Seasonal variations of geofluids from mud volcano systems in the Southern Junggar Basin, NW China

Abstract

Variations in the chemical composition of geofluids and of gas fluxes are significant parameters for understanding mud volcanism and correctly estimate their emissions in carbon species, particularly greenhouse gas, methane. In this study, muddy water and gas samples were collected from the Anjihai, Dushanzi, Aiqigou, and Baiyanggou mud volcanoes in the southern Junggar Basin during the four seasons, around a year. This region hosts the most active mud volcanism throughout China. Gas and water were analyzed for major molecular compositions, carbon and hydrogen isotopes of the gas phase, as well as cations and anions, hydrogen and oxygen isotopes of water. The emitted gases are dominated by CH4 with some C2H6, CO2, and N2. The seasonal changes in the chemical composition and carbon isotopes of emitted gases are not significant, whereas clear variations in the amounts of cations and anions dissolved in the water are reported. These are higher in spring and summer than autumn and winter. The CH4, CO2, and C2H6 fluxes are 157.3–1108 kg/a, 1.8–390.1 kg/a, and 10.2–118.7 kg/a, respectively, and a clear seasonal trend of the gas seepage flux has been observed. In January, the macro-seepage flux of open vents is ≥65 % higher than in April, whereas the micro-seepage flux significantly decreased, probably due to the frozen shallow ground and blockage of soil fractures around the vents by heavy snow and ice during January. This probably causes an extra gas pressure transferred to the major vents, resulting in higher flux of the macro-seepage in the cold season. However, the total flux of the whole mud volcano system is generally consistent around a year.