Transgression and Transformation: Methane Cycling in Thawing Arctic Coastal Landscapes

About one-third of the world’s coastline is classified as permafrost. Increased erosion promotes abrupt thaw along these shorelines, leading to remobilization and enhanced microbial degradation of previously stored organic matter. With rising sea levels, coastal thermokarst lakes may be inundated by seawater, causing a gradual transformation into marine-influenced lagoons. Depending on the connectivity of the lagoon to the open sea, methanogenic communities are exposed to high concentrations of ions, especially sulfate, which promotes the establishment of competitive anaerobic methanotrophic archaea and sulfate-reducing bacteria consortia and thus leads to shifts in the composition and activity of the microbial community (Yang et al., 2023). Previous research focusing on surface sediments from lagoon systems in the Canadian Arctic found the highest total greenhouse gas production in the initial stage of the transition (Jenrich et al., 2025) while surface soil samples from a land-sea transect showed highest methane production rates in the active layer of the intertidal zone (Roy-Lafontaine et al., 2025). However, questions remain regarding the effects of marine inundation on the microbial community and the associated carbon dynamics of erosion-affected coastal environments. Here, we use vertical sediment profile incubations from thermokarst lakes, the coastal ocean, and soils from an intertidal zone near the community of Tuktoyaktuk, located in the Inuvialuit Settlement Region (NWT, Canada). We performed anoxic long-term incubation experiments under in situ (freshwater) and marine conditions to simulate saltwater intrusion. Corresponding methane and carbon dioxide production rates were monitored by monthly measurements. In addition, we analyzed sedimentary pore-water nutrient and metal concentrations, along with bulk organic matter characteristics (TOC, δ¹³C, lability), to examine potential relationships between initial redox conditions, organic matter quantity and quality, and greenhouse gas production. Additionally, we investigated potential shifts in the microbial community during the incubation by 16S rRNA sequencing. Preliminary results of the first months of incubation indicate that freshwater lakes located further away from the coastline show higher production rates under in situ compared to marine conditions. In contrast, negligible production rates were found for a marine-influenced lagoon. This pattern suggests a shift in the microbial community from a dominance of methanogens in freshwater lakes to the establishment of methanotrophs as a consequence of increased marine influence. As a result, rising sea levels may decrease methane emission rates from coastal lakes.

References:

Jenrich, M., et al. (2025). Biogeosciences, 22, 2069–2086.

Roy-Lafontaine, A., et al. (2025) EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-2570.

Yang, S., A et al. (2023). Global Change Biology, 29, 2714–2731.