Methane in soil gas and its transfer to the atmosphere in the Yakela condensed gas field in the Tarim Basin,Northwest China

In this study, by analyzing CH4 concentration and 613CCH4 in soil-gas profiles, the potentials of CH4 gas transfer from ground to atmosphere were studied at four representative sectors in the Yakela condensed gas field in the Tarim Basin, Xinjiang, China. These are： 1） the oil-gas interface sector, 2） fault sector, 3） oil-water interface sector, 4） an external area. Variation in CH4 in soil-gas profiles showed that CH4 microseepage resulted from the migration of subsurface hydrocarbon from deep-buried reservoirs to the earth＇s surface. It was found that CH4 from deep-buried reservoirs could migrate upwards to the surface through faults, fissures and permeable rocks, during which some CH4 was oxidized and the unoxidized methane remained in the soil or was emitted into the atmosphere. The lowest level of CH4 at the soil-gas profile was found at the CH4 gas-phase equilibrium point at which the CH4 migration upwards from deep-buried reservoirs and the CH4 diffusion downwards from the atmosphere met. The 613CcH4 and ethane, propane in soil gas exhibited thermogenic characteristics, suggesting the occurrence of CH4 microseepage from deep-buried reservoirs. A linear correlation analysis between CH4 concentrations in soil gas and temperature, moisture, pH, Eh, Ec and particle size of soil indicated that both soil Eh and soil temperature could affect CH4 concentration in soil gas while soil pH could indirectly influence soil methanotrophic oxidation via impacting soil Eh.

Variation of CH_4 concentrations recorded in Dunde ice core bubbles

GREENHOUSE gases such as CH4 in the ancient atmospheric concentration can be reconstructed by extracting and analyzing the gases trapped in polar ice core bubbles. It is a new attempt to extract and analyze the CH4 concentration in Dunde ice core for the study of mountain glacier. Samples were taken from the about 140-m ice core drilled in 1987 in Dunde ice cap in Qilian Mountain. The ice core has been well preserved in cold room. 12 samples were taken from the upper 95 m, and cut to 15 cm in length and 5cm in diameter. CH4 concentrations were measured in CNRS Laboratory of Glaciology, Grenoble, France.

Effects of elevated atmospheric CO_2 concentration and temperature on the soil profile methane distribution and diffusion in rice–wheat rotation system

The aim of this experiment was to determine the impacts of climate change on soil profile concentrations and diffusion effluxes of methane in a rice-wheat annual rotation ecosystem in Southeastern China. We initiated a field experiment with four treatments：ambient conditions（CKs）, CO2 concentration elevated to - 500 μmol/mol（FACE）,temperature elevated by ca. 2°C（T） and combined elevation of CO2 concentration and temperature（FACE ＋ T）. A multilevel sampling probe was designed to collect the soil gas at four different depths, namely, 7 cm, 15 cm, 30 cm and 50 cm. Methane concentrations were higher during the rice season and decreased with depth, while lower during the wheat season and increased with depth. Compared to CK, mean methane concentration was increased by 42%, 57% and 71% under the FACE, FACE ＋ T and T treatments, respectively, at the 7 cm depth during the rice season（p 〈 0.05）. Mean methane diffusion effluxes to the 7 cm depth were positive in the rice season and negative in the wheat season, resulting in the paddy field being a source and weak sink, respectively. Moreover, mean methane diffusion effluxes in the rice season were 0.94, 1.19 and 1.42 mg C/（m^2·hr） in the FACE,FACE ＋ T and T treatments, respectively, being clearly higher than that in the CK. The results indicated that elevated atmospheric CO2 concentration and temperature could significantly increase soil profile methane concentrations and their effluxes from a rice-wheat field annual rotation ecosystem（p 〈 0.05）.

Dasuopu ice core record of atmospheric methane over the past 2000 years

The concentrations of CH4 in the atmosphere over the past 2000 years have been deduced by extracting and analyzing the air in bubbles embedded in the Dasuopu ice core, Qing-hai-Tibetan Plateau. Upon analyzing 57 ice core samples we found that the concentration of CH4 200 years ago and earlier was 0.85 μ mol· mol-1 or about 40% of present atmospheric CH4 levels over Qinghai-Tibetan Plateau. A rapid and significant increase of atmospheric CH4 started about 200–250 a ago. For a given age before 19th century, the Dasuopu CH4 concentrations were about 15%-20% higher than those in Antarctic and Greenland references. It was also found that the Dasuopu CH4 concentrations changed more frequently, and its fluctuations could reflect the temperature change sensitively.