Effect of Sea Water Inundation on CO₂ and CH₄ Production of Thawing Coastal Permafrost Near Utqiaġvik, Alaska

Rising sea levels and changing marine dynamics are increasing the inundation of previously terrestrial permafrost, accelerating thaw and altering microbial carbon cycling. On the Arctic Coastal Plain of Alaska, permafrost features like drained lake basins (DLBs) and uplands (ULs), offer distinct redox conditions and formation histories that may influence the carbon cycling response to sea water inundation. This study investigates changes in CO 2 and CH 4 production during potential seawater inundation using ex situ anaerobic incubations of soils from a DLB and a UL near Utqiaġvik, Alaska. Results showed that CO 2 and CH 4 production was, respectively, up to 2.2 and 3.3 times higher in the DLB site than in the UL site with the salt‐less control treatment. The addition of artificial sea water inhibited CH 4 production at both sites. CO 2 production increased in the active and permafrost layers of the UL site, decreased in surface layers of both sites, and remained unaffected by saltwater treatments in the permafrost layers of the DLB site, likely due the presence of marine sediments in the DLB. Carbon availability, microbial adaptation, and electron acceptors are potential factors for the CO 2 and CH 4 response. Overall, the results of this study showed that anaerobic CO 2 production responded differently to sea water at different landscape positions and formation histories while CH 4 was inhibited independently of the landscape position. Those results highlight the need to consider local hydrology and landscape history in future GHG projections for coastal permafrost.