Permafrost thaw increases CH4 fluxes in disturbed Yedoma tundra landscapes in Western Alaska

Permafrost thaw features –such as thermokarst lakes, retrogressive thaw slumps, and thermo-erosional gullies– are becoming increasingly widespread across the Arctic as a direct result of warming global temperatures. Permafrost underlies 15% of the land surface area in the Northern Hemisphere, and globally is estimated to store substantially more carbon (1,300 Pg of C) than all the world's forests. Despite the massive amount of permafrost soil carbon, little is known about the processes that spur microbial respiration as the soils transition to a post-thaw state, resulting in increased contributions of greenhouse gas emissions. Understanding these processes is particularly important in regions underlain by late Pleistocene, ice-rich Yedoma deposits that contain large amounts of buried, poorly decomposed organic matter. To investigate the potential greenhouse gas (GHG) contribution of these thaw features in Yedoma landscapes, we sampled carbon dioxide and methane gas fluxes across eight thaw transects on the Baldwin Peninsula (Western Alaska) in the summers of 2023 and 2024. We used manual chamber measurements to measure the in-situ GHG fluxes and recorded site parameters to categorize the extent of thaw disturbance (including topography, active layer depth, soil moisture, vegetation, and more). 

 

From these measurements, we found that permafrost thaw increased CH₄ fluxes exponentially relative to adjacent undisturbed tundra, with mean fluxes ranging from 0.6 to 7.0 mg CH₄-C m^-2 d^-1 and 25.6, 67.4, 71.9 mg CH₄-C m^-2 d^-1 in recently drained thermokarst lake basins, thaw ponds, and thermo-erosional gullies, respectively. Almost all sites were net sources of methane to the atmosphere, with the exception of two upland measurements in minimal thaw disturbance landscapes (CH₄ fluxes were < -0.3 mg CH₄ m^-2 d^-1 representing net CH₄ oxidation). Across all landscapes included in the sampling, retrogressive thaw slumps, representing freshly exposed Yedoma deposits, had the highest mean CH₄ flux among the other disturbed permafrost landforms (159 mg CH₄-C m^-2 d^-1). Though undisturbed upland sites are generally thought to be net sinks of carbon, our measurements show that methane emissions were ubiquitous – even in well aerated sites, suggesting complex or inhomogeneous subsurface conditions. In these undisturbed sites, ecosystem respiration fluxes from sites that had vegetation were not significantly different than those with bare soil, suggesting that it was not the upland plant life driving net GHG patterns. Rather, we propose that the underlying carbon-rich Holocene soils and late Pleistocene Yedoma deposits provide a consistent trickle of methane and CO₂ to the surface. This study provides field data on CH₄ and CO₂ fluxes across multiple thaw landforms in a Western Alaska Yedoma landscape, highlighting the potential role of deep carbon mobilization across a gradient of disturbance.