Methane Seeps in West Siberian Middle Taiga River Floodplains: Origin and Fluxes

Abstract:

Monitoring of methane emission from various sources is necessary for prediction of global climate changes. Methane seepage is considered as the second most important natural methane source after wetlands. There two sources of methane, seeping in natural environments: hydrocarbon reservoirs and melting permafrost, which releases both methane and organic matter converted into methane by microorganisms. Both these sources are common in the middle taiga of Western Siberia, where many active seeps (from single ones to fields with an area of hundreds of m²) were found in the river floodplains. Another possible CH₄ source in this region is widespread raised bogs. Methane from them can move laterally through the groundwater and released as bubbles in seeps located lower in the landscape.

To quantify emissions and test the hypotheses on the origin, gas samples from seeps were taken in the region during the 2019-2020 field campaigns, the CH₄ concentration and the stable isotope ratios of carbon and hydrogen in methane were measured. Laboratory anaerobic incubation of the peat sampled from the bogs closest to the seeps was carried out to obtain the stable isotope ratios of CH₄ produced in them. The methane flux from the Bolshaya Rechka River seep field and in upland ecosystems around it was measured by the static chamber method. The concentration of dissolved CH₄ in the water leaking out from the seeps, as well as in the rivers (both with and without seeps) and in the bogs closest to the seeps was also measured.

Stable isotope values for methane (δ¹³C from -75 ‰ to -97 ‰) show that CH₄ in seeps is biogenic, which excludes the hypothesis of the CH₄ uplift from hydrocarbon reservoirs. The concentrations and δ¹³C of methane from seeps and wetlands did not differ significantly. δ¹³C for methane produced in incubation experiments was on the upper bound of the range for gas from seeps (-75 ± 1.2 ‰).

Methane fluxes from the seep field varied from 15 to 2727 mg·m^-2·h^-1 with a median of 340 mg·m^-2·h^-1. Methane microseepage from upland ecosystems surrounding the floodplain with a seep field was not found. The concentration of dissolved CH₄ in rivers with seeps was 10 times higher than in rivers without them. Thus, these seeps are a significant source of methane into the atmosphere.

This study was supported by a grant of the Russian Science Foundation (19-77-10074).