Monthly temperature variation of the soil upper active layer.
The monthly temperature variations of the soil upper active layer and low-level air are shown in Fig. 2 based on daily monitoring average data through 2017 and 2018 at the gas hydrate drilling area in the Muli permafrost of the Qilian Mountains. The soil temperature varied with the atmospheric temperature, but had a smaller variation range than the corresponding atmosperic temperature. The soil temperature in winter had the lowest around − 16℃, and the one in summer had the highest around 16℃. The soil temperature was above 0℃ in summer (June-August) and below 0℃ in winter (December-Febuary), and increased gradually in spring (March-May) and decreased in autumn (September-November). The soil temperature was first observed above 0℃ in the early April, but always greater than 0℃ between May and Octerber which is main period of permafrost thawing.
Methane content and carbon isotopic composition of gas effusion from the gas hydrate drilling well DK-8.
The monthly variations in methane content and carbon isotopic composition of gas effusion from the well DK-8 during the period from January to August in 2017 are shown in Fig. 3. The measured methane contents showed a large variation from 1.904 ppm to 8.530 ppm, with the average of 2.675 ppm. The methane was constant at a low concentration of 1.967 ± 0.130 ppm from January to March and of 2.015 ± 0.108 ppm from June to August, whereas it increased obviously in April–May with the average of 4.409 ppm. The measured carbon isotope compositions of methane (δ13CCH4) were between − 49.6‰ and − 34.6‰ with the average of -45.2‰, simillar as the range of the headspace gases from gas hydrate-bearing drill cores retrieved from the depth of 104.6m to 397.99m of the DK-8 (generally from − 38.3‰ to -53.2‰ with the average of -43.6‰)23. They are charcterized by less negative δ13CCH4 values, which are typical of thermogenically-derived methane source (-48‰ to -35‰)33.
The monthly variation of the carbon isotopic composition was almost parallel with that of the methane content (Fig. 3a). And a positive relationship was observed between the methane contents and δ13CCH4 values (Fig. 3b). Generally, the higher content of methane had less negative δ13CCH4 vaule. The methane concentration suddenly increased to 2.920 ppm at the early April with δ13CCH4 of -39.9‰, and peaked at at 8.530 ppm in early May with δ13CCH4 of -40.8‰, and increased to 4.201 ppm at the late May with δ13CCH4 of -34.6‰. The less negative δ13CCH4 values indicated that the increase of methane concentration was attributed to release of substantial proportion of accumulated gas hydrate when the temperature rised above 0℃. Samples collected in late May had the highest δ13CCH4 whereas samples in June-July had the lowest δ13CCH4. The more negative δ13CCH4 values in June-July indicated bacterially-derived mechane that also contributed to the methane emission under the influence of elevated temperatures.
The carbon isotope separation factor (εC) between δ13CCO2 and δ13CCH4 of gas hydrates found in the Muli permafrost region ranged from 20‰ to 40‰, with values most commonly around 30‰ to 40‰ (Fig. 4). The εC values of gas hydrates can be indicative of thermogenically-derived methane, due to the distiguishing isotopic signature from the εC values associated with methanogenesis generally more than 40‰34. The εC vaules of gas effusion from the well DK-8 ranged between 29.2‰ and 39.5‰, especially the εC vaules in April-May closer to 30‰, further proving the release of gas hydrates. The exceptions of εC vaules (44.4 ~ 46.3‰) in June-July were more than 40‰, also indicating the contribution of biogenic methane.
Methane content and carbon isotopic composition of low-level air.
The monthly variations in methane content and carbon isotopic composition of low-level air in 2017–2018 are shown in Fig. 5a. The methane concentraions of low-level air samples ranged from 1.880 ppm to 2.048 ppm with an average of 1.933 ppm. The average of methane content was 1.933 ppm, which was slightly higher than that of the Waliguan station in Qinghai through 2017 (1.912 ppm) and significantly higher than the globally averaged methane content (1.859 ppm) reported by WMO in 2017. The δ13CCH4 values of low-level air samples were between − 50.4‰ and − 45.9‰, with an average of -48.7‰. The δ13CCH4 values in 2018 were slightly higher than in 2017. There is no sighnificant correlation between the methane concentrations and the carbon isotopic compositions. However, samples collected in summer had higher methane concentrations with more negative δ13CCH4 values. The εC values of low-level air ranged from 38‰ to 46‰, with an average of 41.5‰. It is worth mentioning that the methane content and δ13CCH4 value, as well as the εC value exhibited consistent variation at early April, characterized by higher CH4 concentration, less negative δ13CCH4 and lower εC value. This reflected the effect of methane emission from the gas hydrate drilling wells on the local low-level air CH4 concentration.
The methane content exhibited seasonal variation trend, with the highest mean values in summer, followed by spring and autumn, and the lowest mean values in winter (Fig. 5b). Samples collected in Summer had higher mean δ13CCH4 than in spring with a wider δ13CCH4 range from − 50.4‰ to -47.1‰, and had the highest mean εC values of 43.7‰ with a εC range between 40.5‰ and 46.3‰. The carbon isotopic compositions indicate the increase of CH4 concentration in low-level air was mainly contributed by biogenically derived methane. The obvious difference is that samples collected in autumn and winter had higher δ13CCH4 but lower εC than in summer, indicating non-negligible contribution from thermogenically derived methane.
Carbon isotopic composition of methane from the upper active layer of soil.
The results show the δ13CCH4 values of the upper active layer of soil through 2017 and 2018 ranging between − 53.2‰ and − 39.9‰ with an average of -47.7‰, and the εC values ranging between 28.6‰ and 47.9‰ with an average of 40.3‰. The montly variation trend of εC values of soil samples was opposite to that of the δ13CCH4 value (Fig. 6a). The εC value is higher when the δ13CCH4 value is lower, vice versa.This negative correlation reflects seasonal variation of methane source. Samples collected in spring and summer had lower δ13CCH4 values, whereas samples collected in autumn and winter had higher δ13CCH4 values (Fig. 6b). Especially, samples in winter had δ13CCH4 values heavier than − 50‰ and εC values less than 40‰, indicating the methane was dominated by thermogenic origin.
The combination plots of δ13CCH4 and δ13CCO2 with isotope fractionation lines (εC) are shown in Fig. 7, differentiating the major methane sources, methanogenic pathways and methane oxidation. The carbon isotope fractionation factor for methanogenis predoniminantly of carboate reduction most commonly from 49‰ to 100‰, with values most commonly around 65‰ to 75‰, and that dominated by by fermentation of methylated substrates are distinctively lower with εC values typically ranging between 40‰ to 55‰34. In comparison, the methane associated with thermogenic gas hydrates emission has distinctly lower εC values between 20‰ and 40‰, with the corresponding δ13CCH4 values heavier than − 50‰. The soil samples are plotted near the range of methane originated from methyl oxidation and gas hydrates, without noticeable CH4 oxidation. Some of samples collected in spring and summer plotted in the methyl oxidation zone indicated the soil methane being dominated by methanogenic source, and the others plotted between the zone of methyl oxidation and gas hydrates indicated another contribution of thermogenic production. In contrast, samples collected in winter were almost plotted in the zone of gas hydrates, indicating the soil methane being dominated by thermogenically-derived source. The two samples in autumn showing rising δ13CCH4 value and lowering εC from september to october indicated a decrease in bacterial activity and an increase in thermogenic production during the transition from summer to winter.