Changing greenhouse gas production within a thermokarst lagoon system, Reindeer Island, Mackenzie Delta, Canada

The permafrost carbon pool is an important storage of the terrestrial carbon cycle that is at risk as the Arctic rapidly warms. Accordingly, in 2019, the United Nations Environmental Program identified permafrost thaw as one of the top five emerging environmental issues of global concern (UNEP, 2019).\nIn addition to increasing microbial decomposition of organic material and greenhouse gas release, permafrost thaw also leads to surface changes. Thermokarst lakes and basins are the result of the decrease in soil volume by melting ice in the subsurface. Rising sea levels and coastal erosion lead to the flooding of thermokarst lakes or drained lake basins along the ice-rich permafrost coasts of Siberia, Alaska and Canada, leading to the formation of thermokarst lagoons. These Arctic lagoons form a transition zone between the terrestrial and marine permafrost regime and represent an ideal research object for how permafrost carbon is affected by increasingly marine conditions. Due to current and future climate change in the Arctic, it is expected that the formation and development of thermokarst lagoons will accelerate (Jenrich et al. 2021). So far, thermokarst lagoons and their role in climate change have hardly been explored. To investigate the greenhouse gas production under varying degrees of seawater influence, and thus to assess whether the organic material in thermokarst lagoons is degraded on different temporal scales, we incubated the surface sediment below the lagoons with artificial sea water at two concentrations (brackish 13 g/L and marine 36 g/L) anaerobic at 4°C for 1 year. Here brackish conditions are considered as near natural conditions and represent the greenhouse gas production in the current state, while marine conditions represent the greenhouse gas production after the transition into a subsea state. \nFirst results of the incubation experiment show that the greenhouse gas production is depending more on the location, thus microbial community and/or carbon degradability, than the salinity treatment. Highest methane and carbon dioxide production was measured at location 13, which is the youngest lagoon, least connected to the sea.\nIn conclusion, we expect that coastal permafrost erosion is leading to higher sediment and organic carbon input and newly formed thermokarst lagoons produce more greenhouse gases than older, more connected lagoons.

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