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Origin of methane seeping in West Siberian middle taiga river floodplainsтезисы доклада
- Авторы: Sabrekov A., Terentieva I., Filippov I., Glagolev M., Litti Y.
- Сборник: "Focus Siberian Permafrost – Terrestrial Cryosphere and Climate Change" International Online Symposium (Institute of Soil Science, Universitat Hamburg 24 – 25)
- Тезисы
- Год издания: 2021
- Место издания: Universitat Hamburg Hamburg
- Первая страница: 82
- Последняя страница: 82
- Аннотация: Recently, numerous methane seeps were found in West Siberian middle taiga (WSMT) river floodplains. Seepage occurs on unvegetated flat areas of saturated quicksand deposits and on the bottom of river stream beds. In order to estimate the importance of WSMT seeps for the regional methane budget, it is necessary to reveal the origin of hydrocarbons emitting from them. Three hypotheses of origin were considered for seeping methane: uplift of ancient thermogenic methane from oil and gas reservoirs, release of ancient biogenic methane from degrading relict permafrost in Oligocene deposits, horizontal transport of modern biogenic methane from raised bogs through ground waters in Quaternary deposits. Study sites were chosen in a way to cover both banks of Ob’ and Irtysh in both north-south (250 km) and east-west directions (400 km) using an available road net. For each site, the following samples were taken in August and September of 2019-2020: gas from seeps (for methane concentration, stable isotope ratios of C and H in methane, and radiocarbon content in methane); water flowing from seep (for pH, electrical conductivity (EC), dissolved methane and oxygen concentrations); water from streams and rivers all over the region (for dissolved methane concentration, pH and EC). Water samples were also taken from wetlands (at the depth of 1-2 meters) to measure dissolved methane concentration and stable isotope ratios of C and H in methane. Water from three boreholes (25-, 70- and 210-meters deep household wells) in Shapsha village was taken to measure pH, EC, methane concentration, stable isotope ratios of C and H, and radiocarbon content in a gas dissolved in groundwater. Obtained results argue in favour of the third hypothesis of origin. Seeping methane is modern and biogenic. Stable isotope ratios for the seep gas of the highest methane concentration correspond to the methane sampled in wetlands. With linear depleting in heavier isotopes, methane concentration also linearly decreases. Linear correlation between delta13C(CH4) and deltaD(CH4) with a slope of 0.310 ± 0.019 was observed for seep methane. This “local biogenic methane line” could show that a) methane in all seeps is of the same origin; b) this fractionation is mainly driven by gas/liquid phase equilibration. Methane from groundwater had close values of delta13C(CH4) and deltaD(CH4) with methane from seeps. This fact could show that there is intensive water exchange between Oligocene and Quaternary aquifers and surface seepage can occur from both. This idea explains seep finding far in the Irtysh floodplain, where several river channels flow between the wetland on watershed (or terrace) and seeps. Dissolved methane concentration in seeps and rivers of the study region reaches high values (up to 61% of the total saturation capacity under atmospheric pressure) and correlates with EC of water. It provides clear evidence that methane transport is directly linked to groundwater dynamics across the whole region. This study was supported by a grant of the Russian Science Foundation (№ 19-77-10074).
- Добавил в систему: Глаголев Михаил Владимирович