Methane emission from seeps of West Siberian middle taiga river floodplains

Natural gas seepage is considered as a strong methane source influencing future climate predictions and creating local fire and explosion hazards. Recently numerous methane seeps were found in West Siberian middle taiga (WSMT) river floodplains. Seepage occurs in unvegetated spots and areas of saturated quicksand deposits and in the bottom of river stream beds. Based on location and size seeps found in WSMT can be classified in three groups: 1) single seeps and seep puddles away from river stream bed, 2) seep chains along the river streams, including seeps inside stream bed, 3) seep fields with numerous holes, funnels and craters with area up to several thousands m². Origin of methane in these seeps is still not fully understood and methane emission from them is also not quantified. We aimed to fill these gaps during 2019-2020 August-September field campaigns.

To reveal the origin of seeping gas hydrocarbons concentration, methane carbon and hydrogen stable isotope ratios and methane radiocarbon concentration were measured in a gas sampled throughout the study region (250 km in north-south and 400 km in east-west directions). To compare characteristics of methane from seeps and from wetlands gas dissolved in wetland pore water and peat for incubation studies were sampled on a depth of 1-2 m in three representative bogs near seeps. During anaerobic peat incubation the dynamics of methane (including carbon stable isotope ratios), hydrogen, carbon dioxide and low-molecular fatty acids concentrations were monitored. Methane emission was estimated in the same three bogs using both static chamber method (provided a benchmark) and ecosystem-scale inverse modelling (backward Lagrangian simulation).

Obtained data indicate modern biogenic origin for the methane seeping in WSMT. Similarity in isotope signatures between methane from wetlands and seeps suggests lateral transport of methane through groundwater from raised bogs to seeps in floodplains. Methane produced in the upper layer of raised bogs emits to the atmosphere mainly through the root transport while in deeper layers vertical methane migration is limited. Raised bogs are widely developed in WSMT and cover about half of the total region area. We conclude that methane seeping observed in WSMT floodplains is caused by the lateral groundwater transport downward from the watershed to the floodplain.

Methane emission estimates for three seep fields made by chamber method and inverse modelling were in a good correspondence, although inverse modelling fluxes are 20-40% higher. Methane flux for investigated fields ranges from tens to hundreds mgCH₄·m^-2·h^-1 with a strong difference (up to order of magnitude) between different fields.

This study improves understanding of the insufficiently investigated element of the methane biogeochemical cycle – transport through the groundwater when methane avoids oxidation in the unsaturated surface layer of the wetland. Emission from seeps can make a valuable contribution to the regional methane flux. Potential reaching of a lower explosive limit for methane concentration should be taken into account during planning of groundwater use.

This study was supported by a grant of the Russian Science Foundation (№ 19-77-10074).