Caratteristiche e varietà dei gas enclatrati negli idrati di gas naturale recuperati nel Lago Baikal

Empirical diagrams of hydrate-bound gases. (a) C1/(C2 + C3) plotted against C1 δ13C, based on the classification of Milkov and Etiope20,000 samples. Org. Geochem. 125, 109–120. https://doi.org/10.1016/j.orggeochem.2018.09.002 (2018)." href="/articles/s41598-023-31669-7#ref-CR13" id="ref-link-section-d58456844e941">13; (b) C1 δ2H plotted against C1 δ13C, based on the classification of Milkov and Etiope20,000 samples. Org. Geochem. 125, 109–120. https://doi.org/10.1016/j.orggeochem.2018.09.002 (2018)." href="/articles/s41598-023-31669-7#ref-CR13" id="ref-link-section-d58456844e957"13; and (c) C2 δ13C plotted against C1 δ13C, based on the classification of Milkov15. The data for Malenky, Bolshoy, Malyutka, P-2, K-0, K-2 and Goloustnoe are sourced partly from Hachikubo et al.33. The data for Kedr and Kedr-2 are sourced partly from Hachikubo et al.41./p>

Figure 2a shows the relationship between C1 δ13C and C1/(C2 + C3) plotted in the empirical diagram20,000 samples. Org. Geochem. 125, 109–120. https://doi.org/10.1016/j.orggeochem.2018.09.002 (2018)." href="/articles/s41598-023-31669-7#ref-CR13" id="ref-link-section-d58456844e1064"13. More than 20 of the 60 total sites have C1 δ13C between − 68‰ and − 65‰ and C1/(C2 + C3) concentrated around 1000–5000, which means that microbial gas is enclathrated in more than one-third of the hydrate-bearing sites in Lake Baikal. However, along the mixing line from the microbial to thermogenic regions, C1 δ13C increases with a decrease in C1/(C2 + C3), passing through the mixed region of microbial and thermogenic gases to thermogenic gas (e.g., K-4, PosolBank, Kedr, and Kedr-2). For the eight sII hydrate data points, C1/(C2 + C3) is nearly constant at 6–7. Furthermore, C1 δ13C seems independent of the crystallographic structure at the same sites but differs considerably in K-3 and K-pockmark. This is because the hydrate-bearing sediment cores are different, even at the same site, indicating that the characteristics of the hydrate-bound gas can change markedly with slight differences in location. Gorevoy Utes43,44 is one of the two oil seep sites and plots in the field of secondary microbial gas (Fig. 2a)9. Another point, ZelenSeep, also plots near the Gorevoy Utes. Most of the data plotted for the thermogenic origin overlap with the field of secondary microbial gas./p>

Figure 2b shows the relationship between C1 δ13C and C1 δ2H, which is also plotted in an empirical diagram20,000 samples. Org. Geochem. 125, 109–120. https://doi.org/10.1016/j.orggeochem.2018.09.002 (2018)." href="/articles/s41598-023-31669-7#ref-CR13" id="ref-link-section-d58456844e1130">13. The isotopic fractionation of C1 between the gas and hydrate phases is negligible when considering gas origins using a diagram45. C1 δ13C tends to increase with C1 δ2H. In a diagram by Whiticar12, hydrate-bound C1 in Lake Baikal is interpreted to be of microbial origin via methyl-type fermentation31,32,33,46. However, the latest diagram by Milkov and Etiope20,000 samples. Org. Geochem. 125, 109–120. https://doi.org/10.1016/j.orggeochem.2018.09.002 (2018)." href="/articles/s41598-023-31669-7#ref-CR13" id="ref-link-section-d58456844e1165"13 shows that most of the C1 δ13C values below − 60‰ overlap completely with the microbial origin via CO2 reduction and may possibly be of an early mature origin. Therefore, it is difficult to determine the origin of C1 in Fig. 2b./p> 10%) of C3 and C4, whereas in Lake Baikal the ratio of C3 and C4 in the hydrate-bound gas is < 0.5% in the sII hydrates (Table S1). Therefore, C1 and C2 mixed-gas systems are responsible for the appearance of sII in Lake Baikal. Pure C1 and C2 hydrates each form sI, but in C1 and C2 mixed-gas systems, sII appears at certain mixing ratios36,37. In Lake Baikal, sII hydrates, in which the hydrate-bound gas was 85% C1 and 15% C2, were retrieved at Kukuy K-2 in 200531,32,33,46. Generally, the sII hydrate is adjacent to the sI hydrate with 1–4% C2, forming a "double structure". Current understanding of the process is that the formation processes of double-structure gas hydrates in Lake Baikal have been discussed extensively34,46,47,48. The formation of sI hydrates blocked the gas supply channel, causing dissolution of sI hydrates and secondary formation of sII hydrates by enrichment of C2 from the dissociated gas34,47. A detailed investigation of the sII hydrates at Kedr and Kedr-2 revealed that besides C2 also C3, i-C4, n-butane (n-C4), and neopentane (neo-C5, 2,2-dimethylpropane) are enriched in the crystals41. The sII hydrates are identified at eight sites: Kukuy K-2, K-3, K-4, K-10, and K-pockmark in the central sub-basin and PosolBank, Kedr, and Kedr-2 in the southern sub-basin. The C1/(C2 + C3) ratios of these hydrate-bound gases are concentrated at approximately 6–7 (Fig. 2a), and the contribution of C3 is negligible compared to that of C2 (Table S1). The homogeneous gas composition of sII hydrates over a wide area of Lake Baikal can be explained by the decomposition processes of C1 and C2 mixed gas hydrates and the concentration of C241,49./p>  − 42‰), C3 δ13C is widely distributed from − 20 to + 10‰, suggesting the effect of biodegradation. For example, C3 is selectively affected by microbial alteration and exhibits anomalous C3 δ13C50. C1-rich dry gas, large C1 δ13C (− 55‰ to − 35‰), and large CO2 δ13C (> + 2‰) have been proposed as characteristics of secondary microbial C19. The molecular and stable isotope compositions of CO2 in the hydrate-bound gas are unknown; however, the CO2 δ13C in the sediment gas around the hydrate crystals reaches + 20‰ (Kedr) and + 30‰ (Kedr-2)41, indicating the generation of secondary microbial C1. With two exceptions (Kukuy K-2 and K-10), the hydrate-bound gases of the sII crystals plot in the area of secondary microbial gases in Fig. 2a. Thus, at most sites in Lake Baikal, where thermogenic gas is supplied from a deeper sediment layer, hydrocarbons heavier than C3 are selectively and microbially degraded, resulting in the appearance of C1 and C2 mixed gas, further dissociation of sI hydrate, and the formation of sII hydrate adjacent to sI./p>

The C1 δ2H of hydrate-bound gas in marine sediments is generally concentrated between approximately − 200‰ to − 190‰ for microbial gas and is greater for thermogenic gas, reaching approximately − 140‰ for gas hydrates retrieved offshore of Vancouver Island and Costa Rica15. The distribution of C1 δ2H of the thermogenic gas ranges from − 300‰ to − 100‰11 and from − 350‰ to − 100‰20,000 samples. Org. Geochem. 125, 109–120. https://doi.org/10.1016/j.orggeochem.2018.09.002 (2018)." href="/articles/s41598-023-31669-7#ref-CR13" id="ref-link-section-d58456844e1586"13. In addition, C1 δ2H tends to increase with C1 δ13C11,12. The factors that determine the C1 δ2H of thermogenic gas have not yet been clarified; however, it can be assumed that hydrogen isotope exchange occurs between C1 and environmental water. Based on the effect of temperature on the hydrogen isotope fractionation between C1 and hydrogen, and between hydrogen and water51, the hydrogen isotope fractionation between C1 and water can be expected to be smaller at higher temperatures. If the thermogenic gas produced by the decomposition of organic matter exchanges isotopically with environmental water during decomposition, the C1 δ2H of thermogenic gas in the deep sediment layers becomes greater than that of microbial gas produced in shallower sediment layers./p>

Although little information is available on microbial C3, a mechanism has been proposed by Hinrichs et al. for its formation from acetate and hydrogen58, in which it has been noted that C3 δ13C was greater than C2 δ13C, and Fig. 3b satisfies this relationship. In the area of microbial C2 where C2 δ13C is below − 42‰, C3 δ13C is also relatively low, ranging from − 40‰ to − 30‰, indicating that microbial C3 is more depleted in20,000 samples. Org. Geochem. 125, 109–120. https://doi.org/10.1016/j.orggeochem.2018.09.002 (2018)." href="/articles/s41598-023-31669-7#ref-CR13" id="ref-link-section-d58456844e1975"13C than thermogenic C3./p>

20,000 samples. Org. Geochem. 125, 109–120. https://doi.org/10.1016/j.orggeochem.2018.09.002 (2018)./p>

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