Вестник МГТУ. 2018, том 21, № 1.

Сорохтин Н. О. и др. Коромантийная ветвь глобального цикла углерода… 74 a mix of light and heavy carbon isotopes since the thermal effect of the 12 С isotope fractionation reaction as compared with the 13 С isotope reaches 0.412 kcal/g [11]. This effect tends to the lightening of carbon in the resultant methane. Thus, the exchange isotope reaction is as follows: 12 СО 2 + 13 СН 4 → 13 СО 2 + 12 СН 4 + T ○ C (51) and evolves from the left to the right. The deviation of the methane isotope signature in black smokers is usually δ 13 C ≈ –13 to –14 ‰ while the compositions of 3 HCO − and СО 2 dissolved in oceanic waters show values close to δ 13 C = –5.5 ‰ [35]. It is hence evident that the isotope exchange reaction between carbon dioxide and methane, as it follows from the reaction (51), proceeds towards the reduction of δ 13 C in methane. Further, during the processing by methane- absorbing bacteria, the carbon composition becomes additionally lightened in the formed organic matter (С org ) of bacterial communities. The organic matter acquires extremely low shift values of δ 13 C org to –50 ‰ and even to δ 13 C org ≈ –80 ‰. The same phenomenon apparently may account for the origin of minimums in the distribution of δ 13 C org in marsh gases of Quaternary sediments and in shale gas deposits. Conclusion Studying the crust-mantle carbon cycle processes, the issue of carbon content in the Earth mantle has to be touched on. It cannot provide an unambiguous response, but the solution lies in the plane of revealing indirect features that allow some degree of confidence. Thus, in mafic eruptive rocks, dispersed carbon demonstrates negligible concentrations of 10 to 100 ppm and deficiency of heavy isotope, δ 13 C = –22–27 ‰. The carbon contained in the Earth crust is heavier, δ 13 C = –3–8 ‰ [40] while the values are typical of isotope shifts in diamonds. The tholeiite basalts of oceanic rifts contain 20 to 170 ppm of carbon with isotope shifts of ca. –5 ‰ [41]. Alongside, two factors shall be considered that significantly cut down the values of free carbon presence in the Earth mantle. On the one hand, part of it is in atomic state there and composes the crystal lattice of silicates [42]. On the other hand, some certain portion of carbon represents a product transferred by the mantle convective currents from the plate underthrust zones to the rift systems and participates in the crust-mantle exogenic carbon cycle. The given data show that the Earth mantle may contain a negligible carbon amount, and its total concentrations may significantly differ from those accepted downwards. By the data from [43], the carbonates of the Earth crust bind ca. 3.91·10 23 g of СО 2 and ca. 1.95·10 22 g of organic carbon (С org ). A significant portion of this substance is deposited as sediments on the seabed and slopes of continents and participates in the conveyor process of crust-mantle carbon cycle together with a drift of lithospheric plates and generation of convergent and divergent structures at their borders. The above-described fact that a wide spectrum of hydrocarbon gases from methane (CH 4 ) and ethane (C 2 H 6 ) to propane (C 3 H 8 ) and butane (C 4 H 10 ) is present in the rift zones draws special attention. Complex hydrocarbons alone in a free state at high PT conditions become instable and tend to decompose to simpler ones down to methane (CH 4 ). This indicates the generation of the listed compounds in near-surface and low- temperature settings, but not due to their removal from the deep mantle. This variety of hydrocarbon compositions may apparently be accounted for by the fact that, under high temperature and pressure values of the dry mantle, metal carbides remain stable, and their decomposition starts only when the hydration levels are reached, i. e. below 400 ○ C. The analysis of isotope geochemical data demonstrates that, in the rift zone, together with a wide spectrum of hydrocarbon gases, there is an effect of intense fractionation of carbon isotopes, which scatter of values may vary in a very wide range. It is related to the fact that, alongside with the in-situ process, convective currents add carbon here from the subduction zones with their own isotope features and another genetic type. For example, this may result both in the contamination of radiocarbon analysis data and meaningless age values. The studied processes allow concluding that the crust-mantle carbon cycle is related to the formation of this element in one geodynamic setting and its transfer due to the drift of lithospheric plates to another. As a result, it undergoes multi-stage transformation from the chemogenic state to the biogenic one and vice versa, and submersion to the mantle at the levels of its convective mixing and rise to the surface through rift zones (Fig. 1). Almost all carbon has an exogenic origin. This process is tightly related to the crust-atmospheric carbon cycle since it is primarily supplied by carbon dioxide and products of transformation (carbides, carbonates, hydrocarbonates, organic substance). Together they form a global carbon cycle in the nature. The amount of abiogenic hydrocarbon gases generated by the described methods cannot provide the formation of large gas and oil-and-gas fields since their significant part is transferred to the atmosphere. Just some hydrocarbon compounds may deposit in oceanic sediments and form gas-hydrate beds [34]. One of the main conclusions of this investigation represents a factor of no need to involve a large amount of water for the implementation of physical and chemical transformations of crustal substance in the mantle asthenosphere. The data in the paper allow inferring that the notion of the global carbon cycle, which was originally proposed by the academician A. Ye. Fersman in the 1920s has to be expanded by including processes