A Gas-Emission Crater in the Erkuta River Valley, Yamal Peninsula: Characteristics and Potential Formation Model

geosciences

Article A Gas-Emission Crater in the Erkuta River Valley, Yamal Peninsula: Characteristics and Potential Formation Model

Evgeny Chuvilin 1,* , Julia Stanilovskaya 2, Aleksey Titovsky 3, Anton Sinitsky 4, Natalia Sokolova 1, Boris Bukhanov 1 , Mikhail Spasennykh 1, Alexey Cheremisin 1 , Sergey Grebenkin 1, Dinara Davletshina 1 and Christian Badetz 2

1 Center for Hydrocarbon Recovery, Skolkovo Institute of Science and Technology (Skoltech), Skolkovo Innovation Center, 3 Nobel Street, Moscow 121205, Russia; [email protected] (N.S.); [email protected] (B.B.); [email protected] (M.S.); [email protected] (A.C.); [email protected] (S.G.); [email protected] (D.D.) 2 Total, 2 Jean Miller, La Defense, 92078 Paris, France; [email protected] (J.S.); [email protected] (C.B.) 3 Department of Science and Innovation of the Yamal-Nenets Autonomous District (YNAO), Salekhard 629008, Russia; [email protected] 4 Arctic Research Center of the Yamal-Nenets Autonomous District, Salekhard 629008, Russia; [email protected] * Correspondence: [email protected]

Received: 27 January 2020; Accepted: 6 May 2020; Published: 8 May 2020

Abstract: Methane is a powerful greenhouse gas, and the abrupt degassing events that recently have formed large craters on the Russian Arctic Yamal and Gydan Peninsulas have caused major concern. Here we present field data on cover sediments and evolution of a gas-emission crater discovered in the Erkuta–Yakha River valley in the southern Yamal Peninsula in June 2017. The crater is located south of other similar craters discovered over the past decade in northern West Siberia. Data were collected during a field trip to the Erkuta crater in December 2017 which included field observations and sampling of permafrost soil and ground ice from the rim of the crater. All soil and ice samples were measured for contents of methane and its homologs (ethane and propane) and carbon dioxide. The contents of carbon dioxide in some samples are notably higher than methane. The strongly negative δ13C of methane from ground ice samples ( 72%) is typical of biogenic hydrocarbons. − The ratio of methane to the total amount of its homologs indicate a component of gases that have migrated from a deeper, thermogenic source. Based on obtained results, a potential formation model for Erkuta gas-emission crater is proposed, which considers the combined effect of deep-seated (deep gas migration) and shallow (oxbow lake evolution and closed talik freezing) causes. This model includes several stages from geological prerequisites to the lake formation.

Keywords: permafrost; Yamal; talik; freezing; methane; gas emission; crater; proposed formation model

1. Introduction Methane is a powerful greenhouse gas, and the recent abrupt events of degassing and crater formation on the Yamal and Gydan Peninsulas (Figure1) have caused major concern that a warming Arctic may lead to increased thawing of permafrost and gas emissions. In addition, exploration and development of oil and gas ﬁelds in northern West Siberia, with construction of production, transportation and support facilities upon permafrost, face problems due to the harsh Arctic climate, low negative temperatures of air and ground and complex periglacial processes like frost heaving,

Geosciences 2020, 10, 170; doi:10.3390/geosciences10050170 www.mdpi.com/journal/geosciences Geosciences 2020, 10, 170 2 of 16 thermokarst, thermal erosion. Processes in shallow permafrost may lead to the formation of gas-emission craters, a phenomenon discovered during exploration for the past decade in the Arctic West Siberia. The first methane-leaking crater (Yamal Crater) was found in the Yamal Peninsula 42 km from the Bovanenkovo gas field (Figure1), and that discovery was followed by several more from the Yamal and Gydan Peninsulas [1–3]. The Yamal Crater has been better documented [4–7] than the others, which are only imaged in few photographs, but the available data are insufficient to concede about the conditions and causes of its origin. The crater origin has been unanimously attributed to an explosive gas emission event, but the origin of the gas remains a subject of discussions. The proposed hypotheses explaining the formation of the Yamal Crater, and other craters in the Yamal and Gydan Peninsulas, can be divided into two main groups that invoke either deep-seated or shallow causes. The deep-seated causes of crater formation [7–9] include increased deep heat flux, upward migration of deep gaseous fluids through fault zones and fault intersections to shallow permafrost and dissociation of intrapermafrost gas hydrates driven by ascending heat and gas flows. These processes can produce reservoirs of pressurized gas in shallow permafrost which can release explosively, break up the frozen cap and form a crater. The hypothesized shallow prerequisites of crater formation are associated with permafrost temperature variations under the effects of climate and surface heat transfer. These models involve permafrost is warming from above and the ensuing destabilization of relict gas hydrates in the upper permafrost [2,5]. Another possible explanation for crater formation is that freezing of a closed talik (a body of unfrozen soils in permafrost) produces a confined hydraulic system with gradually increasing gas pressure and a pingo-like feature prone to collapse by explosive emission of the pressurized gas [10]. Understanding the formation mechanisms of gas-emission craters requires collecting and analyzing new relevant field data. At the time being, only a few panoramic photographs of craters other than the Yamal Crater (Figure1) and that in the Seyakha River valley [ 11] are available. Any new data on the crater structure and evolution, on gas chemistry and stable isotopes, as well as on the lithology of permafrost in the area are of scientific value. Field data from crater evolution is particularly important because the craters have a very short lifetime (a few months to one or two years) before transforming into lakes. However, drilling and geophysical surveys from the craters are lacking as the craters are most often located in hardly accessible remote areas far from infrastructure. Here we provide substantive background on the cover sediments of the Erkuta crater in the southern Yamal Peninsula (Figure1) and propose a formation model that could provide a basis for further assessment with more field data.

2. Erkuta Crater and Its Evolution The Erkuta crater formed in the winter of 2016–2017 in the southern Yamal Peninsula (Figure1), in typical conditions of continuous permafrost described in a number of publications [12,13]. Permafrost mainly consists of interbedded Quaternary (middle-upper Pleistocene and Holocene) sand, silty sand, sandy silt and clay silt with lenses of gas-saturated rocks and cold saline groundwater (cryopegs). Ice contents in shallow permafrost can reach 40%–50%, mainly in fine-grained sediments. Even though the long-term mean ground temperatures of the study area are 5 to 6 C and permafrost is around − − ◦ 200-m-thick; the zero ◦C isotherm can be even 100–150 m deeper due to salinity of soils and the presence of cryopegs. The seasonal thaw depth is about 0.7–1.2 m. The territory abounds in ground ice of various genetic types, seasonal and perennial frost heaves, peatlands, swamps and thermokarst lakes that occupy 20% of the area, including lakes with crater-like features possibly associated with gas emission through faults and fractures on the bottom. The gas-emission crater formed in the Erkuta–Yakha River valley filled with floodplain and riverbed alluvial sand and silt enclosing plant detritus. There are many oxbow lakes in the crater area, some close to the crater (Figure2). Geosciences 2020,, x FOR PEER REVIEW 3 of 17

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Figure 1. Location map of the Erkuta crater and oil and gas fields in the Yamal Peninsula.

The crater was discovered in June 2017 by people from the Erkuta polar station on the floodplain of the Erkuta River, 30 km east of the station. The area is of interest to biologists for its proximity to the nestingFigureFigure- 1.place Location 1. ofLocation falcons. map of map Two the ofErkuta years the Erkuta craterbefore craterand, the oil and terrain and oil gas and was fields gas absolutely ﬁeldsin the inYamal the flat, Yamal Peninsula as witnessed Peninsula.. by Dr. A. Sokolov in a TV broadcast (“Vesti Yamal”) of 30 June 2017. The crater was discovered in June 2017 by people from the Erkuta polar station on the floodplain of the Erkuta River, 30 km east of the station. The area is of interest to biologists for its proximity to the nesting-place of falcons. Two years before, the terrain was absolutely flat, as witnessed by Dr. A. Sokolov in a TV broadcast (“Vesti Yamal”) of 30 June 2017.

Erkuta crater

Figure 2. HelicopterHelicopter view view of of the the Erkuta Erkuta crater crater and and adjacent adjacent territory territory in insummer summer 2017. 2017. Photo Photo by bythe theYamal- NenetsYamal-Nenets Autonomous Autonomous District District (YNAO) (YNAO) government. government.

The crater was discovered in June 2017 by people from the Erkuta polar station on the ﬂoodplain of the Erkuta River, 30 km east of the station. The area is of interest to biologists for its proximity to Figure 2. Helicopter view of the Erkuta crater and adjacent territory in summer 2017. Photo by the Yamal- Nenets Autonomous District (YNAO) government.

Geosciences 2020, 10, 170 4 of 16 the nesting-place of falcons. Two years before, the terrain was absolutely flat, as witnessed by Dr. A. Sokolov in a TV broadcast (“Vesti Yamal”) of 30 June 2017. The team of biologists led by Sokolov observed heaving of the previously flat surface, as well as cracks in soil, during field works in July 2016, a year before the crater was first discovered in June 2017. The newly formed crater had a cylindrical shape, 10 to 12 m in diameter, with smooth walls was Geosciences 2020,, x FOR PEER REVIEW 4 of 17 20 m deep (A. Sinitsyn, Personal Communication). The original heave mound was not fully eliminated (Figure2): the crater was encircled by a 2–3 m high parapet-like ridge, with its slopes covered with silt GeosciencesThe team 2020 of,, biologists x FOR PEER REVIEWled by Sokolov observed heaving of the previously flat surface, 4as of 17well as andcracks clay in siltsoil, ejected during during field works the explosive in July gas2016, emission. a year before the crater was first discovered in June 2017.In The DecemberT henewly team formed of 2017, biologists acrater field led had by trip Sokolov a to cylindrical the observed Erkuta shape, heaving crater 10 (Figureofto the12 previouslym3) in was diameter, organized flat surface, with jointly assmooth well byas walls the governmentwas cracks 20 m deepin ofsoil, the (A. during Yamal–Nenets Sinitsyn, field works Personal Autonomous in July Communication). 2016, a District,year before the The the Total original crater SA was and heave first the discovered mound Skolkovo was in Institute Junenot fully of 2017. The newly formed crater had a cylindrical shape, 10 to 12 m in diameter, with smooth walls Scienceeliminated and (Figure Technology 2): the (Skoltech). crater was The encircled field work by includeda 2–3 m high sampling parapet of soil,-like ice ridge and, waterwith its from slopes the was 20 m deep (A. Sinitsyn, Personal Communication). The original heave mound was not fully cratercovered rim. with By silt that and time, clay thesilt southernejected during wall ofthe the explosive crater had gas collapsedemission. and the diameter increased eliminated (Figure 2): the crater was encircled by a 2–3 m high parapet-like ridge, with its slopes to 17.5 m. The crater was partly filled with water which made an up to 8-m-deep lake covered by coveredIn December with silt 2017, and clay a field silt e jected trip to during the Erkutathe explosive crater gas (Figure emission. 3) was organized jointly by the ~1-m-thick ice under ~0.8 m of snow. A part of the parapet-like ridge remained around the crater next governmentIn December of the Yamal 2017,– Nenets a field tripAutonomous to the Erkuta District, crater the (Figure Total 3) SA was and organized the Skolkovo jointly byInstitute the of toScience a lakegovernment and (Figure Technology 4ofa). the Yamal (Skoltech).–Nenets TheAutonomous field work District, included the Total sampling SA and of thesoil, Skolkovo ice and Institutewater from of the craterScienceIn rim June. By and 2018, th Technologyat time, the crater the (Skoltech). southern was imaged The wall field of by workthe a dronecrater included had survey sampling collapsed (Figure of andsoil,4b), icethe which and diameter wat showeder from increased t furtherhe to degradation17.5 m.crater The rim crater of. By its th was wall.at time, partly The the deepfilled southern craterwith wall water of of a yearthe which crater before made had became collapsedan up to almost and8-m the-deep fully diameter lake filled covered increased with water by to ~1 and-m- transformedthick17.5 ice m.under The into crater~0.8 a lakem was of semi-circledsnow.partly filled A part with by of awaterth remnante parapet which 2–3 made-like m ridgean parapet up remainedto 8- (Figurem-deep around4 lakeb). covered The the crater crater by ~1 wall next-m- was to a deformedlake (Figurethick ice by 4a)under thermal. ~0.8 erosion m of snow. and A slumping part of the in parapet its outer-like part ridge but remained remained around vertical the crater in the next inner to a part. lake (Figure 4a).

FigureFigureFigure 3. 3. PanoramicPanoramic 3. Panoramic view view view of of ofthe the the Erkuta ErkutaErkuta crater crater in in December December 2017. 2017. 2017. Photo Photo Photographgraphgraph by J.V.by by J.V.Stanilovskaya. J.V. Stanilovskaya. Stanilovskaya.

(a) (b) Figure 4. A lake (a) (nexta) to the remnant parapet-like ridge around the Erkuta(b) crater (December 2017, photograph by E. Chuvilin) and a panoramic view (b) of the Erkuta crater (summer 2018, photograph by FigureA. 4.Sokolov). 4. AA lakelake ( a (a)) next next to to the the remnant remnant parapet-like parapet-like ridge ridge around around the the Erkuta Erkuta crater crater (December (December 2017, 2017, photographphotograph byby E.E. Chuvilin)Chuvilin) andand aa panoramicpanoramicview view ( b(b)) of of the the Erkuta Erkuta crater crater (summer (summer 2018, 2018, photograph photograph by byA. Sokolov). A.In Sokolov). June 2018, the crater was imaged by a drone survey (Figure 4b), which showed further degradation of its wall. The deep crater of a year before became almost fully filled with water and 3. FieldtransformedIn June Data 2018, into the a lake crater semi was-circled imaged by a remnant by a drone 2–3 msurvey parapet (Figure (Figure 4b)4b)., The which crater showed wall was further deformed by thermal erosion and slumping in its outer part but remained vertical in the inner part. degradatFieldion studies of its inwall December. The deep 2017 crater revealed of a ayear layer before of massive became ground almost ice, fully 3–4 mfill ofedvisible with water thickness, and transformed into a lake semi-circled by a remnant 2–3 m parapet (Figure 4b). The crater wall was in the3. inner Field wallData of the crater (Figure5a,b). deformed by thermal erosion and slumping in its outer part but remained vertical in the inner part. Field studies in December 2017 revealed a layer of massive ground ice, 3–4 m of visible thickness, 3. Fieldin the Data inner wall of the crater (Figure 5a,b). Field studies in December 2017 revealed a layer of massive ground ice, 3–4 m of visible thickness, in the inner wall of the crater (Figure 5a,b).

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(a) (b) FigureFigure 5. Sampling 5. Sampling points points( (reda) (red circles) circle ins) thein the inner inner wall wall of of the the Erkuta Erkuta crater, crater December, (Decemberb) 2017. 2017. (a) fragmentfragment of the sampling site, (b) numbers of sampling points correspond to sample numbers in Table 1. Red dashed of theFigure sampling 5. Sampling site, (b points) numbers (red circle of samplings) in the inner points wall correspond of the Erkuta to crater sample, December numbers 2017 in. Table(a) fragment 1. Red line in panel b delineates the ice top. Photograph by E. Chuvilin, December 2017. dashedof th linee sampling in panel siteb delineates, (b) numbers the of icesampling top. Photograph points correspond by E. to Chuvilin, sample numbers December in Table 2017. 1. Red dashed lineIce atin panelthe sampling b delineates site the was ice generallytop. Photo graphtransparent by E. Chuvilin, and pure December, but locally 2017. enclosed 1–2-cm-thick Ice at the sampling site was generally transparent and pure, but locally enclosed 1–2-cm-thick layers of fine-grained soil (Figure 6a,b) and a minor amount of intricately shaped 1–3 mm gas bubbles. layers of ﬁne-grainedIce at the sampling soil (Figure site was6a,b) generally and a minortransparent amount and ofpure intricately, but locally shaped enclosed 1–3 mm1–2-cm gas-thick bubbles. The clear massive ice was topped by a dirty gray ice layer of a few centimeters thick with abundant layers of fine-grained soil (Figure 6a,b) and a minor amount of intricately shaped 1–3 mm gas bubbles. The clearsoil inclu massivesions. ice was topped by a dirty gray ice layer of a few centimeters thick with abundant soil inclusions.The clear massive ice was topped by a dirty gray ice layer of a few centimeters thick with abundant soil inclusions.

(a) (b) Figure 6. Massive ground( icea) in the crater wall (December 2017). Photograph(b) by E. Chuvilin. (a) the surface of ice, (b) fresh crush cut of ground ice with soil inclusions. FigureFigure 6. Massive 6. Massive ground ground ice ice in in the the cratercrater wall wall (December (December 2017). 2017). Photo Photographgraph by E. byChuvilin. E. Chuvilin. (a) the surface (a) the surfaceofThe ice of ,groundice, (b) fresh (b) fresh icecrush was crush cut overlaof cut ground ofin ground byice with frozen ice soil with siltinclusions. soilwith inclusions. organic inclusions (plant remnants). The sediments had cross and wavy stratification common to alluvial facies (Figure 7a,b). Some organic The ground ice was overlain by frozen silt with organic inclusions (plant remnants). The Thelayers ground and lenses ice was were overlain a few centimeters by frozen siltthick with and organic a few cm inclusions to tens of (plantcentimeters remnants). long. The sediments had crosssediments and wavyhad cross stratification and wavy stratification common to common alluvial to facies alluvial (Figure facies7 a,b).(Figure Some 7a,b). organic Some organic layers and layers and lenses were a few centimeters thick and a few cm to tens of centimeters long. lenses were a few centimeters thick and a few cm to tens of centimeters long. The sandy crater wall was locally cut by discontinuous branching fractures partly filled with ice, often within zones of iron impregnation (Figure7a). The fractures possibly formed under stress produced by fluid (water or gas) pressure from underlying sediments. Geosciences 2020,, x FOR PEER REVIEW 6 of 17

Geosciences 2020The, sandy10, 170 crater wall was locally cut by discontinuous branching fractures partly filled with ice,6 of 16 often within zones of iron impregnation (Figure 7a). The fractures possibly formed under stress produced by fluid (water or gas) pressure from underlying sediments. The ejectedThe ejected clay clay silt silt on on the the outer outer crater crater wallwall was originally originally wet wet and and was was deposited deposited in a floodplain in a ﬂoodplain environment;environment; it di itﬀ differedered markedly markedly from from the the overlying overlying organic organic-rich-rich alluvial alluvial sand. sand.

(a) (b)

FigureFigure 7. Interbedded 7. Interbedd sandyed sand andy and silty silty soils soils with with inclusions inclusions of of organic organic matter matter inin thethe cratercrater wall (December 2017).2017). Photograph Photograph by by E. E. Chuvilin. Chuvilin. ( (aa)) discontinuous discontinuous branching branching fractures, fractures, (b) wavy (b) stratification wavy stratiﬁcation of organic of organicmaterial material..

ShrubsShrub ons theon the remnant remnant slope slope facing facingthe the lakelake do not not differ differ much much from from the the surrounding surrounding lowland lowland vegetation. Therefore, heaving shortly preceded the gas emission event and did not cause substantial vegetation. Therefore, heaving shortly preceded the gas emission event and did not cause substantial changes to the vegetation. Otherwise, at years-long heaving, vegetation would have adapted to new changes to the vegetation. Otherwise, at years-long heaving, vegetation would have adapted to new conditions. Shrubs in the Arctic tundra are usually lower on topographic highs than on the plainland conditions.and lack Shrubs from many in the areas Arctic where tundra only aregrass usually can grow lower. on topographic highs than on the plainland and lack from many areas where only grass can grow. 4. Laboratory Data 4. Laboratory Data 4.1. Samples 4.1. Samples The samples collected during the December 2017 field trip to the crater included frozen soil from Thethe northern samples wall, collected fine-grained during material the December ejected from 2017 the field crater trip and to theice (Table crater 1). included frozen soil from the northern wall, fine-grained material ejected from the crater and ice (Table1). Table 1. List of soil and ice samples (see Figure 5).

Sample No. BriefTable Description 1. List of soil and ice samples (seeSampling Figure5). Point Sample 2 Loam ejected from the crater On the surface near the crater Sample No. Brief Description Sampling Point Ice with soil inclusions (less Sample 6 0.5 m above the lake ice surface Sample 2 Loamthan ejected1%) from from ice the lens crater On the surface near the crater Ice with soil inclusions (less than Sample 6 Ice with soil inclusions (up 0.5 m above the lake ice surface Sample 7 1%) from ice lens 0.5 m above the lake ice surface to 2%–3%) from ice lens Ice with soil inclusions (up to Sample 7 Ice with soil inclusions (up 0.5 m above the lake ice surface Sample 8 2%–3%) from ice lens 0.5 m above the lake ice surface Iceto 2 with%–3%) soil from inclusions ice lens (up to Sample 8 0.5 m above the lake ice surface 2%–3%) from ice lens From the crater’s inner sidewall. Sampling point at 1.2 m Sample 9 Sand From the crater’s inner sidewall. Sampling point at 1.2 m Sample 9 Sand above the lake ice surface Sample 11 Pure ice A lens of massive iceabove in the the crater's lake ice sidewall; surface The middle Sample 11 Pure ice A lens of massive ice in the crater’s sidewall; The middle partpart of the of thelens lens is free is free from from visible visible inclusions inclusions of of soils. soils. Sample 13 Pure ice SamplingSampling point point at at 0.5 0.5 m m above above thethe lakelake ice ice surface surface and and 2 m2 to Sample 13 Pure ice m to thethe right of sample No.6No.6 Ice from a lens of massive underground ice in the crater’s sidewall. The middle part of the ice intrusion with visible Sample 21 Pure ice (1%–2%) inclusions of soils. Sampling point at 1 m below the top of the ice lens At a contact between a lens of underground ice and Sample 22 Ice with soil additives overlaying frozen soil At a contact between the massive underground ice and the Sample 23 Ice with soil additives lake ice in the crater Geosciences 2020, 10, 170 7 of 16

The particle size distribution of soil samples and soil inclusions in ground ice was analyzed in the laboratory of Fundamentproekt (Table2). The particles sizes were determined by the pipette method in silt and clay and by sieve analysis in sand.

Table 2. Particle size distribution and air-dried density of soil samples.

Particle Size Distribution, % Air-Dry Density of Sample No. Type of Soil 1–0.05 mm 0.05–0.002 mm <0.002 mm Soil, g/cm3 Sample 2 16.2 67.8 16.0 2.66 Light clay silt Sample 7 40.5 35.7 23.8 2.7 Light silt Sample 8 87.0 9.3 3.7 2.66 Silty sand Sample 9 94.5 3.9 1.6 2.64 Silty sand

The particle sizes of soil over the ground ice mainly correspond to silty sand while the ejected material is mainly light clay silt and silt, with particle sizes similar to those of soil inclusions in visible ground ice. The mineralogy of soil samples was analyzed at the Geological Faculty of the Lomonosov Moscow State University on a Rigaku ULTIMA-IV X-ray diﬀractometer (Japan) [14]. The samples consist mainly of quartz and feldspar minerals (microcline and albite), about 80%–90% in total. Quartz percentages reach 61.8% and 74.3% in silty sand samples 8 and 9, respectively, notably more than in clay silt sample 2 (~45.2%). The feldspar minerals are 32.3% in sample 2, 23.9% in sample 8 and about 15.4% in sample 9. Clay minerals are mainly mixed-layer illite–smectite, from 5.8% in sample 9% to 10.2% in sample 2. Other minerals occur in minor amounts (1% or less). Samples 2 and 8 share similarity in mineralogy, with similar amounts of microcline, illite, smectite and kaolinite, and thus were originally involved in similar deposition processes. The soil and ice samples were analyzed for the contents of soluble salts by using soil–water extracts prepared from 100 g of dry substance. Soil-free massive ground ice was analyzed in the molten state. The total percentage of salts did not exceed 0.1% in soil samples and was in a range of 40–173 mg/L in ice samples. The predominant major ions were alkaline metals (Na+K) and Mg2+ as 2+ cationsGeosciences and 2020 SO4,, x FORand PEER Cl− REVIEWas anions (Figure8). 8 of 17

FigureFigure 8. 8.Concentrations Concentrations of water water-soluble-soluble salts salts in in samples samples (mg/ (mgL):/ L):Lake Lake = water= water from fromcratercrater lake; Ice lake; Ice== samplesample 22; 22; Lean Lean clay clay (surface) (surface) = sample= sample 2; Sand 2; Sand = sample= sample 9. 9.

The contents of unfrozen pore water in samples of erupted material (sample 2) and sand from the crater wall (sample 9) were determined from pore water activity [15]. The amount of liquid water in the sand samples decreased abruptly at temperatures below −1℃ and was less than 1% at −4℃. The clay silt sample (2) contained more unfrozen water than the sand sample (9): ~4% at −4℃ and around 3% at −10℃. We compared major-ion chemistry (Table 3) and stable isotopes of water in ice samples from the crater wall and in crater lake water. The ice samples (11 and 13) were recovered from the middle part of the ice lens free from visible soil inclusions. Prior to analyses, ice samples were melted, and the obtained water was collected without filtering.

Table 3. Major-ion chemistry of water samples (analyzed at the Laboratory of Hydroisotopes).

Sample Na+, mg/L K+, mg/L Ca2+, mg/L Mg2+, mg/L Cl-, mg/L SO42-, mg/L Water from crater lake 9.9 2.4 27.0 11.0 2.8 21.0 Ground ice 2.4 1.0 4.2 1.2 1.4 0.67

The salinity of lake water is higher than that of ground ice: contents of some major ions are 7 to 11 times higher (Table 3), especially SO42−, Mg2+ and Ca2+. The oxygen and hydrogen isotope compositions of water from the lake and ice are also different: those of ground ice are more depleted than in surface water from the crater (Figure 9). These compositions indicate that the mean annual temperature was 7 to 10 °C lower than now when the ice was forming.

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The contents of unfrozen pore water in samples of erupted material (sample 2) and sand from the crater wall (sample 9) were determined from pore water activity [15]. The amount of liquid water in the sand samples decreased abruptly at temperatures below 1 °C and was less than 1% at 4 °C. − − The clay silt sample (2) contained more unfrozen water than the sand sample (9): ~4% at 4 °C and − around 3% at 10 °C. − We compared major-ion chemistry (Table3) and stable isotopes of water in ice samples from the crater wall and in crater lake water. The ice samples (11 and 13) were recovered from the middle part of the ice lens free from visible soil inclusions. Prior to analyses, ice samples were melted, and the obtained water was collected without ﬁltering.

Table 3. Major-ion chemistry of water samples (analyzed at the Laboratory of Hydroisotopes).

+ + 2+ 2+ 2 Sample Na , mg/LK , mg/L Ca , mg/L Mg , mg/L Cl−, mg/L SO4 −, mg/L Water from crater lake 9.9 2.4 27.0 11.0 2.8 21.0 Ground ice 2.4 1.0 4.2 1.2 1.4 0.67

The salinity of lake water is higher than that of ground ice: contents of some major ions are 7 to 2 2+ 2+ 11 times higher (Table3), especially SO 4 −, Mg and Ca . The oxygen and hydrogen isotope compositions of water from the lake and ice are also diﬀerent: those of ground ice are more depleted than in surface water from the crater (Figure9). These compositions indicate that the mean annual temperature was 7 to 10 ◦C lower than now when theGeosciences ice was 2020 forming.,, x FOR PEER REVIEW 9 of 17

FigureFigure 9.9. OxygenOxygen andand hydrogenhydrogen isotopicisotopic compositionscompositions of waterwater sampledsampled fromfrom thethe cratercrater lakelake andand groundground ice.ice.

4.2. Structure and Texture of Soil and Ice Samples 4.2. Structure and Texture of Soil and Ice Samples TheThe structurestructure andand texturetexture ofof soilsoil andand iceice samplessamples werewere analyzedanalyzed atat Skoltech.Skoltech. TheThe microstructuremicrostructure ofof frozenfrozen soilsoil waswas studiedstudied inin replicareplica samplessamples (imprints(imprints ofof freshfresh fracturefracture planesplanes onon aa plexiglassplexiglass ﬁlm)film) underunder an an optical optical microscope microscope (at Skoltech) (at Skoltech) and a scanning and a scanning electron microscopeelectron microscope (at Moscow (at University). Moscow TheUniversity techniques). The for technique preparings for replica preparing samples replica and their samples optical and and their electron optical microscopy and electron were microscop reportedy inwere a number reported of publicationsin a number [of16 ,publi17]. Icecations structure [16,17]. and Ice texture structur weree and studied textur ine thin were sections studied between in thin crossedsections Polaroids; between the crossed thin sections Polaroids were; the prepared thin sections following were the standard prepared procedures following [ 17 the]. standard procedures [17]. Soil microstructure was studied in sand from the crater wall (sample 9) and in ejected clay silt (sample 2). According to reflected light optical microscopy, the sand sample (9) mostly consists of 0.1–0.25 mm subrounded isometric quartz particles, with lesser amounts of fine-grained material, lenses and layers of more or less strongly degraded organic remnants, distinct 1–2 mm brown organic inclusions, as well as black organic–mineral concretions of silty sand and decayed organics. SEM images (Figure 10) highlight the morphology of quartz grains, with signatures of brittle fracture and dissolution (Figure 10a) and with organic inclusions of different sizes, shapes and decay degrees (Figure 10b). Finer silt or clay particles make continuous or discontinuous films and clusters on the surface of sand particles.

(a) (b)

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Figure 9. Oxygen and hydrogen isotopic compositions of water sampled from the crater lake and ground ice.

4.2. Structure and Texture of Soil and Ice Samples The structure and texture of soil and ice samples were analyzed at Skoltech. The microstructure of frozen soil was studied in replica samples (imprints of fresh fracture planes on a plexiglass film) under an optical microscope (at Skoltech) and a scanning electron microscope (at Moscow University). The techniques for preparing replica samples and their optical and electron microscopy Geosciences 2020, 10, 170 9 of 16 were reported in a number of publications [16,17]. Ice structure and texture were studied in thin sections between crossed Polaroids; the thin sections were prepared following the standard proceduresSoil microstructure [17]. was studied in sand from the crater wall (sample 9) and in ejected clay silt (sampleSoil 2). Accordingmicrostructure to reflectedwas studied light in opticalsand from microscopy, the crater wall the sand(sample sample 9) and (9) in mostlyejected clay consists silt of (sample 2). According to reflected light optical microscopy, the sand sample (9) mostly consists of 0.1–0.25 mm subrounded isometric quartz particles, with lesser amounts of fine-grained material, 0.1–0.25 mm subrounded isometric quartz particles, with lesser amounts of fine-grained material, lenses and layers of more or less strongly degraded organic remnants, distinct 1–2 mm brown organic lenses and layers of more or less strongly degraded organic remnants, distinct 1–2 mm brown organic inclusions,inclusions as, wellas well as as black black organic–mineral organic–mineral concretions concretions of silty silty sand sand and and decayed decayed organics. organics. SEMSEM images images (Figure (Figure 10 10) highlight) highlight thethe morphologymorphology of of quartz quartz grains, grains, with with signatures signatures of brittle of brittle fracturefracture and and dissolution dissolution (Figure (Figure 10 10a)a) and and withwith organic inclusions inclusions of of different different sizes, sizes, shapes shapes and and decay decay degreesdegrees (Figure (Figure 10b). 10b). Finer Finer silt silt or or clay clay particles particles make continuous continuous or or discontinuous discontinuous films films and and clusters clusters on theon surfacethe surface of sandof sand particles. particles.

(a) (b)

Figure 10. SEM images of sand sample 9 at diﬀerent magniﬁcation factors, with signatures of fracture and dissolution (a) and organic inclusions (b).

The microstructure of clay silt ejected from the crater (sample 2) was also examined under the optical and electron microscopes. Reflected light optical microscopy revealed quite uniform silt and clay particles with rare sand grains and fuzzy dark brown organic inclusions. SEM images of different magnifications (Figure 11) resolve fine sand and coarse silt particles (and their replica imprints)Geosciences cemented 2020,, x FOR by PEER clay REVIEW silt at 500 (Figure 11a) and 5 µm to 20 µm particles at 2000 (Figure10 of 17 11 b). × × The 2000 images reveal orientations of mineral matrix particles delineated by sericite flakes, as well × Figure 10. SEM images of sand sample 9 at different magnification factors, with signatures of fracture as organic inclusions easily spotted due to their particular shapes. and dissolution (a) and organic inclusions (b).

(a) (b)

FigureFigure 11. 11.SEM SEM images images of of clay clay silt silt sample sample 22 atat didifferentﬀerent magnification magniﬁcation factors factors:: general general view view (a) and (a) and detaildetail image image (b). (b).

TheThe ice macrostructuremicrostructure of was clay studied silt ejected in samplesfrom the 6,crater 8 and (sample 11 of ground 2) was also ice andexamined sample under 22 from the the topoptical of the iceand lens, electron at the microscopes. ice–soil contact Reflected (Table light1). optical Samples microscopy 6 and 11 revealed are generally quite uniform similar: silt massive and transparentclay particles ice with with chains rare sand of raregrains 0.3–0.5 and fuzzy cm air dark bubbles brown andorganic thin inclusions. ﬂaky layers SEM of images silt and of clay different particles magnifications (Figure 11) resolve fine sand and coarse silt particles (and their replica imprints) (Figure 12a). Clusters of mineral particles occur also at ice crystal boundaries. Mineral inclusions cemented by clay silt at ×500 (Figure 11a) and 5 μm to 20 μm particles at ×2000 (Figure 11b). The ×2000 images reveal orientations of mineral matrix particles delineated by sericite flakes, as well as organic inclusions easily spotted due to their particular shapes. The ice macrostructure was studied in samples 6, 8 and 11 of ground ice and sample 22 from the top of the ice lens, at the ice–soil contact (Table 1). Samples 6 and 11 are generally similar: massive transparent ice with chains of rare 0.3–0.5 cm air bubbles and thin flaky layers of silt and clay particles (Figure 12а). Clusters of mineral particles occur also at ice crystal boundaries. Mineral inclusions are more numerous than air bubbles. Ice crystals in samples 6 and 11 appear as exceeding 5–7 cm in size, but the actual size is difficult to estimate because only crystal fragments fit into the thin sections (Figure 12b).

(a) (b)

Figure 12. Ice sample 6: general view (a) and structure under polarized light (b).

Unlike samples 6 and 11, ice sample 8 (Figure 13a) encloses a layer of silty sand (Table 2) in pure ice almost free from soil particles and air bubbles. The ice crystals are fine (few mm) along the soil layer and coarser (2–3 cm) away from it (Figure 13b). Although the true size of the ice crystals remains unknown because of the limited thin section size, they may be commensurate with those in samples

Geosciences 2020,, x FOR PEER REVIEW 10 of 17

Figure 10. SEM images of sand sample 9 at different magnification factors, with signatures of fracture and dissolution (a) and organic inclusions (b).

(a) (b)

Figure 11. SEM images of clay silt sample 2 at different magnification factors: general view (a) and detail image (b).

The microstructure of clay silt ejected from the crater (sample 2) was also examined under the optical and electron microscopes. Reflected light optical microscopy revealed quite uniform silt and clay particles with rare sand grains and fuzzy dark brown organic inclusions. SEM images of different magnifications (Figure 11) resolve fine sand and coarse silt particles (and their replica imprints) cemented by clay silt at ×500 (Figure 11a) and 5 μm to 20 μm particles at ×2000 (Figure 11b). The ×2000 images reveal orientations of mineral matrix particles delineated by sericite flakes, as well as organic inclusions easily spotted due to their particular shapes. The ice macrostructure was studied in samples 6, 8 and 11 of ground ice and sample 22 from the top of the ice lens, at the ice–soil contact (Table 1). Samples 6 and 11 are generally similar: massive Geosciences 2020, 10, 170 10 of 16 transparent ice with chains of rare 0.3–0.5 cm air bubbles and thin flaky layers of silt and clay particles (Figure 12а). Clusters of mineral particles occur also at ice crystal boundaries. Mineral inclusions are aremore more numerous numerous than than air bubbles air bubbles.. Ice crystals Ice crystals in samples in samples 6 and 6 11 and appear 11 appear as exceeding as exceeding 5–7 cm 5–7 in cm size in, size,but the but actual the actual size sizeis difficult is diﬃcult to estimate to estimate because because only only crystal crystal fragm fragmentsents fit ﬁt into into the the thin thin sections (Figure 1212b).b).

(a) (b)

Figure 12. Ice sample 6: g generaleneral view view ( (aa)) and and structure structure under under polarized light ( b).

Unlike samples 6 and 11, i icece sample 8 (Figure 13a)13a) encloses a layer of silty sand ( (TableTable2 2)) inin purepure ice almost free from soil particles and air bubbles.bubbles. The The ice ice crystals are are fine ﬁne (few mm) along the soil layer and coarser (2–3(2–3 cm) away from it (Figure 1313b).b). Although the true size of the ice ice crystals crystals remains unknownGeosciences because 2020,, x FOR ofof thethe PEER limitedlimited REVIEW thinthin sectionsection size,size, they they may may be be commensurate commensurate with with those those in in11 samples samples of 17 6 and 11. The soil layer, in its turn, encloses numerous small 3–5-mm-long, and up to 2-mm-thick lenses 6 and 11. The soil layer, in its turn, encloses numerous small 3–5-mm-long, and up to 2-mm-thick of ice with ﬁne crystals (fractions of mm). lenses of ice with fine crystals (fractions of mm).

(a) (b)

FigureFigure 13. 13.Ice Ice sample sample 8: 8: general general view ( (aa)) and and structure structure under under polarized polarized light light (b). (b).

IceI samplece sample 22 22 from from the the top top of of the the iceice lenslens didiffersffers markedly markedly in in color color from from the the other other ice icesamples samples (Figure(Figure 14a). 14a). It It enclosesencloses numerous numerous scattered scattered small small soil particles soil particles and air andbubbles air which bubbles make which it looking make it lookinglike d likeirty dirtyopaque opaque ice. Th ice.e ice The crystals ice crystals are fine are (2– fine4 mm) (2–4 and mm) generally and generally isometric isometric (Figure 14b). (Figure The 14 b). Thesand sand-silt-silt material material occur occurss both both along along the the boundaries boundaries of ofice icecrystals crystals and and inside inside them. them. The Theice crystals ice crystals in this sample are finer than in the three other samples possibly because they nucleated and grew in in this sample are finer than in the three other samples possibly because they nucleated and grew in the presence of mineral components in the medium which provide numerous centers of the presence of mineral components in the medium which provide numerous centers of crystallization crystallization but is unfavorable for the formation of large ice crystals. but is unfavorable for the formation of large ice crystals.

(a) (b)

Figure 14. Ice sample 22: general view (a) and structure under polarized light (b).

4.3. Gas Analyses The gas component was analyzed in sample 9 of sandy permafrost and samples 6, 21, 22 and 23 of ground ice. Intrapermafrost gas from sample 9 was extracted by 150 ml syringes from thawing ~50 g specimens in a concentrated NaCl solution following the standard technique [18], using pure nitrogen or helium as carrier gas. The permafrost soil and ice samples were delivered to Skoltech (Moscow) from the Erkuta crater in thermal containers. The gas composition was analyzed at the Geological Faculty of the Lomonosov Moscow State University (Moscow) and at the Hydroisotope Laboratory (Germany). At the Moscow University, the composition of hydrocarbon gases was studied on a Thermo Finnigan Trace GC Ultra gas chromatograph. The analytical work in Germany included analyses of intrapermafrost gas on a Thermo Scientific DELTA V Plus (IRMS) mass spectrometer (Waltham, MA USA), as well as

Geosciences 2020,, x FOR PEER REVIEW 11 of 17

6 and 11. The soil layer, in its turn, encloses numerous small 3–5-mm-long, and up to 2-mm-thick lenses of ice with fine crystals (fractions of mm).

(a) (b)

Figure 13. Ice sample 8: general view (a) and structure under polarized light (b).

Ice sample 22 from the top of the ice lens differs markedly in color from the other ice samples (Figure 14a). It encloses numerous scattered small soil particles and air bubbles which make it looking like dirty opaque ice. The ice crystals are fine (2–4 mm) and generally isometric (Figure 14b). The sand-silt material occurs both along the boundaries of ice crystals and inside them. The ice crystals in this sample are finer than in the three other samples possibly because they nucleated and grew in Geosciencesthe2020 presence, 10, 170 of mineral components in the medium which provide numerous centers of 11 of 16 crystallization but is unfavorable for the formation of large ice crystals.

(a) (b)

FigureFigure 14. Ice 14. sample Ice sample 22: 22: general general view view ( a)) and and structure structure under under polarized polarized light ( lightb). (b).

4.3. Gas4.3. Analyses Gas Analyses The gasThe component gas component was was analyzed analyzed in samplein sample 9 9 of ofsandy sandy permafrost and and samples samples 6, 21, 6, 22 21, and 22 23 and 23 of of ground ice. Intrapermafrost gas from sample 9 was extracted by 150 ml syringes from thawing ~50 ground ice. Intrapermafrost gas from sample 9 was extracted by 150 ml syringes from thawing ~50 g g specimens in a concentrated NaCl solution following the standard technique [18], using pure specimensnitrogen in a concentratedor helium as carrier NaCl gas. solution following the standard technique [18], using pure nitrogen or helium asThe carrier permafrost gas. soil and ice samples were delivered to Skoltech (Moscow) from the Erkuta crater Thein permafrostthermal containers. soil and The ice gas samples composition were was delivered analyzed at to the Skoltech Geological (Moscow) Faculty of from the Lomonosov the Erkuta crater in thermalMoscow containers. State University The gas (Moscow) composition and at was the Hydroisotope analyzed at L theaboratory Geological (Germany). Faculty At the of theMoscow Lomonosov University, the composition of hydrocarbon gases was studied on a Thermo Finnigan Trace GC Ultra Moscow State University (Moscow) and at the Hydroisotope Laboratory (Germany). At the Moscow gas chromatograph. The analytical work in Germany included analyses of intrapermafrost gas on a University,Thermo the composition Scientific DELTA of hydrocarbon V Plus (IRMS) gases mass was spectrometer studied on (Waltham, a Thermo MAFinnigan USA), Trace as well GC as Ultra gas chromatograph. The analytical work in Germany included analyses of intrapermafrost gas on a Thermo

Scientiﬁc DELTA V Plus (IRMS) mass spectrometer (Waltham, MA USA), as well as determination of major-ion chemistry and stable isotopes in ground ice and crater lake water. The analytical results are summarized in Tables4 and5.

Table 4. Contents of methane and its homologs in ground ice and permafrost samples.

3 3 3 Sample CН4, cm /kg C2Н6, cm /kg C3Н8, cm /kg Data of Moscow University Sample 6 1.67 - - 4.74 0.03 - Sample 22 1.64 0.03 0.01 1.6 - - Sample 23 0.8 0.08 0.08 Data of Hydroisotope Lab Sample 21 9.3 3.2 2.2 Sample 22 94 1.7 0.8

Table 5. Contents of carbon dioxide and methane and its homologs in ground ice and permafrost samples from the crater wall (by Hydroisotope Lab).

Sample CO2,% CH4, ppm C2H6, ppm C3H8, ppm Sample 9 6.0 8 - - Sample 21 - 20 7 5 Sample 22 1.76 434 8 4

Methane was present in all samples (Tables4 and5): mostly a few cm 3 per 1 kg of soil or ice. Its content reached 94 cm3/kg at the top of ground ice (sample 22) but was about 10 times less in sample 21 from the middle of the ground ice lens (Table4). All samples except for 21 contained much more C О2 Geosciences 2020, 10, 170 12 of 16 than methane (Table5), which may be evidence of cryogenic concentration in frozen sand, e.g., during freezing of a talik [10]. The contents of ethane and propane (methane homologs) varied from fractions to 2–3 cm3/kg (Table4). The ratios of methane to its homologs in ice samples were generally from 2 to 40 and indicated the presence of both biogenic methane and a component associated with sediment maturation (deep gas). The carbon isotope composition of methane in ground ice (δ 13C = 72%) − analyzed at the Hydroisotope Laboratory (Germany) likewise suggests biogenic origin of the gas.

5. Formation Model of the Erkuta Crater The obtained results have implications for the formation mechanism of the Erkuta crater, which formed on the site of the paleo-channel of the Erkuta–Yakha River. The contours of the dried riverbed can be seen in a photograph of 2017 (Figure2), as well as in a satellite image of 2013 (Figure 15). As a result of evolution, the paleo-channel gradually turned into an oxbow lake, which continued to degrade and split up into several small drying lakes. Then a heaving mound began to form within one of these dried-up lakes. Contour of a mound is a bright spot (probably slightly elevated and drained soils) against the background of a dark thawed water-saturated soils, which can be distinguished in a satellite image of the beginning of summer 2013 (Figure 15). Geosciences 2020,, x FOR PEER REVIEW 13 of 17

Figure 15. SatelliteSatellite image of the Erkuta crater area at the beginning of summer 2013.2013.

The thermal effect effect of such lakes often produces a zone of unfrozen rocks (a talik) underneath. underneath. The lake sediments withinwithin the taliks contain organic matter re recycledcycled by microorganisms with release of biogenic methane methane.. Additionally, Additionally, gases can penetrate into t thehe lake sediments from deep subsurface through permeable deformed zones.zones. The talik beneath the Erkuta crater was most likely closed, given that the permafrost thickness in the area is about 200 m deep andand thethe lakelake waswas small.small. The lake was gradually shoalingshoaling and and shrinking shrinking whereby whereby the the talik talik was was freezing freezing from from below below and fromand thefrom sides, the whichsides, causedwhich causedstress buildup stress inside buildup the inside remaining the confinedremaining talik confined [19,20]. talik The stress[19,20] released. The stress explosively released by eruptionexplosively of theby gas–water–soil eruption of the mixture gas–water from– thesoilfreezing mixture talik from and the the freezing ensuing talik formation and the of the ensuing crater information its place. of the crater in its place. Based on our results and available information, we propose the following conceptual model for formation of the Erkuta crater, (Figure(Figure 1616):): Stage I: A lake is underlain by a talik, with periodic inputs of organic matter into the lake in summer seasons. seasons. The The organic organic matter matter in in the the lake lake sediments sediments is isrecycled recycled microbial microbiallyly with with generation generation of ofbiogenic biogenic methane methane which which is isaccumulated accumulated in in winter winter and and emitted emitted into into the the air air in in sp spring.ring. The gaseousgaseous component of of the the sediments sediments in the in talik the increases talik increase additionallys additionally due to migration due to of migration deeper thermogenic of deeper gasesthermogenic along faults gases and along fractures faults inand the fractures crust. Emission in the crust. of deep-seated Emission of gases deep from-seated Arctic gases lakes from isknown Arctic fromlakes severalis known other from areas several [1,2 ,other11]. areas [1,2,11]. Stage II:II: OnsetOnset of talik freezing takes placeplace afterafter thethe lakelake hadhad drieddried out.out. The talik undergoesundergoes confinedconfined freezing and becomesbecomes saturatedsaturated withwith biogenicbiogenic andand deep-seateddeep-seated thermogenicthermogenic gas.gas. Stage III: Confined freezing of the talik and buildup of cryogenic pressure takes place. Freezing of gas-saturated pore moisture under gas pressure at this stage has been studied previously by thermodynamic modeling in laboratory experiments [21,22]. As the lake is drying, the sub-lake unfrozen sediments are freezing from the top and from the sides, which leads to cryogenic gas concentration and stress buildup in the freezing closed talik. Gas- bearing sediments in the latter are confined by the surrounding ice-rich sediments, which increases gas pore pressure in the talik. The pressure may lead to ductile deformation of the permafrost cap above the talik if it exceeds the overburden pressure. At this stage, gas, water and soil in the residual talik can start to stratify. Stage IV: Stratification of gas, water and soil in the residual talik and heaving. This process occurs in fairly homogeneous alluvial deposits. represented by sandy and silty sediments. As a result of stratification, heavier and denser soil stays on the bottom, while the light volatile gas component rises to the top; liquid water is in the middle. The layers of predominant soil, water and gas components are separated by dash lines in the figure, which are drawn tentatively because each layer contains some amounts of other components. If the pressure buildup is slow, the frozen cap can deform ductily, producing long-lasting heaves on the surface, pingo-like structures. However, if pressure increases rapidly, no slow heaving occurs long before the stress release.

Geosciences 2020, 10, 170 13 of 16

Stage III: Confined freezing of the talik and buildup of cryogenic pressure takes place. Freezing of gas-saturated pore moisture under gas pressure at this stage has been studied previously by thermodynamic modeling in laboratory experiments [21,22]. As the lake is drying, the sub-lake unfrozen sediments are freezing from the top and from the sides, which leads to cryogenic gas concentration and stress buildup in the freezing closed talik. Gas-bearing sediments in the latter are confined by the surrounding ice-rich sediments, which increases gas pore pressure in the talik. The pressure may lead to ductile deformation of the permafrost cap above the talik if it exceeds the overburden pressure. At this stage, gas, water and soil in the residual talik can start to stratify. Stage IV: Stratification of gas, water and soil in the residual talik and heaving. This process occurs in fairly homogeneous alluvial deposits. represented by sandy and silty sediments. As a result of stratification, heavier and denser soil stays on the bottom, while the light volatile gas component rises to the top; liquid water is in the middle. The layers of predominant soil, water and gas components are separated by dash lines in the figure, which are drawn tentatively because each layer contains some amounts of other components. If the pressure buildup is slow, the frozen cap can deform ductily, producing long-lasting heaves on the surface, pingo-like structures. However, if pressure increases rapidly,Geosciences no slow 2020 heaving,, x FOR PEER occurs REVIEW long before the stress release. 14 of 17

FigureFigure 16. Formation 16. Formation of theof the Erkuta Erkuta crater crater in in severalseveral stages: I:I a: alake lake and and a talik a talik underneath; underneath; II: onset II: onset of talik of talikfreezing after after the the lake lake has has dried dried out; out; III: confinedIII: confined freezing freezing of the talik of the and talik buildup and buildupof cryogenic of cryogenic pressure. pressure.Arrows Arrows show showthe expulsion the expulsion direction direction of gas (blue of gas dots); (blue IV dots);: stratificationIV: stratification of gas, water of gas,and watersoil in andthe soil infreezing the freezing talik and talik frost and frostheaving; heaving; V: collapseV: collapse of the of frozen the frozen cap above cap above the talik the talik by pressurized by pressurized gas gas (cryovolcanism);(cryovolcanism); VIVI: :active active eruption eruption of ofthe the gas gas–water–soil–water–soil mass mass and onset and onsetcrater craterformation; formation; VII: endVII of : eruption and crater formation; VIII: lake formation as crater becomes filled with water. end of eruption and crater formation; VIII: lake formation as crater becomes filled with water. Stage V: Pressure increase in the talik saturated with water and gas and explosive pressure release breaking through the frozen cap. This phenomenon is known as cryovolcanism [10,23]: eruption of water, fluids and liquefied soil triggered by overpressure in a freezing confined or open water-bearing system. The collapse of the frozen cap may be accompanied by outpouring of water or mud. No explosion occurs if the amount of gas in the talik is small, but the gas–water–soil mass from the talik erupts explosively and becomes dispersed, together with the frozen cap debris, in the presence of a gas cap. Stages VI and VII: Progress of cryovolcanism. At stage VI, the gas–water–soil mixture erupts vigorously, and a crater starts to form. The explosive gas emission breaks up the meters thick frozen cap and disperses its material around the cryovolcano vent. A part of the ejected material falls near the crater and produces a parapet-like ridge rising above the surface around the crater, while some other part obviously falls back into the crater and gradually sinks to the bottom. As the eruption continues, the ejected unfrozen soil falls over the debris of the frozen cap. The soil, water and gas components of the talik, which were previously stratified during the confined freezing, mix again when erupting (Figure 16; Stage VI). The level of the gas–water–soil mixture in the crater gradually decreases and the crater walls emerge. At stage VII, the eruption stops and leaves a crater, with its

Geosciences 2020, 10, 170 14 of 16

Stage V: Pressure increase in the talik saturated with water and gas and explosive pressure release breaking through the frozen cap. This phenomenon is known as cryovolcanism [10,23]: eruption of water, fluids and liquefied soil triggered by overpressure in a freezing confined or open water-bearing system. The collapse of the frozen cap may be accompanied by outpouring of water or mud. No explosion occurs if the amount of gas in the talik is small, but the gas–water–soil mass from the talik erupts explosively and becomes dispersed, together with the frozen cap debris, in the presence of a gas cap. Stages VI and VII: Progress of cryovolcanism. At stage VI, the gas–water–soil mixture erupts vigorously, and a crater starts to form. The explosive gas emission breaks up the meters thick frozen cap and disperses its material around the cryovolcano vent. A part of the ejected material falls near the crater and produces a parapet-like ridge rising above the surface around the crater, while some other part obviously falls back into the crater and gradually sinks to the bottom. As the eruption continues, the ejected unfrozen soil falls over the debris of the frozen cap. The soil, water and gas components of the talik, which were previously stratified during the confined freezing, mix again when erupting (Figure 16; Stage VI). The level of the gas–water–soil mixture in the crater gradually decreases and the crater walls emerge. At stage VII, the eruption stops and leaves a crater, with its diameter commensurate with that of the residual closed talik prior to the emission. The ice-rich debris of the cap and the ejected talik sediments are scattered around the crater and cover its bottom. Note that stages V, VI and VII follow one after another in a few hours to days. Stage VIII: Stable evolution of the crater and its gradual transformation into a different landform. As the ejected material becomes involved into seasonal freezing–thawing cycles, the crater becomes filled with water in a few months and transforms into a circular lake surrounded by ejected material. The suggested model explains the crater formation as a result of gas generation and accumulation in a sub-lake talik and the evolution of the talik exposed to confined freezing as the lake is drying out. Gas accumulation in the talik is additionally maintained by ascend of deep fluids migrating upwards through permeable faulted and fractured bedrock.

6. Discussion The data on the structure and composition of soil samples from the crater wall, as well as the proposed conceptual model for formation of the Erkuta crater, characterize it as a feature of explosive gas emission. Apparently, the crater formed in the place of a freezing closed talik under a dried lake by explosion of pressurized gas in the confined unfrozen sediments. The formation of the crater was preceded by rapid heaving within one or two years, judging by remnants of a mound detected in the first helicopter view of the area (Figure2). The crater had a shape of a vertical cylinder with smooth walls, possibly because the explosion broke the frozen cap above the talik and mobilized the unfrozen soil–water–gas mass from the talik. A similar process of cryovolcanism has been proposed for formation of the Yamal crater [10]. In our case, we also consider the talik zone freezing. However, the essential role in the gas accumulation process (unlike the model [10] for the Yamal crater) is played not by the biogenic gas generated in the bottom sediments of the thermokarst lake, but by the deep gas entering through the permeable zones. The presence of fractures partly filled with ice on the crater walls (Figure7) also indicates that the freezing talik underwent buildup and partial release of stress. Our results indicate that the gas accumulated in and emitted from the talik came from two sources: it was biogenic gas resulting from microbially mediated decay of organic matter in lake sediments and thermogenic gas that migrated from deeper hydrocarbon reservoirs through permeable deformed bedrock. The proposed formation model of the Erkuta crater, unlike that of cryovolcanism suggested for the Yamal Crater [10], includes the contributions of thermogenic gas that had migrated along faults and fractures from deeper hydrocarbon reservoirs in addition to biogenic gas that formed within the talik. In general, proposed formation model for the Erkuta crater associated with the emission of gas, which is accumulated in shallow permafrost. Its main feature is the consideration of the combined Geosciences 2020, 10, 170 15 of 16 influence of deep-seated (deep gas migration) and shallow (oxbow lake evolution and closed talik freezing) causes in the process of Erkuta gas-emission crater formation. This vision is fundamentally different from the models of other authors, where only one prerequisites type of crater formation is considered: either deep-seated causes [7–9] or only shallow ones [2,5,10].

7. Conclusions The study presents exceptional data on the Erkuta gas-emission crater which was discovered in the summer of 2017 in the floodplain of the Erkuta–Yakha River on the Yamal Peninsula, south of all other craters of this kind found in the North of West Siberia for the past decade. The main value of the research was the timely organized field trip to the crater in December 2017, which allowed collecting field data and sampling soil, ice and water before the crater became fully filled with water. The lifetime of these features being very short (<2 years), the soil and ground ice collected in December 2017 are the only samples suitable for laboratory analyses. The study provides field data on the crater evolution in 2017–2018 and laboratory results for samples of frozen soil and ground ice from the crater walls. The crater formation was preceded by rapid heaving (within 1–2 years) detectable in aerial photographs. The presence of fractures partly filled with ice on the crater walls records buildup and partial release of stress in the freezing talik. The carbon isotope composition of the gas component in ground ice proves the biogenic origin of methane in the surrounding permafrost (δ13C = 72%). The presence of ethane and propane indicates − that deep-seated gases generated during sediment maturation processes may have been involved into the gas-emission event. The higher contents of carbon dioxide compared to methane in several samples confirms the assumption of cryogenic concentration, which usually occurs during freezing of taliks. The results are used to model the formation of the Erkuta gas-emission crater in shallow permafrost caused by the evolution of a talik under a dry lake, assuming a deep gas flow into the unfrozen zone. The model describes the crater evolution in several stages from geological prerequisites to the formation of a new lake-like landform.

Author Contributions: Conceptualization, methodology, supervision, E.C., J.S.; organization and field work, J.S., A.T., A.S., E.C., B.B., M.S., A.C., C.B.; laboratory research and analysis, E.C., N.S., J.S., B.B., S.G., D.D.; writing manuscript and editing, E.C., N.S., J.S., D.D., B.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the energy company Total SA (France), Russian Science Foundation (grant 18-77-10063) with logistic support of the reconnaissance field trip by the Arctic Research Center of the Yamal-Nenets Autonomous District (Russia). Acknowledgments: The study was carried out jointly by the Skolkovo Institute of Science and Technology (Moscow), the Total Company (France) and the Department of Science and Innovations of the Yamal–Nenets Administrative District. We thank anonymous reviewers for their insightful comments and suggestions that helped us to improve the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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