Spatiotemporal landscape dynamics drive organic matter mobilization and degradation in thermokarst terrain of the southern continuous permafrost zone

The permafrost carbon cycle in Arctic coastal plains is strongly affected by abrupt thaw processes such as lake and hillslope thermokarst, which are expected to intensify under continued climate warming. Organic matter (OM) degradation may increase if taliks beneath drained lake basins (DLBs) fail to refreeze and retrogressive thaw slump activity accelerates. Thus, we investigated a coastal transect on the Baldwin Peninsula in northwest Alaska spanning a Yedoma upland, a thermokarst lake, a semidrained lake, a DLB, and a nearshore marine unit in proximity of a thaw slump. Using a 70-year aerial imagery analysis (1952–2022) with biogeochemical investigations of sediment cores (0–1 m), we assessed landscape and OM dynamics. We identified two dominant OM mobilization and decomposition trajectories. First, talik conditions favor OM degradation beneath the lake and the semidrained lake basin, where the talik did not completely refreeze for approximately 60 years following partial drainage. Second, thaw slump driven erosion mobilizes prealtered OM from the inland and transfers it into the marine environment, where further degradation occurs during sediment transport and reworking. In contrast, the upland and refrozen DLB currently preserve and sequester OM; yet, high ground ice contents render these systems vulnerable to future abrupt thaw.