larix cajanderi
. Its phytomass reserves barely exceeded 6.00 kg m−2
, which is formed mainly due to green, lignified aerial parts, roots and moss-lichen complexes. Stocks of dead organic matter are 3.00 kg m−2
, and litter is 2.50 kg m−2
. In the northern taiga zone, they are considered to be the forests with the lowest production, with about 11.50 kg m−2
per year [38
]. As a result, carbon accumulation rates are low in the sediments of the upper Ice Complex. The main part of the organic matter is stored on the surface as a humus horizon of the soil, which undergoes degradation and weak decomposition, and the mineralization rate in such natural conditions remains low.
There is no doubt that the continental [1
] genesis of the formation of the lower Ice Complex (A1) and its archaic past, compared with the upper Ice Complex (B1), affected the loss, impossibility of renewal and accumulation of plant organic matter. The cycling of the biogenic elements was interrupted by the rapid accumulation of sandy loam layers and sediment deposits [5
] and most likely by the removal of organic layer beyond the meadow steppes [39
] due to aeolian processes. We estimated the amount of ancient buried carbon at 17.66 kg m−2
for a thickness of 1.2 m.
Permafrost degradation caused by global warming is not always a catastrophic consequence for the ecumene. To justify this point of view, we give a few well-founded examples. Siberian larch forests occupy 40% of all coniferous wood in Russia, which develop on permafrost. The amount of accessible nutrients for trees increases with permafrost degradation, which causes an increase in their biomass [40
]. An increase in the average annual temperature and rainfall contributes to the advancement of natural zones deep into the north, expanding the agricultural area and changing the structure of the soil cover with an increasing share of organic matter due to an increase in the productivity of vegetation [41
The permafrost zone has changed repeatedly, giving way to periods of warming and cooling, which favoured the formation and accumulation of permafrost in the past. With a certain degree of probability, it can be assumed that the multi-tiered features of the Batagay megaslump and their heterogeneous origin are the results of the participation of several geological factors:
prolonged permafrost conditions of the territory with cryogenic and exogenous processes occurring in it;
the transfer of aeolian material and its accumulation;
the impact of the Quaternary climate and paleoclimate;
the movement and deformation as a result of the tectonic influence of the territory. That means the Adycha–Taryn general snap has or had an impact on the formation of the Batagay megaslump in conjunction with the other listed predictors. This is because the formation of such a large-scale current geocryological object in the North of Yakutia could not have taken place without the participation of intense seismotectonic conditions of the territory.
The anthropogenic activity was also one of the significant reasons for the formation of the megaslump. The economic and industrial development of the northern territories has never left an integral ecosystem behind, especially in such vulnerable natural conditions as boreal forests or tundra landscapes, which undoubtedly provoked the degradation of the Batagay Ice Complex through the destruction of vegetation and soil cover. Cutting down forests for timber for consumer needs, including the destabilizing pyrogenic situation at the site, has also had an impact.