Raised bogs are the key source of methane in West Siberia terrestrial seeps

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.