Formation of the Yamal Crater in Northern West Siberia: Evidence from Geochemistry

geosciences

Article Formation of the Yamal Crater in Northern West Siberia: Evidence from Geochemistry

Sergey Vorobyev 1, Andrey Bychkov 1 , Vanda Khilimonyuk 1,*, Sergey Buldovicz 1, Evgeny Ospennikov 1 and Evgeny Chuvilin 2,*

1 Department of Geology, Lomonosov Moscow State University (MSU), GSP-1, Moscow 119991, Russia; [email protected] (S.V.); [email protected] (A.B.); [email protected] (S.B.); [email protected] (E.O.) 2 Skolkovo Institute of Science and Technology (Skoltech), Moscow 121205, Russia * Correspondence: [email protected] (V.K.); [email protected] (E.C.)

Received: 24 October 2019; Accepted: 11 December 2019; Published: 14 December 2019

Abstract: In the framework of this work, studies on the Yamal crater formed as a result of a cryogenic eruption of a water-gas fluid were carried out. The structure and variations of the composition of the geochemical field along the section of the upper horizons of permafrost are considered on the basis of field work, including the drilling of boreholes near the crater. The obtained regularities of the distribution of chemical elements, and gases between the mineral component of the soil and meltwater, suggest that permafrost at the site of the funnel are the remains of a sub-lake paleo-talik, from which mineralized water and gases were expulsed into the yet unfrozen reservoir that previously existed at this place. The component composition of gases suggests that they are products of biochemical processes similar to those that occur in modern peatlands. The δ13C value for methane extracted from the sediments of the near-contact zone of the Yamal crater was found to be 76%. The predominance − of high molecular weight normal alkanes in frozen bitumen indicates that the original organic substrate which was buried contained remains of higher vegetation. The Yamal funnel was formed by the sediment’s “explosion” while the water-gas fluid was released. The volume of the ejected sediment amounted to about 220 thousand m3.

Keywords: Yamal Peninsula; Yamal crater; gases in permafrost; geochemical studies; talik; gas generation; methane; carbon dioxide; freezing; gas outburst

1. Introduction A cylindrical crater, at least 50 m deep and 20–25 m in diameter (Figure1), was discovered in June 2014 in the Yamal Peninsula 30 km south of the Bovanenkovo oil-gas-condensate field. The crater has been described in detail in previous publications [1–5]. It formed in the place of a large pingo produced by long-term lateral back freezing of a paleo-talik beneath a past thaw lake on the bottom of a thermokarst depression [5]. The authors [5] have examined in sufficient detail the hypothesis of the formation of a large lake talik, as well as its long-term freezing, based on mathematical modeling of heat transfer in rock strata. In the same place [5], the explosion conditions of the formed heaving mound with the formation of a crater are considered and the stages of this process are described. The existence of thick (over 100 m) or through taliks beneath large drained lake basins of the North of Western Siberia has been confirmed by modern geophysical studies [6].

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Figure 1. A fragment ofof aasatellite satellite image image of of the the Earth’s Earth’s surface surface within within the the Yamal Yamal Peninsula Peninsula indicating indicating the locationthe location of the of crater the crater (left) (left) and a and view a ofview the explosionof the explosion crater from crater a helicopterfrom a helicopter (right). 1, (right). 2, 3 contours: 1, 2, 3 cratercontours: vents, crater explosion vents, explosion funnels, and funnels, parapet and of parapet parapet, of respectively. parapet, respectively. Named points Named mark points wells mark and their numbers. wells and their numbers. Most likely is that the crater originated from a sudden outburst of pressurized gas upon collapse The crater has been described in detail in previous publications [1–5]. It formed in the place of a of its subsurface reservoir, judging by the absence of visible signatures that would indicate a thermal large pingo produced by long-term lateral back freezing of a paleo-talik beneath a past thaw lake on effect on the rocks. Abrupt decompression led to avalanche-like degassing and eruption of the unfrozen the bottom of a thermokarst depression [5]. The authors [5] have examined in sufficient detail the gas reservoir. The gas release was possibly triggered by instantaneous crystallization of supercooled hypothesis of the formation of a large lake talik, as well as its long-term freezing, based on gas-saturated water or by dissociation of sub-pingo gas hydrates. mathematical modeling of heat transfer in rock strata. In the same place [5], the explosion conditions In June 2015, a team from Moscow University (Department of Geology) undertook a field trip of the formed heaving mound with the formation of a crater are considered and the stages of this to study the crater and its vicinities by geocryological, geochemical, geophysical, and radiometric process are described. The existence of thick (over 100 m) or through taliks beneath large drained methods, with the support of the Innopraktika and NOVATEK companies [5]. The cryostratigraphy lake basins of the North of Western Siberia has been confirmed by modern geophysical studies [6]. and composition of permafrost in the area were studied along a profile that traversed the crater in the Most likely is that the crater originated from a sudden outburst of pressurized gas upon collapse North East direction (Figure1), with core sampling obtained from seven 10 to 17 m deep boreholes. of its subsurface reservoir, judging by the absence of visible signatures that would indicate a thermal According to the core data, the section consists of marine, lacustrine, fluvial, and slope wash clay, effect on the rocks. Abrupt decompression led to avalanche-like degassing and eruption of the clay silt, and silt that enclose poorly degraded peat and remnants of shrub roots and twigs. unfrozen gas reservoir. The gas release was possibly triggered by instantaneous crystallization of 2.supercooled Methods gas-saturated water or by dissociation of sub-pingo gas hydrates. In June 2015, a team from Moscow University (Department of Geology) undertook a field trip to studyThe the boreholes crater and were its sampledvicinities every by geocryolog ~1 m and theical, retrieved geochemical, core wasgeophy cleanedsical, from and materialradiometric that contactedmethods, with the hole the walls.support Sampling, of the Innopraktika storage, and and analysis NOVATEK were performed companies in [5]. full The compliance cryostratigraphy with the guidelinesand composition governing of permafrost the order ofin geologicalthe area were and studied geochemical along work a profile in the that search traversed and exploration the crater ofin hydrocarbonthe North East deposits. direction The (Figure time from 1), with selection core tosampling receipt of obtained analytical from data seven did not 10 exceedto 17 m 10 days.deep Theboreholes. gas component of permafrost was studied in 10 cm long core samples from specified depths placed in 0.5According L glass vessels, to the which core data, were thenthe section filled with consists a saturated of marine, NaCl lacustrine, solution tofluvial, a volume and measuredslope wash to anclay, accuracy clay silt, of and 0.5 mL,silt that and someenclose salt poorly was added degraded to compensate peat and remnants dilution by of meltwater. shrub roots The and vessels twigs. were sealed under metal lids covered with plastic, turned upside down, and transported to a laboratory. The2. Methods selected samples were represented by loamy and clayey soils with ironing spots and inclusions of individual lenses of plant detritus of the Middle-Late Pleistocene (m III2–3) and Holocene deposits [7]. The boreholes were sampled every ~1 m and the retrieved core was cleaned from material that At the same time, samples of surrounding rocks were collected in plastic bags for chemical analyses. contacted the hole walls. Sampling, storage, and analysis were performed in full compliance with the The species and contents of gases were analyzed in 118 and 103 samples, respectively. guidelines governing the order of geological and geochemical work in the search and exploration of The contents of methane and carbon dioxide were measured in drill holes twice a day during the hydrocarbon deposits. The time from selection to receipt of analytical data did not exceed 10 days. whole field campaign by a Seitron gas analyzer with a range of 1 to 10,000 ppm for CH4 and an optical The gas component of permafrost was studied in 10 cm long core samples from specified depths interferometer with a range of 0.1 to 6 vol.% for CO2. The gas flux from the boreholes was estimated placed in 0.5 L glass vessels, which were then filled with a saturated NaCl solution to a volume by air expulsion from a vessel of known volume (5 L) using a gas analyzer which was placed inside. measured to an accuracy of 0.5 mL, and some salt was added to compensate dilution by meltwater. The vessels were sealed under metal lids covered with plastic, turned upside down, and transported to a laboratory. The selected samples were represented by loamy and clayey soils with ironing spots

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In the laboratory, gas was retrieved from sealed samples by thermal-vacuum degassing (the TVD method). During thermal vacuum degassing in a vacuum, each sample heated to 65–75 ◦C. The evaluated gas volume and the dry rock weight were used to calculate gas and ice contents in permafrost. Total core gas content varied from 17 to 233 mL/L depending on the location of the sampling points with respect to the crater walls and the ice content. The average gas content of samples was 99.5 mL/L. Gas species were identiﬁed by gas chromatography. All samples were analyzed for inorganic (He, H, CO2, O2, and N2) and organic (CH4 and its 40 homologs) gases. It was impossible to constrain the gas volume per unit weight of rocks because of ice content variations; the gas contents were recalculated per unit volume of undisturbed permafrost. Table1 lists the maximum and minimum contents of main gas components in permafrost around the crater.

Table 1. Contents of intrapermafrost gases around the Yamal crater.

He CO2 H2 O2 N2 CH4 C2H6 Parameter Relative to Total Gas, Vol.% 3 3 Cmax 1.3 10 40.3 13.3 19.81 89.1 13.2 2.5 10 × − × − C 4.8 10 3 0.48 0.00014 0.03 45.4 7.1 10 4 8.5 10 6 min × − × − × − Relative to Permafrost, Vol.% 4 4 Cmax 1.7 10 7.8 1.18 1.84 11.6 1.2 2.1 10 × − × − C 4.2 10 5 0.04 1.8 10 5 0.0012 1.79 2.6 10 4 4.6 10 6 min × − × − × − × −

Major-element contents in rock samples were analyzed using the X-ray ﬂuorescence (XRF) method, and trace elements were determined by mass spectrometry with inductively coupled plasma. (ICP-MS). Processing and mapping of geochemical data was performed by means of GIS “Gold-Geochemist”, developed at the Department of Geochemistry of Lomonosov Moscow State University.

3. Results

3.1. Chemical Aureoles around the Yamal Crater

3.1.1. Lithochemical Aureoles The rocks within the sampled section were found to be compositionally uniform and the patterns of major and trace elements were observed to have no spatial relation to the crater wall contours. According to cluster analysis, the samples fall into two groups located south and north of the crater (Figure2). The North group includes deeper samples from the northern and central parts of the profile where ice contents in permafrost exceeds 75%, and the South group includes samples from the southern profile part and shallower samples from the north, all of which have lower ice contents. The contents of major and trace elements in the two groups of samples were found to have no statistical difference, but the composition difference shows up in deviations of group average element contents (Cij) from the average values over the whole profile (Ci). This difference was normalized to the standard deviation si in order to obtain dimensionless values (Figure3). Geosciences 2019, 9, x FOR PEER REVIEW 4 of 10

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Figure 2. Classification of permafrost samples from around the Yamal crater.

The North group includes deeper samples from the northern and central parts of the profile where ice contents in permafrost exceeds 75%, and the South group includes samples from the southern profile part and shallower samples from the north, all of which have lower ice contents. The contents of major and trace elements in the two groups of samples were found to have no statistical difference, but the composition difference shows up in deviations of group average element contents (C) from the average values over the whole profile (C ). This difference was normalized to the standard deviationFigure 2. 𝑠Classiﬁcation in order toof obtain permafrost dimensionless samples from values around (Figure the Yamal 3). crater. Figure 2. Classification of permafrost samples from around the Yamal crater.

contents (C) from the average values over the whole profile (C). This difference was normalized to

the standard deviation 𝑠 in order to obtain dimensionless values (Figure 3).

Figure 3.3. Major- and trace-element contents in samplessamples from the South and North groups’groups’ deviationdeviation fromfrom thethe proﬁleprofile average.average. CCii and S i indicateindicate the the average average element element content and standard deviation alongalong thethe section.section. Cij indicatesindicates the the average average content content of of the the el elementsements in the classification classiﬁcation group.

The curvescurves inin FigureFigure3 show3 show that that the the ice-rich ice-rich samples samples of theof the North North group group contain contain more more Si, Ca, Si, and Ca, Naand than Na than those those of the of the South South group group but but less less Al, Al, Mg, Mg, Fe, Fe, and and K K (which (which are are common common toto clay minerals). TheThe mirror symmetrysymmetry ofof the curves about the profileprofile average line indicates that the elements removed during freezing from the ice-rich rocks of the NorthNorth group werewere redepositedredeposited in thosethose ofof thethe SouthSouth group. Since the total element contents always appr approachesoaches 100%, the loss of mobile compounds leads to a relativeFigure 3. increase Major- and of less trace-element mobile ones, contents which in explainssamples from the higher the South concentrations and North groups’ of Si, Ca,deviation and Na in the Northfrom the group profile samples. average. The Ci and distribution Si indicate of the elements average element between content the mineral and standard phase ofdeviation frozen along rocks and ice wasthe mostsection. contrasted Cij indicates for the calcium average within content the of studiedthe elements section. in the classification group. The zones of high and low contents of Ca in the mineral and ice components of the samples are The curves in Figure 3 show that the ice-rich samples of the North group contain more Si, Ca, located in different parts of the profile (Figure4), which is evidence of Ca partitioning between the and Na than those of the South group but less Al, Mg, Fe, and K (which are common to clay minerals). water and mineral phases when the paleo-talik was freezing back. Unfrozen water contains elements The mirror symmetry of the curves about the profile average line indicates that the elements removed released from ice-rich rocks, i.e., some water-active mobile compounds became leached from minerals during freezing from the ice-rich rocks of the North group were redeposited in those of the South in permafrost, leaving quartz with minor amounts of plagioclase and clay minerals. group. Since the total element contents always approaches 100%, the loss of mobile compounds leads

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to a relative increase of less mobile ones, which explains the higher concentrations of Si, Ca, and Na in the North group samples. The distribution of elements between the mineral phase of frozen rocks and ice was most contrasted for calcium within the studied section. The zones of high and low contents of Ca in the mineral and ice components of the samples are located in different parts of the profile (Figure 4), which is evidence of Ca partitioning between the water and mineral phases when the paleo-talik was freezing back. Unfrozen water contains elements Geosciencesreleased2019 from, 9, 515ice-rich rocks, i.e., some water-active mobile compounds became leached 5from of 10 minerals in permafrost, leaving quartz with minor amounts of plagioclase and clay minerals.

FigureFigure 4.4. DistributionDistribution of Ca in mineral and ice phasesphases of rocks around the Yamal crater.

TheThe partitioningpartitioning ofof elementselements betweenbetween thethe waterwater andand mineralmineral phases of the samples provides evidenceevidence that that ice-rich ice-rich rocks rocks ( >(>75%75% iceice contents)contents) belongbelong toto the paleo-talik. Gases and saline water were expulsedexpulsed fromfrom thethe taliktalik intointo thethe yetyet unfrozenunfrozen reservoirreservoir beneathbeneath thethe pingo,pingo, andand the eruption of fluid ﬂuid (water(water and and gas) gas) increased increased the the salinitysalinity ofof rocksrocks aroundaround thethe crater.crater. TheThe salinity increase left an imprint inin the the composition composition ofof waterwater whichwhich draineddrained thethe rocksrocks andand acquiredacquired salinitysalinity markedlymarkedly above the local background,background, despite despite dilution dilution with with rainfalls rainfalls and and melting melting snow snow after theafter eruption. the eruption. An increase An increase in surface in watersurface salinity waterin salinity puddles in nearpuddles the craternear the was crater also was recorded. also recorded. The volumes The volumes of unfrozen of unfrozen water and water rock canand be rock preliminarily can be preliminarily estimated estimated from the from mass the balance mass ofbalance elements of elements in the water in the and water mineral and mineral phases. Accordingphases. According to estimates, to estimates, the water/ rockthe water/rock ratio for clastic ratio sediments for clastic in sediments the paleo-talik in the is approximatelypaleo-talik is 1000approximately to 10,000. 1000 to 10,000.

3.1.2.3.1.2. GasGas AureolesAureoles

TheThe intrapermafrostintrapermafrost gas phase in in the the crater crater area area comprises comprises mainly mainly N2 N, CO2, CO2, O22,O, H,2 ,and H, andCH4. CH The4. Themaximum maximum contents contents of CO of CO2, H2,H2, and2, and CH CH4 are4 are orders orders of of magnitude magnitude higher higher than than the the recorded background.background. Oxygen Oxygen varies varies from from atmospheric atmospheric values values to hundredthsto hundredths of a of percent. a percent. The contentsThe contents of gases of andgases other and elements other elements in the permafrost in the permafrost around around the crater the correlate crater correlate with the with ice contents the ice contents (Table2). (Table 2). Table 2. Contents of gases in the permafrost around the crater. Table 2. Contents of gases in the permafrost around the crater. Gas Content (%) Group Ice Content Ice Content Characteristic Gas Content (%) Group (%) Characteristic N2 CO2 O2 H2 CH4 He (%) N2 CO2 O2 H2 CH4 He Average 78.6 10.8 3.1 2.7 2.3 0.00041 North 81.1 CAveragemin 55.178.6 3.510.8 0.033.1 62.32.7 27.9 2.3 0.00041 0.00008 C 89.1 21.8 7.8 13.1 13.8 0.00015 North 81.1 maxCmin 55.1 3.5 0.03 62.3 27.9 0.00008 Average 64.5 18.4 7.9 0.37 2.7 0.00054 Cmax 89.1 21.8 7.8 13.1 13.8 0.00015 South 53.1 Cmin 45.4 0.5 0.5 0.2 4.7 0.00009 Average 64.5 18.4 7.9 0.37 2.7 0.00054 Cmax 87.7 40.3 19.8 3.3 10.4 0.00012 SouthBackground 53.1 AverageCmin 77.145.4 9.20.5 7.20.5 0.0008 0.2 0.0006 4.7 0.00009 0.0003 Cmax 87.7 40.3 19.8 3.3 10.4 0.00012 The contentsBackground of carbon dioxideAverage and oxygen77.1 in the samples9.2 from7.2 the northern0.0008 part0.0006 of the profile0.0003 are on average twice as low as those from the south. Hydrogen is almost ten times higher, while methane The contents of carbon dioxide and oxygen in the samples from the northern part of the profile does not differ much across the two profile parts; helium is about that of the background level. This are on average twice as low as those from the south. Hydrogen is almost ten times higher, while distribution is inconsistent with CO2,H2, and CH4 inputs from greater depths along faults or fractures that crosscut the pingo. Gases can be observed to have more intricate vertical patterns (Figure5) than major and trace elements. The near-surface zone of high gas saturation reaches a depth of 7 m. The distribution of gas contents in rocks apparently corresponds to that prior to the eruption event: it is highest between 3 and 5 m below the surface and decreases smoothly to the background values above and below this range. Carbon dioxide shows a distribution similar to that of gas saturation and thus may be a key component of the gas mixture expulsed into the unfrozen zone beneath the pingo when the talik was freezing. Geosciences 2019, 9, x FOR PEER REVIEW 6 of 10

methane does not differ much across the two profile parts; helium is about that of the background level. This distribution is inconsistent with CO2, H2, and CH4 inputs from greater depths along faults or fractures that crosscut the pingo. Gases can be observed to have more intricate vertical patterns (Figure 5) than major and trace elements. The near-surface zone of high gas saturation reaches a depth of 7 m. The distribution of gas contents in rocks apparently corresponds to that prior to the eruption event: it is highest between 3 and 5 m below the surface and decreases smoothly to the background values above and below this range. Carbon dioxide shows a distribution similar to that of gas saturation and thus may be a key Geosciencescomponent2019 of, 9, the 515 gas mixture expulsed into the unfrozen zone beneath the pingo when the talik6 was of 10 freezing.

Figure 5. Vertical patterns of gas saturation and contents of CO2, O2, N2, H, and CH4 in rocks around Figure 5. Vertical patterns of gas saturation and contents of CO2,O2,N2, H, and CH4 in rocks around thethe crater. crater.

OxygenOxygen isis significantlysignificantly belowbelow thethe atmosphericatmospheric valuevalue and is less regularly distributed with depth depth comparedcompared to to other other gases, gases, which which may may be be due due to to permeability permeability variations variations in thein the section. section. Zones Zones of lowof low O2 areO2 locatedare located in ice-rich in ice-rich rocks rocks north north of the of crater. the crater. H and H CH and4 have CH4similar have similar patterns patter withns a zonewith ofa zone highest of valueshighest variable values invariable thickness in thickness but running but continuouslyrunning continuously over the lowerover the part lower of the part section of the and section a zone and of lowa zone contents of low encircling contents encircling the crater 4–5the crater m below 4–5 its m top.below its top. TheThe original original distribution distribution of of gases gases in thein areathe area remains remains unknown. unknown. It formed It formed as the talikas the was talik freezing was backfreezing and theback gases and itthe stored gases were it stored expulsed were byexpulsed the freezing by the front, freezing together front, with together saline with water, saline into thewater, yet unfrozeninto the yet sub-pingo unfrozen zone. sub-pingo That zone zone. made That up zone a core made with up gas- a core and with water-filled gas- and porosity water-filled corresponding porosity 4 tocorresponding a zone of low to H a andzone CH of 4lowin theH and gas CH field; in large the gas amounts field; large of these amounts highly of mobile these volatileshighly mobile were releasedvolatiles from were wall released rocks from into wall the atmosphere rocks into the during atmosphere the cryovolcanic during the event. cryovolcanic event. MeasurementsMeasurements ofof gasgas flowflow atat thethe boreholeborehole headhead showshow that it migratedmigrated into the near-surface air forfor 2–32–3 daysdays atat variablevariable rates.rates. TheThe flowflow raterate waswas highesthighest from boreholes in the southern part of the profile where the rocks have lower ice contents and reached 5 10 4 L s 1 m 2 per unit surface of the × − · − · − borehole wall at the end of drilling. The content of methane emanating from drill holes was at least twice as high as the average in the core samples, e.g., methane in the free gas phase from borehole (B.4) was in the range 0.48% to 0.67%, but pore methane was only 0.16% on average. This dramatic difference indicates that gas can migrate through active voids in permafrost and thus penetrate into boreholes from remote parts of the section. The correlation relationships between the components of subsurface gases (Figure6) reveal two groups with opposite trends: (1) CO2,O2, and He and (2) CH4 and H. The gases of group 1 are in Geosciences 2019, 9, x FOR PEER REVIEW 7 of 10 profile where the rocks have lower ice contents and reached 5 × 10−4 L·s−1·m−2 per unit surface of the borehole wall at the end of drilling. The content of methane emanating from drill holes was at least twice as high as the average in the core samples, e.g., methane in the free gas phase from borehole (B.4) was in the range 0.48% to 0.67%, but pore methane was only 0.16% on average. This dramatic difference indicates that gas can migrate through active voids in permafrost and thus penetrate into boreholesGeosciences 2019 from, 9, 515remote parts of the section. 7 of 10 The correlation relationships between the components of subsurface gases (Figure 6) reveal two groups with opposite trends: (1) CO2, O2, and He and (2) CH4 and H. The gases of group 1 are in positive correlation withwith gasgas saturation,saturation, which which indicates indicates that that they they originated originated from from the the same same source, source, i.e., i.e.,the freezingthe freezing sub-lake sub-lake paleo-talik. paleo-talik.

FigureFigure 6. CorrelationsCorrelations between between the the contents contents of of subsurface subsurface gases gases around around the the Yamal Yamal crater. crater. r5% r5% is is the criticalcritical value value of of the the correlation correlation coefficient coeﬃcient for a 5% signiﬁcancesignificance level.

TheThe gases gases in in the the crater crater walls walls are are marked marked by by a a positive positive correlation correlation between CO 2 andand O O22 (Table(Table 1),1), unlikeunlike those those above above faults faults and and mineral mineral deposits deposits or or oil oil and and gas gas fields, ﬁelds, where where they they are are in negative correlation.correlation. The The correlation correlation is is negative negative in in the the latter latter case case because CO 22 isis either either a a product product of of oxidation oxidation reactionsreactions or or a a component of of deep-seated deep-seated gases gases which which release release near near the the surface surface and and thus thus reduce reduce the the contentcontent of of atmospheric gases in the porepore space.space. The The positive correlation between CO 2 and O 2 isis evidenceevidence of their joint input into the crater from the freezing talik. CarbonCarbon dioxide andand nitrogennitrogen areare the the principal principal components components of of the the intrapermafrost intrapermafrost gas gas phase phase in thein thecrater crater area. area. They They reach reach atotal a total content content of of 71% 71% to to 99% 99% and and make make up up aa closedclosed two-componenttwo-component system where an increase of one component corresponds to a decrease of the other. Nitrogen Nitrogen is is an an inert inert gas gas which does does not not participate participate in in reactions reactions and and its its content content in inthe the rocks rocks fully fully depends depends on that on that of CO of2 CO; the2; twothe twoare arerelated related as N as2 N= 287.6= 87.6 − COCO2. The2. The free free term term of of this this relationship relationship corresponds corresponds to to the the initial initial − concentrationconcentration of of nitrogen nitrogen in the the subsurface subsurface gas gas phase, phase, which which is is fully fully consistent consistent with with the the results results of of gas gas surveyssurveys from from different diﬀerent years in northern Russia. Inpu Inputsts of of gas gas from from the the freezing freezing talik talik locally locally make make the the contentcontent of of N N22 almostalmost twice twice as as low. low. The The strong strong positive positive correlation correlation of CO 2 withwith gas gas saturation saturation and and the the negativenegative correlation correlation with nitrogen indicates that CO 2 waswas the the principal principal subsur subsurfaceface gas gas component component in in thethe freezing freezing talik talik and and that that its its pattern pattern controlled controlled the the contents contents of of other other gases gases in in rocks rocks around around the the crater. crater.

3.2.3.2. Origin Origin of of Gases Gases in in the the Yamal Yamal Crater Area TheThe Yamal cratercrater isis located located 60 60 km km from from the the Bovanenkovo Bovanenkovo gas gas field field and and it is it pertinent is pertinent to assess to assess their theirpossible possible relationship. relationship. The gasThe phase gas phase in the in reservoir the reservoir rocks rocks of the of Bovanenkovo the Bovanenkovo fieldcontains field contains 98.9% methane and 0.22% ethane, with only minor N and CO , according to unpublished data collected by 98.9% methane and 0.22% ethane, with only 2minor N22 and CO2, according to unpublished data collectedthe Department by the Department of Geology andof Geology Geochemistry and Geoche of Fuelmistry Deposits of Fuel at MoscowDeposits University.at Moscow University. The carbon isotope composition of the Bovanenkovo methane is about 39.9% δ13C. The rocks in the crater walls The carbon isotope composition of the Bovanenkovo methane− is about −39.9‰ δ13C. The rocks in the store much more CO2,N2, and H2 than those in the gas field. The weighting of the isotopic composition of methane carbon is δ13C = 76%. This difference disproves the hypothesis that the gas that exploded − in the Yamal crater could have come from the Bovanenkovo reservoirs. The gas composition in the samples we analyzed corresponds to that resulting from biochemical processes in paludal environments. The carbon isotope composition of marsh methane, more depleted than that of methane from gas and gas-condensate reservoirs, is controlled by the composition of degrading organic matter and biochemical reactions of methane formation. Geosciences 2019, 9, x FOR PEER REVIEW 8 of 10

crater walls store much more CO2, N2, and H2 than those in the gas field. The weighting of the isotopic composition of methane carbon is δ13C = −76‰. This difference disproves the hypothesis that the gas that exploded in the Yamal crater could have come from the Bovanenkovo reservoirs. The gas composition in the samples we analyzed corresponds to that resulting from biochemical Geosciencesprocesses2019 in, 9paludal, 515 environments. The carbon isotope composition of marsh methane, 8more of 10 depleted than that of methane from gas and gas-condensate reservoirs, is controlled by the composition of degrading organic matter and biochemical reactions of methane formation. The more negative δ13C values of CH may be due to its formation by CO reduction, further The more negative δ13C values of CH4 may be due to its formation by CO22 reduction, further oxidation of CH by methanotrophic bacteria, and fermentation of methylated organic compounds oxidation of CH4 by methanotrophic bacteria, and fermentation of methylated organic compounds without subsequent oxidation. Methane obtained by bacterially-mediated reduction of CO has δ13C without subsequent oxidation. Methane obtained by bacterially-mediated reduction of CO22 has δ13C 13 values reaching −110‰,110%, while the fermentation of methylated dead plantplant materialmaterial givesgives aa δδ13C range − 13 of −50‰50% [8].[8]. In In Siberian Siberian peatlands, peatlands, the range forfor biochemicalbiochemical methanemethane is is −7171 toto −77%77‰ δ 13CC[ [9],9], which − 13 − − includes the δ13 C value of 76% in methane released from the Yamal crater. includes the δ C value of −−76‰ in methane released from the Yamal crater. The origin of hydrocarbon gases at didifferentﬀerent core depths around the crater was inferred from bitumen contents distributed evenly along the section,section, with the relative contents of alkane species given in histograms of Figure7 7.. JudgingJudging fromfrom thethe obviousobvious predominancepredominance ofof high-molecularhigh-molecular alkanesalkanes (C H and higher), buried plant remnants are the primary organic substrate present. Lipids of higher (C1919H4040 and higher), buried plant remnants are the primary organic substrate present. Lipids of higher plants are rich in even-chain fatty acids and tricos tricosaneane produced by decarboxylation of saturated fatty lignoceric acid,acid, aa typicaltypical high-molecular high-molecular organic organic component component of of higher higher plants. plants. The The pristan pristan/fitan/ﬁtan ratio ratio of 0.68of 0.68 (< 1)(<1) indicates indicates reduced reduced euxinic euxinic conditions conditions of of diagenesis diagenesis [10 [10].].

Figure 7. Contents of alkanes (ppm) in the cored section.

Biochemical transformation of organicorganic matter is accompanied by rapid gas generation. Methane, Methane, a major component of the gas phase in degrading buried matter, is producedproduced by anaerobicanaerobic bacteriabacteria (mainly thethe Methanobacterium,Methanobacterium, Methanococus,Methanococus, and MethanosarcinaMethanosarcina genera)genera) fromfrom COCO22,H, H22, acetate, and fatty alcoholsalcohols resulting resulting from from cellulose cellulose fermentation fermentation by other by bacterial other species.bacterial In euxinicspecies. environments, In euxinic theenvironments, reactions are the [11 reactions] are [11] C6HC126HO612O+ 66H + 6H2O2O= 6CO= 6CO2 +2 +12H 12H2 2 CO2 + 4H2 → CH4 + 2H2O CO + 4H CH + 2H O Thus, each cellulose molecule decomposing2 2 → during4 biochemical2 transformation of plant organic matterThus, releases each eight cellulose molecules molecule of hydrogen decomposing while during producing biochemical one CH transformation4 molecule. The of carbon plant organicisotope 13 mattercomposition releases of eightthe reaction molecules products of hydrogen is −75‰ while to −80‰ producing δ C [9]. one CH4 molecule. The carbon isotope compositionMarsh gases of the also reaction store quite products large is amounts75% to of hydr80%ogen,δ13C[ occasionally9]. making up up to 2% of the − − total Marshgas volume gases [12]. also storeWe compared quite large relative amounts percen of hydrogen,tages of gases occasionally isolated making from the up lake up to and 2% marsh of the totalwater gas in the volume depression [12]. We that compared accommodates relative the percentages crater at its of southern gases isolated edge and from found the lakeabout and ten marsh times waterhigher in contents the depression of dissolved that accommodateshydrogen and methane the crater in at marsh its southern water edgecompared and found to that about in the ten lake: times H2 higher contents of dissolved hydrogen and methane in marsh water compared to that in the lake: H2 = 0.0081 and CH4 = 0.46 against H2 = 0.00016 and CH4 = 0.039, respectively. Note that in both cases the samples were from water that contacted the atmosphere, which favors the escape of poorly soluble gases such as hydrogen and methane. Geosciences 2019, 9, 515 9 of 10

3.3. Volume of Exploded Gas The volume of gas that migrated into the unfrozen zone remaining beneath the pingo cannot be reliably constrained from gas contents in the frozen part of the paleo-talik because the original talik underwent a complex evolution and diverse physical processes in the course of long-term lateral freezing. Specifically, it is impossible to estimate the loss of gas by cryogenic concentration at an early stage when the primary lakes still existed. Nevertheless, the volume of gas that exploded and caused the crater formation can be estimated within the first approximation proceeding from the volume of the frozen talik part and its remnant gas saturation. According to estimates of gas redistribution during freezing of methane-saturated sand, up to 75 vol.% of gas can be expulsed into the unfrozen zone while 25 vol.% can remain in the rock [13,14]. In our case, the remnant gas in the sediments, which have fine grain sizes, may occupy 50 vol.%. We estimate that pingo formation involved 220,000 m3 of frozen rock, which is consistent with the volume of ground ice recorded by geophysical surveys of the surroundings of the crater. Laboratory studies of rock physics in the core samples indicate the presence of free gas in the pore space free from ice or liquid water in rocks between the boreholes, as well as gas bubbles in ice inclusions. Gas fills 12.5 vol.% of the pore space in permafrost on average, while the average pore gas pressure is 2 bars. With these values, the gas volume in the frozen part of the paleo-talik at the time when it became closed under normal conditions can be estimated to have exceeded 50,000 m3. About the same amount of gas apparently moved into the unfrozen zone beneath the pingo, given that 50% of the total gas content remained in the frozen part upon the talik freezing. In fact, this volume may have been much greater initially because the rocks around the crater lost much gas during post-eruption degassing, which was observed in the boreholes.

4. Conclusions The trends of gas distribution in permafrost samples from boreholes drilled around the Yamal crater discussed in this work show that the processes responsible for generation of the exploded gas are largely similar to those in modern paludal environments. Partitioning of some major and trace elements between the mineral and ice components indicates that deeper ice-rich rocks in the northern part of the profile may belong to the sublake paleo-talik. Gas from the talik was expulsed by the freezing front into a yet unfrozen zone of sediments beneath the pingo and was made part of a water and gas fluid. Judging by the compositions of the mineral and gas components of permafrost, the crater is located at the edge of the freezing talik. The elements had been partitioning between the liquid and solid phases for a few thousand years while the talik was freezing, and biogenic reactions transformed organic matter. As a result, high-molecular compounds of peat and mud communities converted to fulvic and humic acids, while bacteria metabolized organic matter and formed CO2, CH4, and H2, the main gas species found in ice-rich sediments within the frozen paleo-talik. The results obtained for the upper part of the section for drilled boreholes confirm the presence of cryogenic gas separation during the freezing of a closed thawed rock mass. The gas composition transformations have their own specifics for different depths of the section. Thus, according to estimates of the internal pressure in the talik before the explosion, favorable conditions are created in the lower part of the talik for the formation of carbon dioxide gas hydrates [5].

Author Contributions: Supervision, V.K.; conceptualization and methodology, S.V. and A.B.; field work, V.K., E.O., S.V., and A.B.; laboratory research and analysis, S.B., V.K., S.V., A.B.; E.O., and E.C.; writing and editing, S.V., A.B., V.K., and E.C. Funding: The research was supported partly by the companies Innopraktika and NOVATEK. Conflicts of Interest: The authors declare no conflict of interest. Geosciences 2019, 9, 515 10 of 10

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