ИСТИНА

The subject of the report is the results of a study of a unique natural crater-shaped object in the central part of the Yamal Peninsula, carried out in 2015 by the research team of Lomonosov Moscow State University, Geological Faculty supported by Innopraktika and PAO NOVATEK. Methods Complex studies included field and laboratory investigations: detailed description the territory; drilling 20 m wells for the cryogenic structure study and sampling of soils and ice for determination of physic and mechanical properties; thermometric measurements; topographic works, a complex of geochemical and geophysical studies. Radiological measurements were conducted. Sampling of surface water, soil, frozen and thawed rock, free and adsorbed gases were carried out. Methane and carbon dioxide contents were determined in the wellbores using field gas analyzers and trapping of gases enclosed in frozen soil and ice were performed. Trace elements of soils and ice were determined by ICP-MS. Mathematical modeling of heat exchange processes in the rocks and assessment of the pressure required for a pingo destruction were performed. Results On the basis of the materials obtained, the authors developed their own concept of a cryogenic origin of the Yamal Crater, explaining its formation by the process of cryovolcanic event on the Earth. Cryovolcanism is understood as the explosion-like effusive destruction of pingo with the eruption of a substance under the action of high cryogenic pressures as a result of freezing of closed water and gas saturated systems (a closed talik). It was established that on the site of the crater the injection pingo took place. It was formed as a result of the freezing of the talik after the descent of the ancient lake, which was accompanied by the release of water and dissolved gases from the freezing front into the thawed zone and a significant increase in internal pressure. This pressure at some point led to the destruction of the layer of frozen soils, overlapping the residual part of the talik above. Evaluation of the pressure for the destruction of pingo with a 7–9 m thickness with physical properties of frozen soils leads to 1.0 MPa value. The process of destruction of the pingo was accompanied not only by the scattering of the frozen fragments of the “cover”, but also by explosion of thawed core, which was a water-ground gas-saturated mixture (possibly with some free gas) and formed a cylindrical crater. As a result of solving a thermal modeling, it was found that the cylindrical shape of the residual talik is formed when the intensive lateral freezing of the initial talik during of decreasing of a lake. Frozen sediments and ice are surprisingly rich in gases reaching 20 vol. %. The gaseous component in sediments near the crater differs markedly from natural gas of the Bovanenkovo field in the concentrations of carbon dioxide, nitrogen, hydrogen, and methane homologs. The carbon isotope composition of methane is typical of biogenic hydrocarbons (δ13C = -76‰ PDB). This difference disproves the hypothesis that the gas in the crater would come from deep Bovanenkovo reservoirs. Prevalence of high normal alkanes (above C19H40) indicates that the hydrocarbons are derived from buried plant remnants. The study of the chemical composition showed the difference in the distribution of chemical elements between the mineral component of the soil and ice inclusions, associated with the talik freezing front. These features allowed us to establish the boundary of the ancient lake. The available materials show that the freezing talik before explosive destruction contained a large amount of water saturated with gas, predominantly carbon dioxide. This is due to a peculiar mechanism of cryogenic separation of gases. During the freezing of the pore water, the gas mixture dissolved in the ice is displaced from the formed ice, while part of this gas is fixed in ice in the form of macro-inclusions (bubbles), and the soluble components of the gas mixture (primarily carbon dioxide) dissolve in the liquid phase inside talika. This is facilitated by the ever-increasing pressure during the freezing of a closed soil volume of rocks. At a low temperature on the talik surface, in the presence of a long-lasting pingo, the frozen cap turned out to be too thick to break down under hydrostatic pressure. Calculations show that the equilibrium pressure of the talik core under a 6-8 m thick cap can reach 5 bar, while about 1 MPa bar is required for the breakdown of such a cap. This pressure is close to the invariant point in the H2O-CO2 system (P = 0.982 MPa; T = -1.4 °C) at which liquid water can coexist with ice, carbon dioxide clathrate and gaseous carbon dioxide. The formation of carbon dioxide clathrate is especially possible at the bottom of the talik core where pressure may reach 1.5 MPa. The estimates have shown that the explosive destruction of the pingo is not related to the climatic anomalies of individual years. The explosion itself, presumably, took place in several stages. At the first stage, gas is predominantly ejected from the upper part of the talik and scattering of the frozen fragments of the “cover”. At the second stage of the eruption, degassing (“boiling”) of water filling the talik, accompanied by an intense outpouring of the water-soil mixture from the crater. The final stage of the eruption is the release of gas from the thawed soil both by degassing the pore solution and, possibly, due to the decomposition of carbon dioxide hydrates.