Coupling Processes and Dynamic Mechanisms of Methane Emissions and Surface Catastrophism in Permafrost Regions

The Earth’s cryosphere represents one of the most climate-sensitive and important global carbon reservoirs. Its carbon storage exists not only in the form of organic carbon within permafrost soils, but also as methane and other gases trapped in permafrost layers, underlying sediments, and frozen media. Climate change continues to threaten the stability of this vast carbon pool. The cryosphere is currently retreating at an average rate of approximately 8.7 × 10⁴ km²·a⁻¹, and nearly 80% of boreal forests are located within permafrost-affected regions. Over the past two decades, boreal forest fires have annually affected more than 1% of forested areas, with a continuously increasing frequency. Meanwhile, permafrost degradation has significantly intensified along the southern margin of the Eurasian permafrost zone, particularly in southeastern Siberia and Northeast China. This has triggered a range of geomorphological and geohazard processes, including landslides, debris flows, thermokarst subsidence, and thermokarst lake formation. Redistribution of pore pressure induced by permafrost meltwater migration and the release of stored gases is considered a key internal mechanism driving these enhanced surface responses. Among these gases, methane plays a dominant role, and its generation, migration, and release are critical processes governing permafrost system evolution. To address these scientific issues, a representative study area was selected in the permafrost swamp zone along the K153–K183 section of the Bei’an–Heihe Expressway at the margin of the Sunwu–Jiayin Basin in the northern Xiao Xing’an Mountains, Northeast China (127°17′31″–127°21′24″E, 49°30′57″–49°41′50″N). A comprehensive investigation was conducted focusing on permafrost degradation, methane emissions, and associated environmental effects.

In terms of observations and data acquisition, a multi-parameter monitoring system was deployed, including methane concentration sensors, atmospheric temperature sensors, pore water pressure sensors, atmospheric electric field instruments, and soil temperature sensors, enabling long-term continuous measurements. Integrated geophysical and remote sensing techniques, including high-density resistivity (HDR), ground-penetrating radar (GPR), field drilling, and MODIS data, were used to characterize permafrost distribution and degradation patterns. Sentinel-2 (L1C) imagery, UAV-based observations, and 3D reconstruction techniques were further employed to quantify surface deformation and landscape evolution. For atmospheric processes, AIRS (Aqua satellite) data were used to reconstruct the spatiotemporal distribution of tropospheric methane over Northeast China. A porous-media gas transport model was developed to simulate methane migration and diffusion under permafrost degradation conditions. Regarding fire response mechanisms, Landsat TM and Sentinel-2 data were used to analyze the spatial correlation between methane emissions and wildfires. Laboratory gas–solid frictional electrification experiments were conducted to verify charge generation induced by particle interactions, combined with in situ atmospheric electric field measurements to reveal potential seasonal wildfire triggering pathways. For surface deformation analysis, field monitoring data of landslides and subsidence were combined with laboratory triaxial shear tests and high-pressure methane exposure experiments (including methane hydrate dissociation processes) to systematically investigate deformation mechanisms induced by permafrost degradation. In terms of theoretical modeling, based on the strength reduction method, a fully coupled thermo–hydro–mechanical–chemical (THMC) framework considering methane effects was established. Incorporating methane hydrate dissociation kinetics, a failure model of overburden instability driven by meltwater–gas migration in permafrost degradation zones was developed, followed by numerical simulations.

The main findings are as follows:

(1) Since 2004, permafrost in the Xiao Xing’an Mountains has undergone significant degradation, characterized by a continuous rise of the lower permafrost boundary and a gradual lowering of the upper boundary, with the upward migration of the lower boundary being more pronounced, indicating progressive structural instability.

(2) Tropospheric CH₄ concentrations in Northeast China exhibit significant seasonal variability, with the highest growth rate in spring (6.120 ppbv·a⁻¹ at 600 hPa), followed by autumn and winter (5.830 and 5.470 ppbv·a⁻¹, respectively), and the lowest in summer (4.030 ppbv·a⁻¹). Degrading permafrost regions represent an important methane source, including microbial production, wetland transport, and geological methane stored in frozen layers (metastable and stable methane hydrates, as well as deep thermogenic gas).

(3) Methane emissions in the study area exhibit a distinct three-stage pattern: a short-duration high-intensity release in spring (March–May, ~5 days, surface concentrations exceeding 1000 ppm); a long-duration stable release in summer (June–August, >2 months, up to ~453 ppm); and a short episodic release in autumn (September–November, ~3 days cycle, up to ~326 ppm).

(4) In addition to lightning and anthropogenic ignition sources, atypical wildfires occur in permafrost swamp regions during spring. These fires are predominantly observed in low-temperature, high-humidity lowland areas, showing a significant correlation with methane emissions (maximum correlation coefficient up to 0.7). Methane release and associated near-surface charge accumulation enhance static electricity buildup, facilitating ignition of combustible gases and vegetation residues. Under high methane concentrations, lightning or anthropogenic sparks more readily trigger combustion. Moreover, methane–aerosol interactions (dust, sea salt, sulfate, and black carbon) enhance radiative forcing, further intensifying regional warming and fire susceptibility.

(5) With continued permafrost degradation, subsurface methane and water migrate upward, forming localized high pore-pressure zones, exhibiting a “confined gas pressure accumulation effect.” Increased pore pressure and changes in permeability reduce effective stress, frictional strength, and cohesion, leading to structural failure and triggering landslides, differential settlement, and localized uplift. Results indicate that when the water–gas reduction coefficient decreases from 1.0 to 0.72, the slope stability factor reduces from 1.16 to 0.94; when methane pressure increases from 0.11 to 2.14, the failure influence radius increases from 1.62 to 2.52.