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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 sub-sea 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 the 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 g−1 C in younger, less connected lagoons with high-quality organic matter, leading to up to 18 times higher greenhouse gas (GHG) production (in CO2 equivalents) compared to open lagoons. CO2 production is higher under marine conditions (3.8–5.4 mg CO2 g−1 C) than under brackish conditions (1.7–4.3 mg CO2 g−1 C). 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.

Climate warming in the Arctic results in thawing permafrost and associated processes like thermokarst, especially in ice-rich permafrost regions. Since permafrost soils are one of the largest organic carbon reservoirs of the world, their thawing leads to the release of greenhouse gases due to increasing microbial activity with rising soil temperature, further exacerbating climate warming. To enhance the predictions of potential future impacts of permafrost thaw, a detailed assessment of changes in soil characteristics in response to thermokarst processes in permafrost landscapes is needed, which we investigated in this study in an Arctic coastal lowland. We analysed six sediment cores from the Arctic Coastal Plain of northern Alaska, each representing a different landscape feature along a gradient from upland to thermokarst lake and drained basin to thermokarst lagoon in various development stages. For the analysis, a multiproxy approach was used, including sedimentological (grain size, bulk density, ice content), biogeochemical (total organic carbon (TOC), TOC density (TOCvol), total nitrogen (TN), stable carbon isotopes (δ13C), TOC/TN ratio, mercury (Hg)), and lipid biomarker (n-alkanes, n-alkanols, and their ratios) parameters. We found that a semi-drained state of thermokarst lakes features the lowest OC content, and TOC and TN are generally higher in unfrozen deposits, hinting at a more intact state of organic matter. Indicated by the average chain length (ACL), δ13C, Paq, and Pwax, we found a stronger influence of aquatic organic matter (OM) in the OM composition in the soils covered by water compared to those not covered by water. Moreover, the results of the δ13C, TOC/TN ratio, and CPI indicate that the saline deposits contain stronger degraded OM than the deposits not influenced by saltwater. Additionally, we found positive correlations between the TOC and TOCvol and the Hg content in the deposits. The results indicate that thermokarst-influenced deposits tend to accumulate Hg during thawed periods and thus contain more Hg than the upland permafrost deposits that have not been impacted by lake formation. Our findings offer valuable insights into the dynamics of carbon storage and vulnerability to decomposition in coastal permafrost landscapes, reflecting the interplay of environmental factors, landform characteristics, and climate change impacts on Arctic permafrost environments.

In Arctic coastal lowland regions such as northernmost Alaska, thermokarst landscapes are often underlain by saline deposits, a factor frequently overlooked when assessing permafrost thaw risks. To evaluate the influence of thaw and salinity on organic matter degradation and landscape dynamics, we analyzed six sediment cores from representative landforms near UtqiaÄ¡vik (Alaska) using a multiproxy, carbon-focused approach, with emphasis on n-alkane biomarkers. Undisturbed tundra uplands contained well-preserved, organic-rich Holocene sediments (â¼140 cm thick) overlying brackish late Pleistocene deposits, indicating the presence of saline permafrost. Thermokarst lake subsidence into these substrates led to enhanced organic matter degradation, as reflected by lower n-alkane carbon preference index (CPI) values. While West Twin Lake talik sediments exhibited brackish porewater, East Twin Lake sediments were characterized by predominantly saline porewater, indicating the presence of a cryopeg driven by salt-induced thaw-point depression. Lagoonal environments, receiving both terrestrial and lacustrine inputs, accumulate sediments under unfrozen hypersaline conditions, presenting a high potential for organic matter decomposition. Carbon proxy signatures statistically distinguish perennially frozen uplands, unfrozen lake sediments, refrozen drained lake basins, and lagoonal settings. Our results demonstrate that salt-bearing deposits, as found in all investigated sites, are vulnerable to active layer deepening, talik and cryopeg formation, and lake/coastal shoreline erosion. These processes accelerate organic matter degradation and alter landscape trajectories. Our study underscores the need to better understand the role of saline permafrost in Arctic coastal lowlands and its broader implications under ongoing climate change.

Thermokarst lagoons represent the transition state from a freshwater lacustrine to a marine environment, and receive little attention regarding their role for greenhouse gas production and release in Arctic permafrost landscapes. We studied the fate of methane (CH4) in sediments of a thermokarst lagoon in comparison to two thermokarst lakes on the Bykovsky Peninsula in northeastern Siberia through the analysis of sediment CH4 concentrations and isotopic signature, methane-cycling microbial taxa, sediment geochemistry, and lipid biomarkers. We specifically assessed whether sulfate-driven anaerobic methane oxidation (S-AOM) through anaerobic methanotrophic archaea (ANMEs), common in marine sediments with constant supply of sulfate and methane, establish after thermokarst lagoon development and whether sulfate-driven ANMEs consequently oxidize CH4 that would be emitted to the water column under thermokarst lake conditions. The marine-influenced lagoon environment had fundamentally different methane-cycling microbial communities and metabolic pathways compared to the freshwater lakes, suggesting a substantial reshaping of microbial and carbon dynamics during lagoon formation. Anaerobic sulfate-reducing ANME-2a/2b methanotrophs dominated the sulfate-rich sediments of the lagoon despite its known seasonal alternation between brackish and freshwater inflow. CH4 concentrations in the freshwater-influenced sediments averaged 1.34±0.98 µmol g−1, with highly depleted δ13C-CH4 values ranging from -89‰ to -70‰. In contrast, the sulfate-affected upper 300 cm of the lagoon exhibited low average CH4 concentrations of 0.011±0.005 µmol g−1 with comparatively enriched δ13C-CH4 values of -54‰ to -37‰ pointing to substantial methane oxidation. Non-competitive methylotrophic methanogens dominated the methanogenic community of the lakes and the lagoon, independent of porewater chemistry and depth. This potentially contributed to the high CH4 concentrations observed in all sulfate-poor sediments. Our study shows that S-AOM in lagoon sediments can effectively reduce sediment CH4 concentrations and we conclude that thermokarst lake to lagoon transitions have the potential to mitigate terrestrial methane fluxes before thermokarst lakes fully transition to a marine environment.