Thermokarst processes strongly reshape permafrost landscapes in the Arctic, mobilizing permafrost carbon. In this study, we investigated a coastal lowland transect on the Baldwin Peninsula (northwestern Alaska) spanning a sequence of five landscape units representing different stages of thermokarst, from rather intact Yedoma upland (UL) to thermokarst lake (L), semi-drained (SDL) and completely drained lake basins (DLB), and a coastal retrogressive thaw slump (RTS) connecting to the marine environment. Combining high-resolution aerial imagery (1952–2022) with geochemical and biomarker analyses, we reconstructed spatiotemporal landscape evolution over decadal time scales. Based on organic matter (OM) quality (molecular composition and pre-thaw alteration) and near-surface organic carbon inventories (0–1 m), we found that OM degradation and its potential mobilization are strongly controlled by landscape context: persistent talik conditions in the SDL for ~60 years after drainage likely sustained OM degradation, whereas rapid permafrost aggradation in the DLB stabilized existing OM and new sequestration (peat). Accelerating RTS retreat (8 m yr−1, 2020-2022) may drain the thermokarst lake within the next years, releasing labile OM from lacustrine and OC-rich Yedoma deposits. This case study illustrates how thermokarst evolution governs the distribution and potential fate of OM in the continuous permafrost zone, providing empirical evidence of incomplete talik refreezing even after ~60 years of post-drainage exposure (in SDL) and sustained carbon loss. Here, at the southern margin of the continuous permafrost zone and a setting of rather warm ground temperatures, this may indicate a systemic shift of freshly drained DLB systems from temporary carbon sinks (due to refreezing taliks) to persistent sources (due to not refreezing taliks anymore) under ongoing warming.