The expansive plains of western Siberia contain globally significant carbon stocks, with the largest peatland complex in the world overlying the planet’s largest known hydrocarbon basin. Numerous terrestrial methane seeps have been recently discovered on this landscape, located along the Ob and Irtysh River floodplains in hotspots covering more than 2,500 km². The origin of methane from these seeps is a matter of both practical and academic interest. The release of even negligible portions of western Siberia’s vast carbon pool will have global climate implications. We articulated three hypotheses to explain the origin and migration pathways of methane within these seeps: (H1) uplift of Cretaceous-aged methane from deep petroleum reservoirs along faults and fractures, (H2) release of Oligocene-aged methane capped or trapped by degrading permafrost, and (H3) horizontal migration of Holocene-aged methane from surrounding peatlands. We tested these hypotheses using a range of geochemical tools on gas and water samples extracted from seeps, peatlands, and aquifers across the 120,000 km² study area. Seep-gas composition, radiocarbon age, and stable isotope fingerprints favor the peatland hypothesis of seep-methane origin. We identified the dominant metabolic pathways of mid Holocene-aged ¹⁴CH₄, absent of oxidation, all the way from raised bogs to seeps along the floodplains of the Ob and Irtysh Rivers. Observed ¹³C-depletion of methane, along with concentration decreases between source and seeps, could be associated with mixing between two sources with different conditions for methane production: raised bogs with CO₂ reduction methanogenesis and groundwater with acetate fermentation methanogenesis. Our findings highlight the importance of lateral migration between typical boreal landscapes via groundwater, implying intimate connections between them. Lateral migration may result in high methane emissions from groundwater-fed rivers of the region. These methane hotspots could be potentially overlooked in the West Siberia Lowlands and other bog-dominated regions.

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