Rising Arctic Seas and Thawing Permafrost: Uncovering the Carbon Cycle Impact in a Thermokarst Lagoon System in the outer Mackenzie Delta, Canada

Abstract. Climate warming in the Arctic is directly connected to rising sea levels and increasing erosion of permafrost coasts, leading to inland migrating coastlines and the transformation of coastal permafrost lakes into thermokarst lagoons. These lagoons represent transitional zones between terrestrial to subsea permafrost environments. So far, the effect of the transition on the carbon cycle is fairly unknown. In this study, we conducted long-term anoxic incubation experiments on surface samples from thermokarst lagoons with varying degrees of sea connectivity. We also included terrestrial permafrost and active layer as endmembers to investigate variations in carbon dioxide (CO₂) and methane (CH₄) production within lagoon systems and along a land-sea transition transect on Reindeer Island, Northeast Mackenzie Delta, Canada. Results show that CH₄ production peaks at 4.6 mg CH4 gC⁻¹ in younger, less connected lagoons with high-quality organic matter, leading to up to 18 times higher GHG production (in CO₂ equivalents) compared to open lagoons. CO₂ production is higher under marine conditions (3.8 to 5.4 mgCO₂ g^-1C) than under brackish conditions (1.7 to 4.3 mgCO₂ g^-1C). Along a land-sea transect, CO₂ production increased with increasing marine influence. These findings suggest that the landward migration of the sea, resulting in the inundation of permafrost lowlands and thermokarst lakes, may lead to increased GHG emissions from Arctic coasts in the future.